The Pennsylvania State University
The Graduate School
Department of Agronomy
•
Vegetative and Reproductive Development of Corn (Zea mays L.)
at Four Spring Planting Dates
A Thesis in
Agronomy
by
N'guettia René Y.o
•
Submitted in Partial Fulfillment
of the Requirements
for the Degree of
Master of Science
November 1980
l grant The Pennsylvania State University the nonexclusive right
to use this work for the University's own purposes and to make single
copies of the work available to the public on a not-for-profit basis
if copies are not otherwise available.
~ _..
_
."" •.t«-
•
"-"
N'guet'tia
ao l,

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THESE DE "MASTER Of SCIENCE" EN AGRONOMY, NOVEMBRE 1980
(THE
PENNSYLVANIA
STATE
UNIVERSITY)
DEVELOPPEMENT VEGETATIF ET REPRODUCTIF DU MAIS(Zea maya L.)
A QUATRE DATES DE SEMIS AU PRINTEMPS.
Par
N'guettia René YAO
Réswné
Les rendements en grain du maïs
sont influencés par
plusieurs facteurs y compris les caractéristiques physicochimiques
du sol, les facteurs climatiques, les techniques culturales, le
potentiel génétique des hybrides, la résistance aux rœ.ladies et le
contrôle des ennemis de la plante. Nous devons développer une
meilleure compréhension des mécanismes par qui ces différents
teurs influencent le rendement si nous voulons apporter une amé-
lioration systématique à ce rendement de mals. Un essai a été mis
en place en 1979 à la Ferme agronomique de "Pennsylvania State
University" près de Rock Springs avec du mals provenant de deux
groupes de maturité différente pour étudier la croissance végéta-
tive et le développement de la production dans différents environ-
nements associés au retard du semis au printemps. Ensuite pour
déterminer le mécanisme par lequel le retard du semis au printemps
conduit à des rendements en grain reduits. Les résultats de cette
étude étaient utilisés pour aider à identifier les facteurs qui
s'associent pour réduire les rendements pour des semis à des
périodes recommandées.
L'étude avait deux buts :
1) déterminer les relations entre la température de
l'air et le développement de la plante de mals pendant plusieurs
stades de croissance

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-2-
2) déterminer comment les intéractions temps-température
entraînent la réduction du rendement en grain observé avec des
semis
tardif.
Un hybride hatif
(Cornell 281)
et un hybride semi tar-
dif
(Pioneer 3780) de mais ont été semés manuellement à quatre
dates, du 12 mai au 22 juin et demariés à 86500 et 66500 plants/ha
respectivement pour C281 et
P3780 afin d'obtenir des indices
foliaires
(LAI)
équivalents. Les températures du sol et de l'air,
le rayonnement solaire et les précipitations avaient été horaire-
ment enregistrés. Des relevés de température et d'humidité de sol
ont été faits deux ou trois fois par semaine dans le champ expé-
rimental et des échantillons de plantes régulièrement collectés
pour caractériser le développenlent végétatif et reproductif.
La durée du semis à la levée a varié entre 6 et 8 jours
et est plus courte quand la germination et la période de levée ont
coincidé avec des jours de température moyenne élevée. La durée
en jour de la levée à l'initiation des fleurs mâles décroît avec
le retard du semis.
L'accumulation de surface foliaire par plante et le
développement de l'indice foliaire sont une fonction linéaire de
la somme des degrés jours (GDD). La distribution de la matière
sèche dans les différentes parties de la plante est identique pour
les deux hybrides à toutes les dates de semis : les taux presque
constants de 17, 31 et 52 % étant observés respectivement pour les
graines:
les limbes et les tiges avant la mi-floraison.
Le nombre de double-lignes de grains par épi est signi-
ficativement supérieur
(p < 0,05) pour
P3780
(7,54)
que pour C281
(7,39)
quand la moyenne est faite sur toutes les dates de semis.
Le nombre d'ovules par épi croit linéairement pendant le développe-
ment de l'épi, atteignant son maximum près de l'émission des soies
puis décroit
légèrement. Le nombre maximum d'ovules par double
ligne est identique pour les deux hybrides à toutes les dates de
semis variant entre 92 et 109. Le taux de grains effectivement
remplis a varié entre 50 et 67 % du nombre d'ovules présents à
l'émission des soies. Le rendement décroit avec des semis plus
tardifs de 8,16 à 2,75 tonnes/ha pour C281 et de 8,85 à 1,63 tonnes
par ha pour
P.3780. Les rendements très faibles enregistrés pour
les deux dernières dates de semis sont en grande partie dQs à une
faible pollinisation causée par des dégâts d'insectes sur les soies.

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-3-
L'accumulation de matière sèche dans le grain est une
fonction linéaire à la fois du temps en jours et de la somme des
degrés jours. Le taux de remplissage des grains par jour ou par
degré jour est identique pour les deux hybrides et à toutes les
dates de semis. La période effective de remplissage des grains,
dans la majorité des cas, décroit avec des semis plus tardifs:
variant entre 45 et 25 jours : cependant,
la période apparente
de remplissage
(de l'émission des soies à la formation de la cou-
che noire)
augmente. Cette augmentation est due à un retard dans
la formation de la couche noire à la fin de la période de remplis-
sage. Le remplissage des grains a été prématurement arrêté par le
gêle pour les deux dernières dates de semis. Nous avons conclu
que la diminution de la période effective de remplissage avec des
semis plus tardifs est la principale cause de la réduction du
rendement en grain. Puis que le rendement en grain du mals dé-
croit avec des semis plus tardifs et que la production d'ovules
et l'accumulation de matière sèche sont des fonctions linéaires
à la fois de la somme des degrés jours et de la durée en jours :
l'effet de la température n'a pu être dissocié. Trente à cinquante
pour cent du potentiel d'ovules ne sont pas remplis en grains
mOrs. Un travail de recherche doit être fait à l'avenir pour
utiliser ce potentiel important d'ovules afin de maximiser le
rendement du maïs.

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The signatories be10w indicate that they have read and approved
the thesis of N'guettia René Yào.
Date of Signature:
Signatories:
Daniel P. Knieve1, Associate Professor
of Crop Physio10gy, Chairman of
Committee, Thesis Advisor
\\
J:-;"
,
,
J:-;"
( .
. ,--
\\.
.
\\.
, / / James L. Star1ing, Head of ~e Depart-
. /
~e
. /
ment of Agronomy
!')
Richard H. Fox, Associate Professor of
Soi1 Science, Member of Thesis
Canmittee
C. T. Morrow, Associate Professor of
Agricu1tura1 Engineering, Member of
Thesis Committee

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iii
ABSTRACT
Corn grain yields are influenced by many factors including soi1
physical and fertility characteristics, climatic factors, soil and
crop management, hybrid genetic potential, disease resistance, and
pest control.
We must develop better understanding of the mechanisms
hy which these varied factors influence yield if we are to make sys-
tematic improvement in corn yields.
A field experiment with maize from
two maturity groups was conducted in 1979 at The Pennsylvania State
University Agronomy farm near Rock Springs to study corn vegetative
growth and yield development under the different environments associ-
ated with delayed spring p1anting.
In addition to determining the
mechanism whereby delayed spring planting leads to reduced grain yields,
results of this study were used to help identify factors that combine
to limit grain yields with planting at recommended periods.
The study had two objectives:
(1) to determine the relationships
between air temperature and corn plant development during several
growth stages; and (2) to determine how these time-temperature effects
bring about the reduction in yield observed with la te planting.
An early (Cornell 281), and a mid-season (Pioneer 3780) corn
hybrid, were each hand planted at four dates, from 12 May to 22 June
and thinned to 86,500 and 66,500 plants/ha for C28l and P3780, respec-
tively, to produce equivalent leaf area indexes (LAI).
Soil and air
temperatures, radiation, and precipitation were recorded hourly.
Soil
temperature and moisture block readings were also made two to three
times per week at the experimental field.
Plant samples were collected
frequently to characterize vegetative and reproductive development.

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iv
Time to emergence ranged from 6 to 8 days and was shorter where
the germination and emergence period consisted of days with higher
daily average temperatures.
Time (calendar days) from emergence to
tassel initiation decreased with delayed planting.
Leaf area accumulation per plant and LAI development was found
to be a linear function of growing degree days (GDD).
The distribution
of plant dry weight among different plant parts was similar between
hybrids and among planting dates.
Nearly constant percentages of 17,
31, and 52% were observed for sheaths, blades, and stalks, respectively,
before mid-silk.
The number of double kernel rows per cob was significantly higher
(P < 0.05) for P3780 (7.54) than C28l (7.39) when averaged over planting
dates.
Number of kernel sites per ear increased linearly during ear
shoot development, reached a maximum near silking, and then declined
slightly.
The maximum number of kernel sites per double row was found
to be similar for both hybrids at aIl dates and ranged from 92 to 109.
The percentage of kernels that actually filled ranged from 50% to 67%
of the number of kernel sites present at silking.
Final grain yield
decreased with late planting from 8.16 to 2.75 tons/ha for C28l and
from 8.85 to 1.63 tons/ha for P3780.
The very low yields in the last
two planting dates were in a large part caused by poor pollination which,
in return, was caused by insect damage to ear silks during pollination.
Kernel dry matter accumulation was a linear function of both GDD
and calendar days.
Kernel growth rate for GDD and calendar days was
similar for aIl planting dates and hybrids.
The effective filling period
in most cases decreased with delayed planting, ranging from 45 to 25
days.
The apparent filling period, from silking to blacklayer, however,

---

v
increased with late planting due to delayed blacklayer formation at
the end of the filling period.
Grain filling was halted prematurely
by frost at the two last planting dates.
It was concluded that the
decrease in effective filling period with delayed planting was the
primary cause of reduced grain yield.
Since corn grain yield declined with delayed planting and kernel
site production and kernel dry matter accumulation were linea'r functions
of both GDD and Julian days, the temperature effect remained undeter-
mined.
Thirty to fifty percent of the potential kernel sites did not
fill to mature kernels.
Future research work should be done to utilize
the large number of potential kernel sites for higher grain production.
"~.......L ,.•.

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vi
TABLE OF CONTENTS
ABSTRACT. . . .
iii
LIST OF TABLES.
viii
LIST OF FIGURES
xii
ACKNOWLEDGEMENTS ...
xiv
Chapter
l
INTRODUCTION . . •
1
II
LITERATURE REVIEW
4
A.
Developmental Aspects of Corn
4
1.
Date for Corn Planting in Pennsylvania,'
4
2.
Emergence . . . •
8
3.
Tasse! Initiation . • . .
9
4.
Leaf Development. . . . .
11
a.
Rate of leaf initiation and
leaf appearance . . . .
11
b.
Leaf expansion. . • . .
12
c.
Leaf area (LA) and leaf area
--(>~,
index (LAI) . . • .
.; ..
r~.--(>~,
.; r~.
13
5.
Ear Shoot Development .
14
6.
Tasselling and Sil king.
16
7.
Kernel Filling. • . . .
17
8.
Whole Plant Dry Matter Accumulation
in Corn . . • . . • . . • . .
20
B.
Heat Unit and Growing Degree Day.
21
C.
Statement of Hypothesis
23
III
MATERIALS AND METHODS . • .
24
A.
Location of the Experimental Field.
24
B.
Experimental Design
24
C.
Plant Establishment
24
D.
Plant Sampling . • .
26
E.
Weather Data. . . .
31
F.
Statistical Methods
34
IV
RESULTS AND DISCUSSION.
35
A.
Microclimate Data.
35
B.
Corn Plant Sampling Results
43
1.
Time to Emergence .
43
2.
Time from Emergence to
Tassel Initiation •
45

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vii
TABLE OF CONTENTS (Continued)
Chapter
3.
Plant Development. .
46
a.
piant population
46
b.
Leaf development
46
c.
Plant dry matter accumulation during
the period from emergence to silking
52
d.
Plant height . .
62
4.
Reproductive Events.
66
C.
Factors Influencing Corn Grain Yield
66
1.
Number of Double Rows (DR) per Cob
66
2.
Kernel Site Accumulation .
71
3.
Ear Length • . . • . . . .
83
4.
Kernel Dry Weight Accumulation
86
D.
Final Harvest . . . . .
98
1.
Final Grain Yields
98
2.
Barrenness . •
103
3.
Harvest Index.
106
V
GENERAL DISCUSSION
108
VI
CONCLUSIONS. . ••
115
APPENDIX A:
Microclimate Data of 1979 Growing Season.
117
APPENDIX B:
Leaf Area and Leaf Area Index Data. • . .
125
APPENDIX C:
Plant Part Dry Weight Data and Correction
Factor from 690
69 C to 104°C Dry Weight.
128
APPENDIX D:
Plant Height Data .
134
APPENDIX E:
Ear Shoot Development:
Kernel site, Row
Number, Kernel Filled and Ear Length Data
136
APPENDIX F:
Kernel and Ear Dry Weight Da ta.
143
LITERATURE CITED . • . . . . • . . . .
147

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viii
LIST OF TABLES
Table
1.
Summary of the results from a 30-year period
(1926-1955) study at State College (central
Pennsylvania) on the probability of frost in
spring ar-d fall (Adapted from Kauffman and
Butler, 1961). . . . . . •
5
2.
Pennsylvania corn maturity zones and their
characteristics with respect to Days to Maturity
and Growing Degree Days (GUO) (Adapted from 1980
Agronomy Guide, Pennsylvania State University) •
7
3.
Summary of the t-test used in the comparison of
soil temperature at 5 cm depth between the
Weather Station and the experimental field;
data used in this test were recorded from
6 June to 6 July 1979 . . . • .
38
4.
Growing degree days (GDD) from emergence to
successive harvest dates, computed from shelter
air ternperature, for use in the analysis of the
vegetative growth. • . • . • . • • • • • • • ••
39
5.
Growing degree days (GDD) from midsilk to successive
harvest dates, computed from shelter air tempera-
tures, for use in the ana1ysis of kernel dry
matter accumulation.
• . . . . . .
• • • •
40
6.
Time in days and GDD from planting to emergence
of both Cornell 281 and Pioneer 3780 planting at
successive dates
. . . • . • • • •
44
7.
Time in days and GDD from emergence to tassel
initiation of both Cornell 281 and Pioneer 3780
planted at successive dates. . •
• • • •
44
8.
Regression analysis on Leaf Area Index versus
growing degree days for two corn hybrids (C28l,
P3780).
The points from the linear accumulation
were used in this analysis • • • • • • • • • • •
49
9.
Test of homogeneity on the regression coefficients
shown in Table 6 .
• • • •
49
10.
Leaf Area Index at midsi1k, plants per row (# P!R),
and average number of leaves above the topmost ear
for two corn hybrids (C28l and P3780) planted at 4
dates in the spring of 1979 . • . . . . . • • • . .
51
.O'r.'i\\·
.
" :

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ix
LIST OF TABLES (Continued)
Table
11.
Percentage of total shoot dry matter in plant parts
(leaf sheaths, leaf blades, and stalks) at midsilk
for two corn hybrids (C28l and P3780) planted at
successive dates in the spring of 1979. • •
51
.
0
12.
Total plant dry (69 C) matter per unit land area
for two corn hybrids (C28l and P3780) planted at
four dates in spring of 1979.
Plants were sampled
at blacklayer for early plantings and at frost for
late plantings. • • • • • • • • • •
63
13.
Summary of significance of F-ratios and degrees of
freedom for analysis of variance of total plant
dry weight per ha for C28l and P3780 planted at
4 dates in the spring of 1979 • • • . • •
63
14.
Visually identifiable reproductive growth stages
of two corn hybrids (Cornell 281 and Pioneer 3780)
planted in spring of 1979 at four dates • • .
67
15.
Number of double rows (DR) per cob for 2 corn
hybrids (C28l and P3780) planted in the spring
of 1979 at 4 dates. • • . • • • ••
. • • •
68
16.
Summary of significance of F-ratios and degree
of freedom for analysis of variance of number of
double rows for two corn hybrids (C28l and P3780)
planted in the spring of 1979 at 4 dates. • • . •
68
17.
Regression analysis of the kernel site accumulation
against GDD from emergence, for C28l. • • • • • • •
74
18.
Test of homogeneity on the regression coefficients
of kernel site accumulation for C28l and P3780 (GDD basis)
74
19.
Regression analysis on the kernel site accumulation
against GDD from emergence for Pioneer 3780 • . . •
75
20.
Test of homogeneity of the regression coefficients of
kernel site accumulation for C28l and P3780 (calendar
day basis). • • • • • • • • • • • • • • • . • •
75
21a.
Regression analysis of kernel site accumulation against
calendar day for C28l
.
76
2lb.
Regression analysis of kernel site accumulation against
calendar day for P3780.
.
.
76

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x
LIST OF TABLES (Continued)
Table
22.
Number of maximum kernel sites per double row
(Max KS/DR), number of kernel sites per double
row present at silking and number of filled
kernels/DR for 2 corn hybrids (C28l and P3780)
planted in the spring of 1979 at 4 dates. • • •
78
23.
Summary of significance of F-ratios and degrees of
freedom for analysis of variance of number of
maximum kernel sites for C28l planted in the spring
of 1979 at 4 dates. • • . . • • • .
• • • .
79
24.
Summary of significance of F-ratios and degrees
of freedom for analysis of variance of number of
maximum kernel sites for Pioneer 3780 planted in
the spring of 1979 at 4 dates • • •
79
25.
Summary of significance of F-ratios and degrees of
freedom for analysis of variance of number of
kernels harvested for Pioneer 3780 planted in the
spring of 1979 at 4 dates • . • • .
81
26.
Summary of significance of F-ratios and degrees
of freedom for analysis of variance of number of
kernels harvested per ha for Corne Il 281 and Pioneer 3780
planted in the spring of 1979 at 4 dates . • • • • .
81
27.
Maximum number of kernel sites and number of filled
kernels per ha for 2 corn hybrids (C28l and P3780)
planted in the spring of 1979 at 4 dates • . .
82
28.
Regression analysis on the kernel dry weight
versus Julian day for Cornell 281
• • • •
91
29.
Test of homogeneity of the regression coefficients
of kernel dry weight accumulation for C28l. •
91
30.
Regression analysis on the kernel dry weight
versus Julian day for Pioneer 3780. • . • . .
92
31.
Test of homogeneity of the regression coefficients
of kernel dry weight accumulation for P3780 • • • .
92
32.
Regression analysis of kernel dry matter accumulation
versus GDD (base 100
10 , 300
30 C) for 2 corn hybrids
(Cornel1 281 and Pioneer 3780)..
• • • • . . • •
93
33.
Test of homogeneity on the regression coefficients of
kernel dry matter accumulation on GDD basis for C28l
and P3780 • • . • • • . • • • • . . • • • . • . . • •
93

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xi
LIST OF TABLES (Continued)
Table
34.
Kernel growth rates on calendar day basis (mg/day)
and GDD basis (mg/GDD) for 2 corn hybrids (C28l and
P3780) planted in the spring of 1979 at 4 dates • .
94
35.
Effective kernel filling period duration in Julian
days and GDD and apparent kernel filling period
duration in days of two corn hybrids (C28l and
P3780) planted in the spring of 1979 at 4 dates.
96
36.
Final dry (104°C) weight per kernel and GDD accumu-
lated per mg dry weight per kernel for two kernel
hybrids (Cornell 281 and Pioneer 3780) planted in
the spring of 1979 at 4 dates • • • . •
97
37.
GDD accumulation per calendar day and time from midsilk
to beginning of effective grain filling (first lag) and
from the end of the effective filling period to blacklayer
formation (second lag) for C28l and P3780 planted at 4
dates in the spring of 1979. •
• • • . • • • • ••
99
38.
Final grain dry (104°C) weight (metric tons/ha)
calculated on grain weight/l.83 m of row (method 1)
of two corn hybrids (Cornell 281 and Pioneer 3780)
planted in spring of 1979 at successive dates. • • •
99
o
39.
Final grain dry (104 c) weight (metric tons/ha) cal-
culated from average grain weight/ear x population basis
(method II) of two corn hybrids (Cornell 281 and Pioneer
3780) planted in the spring of 1979 at successive dates.
101
40.
Summary of significance of F-ratios and degrees of
freedom for analysis of variance of final grain yield
of two corn hybrids (C28l and P3780) planted in the
spring of 1979 at 4 dates. • • •
• • • • • •
102
41.
Percentage barrenness at harvest and LAI at midsilk
for 2 corn hybrids (C28l and P3780) planted in the
spring of 1979 at 4 dates. • . . • • • •
104
42.
Whole plant and grain dry (104°C) weight from 6
plants for 2 corn hybrids (C28l and P3780) planted
in the spring of 1979 at 4 dates • • . • •
104
. 43.
Harvest indexes of 2 corn hybrids (Cornell 281 and
Pioneer 3780) planted in the spring of 1979 at 4 dates
105
44.
Summary of significance of F-ratios and degrees of
freedom for analysis of variance harvest index for 2 corn
hybrids (C28l and P3780) planted at 4 different dates. • .
105

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xii
LIST OF FIGURES
1.
Pennsylvania corn maturity zones; 1, 2, 3, and 4
are the different zones..
• . • •
6
2.
Summary of microclimate data recorded du ring 1979
growing season:
irradiance (ly/day); maximum and
minimum temperatures (oC); and daily precipitation
(mm/day); and irrigation (open bar). . • •
37
3.
Average and Cumulative growing degree days in Oc of
1979 growing season (from May 1 to October 15) over
5 day intervals. • • • . . • •
• • • .
42
4.
Relationship between Leaf Area Index at four succes-
sive planting dates for 2 hybrids and the growing
degree days (oC) accumulated during 1979 growing
season • • • . • • •
• . • • • • •
48
0
5.
Relationship between stalk dry (69 C) weight (g/4 plants)
of two corn hybrids (C28l and P3780) planted at succes-
sive dates inothe ~pring of 1979, and the growing degree
days (base 10 , 30 C) computed from shelter air
temperature. . • • • • • • • ••
• • . • . • •
54
0
6.
Relationship between leaf sheath dry (69 C) weight
(g/4 plants) of two corn hybrids (C28l and P3780)
planted at successive dates, in the 8prin§ of 1979,
and the growing degree days (base 10 , 30 C)
computed from shelter air temperature. • • • • • .
56
0
7.
Relationship between leaf blade dry (69 C) weight
(g/4 plants) of two corn hybrids (C28l and P3780)
planted at successive dates in the s~ring of 1979,
o
and the growing degree days (base 10 , 30 C)
computed from shelter air temperature. • • •
58
8.
Relationship between total plant dry (104°C) weight
(g/plant) for Corneel 281 planted at four dates in
the sprigg of 1979, and the growing degree days
o
(base 10 , 30 C) • . • • • • • • • • • • • • • • •
60
o
9.
Relationship between total plant dry (104 C) weight
(g/plant) for Pioneer 3780 planted at four dates in
the sprigg of 1979, and the growing degree days
o
(base 10 , 30 C) • • • • • • • • . • • • • • • •
61
10.
Relationship between plant height (cm) of two corn
hybrids (C28l and P3780) planted at successive dates
in the sgring of 1979, and the growing degree days
o
(base 10 , 30 C) computed from shelter air temperature
65

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xiii
LIST OF FIGURES (Continued)
11.
Week prior to the beginning of kernel site accumu-
lation with its average GDD/day and its cumulative
irradiance, 103
10
ly (in parentheses), in solid line;
duration of linear kernel site accumulation with
the average daily temperature du ring this period
in dark line for 2 corn hybrids (C28l and P3780)
planted in the spring of 1979 at 4 dates.
70
12.
Relationship between the number of kernel sites
per double row for Cornell 281 (planted at 4 dates)
and the growing degree days (base 10°, 30°C) from
emergence, calculated with shelter air temperatures
72
13.
Relationship between the number of kernel sites
per double row for Pioneer 3780 (planted at 4
dates) and the growing degree days (base 10°, 30°C)
from emergence, calculated with shelter air
temperatures. • • • . • • • • • • • • • • .
73
14.
Re1ationship between the ear length (cm) of
Corne Il 281 planted at four dates, and the growing
degree days (base 10°, 30°C) from emergence • • . •
84
15.
Re1ationship between the ear length (cm) of Pioneer
3780 planted at four dates, and the growing degree
days (base 10°, 30°C) from emergence • • • •
85
16.
Kerne1 dry matter accumulation with time for 2
corn hybrids (C28l and P3780) p1anted in the
spring of 1979 at 4 dates • • . • • • • • •
88
17.
Relationship between the dry (104°c) weight
per kernel for 2 corn hybrids (C28l and P3780)
p1anted at 4 dates in the spring of 1979 and the
growing degree days (base 10°, 30°C) from midsilk
90

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xiv
ACKNOWLEDGEMENTS
The author wishes to express his sincere gratitude to
Dr. D. P. Knievel, Advisor and Committee Chairman, for his direction
of the author's graduate program and for his invaluable assistance
throughout this study.
Appreciation is expressed to Dr. G. W. McKee for providing sorne
research equipment used in the project and for his careful review and
helpful suggestions regarding the preparation of this manuscript.
Appreciation is extended to Drs. R. H. Fox and C. T. Morrow for
their helpful suggestion and critical evaluation of the thesis.
Special appreciation is extended to James L. Starling, Head of
the Department of Agronomy, The Pennsylvania State University, The
African American Institute, and the Government of The Ivory Coast for
providing facilities and financial support to make the author's
graduate program and this research possible.
':',
,
;. l_
•

---

CHAPTER l
INTRODUCTION
Date-of-planting experiments not only allow one to evaluate
the performance of hybrids from different maturity classes, but also
to investigate developmental aspects of corn in the environment.
Significant differences in corn plant development are likely to occur
in different environments created by various planting dates.
Charac-
terization of these differences should contribute to a better under-
standing of corn plant development and of the influences of certain
aspects of the environment on it.
From a practical viewpoint, farmers
frequently have to delay corn planting when spring weather conditions
are poor for early field work or early seeding.
In addition, replanting
is often necessary when poor germination resulting in a poor plant popu-
lation is likely to drastically reduce crop yield.
A better under-
standing of plant response to late planting could lead to management
practices that encourage late planting of corn in certain cropping
systems.
The influence of delayed planting on corn (Zea mays L.) growth
and yield has been studied by a number of workers.
Benoit, Hatfield,
and Ragland (1966) concluded from their research work in Kentucky that
corn final yields indeed decreased with late planting.
Decreases in
yield with delayed planting are commonly reported in Pennsylvania and
in many other states (1980 Agronomy Guide, The Pennsylvania State
University).
The 1980 Agronomy Guide of The Pennsylvania State Uni-
versity states:
the "Ideal planting time is only a few days in duration.
In most years, each day's delay past this period reduces yields by up to

---

2
one bushel per acre per day."
The concept of critical periods of
planting for optimum corn yields is generally accepted throughout the
country (Shaw, 1975).
There is little evidence that demonstrates why
late planting results in reduced yields.
Various researchers including Hunter et al. (1974) and Carr (1976), have
pointed out that corn development and grain yields are influenced by
many factors such as soil physical and fertility characteristics, climat-
ic factors, and soil and crop management.
A single factor or a combina-
tion of factors could result in yield reduction or increases.
It is
likely that different factors contribute to yield reductions from year
to year and location to location so that the biological mechanisms re-
sponsible for reduced yields are difficult to identify and characterize.
One way to try to understand what happens to grain yield at the
end of the growing season is through the analysis of the deve10pment and
size of the components of yield.
An intensive investigation of plant
vegetative development and yield components (plants/ha, ears/plant,
kernels/ear, dry weight/kernel) throughout the growing season should
help identify the factors contributing to an eventual decrease in yield.
The major environmental factors affecting yield development are solar
radiation, temperature, and moisture.
Although there have been many
studies conducted to examine the effects of temperature and solar
radiation on corn, little has been done to try to understand the mech-
anisms through which delayed planting, temperature, and radiation inter-
actions lead to a decline in yield.
In order to develop an understanding
of the mechanisms in which late planting and temperature influence plant
development and grain yield, a field study was conducted during the 1979
growing season at The Pennsylvania State University Agronomy Farm with

---

3
two objectives:
(i)
To determine the relationship between air temperature and
various periods of corn plant development (tassel initiation,
leaf development, ear shoot development, silking, and kernel
filling); and
(ii)
To determine how these time-temperature effects bring about
the reduction in yield observed with late plantings.
, ,
1 l·~·

---

CHAPTER II
LITERATURE REVIEW
The literature on corn growth and grain production has developed
over many years.
Climatic factors, such as temperature and radiation,
affecting corn development have been weIl studied.
A review of
important findings on effects of environmental factors on devel-
opmental aspects of corn yield will help develop a background for the
present research work.
In this chapter, l will review results of
research work about:
(1) the different parts of the corn plant, their
relations to yield, and how they are affected by factors of their
environment; and (2) the concept of heat unit and growing degree days
used in work involving temperature.
A.
Developmental Aspects of Corn
1.
Date for Corn Planting in Pennsylvania
In regions where weather conditions limit the development of crop
plants, sorne of the factors to look at, before establishing any crop,
are the length of the growing season (period safe from frost:
tempera-
ture > OOC or 32 0
32 F), and the Ideal date for planting.
Early planting
is commonly recommended to encourage good canopy development coincident
with the period of high solar radiation and long photoperiods.
Early
planting is also recommended so that full season hybrids which commonly
produce more leaf area per plant than short season hybrids can be grown.
A study conducted at State College, Pennsylvania (central Pennsyl-
vania) over a 30-year period (Table 1) indicates the variation usually
observed in growing season parameters.
This report of 30 years of

---

5
Table 1.
Summary of the results from a 30-year period (1926-1955)
study at State College (central Pennsylvania) on the prob-
ability of frost in spring and fall.
(Adapted from Kauffman
and Butler, 1961.)
Probability
Season
of
Frost
Spring
Fall
.90
April 16
October
30
.75
"
21
"
23
"
.67
"
23
"
10
.33
&y
Dl
"
06
.25
"
03
"
"
03
"
.10
"
08
September 24
records (Kauffman and Butler, 1961), shows that the frost free growing
season in State College extends roughly from May 8 to September 24.
The
growing season in Pennsylvania varies from 90 to 130 days with the loca-
tion (see Pennsylvania corn rnaturity zone rnap:
Figure l, and Table 2)
(1980 Agronomy Guide, The Pennsylvania State University),
A report by
Shaw (1975) on the Growing-Degree Units for Corn in the North Central
Region of the United States of America, shows an ideal average planting
date to be safe from frost.
This date varies with each zone and ranges
from April 18 (in south Missouri) to June 6 (in Minnesota).
It has been
reported that corn can be safely planted 10 to 14 days before the average
date of the last killing frost in the spring (Shaw, 1975 and 1980 Agron-
orny Guide, The Pennsylvania State University).
Corn should be planted
as early as possible to allow the complet ion of the grain filling before
the frost in fall.
Adequate hybrid selection based on both the length
of the corn development cycle and the length of the growing season should
be made.

---

..j~
""~
l:
~
~,
f"
r~
~.
Figure 1.
Pennsylvania corn maturity zones; 1, 2, 3, and 4 are the
different zones.
!'
Q\\
1
r~

---

7
Table 2.
Pennsylvania corn maturity zones and their characteristics
with respect to Days to Maturity and Growing Degree Days
(GDD).
(Adapted from 1980 Agronomy Guide, Pennsylvania State
Universit y. )
Approximate
Pennsylvania
Maturity Zone
Days to Maturity
GDD*
90-95
1600
1
96-100
1825
101-105
2025
2
106-110
2275
111-115
2500
3
116-120
2725
121-125
2950
4
126-130
3175
*GDD ca1cu1ated with "National Weather Bureau" system (OF system).

---

8
2.
Emergence
Early growth of corn seedlings depends mainly on the conditions
of emergence.
This early growth, as one would expect, has an important
influence on final corn grain yield.
Hanway (1966) reported that the corn ernbryo in the seed has five or more
leaves and that the primary roots have been initiated.
After planting, the
seed absorbs water and the young plant begins to grow.
The first
internode elongates until the coleoptile emerges above the ground and
so brings the plant under the influence of light which suppressés
coleoptile growth and stimulates the formation of chlorophyll and leaf
development.
Under moist and warm conditions, the plants emerge within
5 days, but 2 weeks or longer may be required under cool and dry con-
ditions (Hanway, 1966).
Depth of planting has been reported to influence
the length of time from planting to ernergence.
Blacklow in 1972
described the influence of temperature on germination and emergence of
maize (Blacklow, 1972a and 1972b).
He found that optimum growth during
o
0
seedling establishment occurred at 3a e.
A low ternperature of 9 e led
o
to a cessation of growth while a constant 4a e was lethal for corn
plants.
Blacklow (1973) developed a prediction model for germination
and emergence that gave good agreement with measured values under
fluctuating temperatures in controlled environrnents and in the field.
He also pointed out that the verified model supported the hypothesis
that the germinating seed and its elongating axes respond to prevailing
tempera tures wi th no adaptation to preceding
conditions, and that the
system responds within minutes to changes in ternperature.
Iremiren and
Milbourn (1979), investigating sail temperature effects on corn ernergence,
reported that the initial effect of a clear polyvinyl chloride (PVe) mulch

---

9
treatment was to shorten the interval between sowing and emergence and
to improve germination.
They pointed out that maize development was
accelerated from emergence through to the grain maturity stage by an
increase in soil temperature.
In conclusion to this section, two things need to be noted:
(i) an
improvement in germination and emergence can lead to better yield per
unit land area; and (ii) adequate water supply and warm temperatures
near 3ao
3a e are the ideal conditions for better germination and plant
emergence.
3.
Tassel Initiation
The importance of tassel initiation in corn plant development is
due to the fact that leaf and ear primordia initiation are stopped at
that stage.
Therefore, one would expec t a greater number of leaves per
plant if any factor delays tassel initiation without affecting the leaf
initiation rate.
Temperature and photoperiod are two such factors that
have been investigated.
There are reports that the ear shoot and the tassel begin to form
when the plant 'has 8 or more
leaves (Hanway, 1966).
The tassel and its
parts differentiate when the internodes of the stem start to elongate.
Bonnett (1966) reported that tassel development was completed when
anthers dehiced.
There is evidence for a consistent decrease in time
0
0
to tassel initiation as temperature increases from l5 e to 25 e
regardless of photoperiod (eoligado and Brown, 1975).
However, no
difference in tassel initiation time has been reported at temperatures
0
from 25 e to 3aoe, suggesting that the optimum temperature is between
25-3a o
25-3a e.
They reported an increase in leaf number due to a delay in
tassel initiation with long photoperiods.
A similar increase in leaf
. L:~.:I...

---

10
number due to delayed tassel initiation was reported with cool compared
to warm temperature treatments.
Hunter ~ al. (1974) found that the
time to tassel initiation in a growth chamber experiment increased up
to 5 days when the photoperiod was increased from 10 to 20 hours, and
that this response occurred at aIl temperatures studied.
However, the
magnitude of the response was reported to be less at high temperature.
Their findings on the time to tassel initiation were confirmed a year
later by Coligado and Brown (1975).
These researchers reported however
that the relationship was nonlinear.
Coligado and Brown (1975) also
reported an interaction between temperature and photoperiod effects on
the time to tassel initiation.
The delay in tassel initiation with long
compared to short photoperiods (20 vs. 10 hours) and the associated
increase in leaf number helped expIa in the increase in the amount of
vegetative dry matter present at tas sel initiation in the long photo-
period treatments.
Iremiren and Milbourn (1975) conducted a mulching
experiment to study the influence of soil temperature on growth and
development in maize.
They reported that soil rather than air tempera-
tures during the first 5 to 6 weeks after sowing had the most dominant
effects on corn plant growth and subsequently on tassel initiation.
This report confirmed results obtained by Watts (1972) and later sup-
ported by Carr (1977), and Cooper and Law (1977).
In conclusion, soil temperature and photoperiod should be recog-
nized as two major environmental factors affecting the time to tassel
initiation.
In addition, this time to tas sel initiation is very im-
portant in determining leaf number and leaf area per plant in sorne
environments.

---

11
4.
Leaf Development
Leaf number and leaf development are important in light intercep-
tion and photosynthate production by the plant canopy.
Like the other
parts of the corn plant, leaf initiation and expansion are influenced
by climatic factors such as temperature and photoperiod.
a.
Rate of leaf initiation and leaf appearance.
One should make
a distinction between leaf initiation rate and the rate of leaf appear-
ance.
The former concerns the rate of leaf primordia initiation.
Leaf
initiation stops with tassel initiation.
Therefore, a greater number of
leaf initiaIs is expected with delay in tassel initiation as reported
earlier in this chapter.
Milthorpe and Moorby (1974) defined plasto-
chron as the time interval between the appearance of successive leaf
primordia.
There is evidence that leaf initiation rate increases with
temperature.
In 1975, Coligado and Brown reported a relative small
decrease in leaf initiation rate with an increase in photoperiod.
Leaf
appearance rate describes leaf unfolding from the terminal enclosing
sheaths.
lt usually occurs at a lower rate than that of primordia
initiation because leaves require longer than one plastochron to
lUlfold.
Brouwer et al. (1973) reported that temperature appeared to
be a major factor in determining the rate of leaf appearance, while
light quality and intensity were fOlUld to have minor effects.
They
pointed out that up to the time of formation of the Sth visible leaf,
root temperature controlled the rate of leaf appearance, but that air
temperature gradua1ly took over the control of leaf growth.
Cooper
(1979) reported a linear relationship between the rate of leaf emer-
gence and soil temperature up to the l2th leaf stage.
Thereafter, he
o
found that the minimum temperature for leaf emergence was 90
9 C (4S F).
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---

12
Bonaparte (1975) reported that leaves emerged at faster rates at higher
temperatures, and under conditions of high soil fertility.
He observed
a reduction in leaf number in shorter daylength regimes as reported by
many other workers.
He also pointed out that the influence of photo-
period on leaf number was greater with cool nights than warm nights.
b.
Leaf expansion.
Cell division continues after leaf unfolding
but at a declining average rate until the leaf is 25-75% of its final
size (Humphries and Wheeler, 1963).
These workers observed that cell
division and leaf expansion ceased first at the leaf tip.
Light and
temperature were reported to influence cell division in the leaf pri-
mordia so that complete darkness or low irradiance led to an extremely
slow rate of division (Evans, 1963).
There is evidence that soil
nutrients also affect leaf expansion.
For example, high nitrogen
supplies usually lead to large leaves (Elias ~ al., 1979).
Cooper and
Law (1977) in Kenya found that soil temperature differences caused by
mulching had an effect on leaf primordia initiation.
They reported that
higher soil temperature led to greater leaf area production during early
growth and subsequently more leaf area per plant at tasselling.
Brouwer
et al. (1970) suggested that leaf expansion could be influenced by soil
temperature by affecting the water supply to the leaf tissue.
However,
Kleinendorst and other workers (1970) indicated that the tempe rature of
the shoot apical meristem may be more important in early leaf expansion
than that of the root system.
Watts (1971) reported that water stress
led to a sharp decline in leaf elongation at temperatures below SoC
o
(4l F), and that the temperatures of the root and of the meristematic
region interacted in the regulation of leaf expansion (Watts, 1971).

---

13
A number of researchers including Chabot ~ ~ (1979) reported that
variation in the light environment produce changes in leaf anatomical
and biochemical structure which determine net photosynthetic COZ
fixation.
In this regard, Chabot ~ ~ (1979) suggested that leaf
adaptation is determined by integrated light energy rather than
instantaneous flux intensity.
c.
Leaf area (LA) and leaf area index (LAI).
One criterion that
should be met in grain yield comparison among hybrids of differing
maturity is that the different corn hybrids should have the same
potentiality for solar radiation interception on a land area basis.
The crop should therefore have an equivalent leaf area per unit land
area, in order to intercept the same amount of radiation reaching the
canopy.
The evaluation of leaf area index (LAI) was used to rneet the
criteria stated above.
LAI is defined as the total area of one side
of aIl leaves divided by the land area subtending the se leaves.
The
concept was first introduced by Watson (1947, 195Z) in research on the
analysis of plant growth and yield.
Leaf area index (LAI) is the ratio
of the total leaf area (LA) over the land area (A).
LA
LAI
A
Duncan (197Z) stated that interception of light is the primary
function of corn canopies, but the intercepted light should be used
efficiently and this is a function of leaf angle.
He pointed out that
between LAI of 3 and 4 fIat leaves intercept 90% of the light while leaves
at a 150 angle intercept only 75 to 80%.
Hoyt and Bradfield (196Z) reported
that "the net assimilation rate of corn was linear to leaf area index

---

14
when this was less than 2.7 but declined rapidly when it was above that
value.
In a stand with a LAI of 3.3, dry matter produced per square meter
of leaf area from grain initiation to maturity by the top, middle, and
bot tom leaves was of the ratio 4: 2. 2: 1. Il
Eik and Hanway (1966) also
reported a linear relationship between corn grain yield and LAI at
midsilk.
They pointed out, however, that the relationship does not
continue beyond LAI of 3.3.
Formulas have been developed for easy determination of corn leaf
area (McKee, 1964).
Many other methods such as photoelectric and com-
parison techniques have been used for leaf area estimation.
The belt-
type photocell leaf area meter and air flow planimeter have also been
used.
In conclusion, leaf area is important in corn development and
production because it functions in interception of solar energy for
photosynthate production.
Sufficient leaf area is needed to intercept
most of the available solar energy if high productivity per unit land
area is to be realized.
Canopy leaf area is a function of leaf area
per plant and plant population.
Plant leaf area development is influ-
enced by temperature, photoperiod, and light during the leaf initiation
period or later during the leaf expansion.
5.
Ear Shoot Development
Soon after leaf initiaIs form during differentiation of the apical
meristem, auxiliary shoot buds or ear primordia develop in acropetal
succession (Kiesselbach, 1949).
Initiation of these auxiliary shoot
buds ceases, however, when tassel formation begins, as indicated by
elongation of the shoot apex (Bonnett, 1966).
Lateral projections

---

15
initiate from the central axis of the ear primordium; these projections
are the primordia from which the spikelet primordia differentiate.
Ear
shoot development was reported to start 5 weeks after plant emergence
(Hanway, 1966).
Sass and Loeffel (1959) conducted two field experi-
ments in 1954 and 1955 to de termine the relationship between the devel-
opment of auxiliary buds in maize and barrenness.
They reported that
the first evidence of floral transition in the auxiliary buds occurred
43 days after planting.
They cited in their report that the lowest
bud, in the axil of the first foliage 1eaf, is weIl defined 2 to 3 days
after the beginning of germination.
Hunter et al.
(1977) reported on1y a slight influence of photo-
period on spike1et number per ear; Ragland et ~ (1966) also observed
a slight increase in number of kernel rows per ear and a 10% increase
in the number of kernels per row at 14 days after silking with long
photoperiods produced by supplernentary radiation during the middle of
the night.
Cooper and Law (1977) conducted a 4-year field experiment
in Kenya to investigate the effect of importance of soil temperature
in determining the early growth vigor and final grain yie1ds of corn
hybrids.
They planted each year at varying time interva1s to generate
differences in growth temperatures.
They reported that soil temperatures
ear1y in the plant life had p1ayed an important ro1e in determining the
number of potentia1 grain sites initiated without exp1aining the re1a-
tionship in detail.
Hanway (1966) reported that during f10wering and
the period of rapid grain fi11ing, moi sture stress or nutrient deficiency
might result in paar pallinatian and unfi1led kernels.
In summary, ear shoots are initiated ear1y in the plant lifetime.
Their development, which starts appraximately 5 weeks after ernergence,

---

16
is influenced strongly by temperature and slightly by photoperiod.
6.
Tasselling and Silking
Flowering in maize is indicated by the extrusion of anthers from
the spikelets on the tas sel and the ernergence of silks from the husks.
Bonaparte (1975) showed that the tassels on plants which experienced
no period of water stress emerge a few days earlier than those which
were subjected to soil moisture stress.
He reported the association
between total light energy and daylength contributed to the variation
in the time of tas sel emergence.
High plant density was also reported
to result in the lengthening of the interval between anthesis and
silking; the delay in flowering was found to have a negative effect
on the grain yield (Buren et ~,1974).
Prine (1971) showed that
second ear abortion occurred during and just after silk emergence of
the top most ear.
Nishikawa and Kudo (1973) found that 60% of even-
tually barren plants (plants Which failed to produce mature ears) had
normal ear development until silk ernergence.
Tollenaar (1977) sug-
gested that the irradiance intercepted per plant during the flowering
period is a dominant factor determining the continuation of ear growth.
Low amount of intercepted irradiance associated with low photosynthate
production resulting in poor kernel filling, was found to contribute
to a high percentage of barrenness (Tollenaar, 1977).
There is evidence
that ear abortion is also related to intraplant competition for photo-
synthate du ring the flowering period (Hanway, 1962).
The flowering
period is a critical stage where care is needed for better grain dry
matter accumulation.

---

17
7.
Kemel Filling
Kemel filling is an important process in the determination of
grain yield.
The kernel filling period is normally considered to start
at midsilk and end at the blacklayer stage.
The blacklayer stage
indicates that the kernel has reached physiological maturity.
A black-
layer forms at the base of the kemel and it is associated with the
cessation of carbohydrate flow from the pedicel to the kemel.
During
the kernel filling period two important factors, kemel filling rate
and the duration of filling, affect the final grain dry weight as we
will see later in this section.
Studies indicate that the period from silking to blacklayer forma-
tion
contains three different phases of kernel grain development:
(i)
a period (first lag period) of low dry matter accumulation, which was
found to begin at pollination and last for 2 to 3 weeks;
(ii) a period
of linear grain filling which accounts for 90 percent of the dry matter
accumulated; and (iii) a period (second lag period) in which the dry
matter accumulation is not significant, and terminated at blacklayer for-
mation (Johnson and Tanner, 1972; Shaw and Thom, 1951).
Final grain yield
is a resultant of rate and duration of dry matter accumulation.
There are
two methods of measuring corn filling period.
The first method is the
procedure described by Duncan and Hatfield (1964), in which kernels
from the same ear are remove d, dried, and weighed periodically through-
out the grain filling period.
The effective filling period duration
(EFPD) is calculated by dividing rates of kernel dry weight accumula-
tion, during the linear growth period, into mature kernel dry weights.
The second method involves a technique based on blacklayer development.
Daynard and Duncan (1969) reported that a blacklayer formed at the base

---

18
of corn kernels at maturity and represented a visible signal that
kernel dry matter accumulation had ceased.
The apparent grain filling
period duration (AFPD) can be defined, then, as the length of time
interval, on an individual ear basis, from silk emergence (at the time
fertilization occurs) to blacklayer formation.
Several reports have
indicated that a positive relationship exists between the length of
the period from silk to grain maturity and grain yield (Hanway and
Russell, 1969; Daynard, Tanner, and Duncan, 1971; Daynard and Kannenberg,
1976).
Daynard and Kannenberg (1976) suggested that the EFPD might be
even more closely related to yield than the AFPD (Daynard, Tanner, and
Duncan, 1971).
Both length of the filling period and rate of grain growth have
been reported to be affected by environmental factors.
Breuer, Hunter
and Kannenberg (1976) indicated that air temperature had the principal
effect on grain filling period duration.
However, they observed a
photoperiod by temperature interaction on kernel grain filling.
The
interaction was confirmed a year later by a growth chamber study con-
ducted by Hunter, Tollenaar, and Breuer
(1977).
They found that a
longer photoperiod (photoperiods tested were 10 and 20 hours) and
cooler temperature (the temperature was varied from 15 to 300
30 C) treatment
led to highest final plant dry weight.
They argued that the long photo-
period led to more leaf area per plan~ suggesting a greater photosyn-
thate production per plant in their experiments.
They also pointed out
that under cooler temperature there was a longer duration of dry matter
accumulation.
It is important to note that these last two experiments
referred to were conducted in growth chambers so that low irradiance
compared to out-of-doors conditions may have contributed to the observed

---

19
results.
A study with other cereals such as spring oats showed signifi-
cant correlation between grain yield and accumulated solar and sky
radiation during the grain filling period (McKee et al., 1979).
Carter
and Poneleit (1973) found significant yearly differences in number of
growing degree days required to complete the effective grain filling
period.
They also observed a significant inbred by year interaction
for the rate of grain dry matter accumulation.
Similar findings were
reported early in 1971 by Cerning and Guilbot.
Plant density was
reported to affect the EFPD to a limited extent, while both the EFPD
and the kernel growth rate were found to be under genetic control
(Poneleit and Egli, 1979).
These authors indicated that the greater
yield of the hybrid compared to that of the inbred was mainly due to
the higher number of kernels per cob of the hybrid.
Hanway and Russell
(1969) reported that the rate of dry matter accumulation in the grain
was similar for aIl hybrids, years, and populations tested.
They also
confirmed that the EFPD varied with hybrids and that the number of
kernels per unit land area varied with consistent differences among
different hybrids.
A number of workers inc1uding Hunter et al.
(1969)
reported increases in grain yield when tassels were removed at or prior
to silking.
Solar energy that would be intercepted by the tassel
reached the leaves for photosynthate production.
Based on these reports, it is apparent that corn grain yield
depends on the rate and the duration of kernel filling and the number
of kernels filled.
These three components vary widely with management
of environmental factors such as variety, plant population, temperature,
light, photoperiod, soil moisture content, and soil fertility, resulting

---

20
in more or 1ess grain yie1d.
No consistent re1ationship between these
factors has been identified.
8.
Who1e Plant Dry Matter Accumulation in Corn
For a better understanding of the partitioning of photosynthate
within the corn plant, one must eva1uate dry matter accumulation in the
other parts (sta1ks, 1eaf b1ades, and 1eaf sheaths) of the corn plant
in addition to the grain.
Paddick (1944) found that comparable plant
parts were 1arger at maturity in the hybrid than in the inbred, and
that bhe hybrids deve10ped faster than their inbred parents.
A number
of workers inc1uding Hanway (1962 , 1966) reported that differences in
soi1 ferti1ity resu1ted in different rates of dry matter accumulation,
but did not marked1y influence the relative proportions of the differ-
ent plant parts.
Bryant and Blaser (1968) reported that the relative
proportion of the different parts at maturity, varied between an ear1y
and a 1ate hybrid, but was inf1uenced on1y slight1y by differences in
plant populations or row spacing.
Hanway and Russell (1969) observed
simi1ar patterns of dry matter accumulation in the total, above-ground
plant parts in 11 hybrids studied and at different plant densities.
They pointed out that the relative weights of the different plant parts
at any given stage of plant deve10pment were essentia11y the same for
the different hybrids.
Hanway and Russell (1969) indicated that the
1eaves, 1eaf sheaths, sta1ks, and husks attained their final mature
weights at about growth stages (Hanway growth stage system, 1966) 4.0
(48 days), 4.5, 5.0 (66 days), and 5.5, respective1y.
The cob and ear
shank were reported to reach their maximum dry weight at about stage
6.5 (about 18 days after si1king) and to show no 1ater decrease in
weight.
The relative proportions of grain and nongrain plant weights

---

21
were found to va~y widely among hybrids and years, but only slightly
with plant populations, with the grain varying from 35 to 52% of the
total plant weight.
The proportion of grain in rnost hybrids decreased
slightlyas the plant population increased from 38,700 to 58,100
plants/ha (Hanway and Russell, 1969).
Elias, Gagianas, and Gerakis
(1979) confirmed that plant density influenced crop growth, grain yield,
and total biomass per unit land area.
In their experiment, three plant
densities (40, 80, and 120,000 plants/ha) and two levels (80 kg of N/ha
and 170 kg of N/ha) of nitrogen were used.
They found that the highest
biomass (18.9 t/ha) was accumulated by the heaviest fertilized very high
density treatrnent.
B.
Heat Unit and Growing Degree Day
In an attempt to find better ways of estimating temperature effects
on crop growth and yields, various thermal unit formulas have been
develqped to provide appropriate interpretation of agronomie experi-
mental data.
The "National Wea ther Service" method uses the following
formula (Cross and Zuber, 1972; Shaw, 1975; Coelho and Dale, 1980).
(max + min) _ 50 F
GDU
2
Where GDU
Growing degree day unit
max
daily maximum ternperature (oF)
min
daily minimum ternperature (oF)
50 = the base temperature for plant growth
It has been suggested that it would be more accurate to set the
temperature at 50 F when the minimum is below the base (Shaw, 1975).
This allows days wlth maximum values above the base, but with minima
below the base to accumulate GDU.
A number of workers including
!!I v..,~IIl,"I.i6lt :.' . '"
••1,
....... '-.""

---

22
Gilmore and Rogers (1958) found that 300
30 C (86°P) was the optimum maximum
temperature value for corn growth so that temperature values higher than
86°P should be set to 86°P.
Cross and Zuber (1972) compared 22 differ-
ent formulas for computing growing degree units proposed by different
researchers.
One of those is the hourly adjusted average system.
n
24
h '
GDD
E
(E X ~)/24
i=l
j=l iJ
Where GDD
Growing degree days
hr'
X ••
hourly adjusted temperature values
1J
hr
hr
hr
X
if X
< 86 and X
> 50
ij
ij
ij
hr
86 i f X
> 86
ij
=
hr
= 50 if X
< 50
ij
actual hourI y temperature value
j
the hour during the day
i
the day during the growing period
n = total number of days of the experiment
(See Cross and Zuber, 1972, for the entire list of the 22 formulas.)
They found that the adjusted average formulas when computed on either
an hourly or a daily basis explained over 95% of the variation in
flowering date in maize.
Both the Ontario heat unit and heat stress
systems use a quadratic equation to adjust the high temperature to fit
a curvilinear growth response.
When based on average thermal units
the Ontario method was superior for only one of the 23 entries studied
by Cross and Zuber (1972).
They pointed out that the hourly heat stress
system appeared to be the best method but the advantage over the other
systems was slight.
They did not indicate in their research report,

---

23
however, what effect hybrid and variable growth conditions might have
on the results presented.
Mederski, Miller, and Weaver (1973) studied the accumulated heat
unit (AHU) method and the calendar day method for classifying the
maturity of corn hybrids and predicting of occurrence of phenological
events, and indicated that the AHU method of classifying corn hybrids
was superior to calendar days.
Sophisticated methods involving not
only temperature function but also radiation, potential evapotranspira-
tion, and leaf area index, have been developed recently for a better
characterization of corn growth and development (Coelho and Dale, 1980).
C.
Statement of Hypothesis
Based on the findings reviewed in this chapter, three hypothesis
were proposed for the investigation of decline in corn grain yield with
late plantings.
The first hypothesis was that early and medium season
corn will yield the same amount of grain per ha if both are planted at
the same date and at appropriate population to obtain canopy LAI suf-
ficient to intercept 95% or more of the available solar radiation.
The
second hypothesis was that air temperatures during ear shoot develop-
ment and at silking combine to limit kernel number and subsequent grain
yield of late compared to early planted corn.
The third hypothesis was
that rate of grain filling is slower for late compared to early planted
corn and that duration of the grain filling period of late planted corn
is halted prematurely due to cool fall temperatures.
, \\' ~r&.L..~; 0, , ,

---

CHAPTER III
MATERIALS AND METHüDS
A.
Location of the Experimental Field
The experiment was estab1ished during May and June 1979 on The
Pennsylvania State University Agronomy farm at Rock Springs, at
0
0
approximate1y 358 m (1175') altitude, 41 N latitude, and 78 W longi-
tude.
The field was in a large, east-west valley with a ridge (469 m)
on the south side.
The predominant soi1 type was a Hagerstown si1t
loam (Typic Hap1uda1f; fine, mixed, mesic).
The area surrounding the
field plots was covered rnain1y by corn, a1fa1fa, and grass.
B.
Experimental Design
The experimenta1 field covered 0.16 ha (0.4 acre) of land.
A
split-plot design, with four rep1ications, was emp10yed.
The main
plots consisted of 4 p1anting dates whi1e the sub-p10ts inc1uded two
corn hybrids.
Each plot consisted of 8 rows that were 6.7 m (22')
long with 76.2 cm (30") between rows.
C.
Plant Establishment
An ear1y (Come11 281) and an ear1y-medium season (Pioneer 3780)
corn hybrid, classification based on the Pennsylvania maturity zone
classification system (McGahen and Johnson, 1978), were grown.
However,
at The Pennsylvania State University Agronomy farm, Come11 281 and
Pioneer 3780 are re1ative1y ear1y-medium and full season hybrids,
respective1y.
Each hybrid was hand p1anted at approximate1y 2-week
interva1s, beginning in the normal recommended p1anting period for the
area and extending to 1ate June.
Actua1 p1anting dates of 12 and 30 May,

---

25
8 and 22 June, deviated from the intended 2-week interval because of
Inclement rainy weather.
In an effort to obtain a mature plant canopy
with a leaf area index (LAI) of approximately 4, two seeds were planted
per hill at a distance of 13.3 cm (5.25") for Cornell 281 (C28l) and
18.4 cm (7.25") for Pioneer 3780 (P3780).
The target LAI of 4 was
chosen based on a study by Duncan (1972) in which he found a LAI of 4
to be near optimum for solar radiation interception in maize.
Earlier
experiments at the Agronomy farm within various research projects
established the approximate plant population necessary to obtain the
desired LAI on P3780 (M. Boyle, M.S. Thesis, Pennsylvania State
University).
Information on yield, standability, and barrenness of
C28l was obtained from the 1978 Pennsylvania Commerèial Hybrid Corn
Tests Report by McGahen and Johnson.
The crop was hand-thinned to
one plant per hill to obtain plant populations of 86,500 and 66,500
plants/ha for C28l and P3780, respectively.
Thinning dates were
25 June, 6 and 18 July, and 6 August for the planting dates 1, 2, 3,
and 4, respectively.
Muriate of potash (60% K) and ammonium nitrate (34% N) were broad-
cast at 235 kg/ha (210 lbs/acre) and 684 kg/ha (600 lbs/acre), respec-
tively, before spring plowing; a 15-15-15 starter mix of 168 kg/ha
(150 lbs/acre) was placed in the row with a tractor mounted, conven-
tional corn planter, before hand seeding.
The soil test report obtained
before fertilizer was applied in 1979 was pH 7.2, 96 kg/ha of bray Pl
extracted P (86 lbs/acre); 0.28, 0.5, and 12.5 meq/lOO g for K, Mg, and
Ca, respectively.
The CEC was 13.3 meq/mg.
Percentage of CEC saturation
was 2.1, 3.8, 93.9 for K, Mg, and Ca, respectively.
The soil test
results indicate that P was medium, Kwas low-medium, Ca :was excessive,
·,H:.</iJ
,·•...: " l

---

26
and Mg was low.
Furadan lOG was banded over the row at 13.5 kg/ha
(12 lba/acre) for insect pest control.
Weeds were controlled by an
application of atrazine at 2.8 kg/ha active ingredient (2.5 lbs/acre).
Eight moisture blocks (gypsum) were put in the field to monitor
soil moisutre status.
These moisture blocks were placed at 23 cm
(9 inches) depth in the row.
The selection of this depth was based
on Bouyoucos et al.
(1940) and Bouyoucos (1950) suggestions that the
moisture block should be placed in the root zone where the water absorp-
tion ls maximum.
The actual depth of 23 cm was used to correspond with
that suggested in the Splinter corn growth model (Splinter, 1973).
The
location of each moisture block throughout the field was chosen such
that the coverage was approximately uniform over the experimental site,
and also that each planting date treatment contained 2 blocks.
They
were monitored at least 3 times a week.
The readings were taken in ohms
(resistance) and then were converted to bars by the use of standard
curves specifie to each moisture block.
The plots were irrigated when
the soil water potential declined below -2 bars.
The field was irri-
gated only once on June 26.
D.
Plant Sampling
Two different types of measurements were taken during the experi-
ment:
(1) measurements concerning the biological and physiological
aspects of the corn plant; and (2) the environmental factors which
were thought to be associated with corn yield.
Plant measurements
included kernel site number and number of kernel rows per cob; stalk,
leaf sheath, leaf blade, and kernel dry weights; and leaf are a and
plant height.

---

27
The two central rows of each plot were reserved for the final plant
and grain harvest.
The 6 other rows were used for the different sam-
plings.
The outside rows were sampled only when the plants were young
and, therefore, the border effect was still negligible.
Plants were
sampled to determine time of tassel initiation before hand thinning by
carefully digging out one of the two plants per hill.
Four consecutive
plants were sampled per plot beginning 13, la, 8, and 6 days after emer-
gence for the planting dates l, 2, 3, and 4, respectively, and continued
daily until tassel initiation was observed.
The plants had 4 to 5 dis-
played leaves when sampled.
Dug plants were placed in labelled paper
bags, transported to the laboratory on campus, and dissected under a
binocular microscope.
The tassel was considered to be initiated if 50
percent of the observations per hybrid (average over the 4 replications)
satisfied Bonnett's (1966) criteria for tassel formation.
Every second
plant, a total of 4 plants, was sampled per plot for dry matter accumu-
lation by the different parts of the plant (stalks, leaf sheaths, leaf
blades).
Samples were taken twice a week.
In order to avoid canopy
opening, plants were selected on alternative rows starting from the
outside rows towards the inside rows.
In the laboratory the plants were
separated into stalk, sheath, and blade components, put into labelled
paper bags, and dried in a forced draft dryer at 69 0
69 C (157°F) for 1 to 3
weeks, depending on the amount of tissue being dried.
A preliminary ex-
periment consisting of a series of drying and weighing cycles was used
to determine appropriate drying time.
Dried plant sampI es were weighed
to the nearest centigram.
Leaf area was determined by measuring the length and width of
leaves on four different plants per plot twice a week from 3 weeks after
emergence to 1 week after midsilk.
Leaf area (LA) was calculated with

---

28
the formula deve10ped by McKee (1964).
McKee (1964) deve10ped two formulas for corn 1eaf area determina-
tion based on non destructive 1eaf measurements.
The equations are
as fo11ows:
(1)
LA = E(L X W)0.73
(2)
LA = (EL)6.67
Where L
the over-a11 1ength of the 1eaf
W
maximum width of the 1eaf
The mean 1eaf area coefficient (0.73) in Equation 1 was based on
measurements of 1,128 1eaves from 8 corn hybrids.
McKee found that
there was no significant difference in the 1eaf area coefficient due
to plant population or plant variety.
He reported 1itt1e consistent
effect of age and position on the plant on the 1eaf area coefficient.
He a1so proposed using the same 1eaf area coefficient for plants grown
both indoGrs in the greenhouse and outdoors.
He found that the cor-
relation coefficient between measured 1eaf area per plant and the 1eaf
area of a rectangle represented by E(L X W) was 0.9985*** (r.
=
001
0.3211).
McKee reported that the 1eaf area coefficient in Equation 2
gave 1ess precise, but adequate resu1ts.
He found that the EL for each
plant was significant1y corre1ated with EW, r = 0.9011***, and with
the measured 1eaf area, r = 0.9439*** (r.
= 0.3211).
Leaf area
001
index (LAI) was ca1cu1ated with the fo11owing formula:
2
LAI = LA/plot (cm )
6
2
0.445X10
(cm )
6
2
Where 0.445 X 10
(cm) = area of each plot
LA/plot = (average 1eaf area/p1ant)(p1ant population/plot).
..o.-,J
..o.-.J....

---

29
Plant height was measured to the whorl before tasselling and to
the tip of the tassel after tassel emergence.
The measurernents were
taken twice a week on the 4 plants used for leaf area determination.
When the topmost ear was visible, the number of leaves above this was
recorded.
Reproductive development was assessed by ear shoot development
before sil king and by grain development after silking.
For ear shoot
development, observations were made on two of the four plants sampled
for measurement of dry matter accumulation, starting about six weeks
after emergence.
The two topmost ear shoots were studied under the bi-
nocular microscope described earlier.
Number of double rows per ear
shoot and the number of kernel sites per double row were counted.
It was
decided to make the observations on the double row basis because the
initial spikelet primordia started dividing from the base of the ear
shoot giving a pair of rows.
Meanwhile, the row stayed single at the
tip of the ear shoot so that it was easier to count the kernel sites per
paired rows.
Ear length, excluding the shank, was also measured.
Tasselling and midsilk dates were determined by counting the
number of plants with visible tassel rising from the whorl or visible
silks appearing from the husks, in the two central rows per plot.
Fifty
percent tasselling and midsilk dates were reported when 50 percent of
the plants averaged over the 4 replications had tassels or silks.
Samples were collected for evaluation of the kernel grain filling
period weekly starting approximately 20 days after silking.
For these
observations, six ears per plot were sampled once a week on consecu-
tive plants starting from the outside rows towards the inside rows.
Number of double
rows and the number of
filled kernels
per

---

double row were counted.
Total ear length was measured to the nearest
1 mm, then the six ears were put in a labelled cloth bag and dried at
69°C (157°F) in the dryer described earlier.
Dry weight (DWt) of the
ear (cob + kernels) was determined right after removal from the dryer,
three to four weeks after sampling.
Grain from the central part of
each ear was shelled separately.
Seventeen kernels from each of the
six ears/plot were bulked to give 102 kernels, weighed. redried at
104°c (219°F). and reweighed.
The rernaining kernels from the six ears
were bulked and weighed without additional drying.
These weights were
o
adjusted to dryness (104 C) based on percentage moisture calculated for
the 102 kernels dried at 104°C.
A coefficient of correction between
0
dry weights at 69 C and DWt at 104°C was calculated.
The coefficient
0
was then used to calculate the total grain DWt at 104 c from the DWt
0
at 69 C.
Fifty percent blacklayer was determined by looking at 4 kernels
per ear from two ears per plot.
The sampling started when blacklayer
was observed on the same hybrid planted earlier in an adjacent experi-
mental field.
The hybrid was considered to be at 50 percent blacklayer
when four of the eight kernels observed. averaged over the replications,
reached blacklayer.
At blacklayer, plants were harvested to determine final grain and
plant weight. and harvest index (HI).
Ears were harvested from aIl
plants within two successive six-foot sections in the middle of one
of the center rows of each plot.
This allowed a five-foot border on
each end of the harvested row.
In addition, six consecutive plants
were sampled from the middle section of the second center row in each
plot.
The plants were cut at ground level. bulked in burlap bags, and

---

31
0
o
dried at 69 C (157 F) in a forced draft oven.
Final plant DWt and final
grain DWt per plant were determined by weighing the samples after
removal from the dryer.
The ears were shelled and 102 kernels sub-
sampled as described earlier for grain dry weight determination at
o
104 C.
The number of barren plants (plants which failed to produce
ears) was determined by counting the total plants in the row harvested
and the number of plants with ears.
The difference between the two
numbers recorded gave the total number of barren plants.
Sorne of the
plants had apparent ears but failed to produce any grain leading
obviously to an underestimation of the percentage of barrenness.
The
barrenness index (BI) was computed as the ratio of barren plants over
total plants.
From the preceding data, the final grain yield was cal-
cualted in two ways using:
(i) the average grain dry weight per 6
feet of row adjusted to a unit land area basis, and (ii) the average
kernel dry weight, the number of kernels per ear, and the plant popu-
lation with an adjustment for percentage barrenness.
The harvest
index was calculated using the following equation:
GW
HI - PW
[2]
PW
Where HI = Harvest Index
GW = total grain dry weight from 6 plants
PW = total plant dry weight (vegetative and reproductive
parts from the same 6 plants)
E.
Weather Data
The weather data were collected at the Agronomy farm of The Pennsyl-
vania State University Weather Station, near Rock Springs, Il km (6 miles)

---

32
o
0
from campus, at approximately 4l N latitude, 78 W longitude, and 358 m
(1175') altitude.
The station was located in the middle of the farm,
about 300 m (~ 1000') from the experimental site.
Soil data were
collected on both bare and sod covered plots.
The following parameters
were measured hourly:
soil and Weather Bureau Shelter air temperatures,
soil water potentials, precipitation, and global solar radiation.
AlI
the instruments were connected to a Campbell Scientific CR5 data logger
which stored recorded data on printed paper tape and on audio cassette
tape for computer processing.
Thermocouple thermometers (Cu-Cn) were used to measure she~ter air
temperature and bare and sod soil temperatures at 5 cm (2 inches) depth.
Hollinger and Reetz, Jr. (1979 Agronomy Abstracts) reported differences
o
(up to 6.l C) between weather station and crop canopy temperatures when
measured hourly.
The differences however were small when the tempera-
ture hourly measurements were averaged over 24 hours (day and night).
Soil tempe rature was monitored manually at least twice a week in the
experimental field plots at 5 cm depth.
Eight thermocouples used for
this purpose were placed at the same locations as the moisture blocks
described earlier.
Let us point out that these thermocouples and
moisture blocks were placed after the second corn planting resulting
in missing data at the beginning of the growing season.
The 5 cm depth
was chosen for soil temperature measurement because of the importance
of soil temperature near the seedling terminal growing point on plant
emergence and seedling growth (Van Wijk, 1965).
Soi1 temperature data
from the field site were used to evaluate whether the weather station
soil data was representative of the experimental site.

---

33
Gypsum blocks were used for measuring the soil moisture status.
Both bare and sod plot soil water potential was measured at depths of
5 cm (2"), 15 cm (6"), and 30.5 cm (12").
A standard rain gage, tipping bucket transmitter Ne562 , marketed
by Science Associates, Inc., was used to measure rainfall.
An Eppley black and white pyranometer was used for global solar
0
radiation measurement.
The Eppley 180
pyranometer, 50 junction model,
used in the study was recalibrated on May 10, 1979 (by the meteorology
department of the Pennsylvania State University) and observed to
generate an emf of 6.92 mV/ly.
The hourly shelter temperature and the following formula were used
to compute growing degree days (GDD).
n
24
GDD
E
(E
T')/24
[ 3)
i=l
j=l
H
Where T'
adjusted hourly temperature (oC)
H
o
TH when 10 < TH ~ 30 C
0
30 i f TH > 30 c
10 i f TH < 100C
TH
the actual hourly temperature
n = total length of period (days) investigated
j
number of hours during the day
i
days investigated
Two different periods were investigated:
(i) time from emergence
to midsilk for the vegetative development; and (ii) time from midsilk
to blacklayer for grain filling.
This method for calculating growing
degree days was described and evaluated by Cross and Zuber (1972).

---

34
F.
Statistical Methods
Statistical analysis and data conversions were carried out by the
use of library programs, compiled for the IBM computer, model 3033, as
operated by the Computation Center at The Pennsylvania State University.
The Statistical Analysis System (SAS) mean procedure was used for the
computation of the hourly adjusted temperature average needed for the
GDD calculations.
The SAS split-plot method of analysis of variance
(ANOVA) and Duncan's multiple range test procedures were used in
analyzing the experimental data and in describing the statistical sig-
nificant differences.
The SAS General Linear Models procedure (GLM)
was used in running linear regression on sorne of the data (kernel
dry-matter accumulation, accumulation of kernel sites).
Homogeneity
of regression coefficients was tested by the use of the sum of squares
term of the interaction between the independent variable and the class
(treatment) •

---

CHAPTER IV
RESULTS AND DISCUSSION
A.
Microclimate Data
Appendix A, Table 45, contains a summaryof the weather data which
includes the adjusted average, the minimum and maximum shelter air tem-
peratures in oC, global solar radiation (ly/day), and the rainfall in
mm/day.
The plot of these different data is shown in Figure 2.
The
o
highest shelter air t~lperature (32 C) of the season was recorded in
May.
In July and August maximum and minimum temperatures were mostly
0
o
higher than 23 C and l2 C, respectively.
Low radiation was recorded
in late May and early June, and in late September and early October.
The rainfall totaled 521 mm from May through October 15 and was fairly
weIl distributed during the growing season.
The crop received only
one application of 25 mm irrigation water on June 26.
Appendix A, Table 46, summarizes soil
sail temperatures (5 cm depth) at
the two locations (weather station bare plot and the experimental field)
for approximately the same time of the day.
The t-test (Table 3) showed
that there was no significant differences between soil temperatures at
5 cm depth recorded both in the weather station and in the experimental
field.
Since there was no difference in soil
sail temperature between the
two locations the microclimate data from the weather station were con-
sidered appropriate for the study in the field.
Tables 4 and 5 contain the growing degree days computed, respec-
tively, from emergence and midsilk.
The cumulative growing degree days
over 5-day intervals, from 1 May to 15 October, are plotted on Figure
3.
The rate of heat accumulation declined late in the growing season

---

---

37
720
0.0
Hay
Jtme
Ju1y
August
+30
û
o~
1;1 +25
'"
+20
+15
+10
+5
û
o
.~
0
;L
48
c
.;;
u
".~..-. 36
o.;..,
.~
~
'0"0
~ ....
~ ~ 24
;..,
-<
.~
~
Cl
12
May

---

38
Table 3.
Surnmary of the t-test used in the comparison of soil tempera-
ture at 5 cm depth between the Weather Station and the experi-
mental field;
data used in this test were recorded from
6 June to 6 July 1979.
Mean
Standard
Location
Temperature
error
Variances
T
DF
Prob> ITI
Field
22.3
0.807
Unequal
0.767
20
0.4524
Weather
Station
21.4
0.770
Equal
0.767
20
0.4524
For Ho:
Variances are equal, Ft
1.10 with la and la DF,
Prob > F' =
0.8847

---

39
Table 4.
Growing degree days (GDD) from emergence to successive harvest
dates, computed from she1ter air temperature, for use in the
ana1ysis of the vegetative growth.
Julian
Emergence Date
Julian
Emergence
1979 Date
Day
5-20-79
6-5-79
6-14-79
6-29-79
-------------------GDD---------------------
22 June
173
548
331
167
03 Ju1y
184
727
511
347
86
05
186
752
535
371
110
06
187
767
550
386
125
09
190
821
604
(~41
179
10
191
840
623
459
198
13
194
904
688
524
263
16
197
972
755
591
330
19
200
1035
818
654
393
23
204
1118
901
737
476
25
206
1162
946
782
521
26
207
1185
969
805
543
31
212
1285
1069
905
644
02 August
214
1334
1118
954
630
05
217
1401
1185
1021
760
07
219
1444
1228
1064
803
10
222
1513
1296
1133
871
13
225
1561
1344
1181
919
16
228
1608
1392
1228
966
20
232
1682
1465
1301
1040
22
234
1719
1503
1339
1077
23
235
1740
1523
1359
1098
26
238
1803
1587
1423
1162
27
239
1825
1608
1444
1183
29
241
1869
1652
1488
1227
30
242
1891
1674
1510
1249
31
243
1911
1694
1530
1269
06 September
249
2039
1822
1658
1397
12
255
2119
1902
1738
1477
17
260
2215
1998
1834
1573
20
263
2257
2041
1877
1616
27
267
2310
2232
1929
1668
30
270
2353
2136
1972
1711

---

Table 5.
Growing degree days (GDD) from midsi1k to successive harvest dates, computed from she1ter air
ternperatures,
temperatures, for use in the ana1ysis of kerne1 dry matter accumulation.
Midsilk Date
C281
P3780
1979 Date
Julian
7-31-79
8-8-79
8-18-79
8-29-79
8-2-79
8-10-79
8-20-79
8-31-79
-------------------------------------GDD--------------------------------------
31 Ju1y
212
0
0
0
0
0
0
23 August
235
454
271
100
405
227
58
27
239
539
356
185
0
491
312
143
30
242
605
422
251
22
556
378
209
0
05 September
248
731
548
377
148
682
504
335
106
06
249
753
570
399
170
704
526
357
128
10
253
801
618
446
218
752
573
405
176
12
255
833
650
479
250
784
606
437
208
17
260
929
746
574
346
880
702
533
304
20
263
972
789
617
389
923
744
576
347
27
267
1024
841
670
441
976
797
628
399
30
270
1067
884
713
484
1018
840
671
442
E
;.
07 October
277
1181
998
826
598
1132
953
785
556
11
281
1224
1041
869
641
1175
996
828
599
13
283
1245
1062
890
662
1196
1017
B4B
848
620
.,..
o

---

---

42
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---

43
(September and October) due to the low solar beam incident angle and
shorter photoperiod during that period in the Temperate Zone of the
Northern Hemisphere.
A relative constant heat accumulation rate was
recorded from Mid July to early August leading to a
linear
increase
in growing degree days.
This result is evident in the diagram presented
in Figure 3.
During the period of study (1 May to 15 October), a total
0
0
of 2900 growing degree days (base 10 , 30 C) was accumulated.
B.
Corn Plant Sampling Results
1.
Time to Emergence
The time from sowing to plant emergence and the growing degree
o
0
days (GDD, base 10 , 30 C) computed from the available soil temperatures
measured at 5 cm (2") depth are shown in Table 6.
The time to emergence
was similar for both Cornell 281 (C28l) and Pioneer 3780 (P3780);
however, there was a slight difference between times to emergence asso-
ciated with planting date.
The time was 6 da ys (average soil tempera-
o
ture
23.l C) for the third planting and 7 days (average soil tempera-
0
ture
22.6 C) for the fourth planting date.
Higher daily average sail
soil
temperature was associated with a shorter time to emergence, which
agrees with earlier findings by Iremiren and Milbourn (1979).
Number
of sail
soil GDD varied with time to emergence showing that the young plant
did not have to accumulate a certain amount of growing degree days
before rising above the ground level.
The average daily sail
soil tempera-
ture, therefore, is more significant for characterizing the time to
emergence than the GDD based on soil temperature.

---

44
Table 6.
Time in days and GDD from p1anting to emergence of both
Corne11 281 and Pioneer 3780 p1anting at successive dates.
1979
GDD (soil)
Average
P1anting
Time to Emergence
Sowing-Emergence
Dai1y Temperature
Date
(days)
at 5 cm
Oc
May 12
8
May 30
6
June 8
6
138.5
23.1
June 22
7
158.1
22.6
Table 7.
Time in days and GDD from emergence to tas sel initiation of
both Corne11 281 and Pioneer 3780 p1anted at successive dates.
Hybrid
1979
C281
P3780
P1anting
Date
Days
GDD*
Days
GDD
at 5 cm D
at surface
at 5 cm D
at surface
12 May
17
17
30 May
11
223.7
224.8
12
248.5
248.2
08 June
9
198.8
189.5
10
215.6
208.9
22 June
6
103.6
111.6
7
120.5
120.8
*GDD computed from soi1 temperature
D = depth

---

45
2.
Time from Emergence to Tasse1 Initiation
The time from emergence to tasse1 initiation (Table 7) varied from
6 to 17 days with p1anting dates.
However, the differences between
c281 and P3780 within p1anting dates was on1y 1 day.
The time from
emergence to tasse1 initiation was found to decrease with 1ate p1anting.
0
0
Number of accumu1ated GDD (base 10 , 30 C) ca1cu1ated from soi1 temper-
atures (5 cm) for the same period dec1ined a1so with 1ate p1anting.
Accumu1ated GDD at 5 cm depth were 223.7, 198.8, and 103.6 for C281,
and 248.5, 215.6, and 120.5 for P3780 at dates 2, 3, and 4, respective1y.
There were no large differences between the GDD (Table 7) computed from
soi1 temperatures recorded at the surface and at 5 cm depth.
This
sma11 difference suggests that sail surface temperatures did not influ-
ence ear1y seed1ing growth any more than soi1 temperatures at 5 cm depth.
The average dai1y soi1 temperature (5 cm) during each growth period
was higher for the third p1anting date in both hybrids.
They were 20.3,
0
22.1, and 17.30
17.3 C for C281 and 20.7, 21.6, and 17.2 C for P3780 at dates
2, 3, and 4, respective1y.
This dec1ine in time to tas sel initiation
whi1e the soi1 temperature decreased, as observed from dates 3 to 4,
disagrees with an earlier finding by Coligado and Brown (1975) that
there was a consistent decrease in time to tasse1 initiation as temper-
0
0
ature increased from 15
to 25 C regard1ess of photoperiod.
It is
important to point out, though, that their experiments were conducted
in growth chambers at constant temperature whi1e my resu1ts were
observed in the field, where temperature and irradiance 1eve1s vary
wide1y.
The total irradiance accumu1ated between emergence and tasse1
initiation decreased with 1ate p1anting.
The dai1y average irradiance

---

46
(ly/day) during each period was 519.7, 432, and 501.6 ly/day for C28l,
and 531.6, 445.0, and 495.8 ly/day for P3780 at dates 2, 3, and 4,
respectively.
Neither temperature nor radiation provide a clear expla-
nation of the decline observed in the time to tassel initiation with
delayed planting.
3.
Plant Development
a.
Plant population.
The plant stand at harvest varied between
the different planting dates (Table 10).
The number of plants per row
deviated from the expected fifty for C28l and thirty-six for Pioneer
3780 in most cases.
The reduction in plant stand was mainly due to
poor emergence resulting from dry soil surface early in the season.
The average plant population was 86,500 plants/ha for C28l and 66,500
plants/ha for P3780.
b.
Leaf development.
A sWTIffiary of the averages of leaf area per
plant and leaf area index (LAI) per planting date are shown in Appendix
B, Tables 47 and 48.
The LAI data plotted in Figure 4 show that the
LAI increased linearly with growing degree days after an early phase
of increasing LAI accumulation rate, reached a maximum, and then declined
slowly.
The analysis of regression (Tables 8 and 9), which only includes
data from the linear part of the graph, gave a single regression line
for all planting dates of E!ach hybrid.
The regression equations are
as follows:
LAI
0.00451 GDD
1.644
for c28l
[4]
LAI
0.00433 GDD - 1.987
for P3780
[5J

---

---

48
Cornell 281
•
Treat l
•
Treat Il
D Treat III
o Treat IV
01----==::;,::..---------------1
Pioneer 3780
o ~_ _.....
_
--=::'.....--I---
.....
......
--=::'.....--I---
--.....la.---
......
....
--.....la.---
-
....
..
300
600
900
1200
1500
0
Growing Degree Days (base 10 , 30°C) fram emergence

---

49
Table 8.
Regression analysis on Leaf Area Index versus growing degree
days for two corn hybrids (C28l, P3780).
The points from
the linear accumulation were used in this analysis.
Regression
F ratios
Hybrid
Intercept
Coefficient
Significance
R-square
C.V.
Cornell 281
-1.644
0.00451
***
0.870
19.49
Pioneer 3780
-1. 987
0.00433
***
0.967
11.36
*** Significant at 1 percent level of probabilit y.
Table 9.
Test of homogeneity on the regression coefficients shown in
Table 6.
Significance of F-ratios
Source of Variation
d.L
Type IV
Hybrid
1
NS
GDD
1
***
t
GDD* Hybrid
1
NS
*** Significant at 1 percent level of probability.
NS Non-significant at 10 percent level of probability.
t The interaction GDD* Hybrid SS were used to test the homogeneity
of the regression coefficients.

---

50
The model was significant at the 1 percent level of probability for both
hybrids.
The respective coefficients of determination and the coef-
ficients of variation were 0.870 and 19.49 for C28l, and 0.967 and
Il.36 for P3780.
The test of homogeneity of regression coefficients
shows that there was no significant difference between the rates
of LAI accumulation for the two hybrids.
The significance of the
sum of squares due to GDD effects at the 1 percent level of prob-
ability suggests that cumulative heat units were an important factor
influencing leaf development.
Essentially 87.0 percent of the vari-
ation in LAI of c28l was associated with the GDD while 96.7 percent
of the variation in LAI of P3780 was associated with the GDD.
However,
sorne other environmental factors such as radiation, has been reported
to strongly affect leaf development (Evans, 1963).
Watts (1971)
reported a sharp decline in leaf elongation caused by water stress.
Soil fertility was found also to have an important influence on leaf
expansion (Elias et al., 1979).
The LAI was found to reach its
maximum at the midsilk growth stage.
Average values ranged from 3.2
to 4.1 for C28l, and from 3.0 to 3.4 for P3780 (Table 10).
Contrary
to expectations, there were sorne differences in the LAI mostly
associated with the variation in plant stand.
Figure 4 showed that
the leaf growth was strongly associated with the growing degree days
while the lower senescence was independent of the GDD at the end
of the growing season.

---

51
Table 10.
Leaf Area Index at midsi1k, actua1 number of plants per row
(# P/R) , and average number of 1eaves above the topmost ear*
for two corn hybrids (C281 and P3780) p1anted at 4 dates in
the spring of 1979.
1979
Hybrid
P1anting
Date
C281
P3780
LAI
Il P/R
IfL
LAI
Il P/R
IlL
12 May
4.1
48
5.9
3.3
36
6.4
30 May
3.5
46
5.7
3.4
35
6.3
08 June
3.3
41
5.8
3.4
32
6.2
22 June
3.2
43
5.6
3.0
33
6.0
*For aIl p1anting dates and hybrids the range in number of 1eaves
ab ove the ear was 5-7.
Table 11.
Percen tage of total shoot dry matter in plant parts (leaf
shea ths, 1 eaf b1ades, and sta1ks) at midsi1k for two corn
hybrids (C281 and P3780) p1anted at successive dates in the
spring of 1979.
1979 P1anting Date
Plant
Hybrids
Parts
12 May
30 May
08 June
22 June
Average
------------------------%------------------------
sheaths
18.6
17.6
17.4
14.9
17.1
C281
b1ades
33.0
26.9
28.7
28.9
29.4
sta1ks
48.4
55.5
53.9
56.3
53.5
sheaths
18.0
16.9
17.0
15.7
16.9
P3780
b1ades
35.3
31. 5
32.6
31.1
32.6
sta1ks
46.7
51.6
50.4
53.2
50.5

---

52
The number of leaves above the topmost ear was slightly higher
for P3780 (6.4, 6.3, 6.2, and 6.0 leaves) than for C28l (5.9, 5.7,
5.8, and 5.6 leaves) (Table Il).
The actual number of leaves above
the topmost ear ranged from 5 to 7 for aIl dates and hybrids.
It
has been shown in research work that these leaves above the ear are
the main source of photosynthate providing the carbohydrate for
kernel fill (Eastin, 1969).
c.
Plant dry matter accumulation during the period from emergence
to silking.
Dry (69 0
(69 C) weights for the different parts of the plant
above the ground are summarized in Appendix C, Tables 49, 50, and 51.
0
The dry (69 C) weights of stalks, sheaths, and blades are plotted
against the growing degree days in Figures 5, 6, and 7, respectively.
Like dry matter accumulation by plant parts on a calendar day basis
(Hanway and Russell, 1969), the general curve of dry matter accumula-
tion by the stalk, leaf sheath, and leaf blade was sigmoid in shape.
The plotted data showed that the dry weight accumulation rate per degree
day in the sheath (Figure 6) was lower (lower slope) than the dry matter
accumulation rate in the stalk (Figure 5) and the blade (Figure 7).
The graphs also show that the rate of accumulation was relatively higher
in the stalk than in the blade.
At midsilk there was less variation
among planting date treatments in the percentage of total plant dry
weight associated with sheaths than with stalks and blades (Table Il).
Averaging over planting dates and h}brtds, 17, 31, and 52 percellt of
the total plant dry matter at midsilk was associated with sheaths,
blades, and stalks, respectively.
Total plant dry matter (Appendix
C, Table 52) determined at 1040
104 C by a correction of the dry weights

---

---

54
Cornell 281
180
• Treat l
a
.qp.
,.....
•
Treat II
Ul
•
160
C Treat III
•
•
Ul
•
160
C Treat III
•
a
'"'r:::
o Treat IV
.--1
Ci
0
l1l
.--1
Ci
l1l
00
p.
140
•
p.
140
-<T
00
---on
'-"
120
8
:J:
a
'"'
D
:J:
'"'
100
,.....
u
0a-
.D
u
0a-
80
~
;.,
I-l
60
0
~<li
40
'"'
U)
20
0
Pioneer 3780
.EI:J-
220
200
D •
200
D
C· 0
,.....
Ul
180
-00
'"'g
.--1
p.
160
.lb •
160
.lb
-<T
---on
---
'-"
•
'-"
140
'"'
::s:
r
'"'
::s:
a
,.....
120
0
u
0a-
'"
100
'"
'-"
-8
'-"
1-1
80
0
~
..
;.,
1-1
80
0
~
;.,
~
_.D
60
'"'
U)
40
.q,
.a
20
.b
0
0
300
600
900
1200
1500
1800
o
Growing Degree Days (base 10°, 30 C) from Emergence to Midsilk

---

---

56
Corne11 281
,-.
<Jl
• Treat I
..,
70-
•
Treat II
~
.-1
D Treat III
0.
60-
o Treat IV
-<t
--...
00
'-"
,.irP.-~-
00
'-"
50
D
..,
_ 0
..,
_
0
~
a
0
.-...
40-
.-...
u
0
rJIIP
0
'"
\\0
30
80
'-"
8
'-"
;..,
I-l
Ça
20-
§
,.<::
•
Ça
20-
,.<::
..,
<tI
QJ
10
,.<::
8""f1'
,.<::
[/)
_ nrOliflt
-,
,
1
1
1
1
Pioneer 3780
80-
.-...
CIl
..,
70-
~
.-1
0.
60-
~
60-
-<t
--...
8 De.- •
.::J
50-
-I-J
_a~ 0
_a~
-I-J
~
~
40-
.-...
u
0
'"
\\!)
30-
......,
."
'"
\\!)
30-
......,
Ba
;..,
20-
l:J
,.<::
• ~
,.<::
•
..,
<tI
10-
QJ
~
QJ
.a
[/)
feB
,
0
•
•
1
•
•
•
1500
1800
0
300
600
900
1200
1500
Growing Degree Days (base 10 0
10 , 30°C) from Emergence
ta Midsilk

---

---

58
Comell 281
aa
• •
90.
•
90.
,-..
• Treat l
,-..
• Treat
Ol
..,
• Treat II
aa
•
• Treat II
aa
a Treat III
& 80·
a. _
& 80·
a.
IV
ri
0 Treat
0
P-
..;:t
70
o
i
..;:t
70
o
.......
00
'-'
0 •
'-'
0
60 -
..,
,-..
50·
u
a .-
~
,-..
50·
u
a
~
°0'\\
\\0
' - '
40·
~
'"0 30·
"i
'"0 30·
't:l
ID
,
al
't:l
ID
ri
20·
~
D
....ID 10 _
al
i
10 _
al
0-1
.-œ's/'
0
•
•
•
•
•
•
Pioneer 3780
.D •
90 -
~
,-..
Ol
.., 80 •
I::l
0
ID
ri
P-
70
"0
ID
"
ri
P-
70
D
..;:t
.......
00
' - '
60·
•
' - '
60·
0
..,~
•
~
50
Û
0
•
00'\\
\\0
40
'-'
0 ) -
~
'" 30 -
0'"
30
0
al
't:l
8
ID
20.
0
ri
-
al
't:l
8
ID
20.
0
ri
~
la
~
....
0
ID
10·
al
0-1
.!lI
0-1
0
•
•
-.
1
1
•
-.
1
•
0
300
600
900
1200
1500
1800
Growing Degree Days (base 10°, 30°C) fran Emergence
to Midsilk

---

59
0
obtained at 69 C from the different plant parts (see correction factor,
Appendix C, Table 53) is plotted on Figures 8 and 9.
The plot shows a
slow increase in plant dry weight with growing degree days during the
early growth of the crop, which reached a constant growth rate at
about 700 GDD.
The same results were obtained by Hanway and Russell
(1969) on a calendar day basis.
These authors found that maximum dry
matter was reached first in the blades, then the sheaths, and later
the stalks.
In my study, plant part dry weights balanced each other
in such a way that the bulk growth rate was essentially constant over
mu ch of the growing season producing a linear relationship with GDD.
There were sorne differences in the linear rates of total plant dry
weight accumulation versus growing degree days with respect to planting
dates (Figures 8 and 9).
The rates for both hybrids ranked by planting
treatment were as follows:
3rd > 2nd > lst > 4th.
The average plant
dry weights (104°C) at midsilk were 69, 70, 80, and 60 grams/plant for
Cornell 281, and 72, 79, 84, and 64 grams/plant for Pioneer 3780 at
dates l, 2, 3, and 4, respectively.
Plant dry weight at midsilk
increased with delayed planting up to the third date and then decreased
substantially with the fourth planting.
The warm temperatures recorded
in July and èàrly August (Figures 2 and 3) apparently provided a better
enviromnent for vegetative growth of the second and third planting date
treatment, thereby leading to a high arnount of dry matter accumulated
by midsilk.
The data showed that Pioneer 3780 had consistently higher
dry matter per plant than Cornell 281 at aIl growth stages.
This
agrees with the earlier statement made in Chapter III that Pioneer 3780
is a larger hybrid than Cornell 281.

---

60
Cornell 281
DI.
• Treat l
•• Treat
.
Treat II
• Treat I
D Treat III
D Treat
0
Treat IV
t:
0
Treat IV
.~
,.....
....~.-lp..
L··
.-l
p..
.......
••
.......
00
'-"
al;,:
'-"
....
..c:
fil
00
.,-i
Q)
~
J/
~
..
,.....
u
0
-<t
0
.-l
'-"
If
.-l
'-"
»
l-<
Cl
.-l
<Il
....0
E-<
.1
....0
E-<
.J
~
1
0
"'~
•
0
0
300
600
900
1200
1500
Growing Degree Days (base 10°, 30°C) frœl Emergence
to Midsilk
Figure 8.
Re1ationship between total plant dry (104°C) weight
(g/p1ant)
(g/plant) for Cornel1 281 p1anted at four dates in the
spring of 1979, and the growing degree days (base 10°, 30°C).

---

61
Pioneer 3780
• Treat l
•• Treat
Treat II
• Treat
D Treat III
0
Treat IV
-;:J
~H
P.
' -
01J
'-'
90
'"'.<::01J
',-1
80
al
~
Û
70
0
-.j"
0
H
60
'-'
;.-.
'"'
A
50
'"'
H
~0
40
H
30
20
10
0
0
300
600
900
1200
1500
Growing Degree Days (base 10°, 30°C) from Emergence
to Midsilk
Figure 9.
Re1ationship between total plant dry (104°C) weight
(gfp1ant)
(g/p1ant) for Pioneer 3780 p1anted at four dates in the
spring of 1979, and the growing degree da ys (base 10°, 30°C).

---

62
At maturity, total plant dry (690
(69 C) weight ranged from 18.9 to
9.8 mT/ha for C28l and from 16.2 to 8.9 mT/ha for P3780.
It is impor-
tant to point out that plants from the last two planting dates did not
reach blacklayer before the frost.
Total plant dry weight per unit
land area at maturity declined with delayed planting (Table 12).
The
reduction in total plant dry matter per ha with late planting was due
to the decrease in grain dry weight as we will see later in this report.
The analysis of variance on the total biomass (Table 13) shows
that both the planting treatment and the hybrid effects on the total
plant weight per unit land area were significant at the 1% level.
The
interaction between planting dates and hybrids was non-significant
indicating that the dry matter accumulation response of both hybrids
to delayed planting was the same.
Duncan's multiple range test shows
that there was a significant difference at the 5% level,
1evel, between
Cornell
Corne11 281 (14.3 mT/ha) and Pioneer 3780 (12.3 mT/ha) when the total
plant weight per ha was averaged over planting date (Table 12).
d.
Plant height.
Plant height measured in cm (see Appendix D,
Table 54) was plotted against growing degree days in Figure 10.
These
data produced sigmoid curves for both hybrids at aIl dates.
The plant
elongation rate was essentially the same for both hybrids at aIl dates
(Figure 10); however, the fourth planting corn was consistently taller
while the first planting corn was consistently shorter when exposed to
the same number of GDD.
Even though P3780 had agreaterdry matter per
plant it was shorter than C28l plant.
At maturity Cornell 281 measured
2.70 m while Pioneer 3780 was 2.40 m long.
This difference in plant
size might be due to the difference in plant population.
At a popula-
tion of 86,500 plants/ha, C28l plants were so
50
close together that stalk

---

63
0
Table 12.
Total plant dry (69 C) matter per unit land area for two
corn hybrids (C28l and P3780) planted at four dates in
spring of 1979.
Plants were sampled at blacklayer for
early plantings and at frost for late plantings.
1979
Hybrid*
Planting
Date
C28l
P3780
---------------mT/ha----------------
12 May
18.9 a
16.9 a
30 May
14.8 b
14.3 b
08 June**
13.9 c
10.1 c
22 June**
9.8 d
8.9 d
Averaget
Average
14.3
12.3
*Dry weight followed by the same letter within and between columns
are not significantly different (p < 0.05).
**Plants did not reach blacklayer before frost in the fall.
tHybrids are significantly different at P < 0.05.
Table 13.
Summary of significance of F-ratios and degrees of freedom
for analysis of variance of total plant dry weight per ha
for c28l and P3780 planted at 4 dates in the spring of 1979.
Source of Variation
d.L
Significance of F-ratios
Hybrid
1
***
Planting date
3
***
Hybrid * Planting date
3
NS
Error
24
Corrected total
31
***Significance at the 1% level.
NS non-significant

---

---

65
270-
Cornell 281
270-
Cornell
• Treat l
'l-' -
Treat l
'l-' a
240·
• Treat II
• Treat
• •
D Treat III
D Treat
8
210-
0 Treat IV
210-
0 Treat
C.
Su 180-
'-'
...
4'
{b
150-
•.-1
•
Q)
:Il
... 120-
0 -
g
o.
Hg
H
~
90
D
90
60-
o~ -
30-
_e ••
30-
_e
.
0
•
.
Pioneer 3780
0
a.-
250-
1
e.
210-
8
S
180_
Su'-'
a
'-'
... 150.
.c
•
.c
0
bD
'.-1
e
Q)
120-
:Il
...~'Il 90-
H
0\\-
~
a
60
al·
e i •
30-
•
0
•
•
•
1
•
•
0
300
600
900
1200
1500
Growing Degree Days (base 10°, 30°C) from Emergence
to Midsilk

---

66
elongation may have been stimulated by interplant competition for
solar energy.
4.
Reproductive Events
The timing of reproductive growth stages including tasselling,
silking, and blacklayer formation dates, are summarized in Table 14.
The time from ernergence ta midsilk was 87, 64, 65, and 61 days for
C28l and 89, 66, 67, and 63 days for P3780 at successive planting
dates.
Except for the first planting date which had an ernergence ta
silking time interval of nearly 90 days for bath hybrids, there was
little difference in time from emergence ta midsilk between planting
dates for either hybrid.
The time from midsilk ta blacklayer which
is considered ta be the apparent kernel filling period, will be dis-
cussed in detail later in the report.
C.
Factors Influencing Corn Grain Yield
The decline observed in corn grain yield with late planting might
be due ta many factors such as:
number of double rows per ear shoot,
rate and dura tian of kernel site development which affects kernel
number, and rate and duration of kernel filling.
These factors will
be discussed separately.
1.
Number of Double Rows (DR) per Cob
The number of double rows (DR) observed per cob are presented in
Table 15.
The DR tended to be higher for P3780 than for C28l at the
same date.
The averages ranged from 7.1 ta 7.6 DR/cob for C28l and
from 7.4 ta 7.8 DR/cob for P3780.
The actual number ranged from 6 ta
10 DR/cob for aIl hybrids and planting dates.
The analysis of variance

---

67
Table 14.
Visually identifiable reproductive growth stages of two corn
hybrids <Cornell 281 and Pioneer 3780) planted in spring of
1979 at four dates.
Hybrid
1979
C28l
P3780
Planting
Date
50% tas.
50% silk
50% black
50% tas.
50% silk
50% black
----------------------------date----------------------------
12 May
26 Jul
31 Jul
27 Sept
1 Aug
2 Aug
8 Oct
30 May
4 Aug
8 Aug
10 Oct
8 Aug
10 Aug
frost
08 June
13 Aug
18 Aug
frost
20 Aug
20 Aug
frost
22 June
26 Aug
29 Aug
frost
29 Aug
31 Aug
frost
tas. = tasselling
Oct = October
Jul = July
frost
Plants did not reach
Aug = August
blacklayer before
Sept = September
frost in the fall

---

68
Table 15.
Number of double rows (DR) per cob for 2 corn hybrids
(C28l and P3780) planted in the spring of 1979 at 4
dates.
1979
Hybrid
Planting
Date
C28l
P3780
Means
12 May
7.4 b*
7.5 c
7.45 b
30 May
7.6 a
7.8 a
7.70 a
08 June
7.4 b
7.4 b
7.40 b
22 June
7.1 c
7.4 b
7.25 c
Average**
7.39
7.54
*Duncan's multiple range test within column (P < 0.05).
**Hybrid averages significantly different at P < 0.05.
Table 16.
Summaryof significance of F-ratios and degree of freedom
for analysis of variance of number of double rows for two
corn hybrids (C28l and P3780) planted in the spring of 1979
at 4 dates.
Source of Variation
d. f.
Significance of F-Ratios
Corrected Total
111
Madel
7
***
Hybrid (H)
1
Planting date (PD)
3
***
H * PD
3
NS
Error
104
***Significant at 1% level
NS Non-significant at 10% level

---

69
(Table 16) showed that the planting date and hybrid effects on the
number of double rows per cob were significant at the 5 percent level
of probability.
The 30 May planting date had the highest number of
double rows per cob for both hybrids.
A study of the microclimate
data recorded during the week prior to the beginning of kernel site
accumulation, showed that the higher number of double rows was associ-
ated with higher irradiance (ly/day) (Figure Il) but not with higher
temperature (GDD/day).
The beginning of kernel site accumulation
was determined by extrapolating the kernel site accumulation lines
(Figures 12 and 13) to zero kernel site numbers.
It was estimated that
the number of double rows per cob was initiated within 7 days prior
to the beginning of kernel site accumulation.
The results of these
investigations did not reveal a clear relationship between temperature
and kernel row number.
Duncan's multiple range test gave three
classes for each hybrid (Table 15).
This test indicated that there
were significant differences at the 5% level of probability between
planting dates with hybrid.
The interaction between planting dates
and hybrids was non-significant indicating that hybrid response was
the same across planting dates.
When averaged over dates, l observed
7.39 DR/cob for C28l and 7.54 DR/cob for P3780.
These averages were
significantly different at the 5% level (DF = 104, MS = 0.0564639).
The data used in computation of these averages are shown in Appendix
E, Table 55.

---

18.4 GDD
18.8 C
•.(3.7).• ---
-
-
- -
- -
- -
- -
- -
- .
- -
12 May
.....
(3.7)
17.2 GDD
21.4 C
30 May
•
-------
12
30
Corne11 281
(1.1) •
(1.1) -
-------
Corne11
21.3 GDD
20.2 C
•
..
- - - - - - - 6 22 June
(3.3)
15.6 GDD
20.6 C
o
0
- - - - 0
12 May
(2.8)
19.6 GDD
21.3 C
~-----o
30 May
(3.8)
Pioneer 3780
20.9 GDD
20.0 C
A
li.
Ii.
â
-~22Jun
---â22Jun
(3.1)
1
1
l
,
1
l
,
,
,
160
170
180
190
200
210
220
230
240
Julian Days
Figure 11.
Week prior to the beginning of kerne1 site accumulation with its average GDD/day and
its cumulative irradiance, 103
10
1y (in parentheses),
parentheses). in sol id 1ine; duration of 1inear
kerne1 site accumulation with the average dai1y temperature during this period in dashed
1ine for 2 corn hybrids (C281 and P3780) p1anted in the spring of 1979 at 4 dates.
'"
o

---

71
2.
Kernel Site Accumulation
Kernel site accumulation data (Appendix E, Table 56) are plotted
on Figure 12 for C2Bl and Figure 13 for P37BO.
Kernel site accumulation
was linearly related to growing degree days.
After reaching a maximum
which was found to be near midsilk, kernel site number declined, but
the decline was not consistent among planting date treatments.
Similar
graphs were obtained when plotting the data against calendar day.
Regression lines were fitted to the points in the linear part of the
curves.
The analysis of regression (Tables 17, 19, 2la, and 2lb) showed
a high correlation (r > .96 in aIl cases) between the number of kernel
sites accumulated and either the GDD or the calendar clay.
The coeffi-
cient of variation was less than 12.3 in both hybrids.
The test of
homogeneity of the regression coefficients (Tables lB and 20) gave the
grouping presented in Tables 17, 19, 2la, and 2lb.
On both a calendar
day and a growing degree day basis, kernel site accumulation rates
were non-significant between dates 1 and 2 for P37BO while the 30 May
planting was significantly different from dates 1 and 4 for C2Bl.
The
third planting date was not included because there were nbt enough
data taken during the linear accumulation period to calculate an
accumulation rate.
The results showed that kernel site accumulation
rate was highly correlated with and linearly related to either GDD or
calendar day over most of the accumulation period.
Even though there
was a high correlation between number of kernel sites accumulated and
GDD, differences in rate of accumulation were observed among planting
dates.
This suggests that the GDD temperature function used was not
appropriate for aIl planting dates or that other factors interacted
with the thermal unit response or operated independently to influence

---

72
CORNELL281
100
(~-~""""
k~ . •....
(~-~""....
k~ . • •
0....
~
0....
..~
..
, ..... .A~\\,~O
~'1..
,
41·0
/1....
---.
41·0
/1....
P
1
80
~
o
1
"
Cl:
./J
Cl:
1
~
.Cl
g60
Cl
"-
planting date
ln
Q)
....
12 May
--0-·
en
- e - 30 May
-40
08 June
Q)
-"-"
Q)
c:
~
--o-. 22 June
~
22
Q)
--0--
~
20
600
900
1200
1500
GDD C10C, 3OC)
from
Emergence
Figure 12.
Relationship between the number of kernel sites per double
row for Cornell 281 (planted at 4 dates) and the growing
degree days (base 100
10 , 30 0
30 C) from emergence, calculated
with shelter air temperatures.

---

73
PIONEER 3780
•
~~--,.-
100
~i~ 'O-.__~o::t!
__
a
--~-
,,
l
'
l l
',
o
,
80
,
1
1
q
1
,
3:
1
3:
,
0
IJ
0
,
1
c:
,,
~
..'
~ 60
,
..0
,
~
0
p
0
,
planting date
......
,,
(/)
12 May
-·0-
Q)
....
•
- ... 30 May
... 30
en 40
- A _
08 June
--0--
22 June
Q)
c:
~
Q)
~
20
600
900
1200
1500
1800
GDD (10C,30C)
from
Emergence
Figure 13.
Re1ationship between the number of kerne1 sites per double
row for Pioneer 3780 (p1anted at 4 dates) and the growing
degree days (base 10°, 300
30 e) from emergence, ca1cu1ated
with she1ter air temperatures.

---

74
Table 17.
Regression ana1ysis of kerne1 site accumulation against GDD
from emergence, for C281.
P1anting
Regression
F-ratio
Date
Intercept
Coefficient
Significance
R-Square
C.V.
12 May
- 63.53
0.141 b
***
0.999
0.2
30 May
-167.67
0.255 a
**
0.929
12.1
**
0.929
tt
08 June
22 June
- 75.44
0.171 b
***
*** Significant at 1 percent 1eve1.
** Significant at 5 percent 1eve1.
tt Not enough data for the third p1anting date.
The regression coefficients fo11owed by the same let ter are not
significant1y different.
Table 18.
Test of homogeneity on the regression coefficients of kerne1
site accumulation for C281 and P378Q (GDD basis).
Significance of F-ratios
Source of Variation
d. f.
Type IV
C281
P3780
Planting dates
2
NS
***
GDD
1
*
***
t
GDD* Planting dates
2
NS
***
*** Significant at 1 percent 1eve1 of probabi1ity.
t GDD* P1anting dates SS were used to test the hornogeneity of
regression coefficients.

---

75
Table 19.
Regression ana1ysis on the kerne1 site accumulation against
GDD from ernergence for Pioneer 3780.
P1anting
Regression
F-ratio
Date
Intercept
Coefficient
Significance
R-Square
C.V.
12 May
-223.288
0.285 a
**
0.999
l.0
30 May
-171.504
0.239 a
***
0.994
4.1
tt
08 June
22 June
- 77.675
0.159 b
***
0.999
0.6 .
*** Significant at 1 percent 1eve1.
** Significant at 5 percent 1eve1.
tt Not enough data for the third p1anting date.
Table 20.
Test of homogeneity of the regression coefficients of kerne1
site accumulation for C281 and P3780 (ca1endar day basis).
Significance of F-ratios
Source of Variation
d.L
Type IV
C281
P3780
P1anting dates
2
NS
1<1,*
GDD
1
*
***
t
GDD* P1anting dates
2
NS
***
*** Significant at 1 percent 1eve1 of probabi1ity.
** Significant at 5 percent 1eve1 of probabi1ity.
NS Not significant.
t GDD* P1anting date SS were used to test the homogeneity of the
regression coefficients.

---

76
Table 21a.
Regression analysis of kernel site accumulation against
calendar day for C281.
Planting
Regression
F-ratio
Date
Intercept
Coefficient
Significance
R-square
C.V.
12 May
- 506.277
2.942 b
***
0.999
0.3
30 May
-1042.904
5.416 a
**
0.926
12.3
tt
08 June
22 June
- 493.995
2.563 b
***
*** Significant at 1 percent level.
** Significant at 5 percent level.
tt Not enough data for the third planting date.
Rate with the same let ter within a column were not significantly
different at the 5 percent level.
Table 21b.
Regression analysis of kernel site accumulation against
calendar day for P3780.
Planting
Regression
F-ratio
Date
Intercept
Coefficient
Significance
R-square
C.V.
12 May
-1118.581
5.949 a
**
0.999
1.2
30 May
-1002.858
5.132 a
***
0.995
3.6
tt
08 June
22 June
- 567.946
2.826 b
***
0.990
1.6
*** Significant at 1 percent level.
** Significant at 5 percent level.
tt Not enough data for the third planting date.
Rate with the same let ter within a column were not significantly
different at 1 percent level.

---

77
kernel site accumulation.
If the GDD temperature function applied over
aIl environments tested or if temperature was the only factor affecting
kernel site accumulation, the rate per GDD would be the same for aIl
planting dates within a hybride
However, different responses may have
been observed between hybrids because corn hybrids could respond dif-
ferently to the same temperature levels.
The results show that at least
the GDD function used in this study does not have any advantage over time
function (calendar day) for kernel site production.
Maximum number of kernel sites per DR, number of kernel sites/DR
at midsilk, and final number of kernels filled per DR are shown in Table
22.
Maximum number of sites/DR ranged from 92.1 to 104.4 for C28l,
and from 101.8 to 108.8 for P3780.
The analysis of variance showed a
significant effect (P < 0.01) of delayed planting on potential kernel
sites/DR for P3780 (Tables 23 and 24).
Duncan's multiple range test
showed that only the kernel sites/DR for the third date was signifi-
cantly different (P < 0.05) from the others for C28l while the potential
kernel sites per double row was not significantly different between aIl
planting dates for P3780.
The highest number for both hybrids was
recorded on the 30 May planting date.
This high number of potential
kernel sites was associated with higher GDD/day during the kernel site
accumulation period (Figure Il) with 21.4 ± 1.5 GDD/day for C28l and
21.3 ± 1.6 GDD/day for P3780.
These values were 6% higher than the
average GDD/day during kernel site development at the other planting
dates.
There were differences also in the duration of the kernel site
accumulation period (Figure Il).
However, there was no difference in
the maximum number of kernel sites per DR between planting dates within
hybrids, except for the third date for CZ8l.
This result suggests that

---

78
Table 22.
Number of maximum kernel sites per double row (Max KS/DR),
number of kernel sites per double row present at silking
and number of filled kernels/DR for 2 corn hybrids (C28l
and P3780) planted in the spring of 1979 at 4 dates.
1979
Planting
Max
KS/DR
Final
Date
KS/DR
(Midsilk)
Kernel il/DR
-----------------------C28l------------------------
12 May
99.8 a
99.8 a
63.1 a
30 May
104.4 a
97.6 ab
54.5 ab
08 June
92.1 b
91.0 c
47.8 cb*
22 June
98.4 a
93.0 cb
43.0 c*
----------------------P3780------------------------
12 May
104.5 a
103.5 a
70.0 a
30 May
108.8 a
108.8 a
54.3 b
08 June
104.6 a
104.0 a
24.7 c*
22 June
101.8 a
100.5 a
22.6 c*
*Poor pollination due to insect feeding on silks.
Kernel numbers with the same let ter within each column are not
significantly different at the 5% level (Duncan's multiple
range test).

---

79
Table 23.
Sunnnary of significance of F-ratios and degrees of freedom
for ana1ysis of variance of number of maximum kerne1 sites
for C281 p1anted in the spring of 1979 at 4 dates.
Source of Variation
d.L
Significance of F-ratio
Corrected total
39
P1anting dates
3
***
Error
36
*** Significant at 1 percent 1eve1
Table 24.
Summary of significance of F-ratios and degrees of freedom
for analysis of variance of number of maximum kerne1 sites
for Pioneer 3780 p1anted in the spring of 1979 at 4 dates.
Source of Variation
d.L
Significance of F-ratio
Corrected total
35
P1anting dates
3
NS
Error
32
NS = non-significant

---

80
the rate of spikelet (kernel site) production was not a limiting factor
in the determination of the number of potential spikelets available for
kernel filling in this experiment.
In most cases the maximum number
of spikelets was reached Just before or at silking.
The observation data of number of kernels filled per double row on
the topmost ear is shown in Appendix E, Table 57.
The final number of
kernels filled per DR are shown in Table 22.
The number of filled
kernels was significantly lower with late compared to early planting.
The analysis of variance (Table 25) and Duncan's multiple range test
showed significant differences (P < 0.05) between and within hybrids.
Final kernel numbers ranged frorn 43.0 to 63.1 kernels/DR for C28l and
from 22.6 to 70 kernels/DR for P3780.
The two last planting dates in
both hybrids had poor pollination due to insect feeding on silks.
The
extent of insect damage depended upon the timing of severe insect
feeding and silking on an individual ear shoot basis.
Since insect
activity was not monitored quantitatively, the degree of poor pollina-
tion due to the insects cannot be quantified.
However, it seemed that
the third planting of C28l silked early enough to avoid most of the
insect damage (Tables 27, 39, and 43).
The difference in final kernel
number between the two first planting dates in both hybrids, although
non-significant for C28l, was mainly due to the fact that spikelets at
the tip of the cob failed to develop early in the filling period.
The analysis of variance on the total number of harvested kernels
per hectare (Table 26) showed a significant effect of the time of planting
for both C28l and P3780.
Maximum number of kernel sites and number of
filled kernels adjusted to a unit land area by taking into account the
differences in plant population and hybrid barrenness are presented in

---

81
Table 25.
Surnmary of significance of F-ratios and degrees of freedom
for ana1ysis of variance of number of kerne1s harvested for
Pioneer 3780 p1anted in the spring of 1979 at 4 dates.
CZ8l
P3780
Source of Variation
d. f.
F-ratio
d.L
F-ratio
Corrected total
51
63
Planting dates
3
***
3
***
Error
48
60
*** Significant at 1% level.
Table 26.
Summary of significance of F-ratios and degrees of freedom
for analysis of variance of number of kernels harvested per
ha for Corne11 281 and Pioneer 3780 p1anted in the spring
of 1979 at 4 dates.
CZ81
P3780
Source of Variation
d.L
F-ratio
d. L
F-ratio
Corrected total
63
51
P1anting dates
3
***
3
***
Error
60
48
*** Significant at 1% 1eve1.

---

82
Table 27.
Maximum number of kerne1 sites and number of fi11ed kerne1s
per ha for 2 corn hybrids (C281 and P3780) p1anted in the
spring of 1979 at 4 dates.
Hybrid
1979
C281
P3780
P1anting
Date
Max
Harvested
Max
Harvested
6
----------------------10 /ha-------------------------
12 May
55.2
37.2 a
55.2
34.3 ab
30 May
63.1
30.1 bc
54.4
26.5 c
08 June t
56.8
28.0 c
49.8
12.0 d
22 June t
56.5
25.3 c
49.5
11.1 d
t Po11ination reduced by insects feeding on si1ks.
Within and between co1umns, kerne1 number fo11owed by the same 1etter
are not significant1y different (P < 0.05).

---

83
Table 27.
Despite the population differences between hybrids, the
maximum kernel site number per hectare was similar for bath hybrids,
6
55.2 X 10
kernels/ha for planting date treatment 1.
Duncan's multiple
range test (Table 27) showed significant difference (P < 0.05) between
the total number of kernels filled for the first two planting dates of
bath hybrids.
The differences in harvested kernel number for the last
two planting dates can be explained mainly by the insect damage on ear
shoot silks resulting in poor pollination.
For the first two planting
dates in bath hybrids where there was no insect damage, there was no
significant difference (P < 0.05) in the number of kernels filled for
C28l while the difference was significant for P3780.
For the first two
dates, the results presented in Table 27 show that 33% ta 50% of the
spikelets present at midsilk failed ta produce mature grain.
This result
suggests that neither the maximum number of potential kernel sites nor
the kernel site accumulation rate were limiting the final grain yield.
3.
Ear Length
Ear length data (Appendix E, Table 58) are plotted in Figures 14
and 15 against growing degree days.
AlI the data points fall essentially
on one sigmoid curve.
There was no difference in the rate of ear elonga-
tian (slope of lines) during most of the growth period, even though more
GDD accumulated before ear shoot elongation began for the first planted
corn than for ears at later plantings in bath hybrids.
The diagrams
show that there was a high correlation between ear length and GDD.
The
final ear length was quite similar for aIl dates within and between
hybrids.
The average was 22.2 cm for C28l and 23.5 cm for P3780.

---

84
Cornell 281
• Treat l
•• Treat
Treat II
a
• Treat
a Treat III
0 Treat IV
0 Treat
a
i·
aCC
o
a
.-
o
c-.
o
ge-
.-
o
300
600
900
1200
1500
1800
Number of Degree-Days from Emergence
ta Harvest
Figure 14.
Re1ationship between the ear 1ength (cm) of Corne11 281
p1anted at four dates, and growing degree days (base 10°,
30°C) from emergence.
j
.,
~-

---

85
Pioneer 3780
• Treat I
•• Treat
Treat II
• Treat I
D Treat III
0 Treat
0
IV
1 DjJ
1
or/' q. •
..c
lJ
bll
J:::
3
15
!-l
(lj
~
Q)
bll
(lj
1i1
1
~
Ol+~~-""--~~--.,....--...,----r-----t
o
300
600
900
1200
1500
1800
Number of Degree-Days from Emergence
ta Harvest
Figure 15.
Re1ationship between the ear 1ength (cm) of Pioneer 3780
p1anted at four dates, and growing degree days (base 100
10 ,
300
30 e) from emergence.

---

86
4.
Kernel Dry Weight Accumulation
The grain dry weight (104°C) data (Appendix E, Tables 59 and 60)
are plotted against calendar days in Figure 16 and against growing
degree days in Figure 17.
Figure 16 shows a linear relationship
between kernel dry matter accumulation and calendar days over most of
the grain filling period.
Linear regression lines were fitted to the
data and the analysis results are summarized in Tables 28 and 30.
A
linear relationship was found also between the dry weight per kernel
and growing degree days.
The regression analysis results are shown in
Table 32.
AlI the F-ratios of the regression models were significant
at the 1% level of probability.
This means that there was a highly
linear relation between kernel dry weight (104°C) and calendar day or
2
GDD.
The coefficient of determination (r ) ranged, in aIl cases, from
0.968 to 0.999.
Thismeansthat thelinearmodels could explain 96.8 to
99.9 percent of the variation in kernel dry weight.
On a calendar
day basis, the coefficient of variation (C.V.) ranged from 2.15 ta
Il.51 for both hybrids, while on the growing degree day 'basis, it was
8.28 and 6.52 for C28l and P3780, respectively.
As shown in Figure 17,
aIl the kernel dry weight (104°C) data points fall on the same line
for each hybrid, when plotted against GDD.
The kernel growth rates
observed during the linear phase of grain filling are summarized in
Table 34.
Tests of regression coefficient homogeneity (Tables 29, 31,
and 33) showed that there were no significant differences between
slopes within and between hybrids on both a calendar day and GDD basis.
Calculated kernel growth rates were 0.36 mg/GDD '(r '" 0.9864, c.v. '"
28.28) for C28l and 0.38 mg/GDD (r '" 0.9925, c.v. = 6.25) for P3780.
These results suggest that rate of kernel dry matter accumulation was

---

---

88

---

---

.,1'.
plant ing
planting date
-,1'.
Il
12 May
,'0
",'0
()
30 May
à
08 June
/
. ,
à
08
June
/ ,
. ,
<t
22 June
~,'
<t
22 June
I!','
• ~,o
.2
0 ,0
. ,
.,,",
(1)
."o·
c:
,t.
,~
,
~
,
(1)
~
~. 6.,'
......
.-..
CORNELL 281 - - -.- L __ PIONEER 3780
E
~... "/),.,
CORNELL 281 - - -
L __ PIONEER
~
y: .000357X -jf07120.:
.000357X-:;:07120~ t'
Y: .000375X -
.000375X- .09600
en
'-' .1
...~ ,0
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.'
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.00L---2.J0-0---4..J0~0----";6~0:"0---::8~0~0--~1~00;0;--­
o
200
400
600
800
1000
\\J:l
o
G DO
GDD (10C.
(10C, 30C) fram
from
Midsilk

---

91
Table 28.
Regression ana1ysis of kerne1 dry weight versus Julian day
for Cornel1 281.
P1anting
Regression
F-ratio
Date
Intercept
Coefficient
Significance
R-Square
C.V.
12 May
-1.425
0.00654 a
***
0.995
2.18
30 May
-1.559
0.00674 a
***
0.968
10.23
08 June
-1.577
0.00663 a
***
0.995
5.22
22 June
-1.290
0.00510 a
***
0.989
5.58
*** Significant at 1 percent 1eve1 of probability.
a = There was no significant difference between the regression
coefficients.
Table 29.
Test of homogeneity of the regression coefficients of kerne1
dry weight accumulation for C281.
Significance of F-ratios
Source of Variation
D. f.
Type IV
P1anting date
3
NS
Julian day
1
***
Julian* P1anting Datet
Date
3
NS
*** Significant at 1 percent 1eve1 of probabi1ity.
t Ju1ian* P1anting date SS were used to test the homogeneity of
the regression coefficients.

---

92
Table 30.
Regression analysis of kernel dry weight versus Julian day
for Pioneer 3780.
Planting
Regression
F-ratio
Date
Intercept
Coefficient
Significance
R-Square
C.V.
12 May
-1.464
0.00652 a
***
0.955
8.49
30 May
-1. 431
0.00612 a
***
0.991
4.69
08 June
-1.579
0.00650 a
***
0.986
8.59
22 June
-1.571
0.00606 a
***
0.978
Il. 51
***
Significant at 1 percent level of probability
a = There was no significant difference between the regression
coefficients.
Table 31.
Test of homogeneity of the regression coefficients of kernel
dry weight accumulation for P3780.
Significance of F-ratios
Source of Variation
D.L
Type IV
Planting date
3
NS
Julian day
1
***
Julian* Planting date t
3
NS
***
Signif ican t al; 1 percent level of probability
**
Significant at 5 percent level of probability
t
Julian* Planting date SS were used to test the homogeneity of
the regression coefficients.
NS
Non-significant

---

93
Table 32.
Regression analysis of kernel dry matter accumulation versus
GDD (base 100
10 , 300
30 C) for 2 corn hybrids (Cornell 281 and
Pioneer 3780).
Regression
F-ratio
Hybrid
Intercept
Coef ficient
Significance
R-Square
C.V.
C28l
-0.0712
0.00035729 a
***
0.973
8.28
P3780
-0.0960
0.0003732 a
***
0.985
6.52
*** Significant at 1 percent level of probabili ty
a = There was no significant difference between the regression
coefficients.
Table 33.
Test of homogeneity on the regression coefficients of kernel
dry matter accumulation on GDD basis for C28l and P3780.
Significance of F-ratios
Source of Variation
d.L
Type IV
Hybrid
1
*
GDD
1
***
t
GDD* Hybrid
1
NS
* Significant at 10 percent level of probability.
*** Significant at 1 percent level of probability
NS
Not significant
-r GDD* Hybrid SS were used to test the homogeneity of the regression
coefficient.

---

94
Table 34.
Kernel growth rates on calendar day basis (mg/day) and GDD
basis (mg/GDD) for 2 corn hybrids (C28l and P3780) planted
in the spring of 1979 at 4 dates.
1979
Hybrid
Planting
Date
C281
P3780
---------------mg/day---------------
12 May
6.5 a
6.5 a
30 May
6.7 a
6.1 a
08 June
6.6 a
6.5 a
22 June
5.1 a
6.1 a
---------------mg/GDD---------------
AlI dates
0.36
0.37
Within and between co1umns different letters indicate significant
difference between the regression coefficients at the 5% 1eve1.
On the GDD basis there was no difference in grain dry matter accumu-
lation rate.

---

95
not significantly limited by cooler fall temperatures associated with
delayed planting (Figure 3).
The test for significance among regression
coefficients is a conservative test.
The decline in kernel filling rate
from 6.6 to 5.1 mg/day by C28l from the third to the fourth planting
date (Table 34) suggests that cool fall temperatures may have started
to influence grain growth.
No such trend was observed for P3780.
The effective kernel filling period duration (EFPD) was computed
in days and presented in Table 35 with the equivalent GDD.
The EFPD
declined in both hybrids from 43 to 30 days for C28l and 45 to 25 days
for P3780.
Number of GDD accumulated during the effective filling
period declined also with late planting.
As temperatures declined in
the fall (Figure 3) the effective kernel filling period became shorter
for the late plantings.
The final grain dry weight per kernel declined
with delayed planting in both hybrids from 290 to 150 mg (Table 36).
It is obvious that the decline in EFPD had contributed to the reduction
in the dry weight per kernel but no rneasurement was done on the initial
size of kernel to determine whether the volume of the kernel was also
limiting.
The number of GDD accumulated per mg dry weight (Table 36)
varied from 2.62 to 2.98 for C28l and from 2.67 to 2.83 for P3780.
Although final dry weight per kernel and GDD accumulated during the
EFPD dec1ined with delayed p1anting, their relationship was inconsistent.
The apparent kernel fi11ing period duration (AFPD) (Table 35) was
found to increase up to the second planting date then declined rapid1y
in Corne1l
Cornell 281, while the AFPD declined steadily in P3780 with delayed
planting.
It should be noted that plants of the last two planting dates
did not reach blacklayer formation before frost in the fall 50 the AFPD
at these dates was shortened by frost.

---

96
Table 35.
Effective kerne1 fi11i~g period duration in Julian days
and CDD and apparent kerne1 fi11ing period duration in
days of two corn hybrids (C281 and P3780) p1anted in the
spring of 1979 at 4 dates.
Hybrid
C281
P3780
1979
AFPD
EFPD
AFPD
EFPD
P1anting
Date
Days
Days
CDD
Days
Days
CDD
12 May
58
43
827
67
45
742
30 May
63
40
728
66
45
788
t
08 June
58
38
666
56
37
608
t
22 June
47
30
460
45
25
370
t : Plants did not reach b1acklayer before frost
EFPD : Effective fi11ing period duration (period of rapid kerne1 dry weight
accumulation)
AFPD : Apparent fi11ing period duration
(si1king to b1acklayer)

---

97
Table 36.
Final dry (104°C) weight per kernel and GDD accumulated per
mg dry weight grain per kernel for two hybrids (Cornell 281
and Pioneer 3780) planted in the spring of 1979 at 4 dates.
1979
Hybrid
P1anting
Date
C281
P3780
mg
GDD/mg
mg
GDD/mg
12 May
279
2.96
290
2.56
30 May
268
2.72
278
2.83
08 June
254
2.62
240
2.53
22 June
154
2.98
150
2.47

---

98
There are two lag periods in the corn grain filling period.
The
first lag period is the time from midsilk to the beginning of rapid
grain dry matter accumulation while the second lag period is the time
from the end of the effective filling period to blacklayer formation.
In this study the first lag was calculated by determining the time
interval from midsilk date to projected zero grain dry weight date.
The second lag was calculated by taking the difference between the
projected date for final grain dry weight and the date of blacklayer
formation.
The calculated lag periods are shown in Table 37.
The
first lag period ranged from 7 to 13 days for Cornell 281 and from
Il to 17 days for Pioneer 3780.
In a study with 20 hybrids of various
maturity groups, Boyle (1980, M. S. Thesis, AgronomyDept., The Pennsyl-
vania State University) found that the estimated time from silking to
the beginning of kernel dry matter accumulation was similar for aIl
hybrids with an average of Il days ± 2 days standard deviation.
Hanway and Russell (1969) reported a value of 12 days for corn grown
in Iowa.
For Cornell 281 the shorter first 1ag period appeared to be
associated with higher GDD accumulated per day (Table 37).
The method
for estimating lag periods in kernel growth are not precise, however,
so actual relationships cannot be determined with my data.
The second
lag period ranged from 4 to Il days for CZ8l and 3 to Il days for P37BO.
The shorter second lag period for the fourth planting date was prob-
ably due to frost which prematurely stopped kernel filling.
D.
Final Harvest
1.
Final Grain Yields
The final grain harvest data are shown in Table 42 and Appendix F,
o
Table 61.
Final grain dry weight (104 C) per unit land area (ha) is

---

99
Table 37.
GDD accumu1àtion per ca1endar day and time from midsi1k to
beginning of effective grain fi11ing (first 1ag) and from
the end of the effective fi11ing period to b1ack1ayer formation
(second 1ag) for C281 and P3780 p1anted at 4 dates in the bpring
of 1979.
Hybrid
1979
C281
P3780
P1anting
Date
lst 1ag
2nd 1ag
lst 1ag
2nd 1ag
GDD/day
Days
Days
GDD/day
Days
Days
12 Nay
23.0
7
8
17.7
11
11
30 Nay
20.9
12
11
21.1
13
*
08 June
21.2
9
*
18.8
12
*
18.8
*
22 June
15.3
13
*
14.8
17
*
14.8
*
*P1ants did not reach 50% black layer before frost.
O
Table 38.
Final grain dry (10q c) weight (me tric tons/ha) ca1cu1ated
on grain weight/1.83 m of row (method 1) of two corn hybrids
(Corne11 281 and Pioneer 3780) p1anted in spring of 1979 at
successive dates.
1979
P1anting
Hybrid**
P1anting
Date
C281
P3780
----------------NT/ha----------------
12 Nay
6.0 b
8.0 a
30 Nay
6.1 b
6.4 b
08 Junet
June
4.3 c
1.5 d
t
22 June
2.0 d
1.1d
** Within and between co~umns yie1ds fo11owed by the same 1etter are not
significant1y different (P < 0.05).
t Si1ks on ear shoot were damaged at po11ination by insects.

---

100
summarized in Tables 38 and 39 for the two methods of calculation.
The
grain yield decreased from 6.1 to 2.0 mT/ha for c28l and from 8.0 to
1.1 mT/ha for P3780 in the first method (Table 38), and from 8.16 to
2.75 tons/ha for C28l and from 8.85 to 1.63 tons/ha for P3780 (Table
39).
Tt is important to point out that insects damaged the silks as
they emerged from plants of third and the fourth planting dates.
Sub-
sequent poor pollination resulted in the unexpected low grain yields
recorded for both hybrids planted at dates 3 and 4.
Plants of C28l
from the third planting date silked a little before insect activity
became significant 50 the yield reduction was smaller than for the
fourth planting (Tables 38 and 39).
Analysis of variance and mean
separation test results for the data are summarized in Tables 38, 39,
and 40.
The F-ratios associated with the variation in yield associated
withplantingdateandhybrid effects were significant at the 1% level of
probability in the second method (using plant population) while only
planting date had a significant effect on yield in the first method
(which used number of rows/plot).
These results mean that there was
a significant variation in yield due to differences in planting dates
and hybrids for the second method.
The interaction between planting
dates and hybrids was also significant at the 1% level in both methods
suggesting that plant yield response was different among planting dates
and hybrids.
The significance of the hybrid effect and the interaction
can be explained by the insect damage on the ear shoot silks at pollina-
tion for the third planting date.
Plants from C28l of the third planting
date silked before the insect damage became significant while plants
from P3780 silked during the critical period of insect activity resulting
in difference in pollination between the two hybrids at this date.
The

---

101
Table 39.
Final grain dry (104°C) weight (me tric tons/ha) calculated
from average grain weight/ear X population basis (method II)
of two corn hybrids (Cornell 281 and Pioneer 3780) planted
in the spring of 1979 at successive dates.
1979
Hybrid**
Planting
Date
C28l
P3780
---------------mT/ha-----------------
12 May
8.16 a
8.85 a
30 May
7.13 b
7.39 b
t
08 June
6.05 c
2.41 d
2.75 d
1.63 e
These results differ frcm those in Table 38 (probably due to an
underestimated barrenness factor).
** Within and between columns yields with the same let ter are not
significantly different at 5 percent level.
t Silks on ear shoot were damaged at pollination by insects.

---

102
Table 40.
Summary of significance of F-ratios and degrees of freedorn
for analysis of variance of final grain yield of two corn
hybrids (C28l and P3780) planted in the spring of 1979 at
4 dates.
lst Method
2nd Method
F-ratios
F-ratios
Source of Variation
d.L
significance
d.L
significance
Corrected total
31
31
Model
22
22
Replication (rep)
3
NS
3
NS
Planting date
3
***
3
***
Rep* Planting date
9
NS
9
NS
Hybrid
1
***
1
NS
Hybrid* Planting
3
***
3
***
Hybrid* Rep
3
NS
3
NS
Error
9
9
***
Significant at 1% level of probability
NS
Non-significant

---

103
data in Tables 27, 38, 39, and 43 support the preceding argument by
showing relatively high kernel nurnber or yield or harvest index for
the third planting date of c281 compared to the fourth planting date
of C28l and the last two planting dates for P3780.
Duncan's multiple
range test at the 5% level of probability (d.f. = 9, MS = 0.779815)
showed that there was significant difference in the yield between
hybrids for the first planting date in the first method while the
difference was non-significant in the second method.
In addition the
two methods gave different values of the yield.
The yields reported
in Table 39 were calculated from individual ear data from 6 consecutive
plants and adjusted for population and barrenness percentages (Method
II).
Meanwhile, the yields reported in Table 38 were calculated from
6 feet row data and adjusted for length and number of rows per unit
land area (Method 1).
The differences between the two methods of
yield calculation suggest that the percentage of barrenness was under-
estimated, and that one must be careful when using the second method.
As l pointed out in Chapter III, sorne plants had apparent ears but
failed to produce mature kernels resulting in hidden barren plants.
The differences in yields for the last two planting dates were partly
due to insect damage on the ear shoot silks resulting in poor pollination.
2.
Barrenness
Percentage barrenness was measured at final harvest of aIl plots
(Table 41).
The average barrenness percentages were 13.6, 8.0, 3.6,
and 6.5 for c28l and 0.0, 3.7, 3.3, and 1.5 for P3780 at dates 1, 2, 3,
and 4, respectively.
The barrenness was higher in Cornell 281 than in
Pioneer 3780 and the highest percentage was recorded with the highest

---

104
Table 41.
Percentage barrenness at harvest and LAI at midsi1k for 2
corn hybrids (C281 and P3780) p1anted in the spring of
1979 at 4 dates.
1979
Hybrid
P1anting
Date
C281
P3780
%
LAI
%
LAI
12 Nay
13.6
4.1
0.0
3.3
30 May
8.0
3.5
3.7
3.4
08 June
3.6
3.3
3.3
3.4
22 June
6.5
3.2
1.5
3.0
Table 42.
Who1e plant and grain dry (104°C) weight at harvest for 6
plants for 2 corn hybrids (c281 and P3780) p1anted in the
spri.ng of 1979 at 4 dat~s.
Hybrid
1979
C281
P3780
P1anting
Date
Who1e Plants
Grain
Who1e Plants
Grain
--------------------grams/6 p1ants---------------------
12 May
1278
640
1422
780
30 May
1002
525
1257
677
08 June
941
425
885
221
22 June
664
199
780
146

---

105
Table 43.
Harvest indexes of 2 corn hybrids (Cornell 281 and Pioneer
3780) planted in the spring of 1979 at 4 dates.
1979
Hybrid*
Planting
Date
C28l
P3780
12 May
0.50 a
0.55 a
30 May
0.52 a
0.54 a
08 June 'r
0.45 a
0.24 bc
22 June t
0.31 b
0.19 c
* Duncan's multiple range test within and between hybrids (p < 0.05).
t Insect damage of silks and fall frost.
Table 44.
Summary of significance of F-ratios and degrees of freedom
for analysis of variance harvest index for 2 corn hybrids
(C28l and P3780) planted at 4 different dates.
Source of Variation
d.f.
Significance of F-ratios
Corrected total
31
Madel
19
Replication (rep)
3
NS
Planting date
3
***
Rep* Planting date
9
NS
Hybrid
1
***
Hybrid* Planting date
3
***
Error
12
*** Significant at 1% level of probability
NS Non-significant at 10% level of probability

---

106
leaf area index in C281 (Table 41).
Tt should be pointed out that
Cornell 281 had a higher plant density (86,500 plants/ha) compared to
Pioneer 3780 (66,500 plants/ha).
Sass and Loeffel (1959) conducted a
two-year study (1954-1955) with 3 single crosses and 4 inbreds of
maize and found that there was a marked differential response of single
crosses and inbreds ta plant populations (29,650 plants/ha, and 59,300
plants/ha) levels with respect to stalk barrenness.
They concluded
from their study that barrenness is the result of failure of silk
emergence during the pollen-shedding period, rather than the failure
of formation of floral organs.
3.
Harvest Index
Table 42 contains the total plant and grain dry (104°C) weight
used in the calcula tian of the harvest index.
A summary of the average
harvest indexes is presented in Table 43 by hybrid and planting dates.
The analysis of variance (Table 44) showed that the planting date and
the hybrid effects on the harvest index were significant at the 1%
level of probability.
The interaction between planting date and hybrid
was also significant at the 1% level showing that the two factors did
not act independently of each other.
This significant interaction could
be explained by the fa ct that the last two planting dates for P3780 were
severely damaged by insects while only the fourth planting date for
CZ8l was severely damaged.
Duncan's multiple range test (Table 43)
showed non-significant difference between hybrids for the two first
plantings at the 5% level of probability.
The differences in harvest
index for the third and fourth planting dates were mainly due to poor
pollination associated with the insect damage on silks and eventually

---

107
to the fall frost which prematurely halted the grain filling before
blacklayer formation.
Based on the first two planting dates which
were safe from frost and insect damage, the average harvest index was
0.53.
This means that at blacklayer (physiological maturity) , 53%
of the above ground bulk dry (104°C) weight of the corn plant consisted
of grain dry matter.

---

CHAPTER V
GENERAL DISCUSSION
It was found in the experiment that the time to emergence varied
between 6 and 8 days.
Shorter times were associated with warmer soil
temperatures obtained with delayed planting.
This high soil tempera-
ture effect on time to emergence supports the findings by Iremiren and
Milbourn (1979) that the initial effect of a polyvinyl chloride (PVC)
mulch treatment was to shorten the interval between sowing and emer-
gence.
In their experiment, the PVC treatment led to increased soil
temperature which was found to improve germination, compared with chalk
mulch.
The small difference in time to emergence was not considered
to be an improtant factor in yield determination in this study.
Time from emergence to tassel initiation varied widely from 17 to
6 days from normal to delayed planting.
The GDD computed for the period
from emergence to tassel initiation was found to decline with delayed
planting.
The short est time from emergence to tassel initiation was
recorded with the lowest average soil temperature at the 5 cm depth
but the temperature range was so small that l cannot say whether my
result disagrees with the findings by Coligado and Brown (1975) in a
growth chamber study that there was a consistent decrease in time to
0
o
tassel initiation as temperature was increased from 15
to 2S C
regardless of photoperiod.
This result suggests, however, that growth
chamber data cannot be systematically applied to field conditions.
The
reason for this shortening in time to tassel initiation remained
undetermined.

---

109
Leaf area index was found to increase linearly with growing degree
days over most of the leaf expansion period.
There was no significant
difference in the slopes (rate of leaf area accumulation) for the two
hybrids (0.00451 LAI/GDD for C281 and 0.00433 LAI/GDD for P3780).
These
results show that the leaf development was closely related to tempera-
ture (GDD).
Temperature influence on leaf development was reported in
the literature by a certain number of workers such as Brouwer et al.
(1973) and Cooper (1979).
However, the importance of other environ-
mental factors such as radiation for leaf development was also pointed
out (Evans, 1963).
As reviewd by Tollenaar (1977), leaf area index is
one of the factors affecting total assimilate suppl y for crop growth.
He reported that LAI attains its maximum value at or shortly after
silking which was also true in this present study.
Duncan (1972) in a
study found that gross photosynthesis increased with LAI and tended to
level off after LAI of 4.0.
He pointed out the importance of having
adequate crop canopies for the interception of light for photosynthate
production.
He reported a general relationship between planting rate
and grain yield to be one in which there is an initial linear phase
where yield rises in proportion to planting rate.
He found that this ig
followed by a phase where yield per plant decreases and yield per ha
climbs to a plateau.
This occurs, as he pointed out, at whatever plant
population is needed to give a leaf area index of approximately 4.0.
The number of leaves above the topmost ear was found to be slightly
higher in P3780 than in C281.
The difference, however, was not signifi-
canto
The number of leaves above the topmost ear ranged from 5 to 7
leaves for aIl hybrids at aIl dates.
Eastin (1969) reported that the
leaves above the ear were the main source providing carbohydrate for

---

110
kernel fill.
The leaf area (green part) per plant (at midsilk) was
2
4751, 5232, 4478, and 4139 cm
for C28l and 5100, 5405, 5913, and
2
5059 cm
for P3780 for planting dates l, 2, 3, and 4, respectively.
Delayed planting did not affect leaf area per plant substantially.
Therefore, a need for plant density adjustment with delayed planting
within hybrids would not be necessary to obtain equivalent LAI.
Duncan (1972) pointed out that barrenness may occur in plants at
a relatively low plant population, even below those needed for a LAI
of 4.0.
Sass and Loeffel (1959) reported an increase in barrenness
with higher population.
This is supported by my results showing a
higher barrenness in C28l (86,500 plants/ha) than in P3780 (66,500
plants/ha) .
These authors found that barrenness was the result of
failure of silk emergence during the pollen-shedding period, rather
than the failure of formation of floral organs.
The higher percentage
barrenness in my study was also associated with higher LAI within
hybrids.
Apparently, factors contributing to barrenness in this
study were strong enough to influence percentage barrenness with the
differences in LAI observed among plots.
The analysis of dry matter accumulation in plant parts before
silking showed a fairly constant percentage associated with leaf
blades, leaf sheaths, and stalks over planting dates.
There were small
differences between the two hybrids (Table Il).
Bryant and Blaser
(1968) reported that the relative proportion of the different parts
varied significantly between an early and a late hybrid.
They observed
only a slight influence of plant population on these proportions.
Hanway and Russell (1969) observed similar patterns of dry matter
accumulation in the total, above-ground plant parts in Il hybrids

---

111
studied and at different plant densities.
The total plant dry weight
at midsilk was found to increase with delayed planting up to the third
date, then to decline for the fourth date.
Final plant dry weight at
physiological maturity, however, was found to decrease with each suc-
cessive planting date (Table 12).
Warmer temperatures and high irradi-
ances in July could account for the difference in total plant dry
weight obtained at midsilk whil~ grain yield differences among planting
dates explained the difference in results attained at maturity.
The time from emergence to silking was found to be relatively
similar (61-67 days) for both hybrids at aIl dates.
Up to silking,
plant response was similar for both hybrids and aIl planting dates.
However, significant differences were found in the final yields.
l am
ignoring the last two planting dates because of the insect damage dis-
cussed earlier.
For the first method of measurement, the yield was the
same for C28l while it decreased for P3780.
The yield indeed decreased
with delayed planting in the second method, averaging 8.16 and 7.13
tons/ha for C28l, and 8.85 and 7.37 tons/ha for P3780 at dates 1 and 2,
respectively.
Duncan's multiple range test showed that thcre was sig-
nificant difference in hybrid yields in the first method while the dif-
ference was non-significant in the second method.
In addition, the
second method overestimated the yield suggesting that the barrenness
factor was underestimated.
In conclusion the calculated yield values
depend on the yield test used.
A harvest over the row estimates better
the barrenness percentage compared to a harvest of few ears from the
row.
Harvest index was found to be similar for both hybrids at planting
dates 1 and 2, suggesting that at maturity the plants from the second
planting were sma1ler compared to those from the first date.

---

112
Many parameters were investigated, in an attempt to try and explain
why a decrease in yield was observed with late planting.
It was found
that number of double kernel rows per cob varied significantly at the
5% level with hybrid and planting date.
The highest number of double
kernel rows was not found to be associated with either air temperature
or radiation recorded during the week prior to the beginning of kernel
site accumulation (Figure Il).
The cause of the observed response was
not determined.
Spikelet accumulation rate per ear shoot was found to be a linear
function of GDD.
For C28l, there was a significant difference between
the kernel site accumulation rates of ears from the first, second, and
fourth planting date.
Kernel site accumulation rate was not calculated
for the third planting date because of insufficient data points during
the site accumulation period.
Spikelet accumulation was highly cor-
related with temperature (GDD) in both hybrids.
The differences in
slope, however, suggest that either the temperature function used was
not appropriate or sorne other factors interacted in the ear shoot devel-
opment.
Tollenaar (1977) reviewing sink-source relationships du ring
reproductive development in maize, reported that development of upper
ears, up until the cessation of spikelet initiation, is rather unaffected
by environmental factors.
However, Hunter ~ al. (1977) reported a slight
influence of photoperiod on spikelet number per ear.
Cooper and Law
(1977) reported that soil temperatures early in the plant life had
played an important role in determining the number of potential grain
sites.
l found that higher kernel site accumulation rates were associ-
ated with higher GDD per day (Figure Il).
This result suggests that
differences in temperatures led to differences in the plant response

---

113
and subsequent kerne1 site accumulation rates.
The maximum number of
kerne1 sites was simi1ar for both hybrids at aIl dates except for third
p1anting of Corne11 281 which was significant1y lower than the other
samp1ing dates.
The maximum number ranged from 92 to 109 sites/double
row.
There was no significant difference between the number of kerne1s
harvested for C281 at dates 1 and 2 whi1e the difference was signifi-
cant at the 5% 1eve1 for P3780 at the same dates.
On1y 50 to 67 percent
of the spike1ets present at midsi1k produced grain, suggesting that at
midsi1k there were enough kerne1 sites such that spike1et deve10pment
was not a 1imiting factor for grain yie1d in this experiment.
Both grain yie1d and number of kerne1s fi11ed were re1ated to the
amount of radiation accumu1ated during the week prior to si1king and
the two fo11owing weeks.
As the accumu1ated irradiance increased both
the grain yie1d and the number of kerne1s fi11ed increased.
The
accumu1ated irradiance during the three-week period was 9071 and 8649
1y for C281, and 8567 and 8360 1y for P3780 for p1anting dates 1 and
2, respective1y.
The amount of irradiance (ly) accumu1ated per ton of
grain varied with p1anting dates, ranging from 968 to 1213 1y/ton of
grain, suggesting that other factors besides radiation such as temper-
ature, interacted for grain production.
The increase in number of
kerne1s fi11ed and subsequent grain yie1d supports the suggestion that
irradiance intercepted per plant during the f10wering period is a
dominant factor determining the continuation of ear growth (To11enaar,
1977).
Ear 1ength was found to be simi1ar for aIl dates within hybrid
and slight1y different between C281 and P3780, 22.2 cm and 23.5 cm,
respective1y.
Since the actua1 physica1 1ength of the ear shoot was

---

114
similar for aIl treatments of bath hybrids, ear length does not appear
to be associated with observed grain yield differences.
Kerne1 dry matter accumulation was found ta be a 1inear function
of bath calendar day and GDD.
The s10pes of the regression lines were
not significantly different within and between hybrids on either a
calendar day basis or a GDD basis.
Johnson and Tanner (1972) reported
that growth of a corn kerne1 starts immediately after ferti1ization as
a non-1inear function followed by a linear growth phase.
They reported
that up to 90% of the maximum kerne1 dry weight may accumulate during
the linear phase.
Duncan ~ al. (1965) found a significant correlation
of temperature and kernel growth rate.
TIle equations derived from my data were:
KDWt (gram)
0.00035729 GDD - 0.0712
for C281
[6]
KDWt (gram)
0.0003732 GDD - 0.0960
for P3780
[7]
Where KDWt is the kerne1 dry weight (104°C) in grams.
My results suggest that the effective kernel filling period
decreases with delayed planting.
Daynard ~ al. (1971) found that corn
grain yield was more closely related to the effective filling period
duration than ta the kernel growth rate.
They reported the EFPD was
not affected by plant density.
Poneleit and Egli (1979) reported that
EFPD, but not the rate of kernel growth, was influenced ta a limited
extent by plant density and bath were under genetic control.
The appar-
ent kernel filling period from silking ta blacklayer was found to increase
with delayed planting but was prematurely halted by fall frost for later
planting dates.

---

CHAPTER VI
CONCLUSIONS
1.
Corn grain yield significantly declined with delayed planting.
2.
When averaged over planting dates, the number of kernel rows
per cob was significantly different between Cornell 281 and
Pioneer 3780.
3.
Kernel site accumulation was a linear function of temperature
(growing degree days).
Differences in the rates of site accumula-
tion suggested that the temperature function used was not appropri-
ate for ear shoot development, or that factors other than temper-
ature significantly influenced the ear shoot development.
4.
The large differences between maximum kernel sites and number of
kernels filled suggest that at silking there were enough kernel
sites such that the kernel site accumulation was not important in
determining the final number of kernels.
S.
Kernel dry matter accumulation was related to growing degree days
(GDD) and calendar days.
Kernel growth rate in both cases was not
significantly different within and between Cornell 281 and Pioneer
3780.
6.
Effective kernel filling period duration decreased with delayed
planting.
7.
The results of the present study do not agree with the second
hypothesis stating that high air temperatures during ear shoot
development and at silking combine to limit kernel number and sub-
sequent grain yield of late planted corn.
Ear shoot development
and potential kernel sites were found not to be limiting.
The
differences in kernel number was mainly associated with insect
damage, but not to temperatures at silking.

---

116
8.
The results agree partially with the third hypothesis:
Kernel
filling rate was found not to be significantly different between
planting dates as l stated; however, the duration of filling was
halted prematurely due to cool fall temperatures and frost.
9.
The final yield differences within hybrids at similar LAI were
mainly due to:
(a)
The final kernel per ha, which was lower in la te planted
corn because of poor pollination.
(b)
The duration of the effective filling period, which was
reduced by lower air temperatures.
(c)
The differences in barrenness, and also plant stand.
10.
The results of this study reveal that future work should be done
to determine why 30 to 50 percent of the potential kernel sites
failed to produce grain.
This future work should help determine
how one can take advantage of the high potential kernel sites for
maximum grain production.

---

117
APPENDIX A
Microc1imate Data of 1979 Growing Season

---

118
Table 45.
Microc1imate data:
maximum, minimum, and adjusted she1ter
air temperatures (oC); Radiation (Ly/day); and precipitation
(mm/day) recorded during the growing season of 1979.
She1ter
Ca1endar
Julian
Adj. Average T*
min T
maxT
Radiation
Rain
day
day
Oc
Oc
Oc
Ly/day
mm/day
May 1
121
10.9
1.4
13.8
562
0.0
122
13.3
-3.1
19.6
590
0.0
123
13.5
7.7
15.1
115
0.2
124
10.4
7.4
14.1
144
0.0
125
10.9
1.3
14.4
605
0.0
126
13.7
2.7
20.0
461
0.0
127
18.6
6.3
26.9
605
0.5
128
23.9
18.2
29.9
590
0.0
129
25.1
18.9
31.4
605
0.0
130
21.1
16.0
30.9
274
0.2
131
21.3
15.4
28.7
504
0.0
132
20.1
17.6
23.7
274
0.0
133
15.5
11.9
17.8
202
0.0
134
15.4
8.2
21. 8
518
0.2
135
13.6
5.8
20.8
403
0.0
136
12.6
4.1
18.0
590
0.0
137
13.7
0.1
20.1
691
0.0
138
15.0
1.2
22.6
590
0.0
139
16.1
13.2
21.1
360
0.0
140
17.3
13.9
21.3
418
0.0
141
16.7
14.9
19.7
259
0.2
142
14.3
2.8
20.6
648
0.2
143
13.5
11. 9
15.4
86
2.9
144
14.4
12.4
17.5
230
0.2.
145
1L5
9.1
13.3
144
0.0
146
10.0
7.3
10.3
216
0.0
147
10.4
6.4
11.6
115
0.2
148
1L5
4.6
14.6
274
0.5
.
.' ,.
.:

---

119
Table 45 (Continued).
She1ter
Ca1endar
Julian
Adj. Average T*
min T
max T
Radiation
Rain
day
day
Oc
Oc
Oc
Ly/day
mm/day
149
12.6
10.1
15.8
144
0.0
150
14.1
6.7
21.2
475
0.0
151
16.2
11.5
22.7
389
0.0
June 1
152
18.2
15.7
20.3
115
0.0
153
20.2
16.2
25.5
418
0.0
154
16.2
14.2
18.1
216
0.0
155
17.5
9.1
26.4
619
0.0
156
18.2
12.0
27.9
562
1.0
157
16.1
11. 7
21.7
245
1.2
158
21.2
14.2
27.4
504
0.0
159
22.3
20.1
26.3
346
46.3
160
23.2
19.3
27.8
475
0.0
161
23.4
20.8
27.1
346
0.0
162
14.4
7.1
22.0
346
2.4
163
13.4
4. 7
18.6
734
0.0
164
14.6
3.3
21. 5
749
0.0
165
17.4
3.6
25.7
749
0.0
166
21.3
10.3
30.0
662
0.0
167
22.3
13.4
29.5
662
0.0
168
21.2
15.3
27.6
590
0.0
169
18.2
13.3
23.1
418
0.0
170
17.0
7.5
24.2
763
0.0
171
17.9
7.3
24.7
648
0.2
172
15.5
14.0
17.0
144
1.2
173
19.2
14.0
27.1
562
6.0
174
16.2
10.5
21.3
547
0.0
175
11. 7
8.8
17.1
331
0.0
176
14.3
3.0
21.3
778
0.2
177
16.8
3.5
23.3
763
0.0
178
18.2
13.0
24.1
634
0.0

---

120
Table 45 (Continued).
She1ter
Ca1endar
Julian
Adj. Average T*
min T
max T
Radiation
Rain
day
day
Oc
Oc
Oc
day
Oc
Oc
Ly/day
mm/day
179
16.6
11.5
22.9
389
13.9
180
15.7
10.7
23.3
346
37.2
Ju1y 1
181
18.5
14.4
23.3
389
17.8
182
16.7
15.1
19.8
331
23.8
183
16.1
15.3
18.1
115
0.0
184
18.4
13.1
23.9
619
0.0
185
12.3
8.6
17.7
187
15.4
186
12.3
7.2
16.0
461
0.0
187
15.0
4.5
21.6
691
0.2
188
16.4
6.0
24.1
720
0.0
189
18.0
8.9
25.1
662
0.0
190
19.3
11.1
24.7
547
0.0
191
18.5
16.7
20.8
230
7.2
192
20.9
15.7
27.4
562
0.0
193
21.9
17.1
28.3
518
0.0
194
22.0
14.8
28.5
518
0.0
195
22.4
18.8
28.2
432
1.0
196
22.9
19.0
29.6
475
0.0
197
21.8
18.4
28.4
389
4.6
198
22.0
17.6
27.7
619
0.0
199
20.9
15.4
26.7
533
0.0
200
20.2
12.4
27.0
590
0.2
201
19.7
14.5
24.1
230
0.0
202
21. 6
16.0
26.4
446
0.0
203
21.7
18.0
27.1
346
0.0
204
19.9
16.6
25.9
288
14.9
205
21. 9
16.8
28.1
504
0.0
206
22.6
20.0
26.9
360
5.3
207
22.9
17.2
27.0
360
0.0
208
19.5
14.1
25.1
504
0.0

---

121
Table 45 (Continued).
Shelter
Ca1endar
Julian
Adj. Average T*
min T
max T
Radiation
Rain
day
day
Oc
Oc
Oc
Ly/day
mml day
209
19.0
14.3
23.1
230
7.0
210
18.3
15.8
20.2
115
33.8
211
20.8
15.2
26.4
490
0.2
August 1
212
22.5
15.9
29.0
605
0.0
213
24.4
20.1
30.6
418
0.7
214
24.5
19.2
28.1
533
0.2
215
21.9
15.7
28.1
576
0.0
216
22.2
16.1
28.1
518
0.0
217
22.9
16.7
29.2
590
0.0
218
22.6
18.0
28.2
475
0.5
219
20.5
12.7
26.9
590
0.0
220
24.1
20.0
29.9
576
0.0
221
22.2
17.8
26.8
504
0.0
222
22.2
18.3
29.4
302
0.5
223
18.0
15.7
20.4
72
2.2
224
14.3
12.8
15.7
130
0.0
225
15.7
7.5
22.9
619
0.5
226
19.1
15.3
20.6
173
0.0
227
13.4
12.0
15.3
187
0.0
228
14.7
9.8
21.1
533
0.0
229
15.0
7.3
21. 2
572
0.0
230
16.8
15.4
17.7
72
0.2
231
21.1
16.7
26.6
374
0.0
232
20.5
15.6
25.8
446
0.0
233
19.4
15.6
25.6
389
0.0
234
18.2
11.7
25.0
547
0.0
235
20.4
15.3
25.0
115
0.0
236
23.0
21.0
26.9
288
0.0
237
21. 7
16.7
25.0
374
0.0
238
19.1
13.3
23.3
259
0.0

---

122
Table 45 (Continued).
She1ter
Ca1endar
Julian
Adj. Average T*
min T
max T
Radiation
Rain
day
day
Oc
Oc
Oc
Ly/day
mm/day
239
21.6
16.9
25.9
346
4.1
240
21.0
14.9
27.3
475
0.2
241
22.7
20.9
26.8
360
12.7
242
22.1
16.8
28.2
504
0.0
Sept. 1
243
19.8
14.5
27.1
590
0.0
244
19.4
11.5
25.8
504
0.0
245
22.8
20.3
27.1
374
26.4
246
22.0
17.2
27.1
403
1.0
247
21.1
14.9
28.2
475
0.0
248
20.9
17.6
23.9
216
17.8
249
22.1
19.4
26.8
360
22.:3
250
19.1
12.9
24.6
446
0.5
251
13.6
7.9
16.2
216
0.0
252
U.7
3.6
20.2
262
0.0
253
15.8
5.0
23.3
533
0.0
254
16.5
10.7
22.1
360
0.0
255
15.8
8.3
22.6
475
0.0
256
18.9
15.3
22.5
288
1.0
257
19.6
12.6
24.3
115
9.4
258
12.8
6.9
18.6
389
0.0
259
13.9
4.5
21.3
518
0.0
260
15.1
5.7
22.0
504
0.2
261
15.8
7.2
22.9
403
1.2
262
13.9
3.5
17.3
440
0.0
263
13.0
0.0
18.0
504
0.2
264
14.6
13.8
15.8
58
30.7
265
13.8
10.8
14.9
115
1.5
266
11.9
4.3
16.9
518
0.0
267
12.2
2.3
17.5
504
4.3
268
13.2
2.9
19.7
418
0.0

---

123
Table 45 (Continued).
She1ter
Ca1endar
Julian
Adj. Average T*
min T
max T
Radiation
Rain
day
day
Oc
Oc
Oc
Ly/day
mm/day
269
14.6
8.5
23.7
389
0.0
270
15.2
10.0
22.0
446
0.0
271
14.8
13.7
15.7
180
30.2
272
17.3
15.1
20.7
158
0.7
Oct. 1
273
18.1
15.3
21.6
173
0.0
274
16.7
14.3
18.6
101
0.0
275
17.0
14.0
23.5
187
10.6
276
14.2
12.4
15.6
101
31. 7
277
15.6
11.5
19.4
374
0.0
278
11.5
7.2
16.8
29
43.0
279
10.9
4.7
13.3
274
0.5
280
10.0
5.6
9.4
101
2.6
281
10.4
5.2
13.4
245
1.2
282
10.9
2.9
13.4
72
3.1
283
10.0
1.9
6.8
173
0.7
284
10.1
0.0
11.1
374
4.8
285
10.2
5.1
11.2
173
5.0
286
10.0
1.5
8.8
230
2.6
Oct. 15
287
10.0
0.0
7.7
144
0.0
*Adj. Average T is the adjusted average temperature (base 10°, 30°C)
ca1culated with the adjusted average heat system.

---

Table 46.
Soi1 temperature at 5 cm depth recorded at 8 locations in the experiment field at the
nearby weather station from 6 June to 6 Ju1y 1979.
Locations
Ca1endar
Weather
day
1
2
3
4
5
6
7
8
Average
Station
06 JURe
16.0
17.0
19.0
20.0
17.0
17.0
17.0
17.0
17.5
17.7
08
30.0
23.0
31. 0
31.0
31. 0
31. 0
26.0
31. 0
25.5
22.9
13
18.0
18.0
17.0
19.0
21. 0
20.0
19.0
19.0
18.9
19.5
18
22.0
22.0
22.0
23.0
24.0
23.0
22.0
22.0
22.5
23.3
20
23.0
24.0
25. 0
22.0
27.0
26.0
23.0
23.0
24.0
18.3
22
24.0
22.5
27.0
29.0
29.0
28.0
24.0
22.0
25.7
23.4
25
23.5
22.0
20.5
24.0
27.8
24.5
22.5
22.5
23.4
24.1
27
19.2
25.5
22.4
26.5
28.5
25.2
23.5
23.5
24.3
23.9
29
21. 5
21. 0
20.0
23.0
24.0
22.0
20.0
20.5
21.5
21.6
05 Ju1y
21.0
20.0
19.0
19.0
21. 0
20.0
19.0
19.0
19.8
17.9
06
22.0
21.5
20.5
22.5
25.0
23.5
20.0
20.0
21. 9
23.0
1-'
'"t->
~
J>-

---

125
APPENDIX B
Leaf Area and Leaf Area Index Data

---

2
Table 47.
Leaf area (cm ) per plant measured at various dates for 2 corn hybrids (C281 and P3780)
p1anted in the spring of 1979 at 4 dates.
LA/P1ant/P1anting date
C281
Julian
P3780
Julian
Harvest da te
12 May
30 May
08 June
22 June
12 May
30 May
08 June
22 June
----------------------------------------cm~-------------------
----------------------------------------cm~-----------------------------------------
----------------------------
169
98 (10.5)
88(9.7)
184
1638 (19.2)
535 (18.1)
1455 (15 .0)
511 (14.8)
186
2106 (8.6)
650 (13.6)
1670 (7.6)
554 (17.4)
190
341 (9.6)
222 (8.4)
191
2630 (15.0) 1068 (18.4)
2390 (14.4) 1000 (19.1)
194
3107 (13.8) 1290 (25.8)
2844(13.7)
2844 (13.7) 1274(12.9)
1274 (12.9)
200
4295 (14.0)
4295(14.0) 2629 (19.6)
2629(19.6) 1177 (12.5)
1177(12.5)
160 (12.6)
4183 (9.5)
2356 (14.8) 1127 (9.3)
256(10.7)
206
2569 (13 .1)
207
4944(10.4)
4944 (10. 4) 3620 (19.7)
5189 (8.7)
5189(8.7)
3765 (18.9)
3765(18.9)
214
4655 (13.9) 4319 (19.4) 4245 (10.4)
1887 (10.5)
5107 (11.2) 4625 (14.7) 4156 (7.8) 1628(12 .4)
222
4544(9.2)
4178(1.3)
5316(9.9)
3721<14.2)
3721(14.2)
4886 (7.2)
4886(7.2)
5412<9.2)
5412(9.2)
5570 (5.9)
5570(5.9) 3121(9.6)
227
31379 (11.3)
4123(17.1)
228
4533 (10.4) 4452 (10.9) 4467 (11.5)
5297 (8.6)
5472 (9.5)
5994 (8.1)
234
4349 (9.5)
4182 (16.4)
5880 (9.7) 4780(11.0)
235
4220 (7.2)
5681 (7.6)
241
4108 (15.0)
51090:0.4)
5109(:(0.4)
243
4185 (10.1)
5297 (11.7)
249
4046 (9.2)
4755(14.3)
The coefficient of variation (c.v.) of the 1eaf area is in parentheses.
There were 16 plants in each mean.
1-'
N
0\\

---

Table 48.
Leaf Area Index calculated
ca1cu1ated for various periods of growth for 2 corn hybrids (C28l
(C281 and P3780)
p1anted in the spring of 1979 at 4 dates.
P1anting Date
C281
P3780
Julian
Harvest date
12 May
date
12
30 May
08 June
22 June
12 May
30 May
08 June
22 June
Jooe
169
0.1
0.1
184
1.4
0.4
0.9
0.3
186
1.8
0.5
1.1
0.4
190
0.3
0.1
191
2.3
0.4
1.6
0.6
194
2.7
1.1
1.8
0.8
200
3.7
2.2
0.9
0.1
2.7
1.5
0.7
0.2
206
1.9
1.2.
207
4.3 }
3.0
3.4
2.4
214
4.0
3.6 }
3.1
1.6
.l..2
..1..J.
2.9}
1.0
222
3.9
3.5
3.9
2.9
3.2
3.4
3.2
1.9
227
3.0
2.5
228
3.9
3.7
3.3}
3.3 }
3.4
3.4
3.5 }
234
3.2
3.2
3.4
2.8
235
3.5
3.6
241
3.2
3.0
243
3.1
3.0
249
3.1
2.8
The under1ined LAI values indicate the 1eaf area index at midsi1k.
The single bracket indicates that the actua1 LAI at midsi1k fa11s between the two values shawn in
the table.
.....
N
......

---

128
APPENDIX C
Plant Part Dry Weight Data and Correction Factor
frOID 69°C to 104°C Dry Weight

---

Table 49.
Stalk dry (690
(69 C or 157Or)
157er) weight per 4 plants samp1ed at various
variaus periods of growth for 2 corn
hybrids (C281 and P3780) p1anted in the spring of 1979 at 4 dates.
Planting Date
Julian
C281
P3780
Julian
Hal:Yest date
12 May
30 May
08 June
22 June
12 May
30 May
08 June
22 June
-----------------------------------------grams-----------------------------------------
173
0.1
0.1
187
3.::
3.3
0.2
2.5
0.1
190
9.2
0.4
6.4
0.4
194
21.0
2.4
15.7
2.1
197
45.6
6.1
0.4
29.6
4.4
0.2
200
69.9.
69.9
17.0
1.9
50.3
11.9
1.0
204
107.4
28.1
6.2
93.8
18.6
3.2
207
48.8
12.4
0.5
113.1
32.5
5.7
0.4
212
133.2
84.3
24.7
2.1
U4.6
124.6
57.4
22.1
1.5
217
142.1
106.4
37.1
5.4
136.8
76.7
37.6
3.7
219
183.7
97.8
13.4
193.1
71.8
9.2
225
184.7
158.9
160.4
36.~
184.4
165.7
137.4
28.1
228
169.1
52.1
178.2
42.3
232
176.4
167.8
74.2
179.6
164.1
47.3
235
87.7
71.9
239
191.5
125.6
182.4
122.5
242
196.1
137.9
197.2
155.3
249
147.8
156.0
1-'
N
\\J:)

---

Table 50. Leaf sheath dry C690
(69 e
C or 157OP)
157Of) weight for 4 plants samp1ed at various periods of growth for 2
corn hybrids Ce281
(C281 and P3780) p1anted
planted in the spring of 1979 at 4 dates.
P1anting
P1ant1ng Date
e281
C281
P3780
Julian
Harvest date
12 May
30 May
08 June
22 June
U May
30 May
08 June
22, June
date
12 May
30 May
08 June
22 June
12 May
30 May
08 June
22,
-----------------------------------------grams-----------------------------------------
173
2.0
0.4
2.0
0.4
0.2
187
9.2
2.6
0.4
8.5
2.0
0.6
190
14.4
4.0
0.9
13.0
3.6
2.5
194
24.1
11.1
3.5
0.3
19.5
11.0
4.0
0.2
197
30.6
13.7
5.0
0.8
30.0
13.4
7.8
0.6
200
36.2
21. 7
8.4
1.8
35.1
20.9
10.2
1.4
204
43.9
24.2
12.2
2.9
43;9
24.8
11.
Il. 0
2.2
207
29.7
13.7
4.7
30.4
20.6
4.3
2U
212
51.0
40.1
18.6
6.6
48.9
34.5
25.5
5.5
217
52.7
42.5
36.9
10.8
52.7
37.1
34.6
9.6
219
39.6
13 .4
13.4
12.4
225
55.1
50.2
48.0
22.1
61.8
54.1
45.0
22.7
228
26.8
28.7
232
52.5
53.2
60.9
55.3
235
53.9
36.2
62.8
32.5
239
58.3
44.2
62.5
42.4
242
53.5
41.4
63.0
45.8
249
45.5
48.4
t--
....,
i-'
w
o

---

Table 51.
Leaf b1ade dry (69°C or 157°F) weight per 4 plants sampled
samp1ed at various periods of growth for 2
corn hybrids (C281 and P3780) p1anted in the spring of 1979 at 4 dates.
P1anting Date
Julian
C281
P3780
Julian
C281
Harvest date
12 May
30 May
08 June
22 June
12 May
30 May
08 June
22 June
-----------------------------------------grams-----------------------------------------
173
7.8
2.5
7.8
2.0
187
30.5
10.6
2.4
28.0
8.2
1.4
190
44.8
13.7
4.5
41.2
12.9
3.1
194
54.3
20.1
6.2
0.9
50.5
20.3
4.7
0.6
197
73.4
28.1
8.9
1.6
68.2
27.7
7.6
1.1
200
79.7
43.7
13.8
2.8
81.6
42.1
12.5
2.4
204
82.9
52.7
25.5
5.4
92.5
50.6
20.6
4.9
207
93.6
61.4
34.2
8.6
97.2
65.2
80.7
8.3
212
74.7
44.2
15.8
74.8
48.0
13.4
217
87.9
75.4
55.2
21.4
103.6
85.5
61.2
19.5
219
95.0
75.1
34.0
113.6
83.2
32.2
225
89.1
76.9
81.4
53.8
113.1
101.2
98.0
57.2
228
80.6
88.9
59.1
107.2
102.2
70.6
232
87.3
67.9
108.3
69.6
235
66.9
81.1
239
89.8
73.5
111.1
91.9
242
88.8
70.7
112.7
90.6
249
76.2:
96.1
1-'
W
1-'

---

132
Table 52.
Determination of the coefficient of dry weight correction
for vegetative plant parts from 69°C to 104°C.
Temperature
Samp1e 1/
104°c
69°C
Excess H2
H 0
2
-----------------grams------------------
1
94.44
96.50
2.06
2.14
2
115.66
118.51
2.85
2.41
3
90.35
92.48
2.13
2.30
4
107.00
110.16
3.16
2.87
Average
101.86
104.41
2.55
2.43
Dry Wt (104 oC)
0.9757 (DWt 69°C)

---

Table 53.
Total dry (104°C) weight per plant samp1ed at various periods of growth for 2 corn hybrids
(C281 and P3780) p1anted in the spring of 1979 at 4 dates.
P1anting Date
Julian
C281
P3780
Julian
Harvest,date
12 May
30 May
08 June
22 June
12 May
30 May
08 June
22 June
-----------------------------------------grams-----------------------------------------
173
2.4
0.7
2.4
0.6
187
10.5
3.3
0.7
9.5
2.5
0.4
190
16.7
4.4
1.3
14.8
4.1
0.9
194
24.2
8.2
2.3
0.3
20.9
8.1
1.7
0.2
197
36. ~
36.~
11. 7
3.5
0.6
31.2
11.1
2.9
0.4
200
45.3
20.1
5.8
1.1
40.7
18.2
5.2
0.9
204
57.1
25.6
10.7
2.0
56.1
22.9
8.3
1.7
207
34.1
14. i
3.4
31.2
11. 5
3.2
212
48.6
21.3
6.0
40.5
22.1
4.9
217
75.2
54.7
9.1
76.1
48.6
8.0
219
51.8
14.8
46.3
13.1
225
124. C
124.C
82.6
70.7
27.4
132.7
87.0'
68.4
26.3
228
33.6
34.5
232
106.1
103.7
235
46.6
45.2
239
111.2
59.3
102.0
62.6
242
128.6
61.0
131.7
71.1
249
80.8:
86.6
.....
. ,
\\,W
,
\\,W

---

134
APPENDIX D
Plant Height Data

---

Table 54.
Plant height (cm) measured at various periods of growth for 2 corn hybrids (C281 and P3780)
p1anted in the spring of 1979 at 4 dates.
P1anting Date
Julian
C281
P3780
Julian
Sampling Date
12 May
30 May
08 June
22 June
12 May
30 May
08 June
22 June
------------------------------------------cm-------------------------------------------
------------------------------------------cm-------------------------------------------
171
23.4
21.5
176
29.1
26.9
184
38.8
21. 7
37.9
20.7
186
45.2
24.3
38.5
21.2
191
57.0
32.2
50.2
31.1
194
67.8
36.5
58.0
36.9
200
116.4
63.3
87.5
54.1
206
207
204.6
102.6
67.7
153.0
83.7
52.1
214
244.1
172.3
172 .3
91. 5
62.2
213.9
130.5
70.3
48.2
222
260.0
241. 0,
241.0,
207.5
103.7
240.2
223.2
166.7
89.0
227
112.8
93.3
228
270.1
253.7
223.6
241.7
235.9
204.3
234
250.0
165.7
236.1
139.9
235
241. 0
241.0
237.0
241
230.7
209.3
243
257.0
241.6
249
253.9
230.9
f-'
W
VI

---

136
APPENDIX E
Ear Shoot Development:
Kernel site, Row Number,
Kernel Filled and Ear Length Data

---

Table 55.
Number of double rows on the topmost ear samp1ed at various dates for 2 corn hybrids (e281
(C281
and P3780) p1anted in the spring of 1979 at 4 dates.
P1anting Date
C281
P3780
Ju1iarr
Samp1ing Date
12 May
30 May
08 June
22 June
12 May
30 May
08 June
22 June
-----------------------------------# double rows/ear------------------------------------
197
7. S ,*
7.8.*
7.9
201
7.4
8.5
7.5
8.0
204
7.3
7.8
205
7.3
8.4
208
7.5
7.6
7.4
7.5
212
7.3
7.5
213
7.5
7.6
215
7.6
8.1
218
7.6
7.f.
7.0
219
7.5
7.9
7.2,
8.0
222
7. 3
7.5
8.0
7.5
225
7.3
7.8
226
7.9
7.1
7.1
7.8
7.4
7.3
229
7.3
7.6
7.4
7.4
8.0
7.4
230
7.0
7.8
232
7.6
7.8
233
7.0
7.1
7.5
7.6
235
7.2
7.2
7.5
7.4
7.5
7.:
7.5
239
7.3
7.6
7.0
7.9
7.4
7.1
242
7.4
7.4
7.4
7.5
243
6.9
7.4
248
7.8
7.2
7.8
7.3
249
7.6
7.3
7.5
7.2
253
7.;'
7.5
7.3
7.7
7.4
255
7. 2
7.2
7.1
7.2
7.5
1-'
w
...,
.....

---

Table 55 (Continued).
Planting Date
C28l
P3780
Julian
Sampllng Date
12 May
30 May
08 June
22 June
12 May
30 May
08 June
22 June
---------------------------------~Uâouble
---
--------#dou6Ie rowS7ear-------------------------
rowsJear---------
--
----------
260
7.5
7.B
7.8
7.8
7.6
263
7.3
7.3
7.6
7.3
267
7.6
7.6
8.1
7.3
270
7.1
7 .5
7.5
277
7.2
7.3
7.3
7.1
7.4
7.3
7.3
283
7.4
7.0
7.4
7.3
7.3
* range 6-10 double rows/ear
f-'
W
aJ

---

Table 56.
Kernel
Kerne1 sites per double row on the topmost ear sampled
samp1ed at various dates for 2 corn hybrids
Ce281
(e281 and P3780) planted
p1anted in the spring of 1979 at 4 dates.
Planting
P1anting Da te
Julian
e281
P37BO
Julian
P3780
Sampling Date
12 May
30 May
OB
08 June
22 June
12 May
30 May
OB
08 June
22 June
-----------------------------~I
kernel sites/double
-----------------------------~!kerne1
row-------------------------------
197
73.4
53.8
201
84.9
39.0
76.5
28.0
204
94.0
95.5
205
72 .6
49.4
208
95.3
91.8
91. 8
100.1
64.4
212
99.8
104.5
213
104.4
93.3
215
104.0
97.8
21B
218
97.4
89.0
219
92.2
97.6
100.9
105.3
222
95.0
90.0
10B.8
108.8
101.0
225
91.5
100.8
226
92.8
B5.6
85.6
B5.1
85.1
101.6
103.7
71. 6
229
88.6
94. 8
94.8
90.9
103.5
99.3
103.8
230
95. 4
95.4
80.6
232
92.4
98.6
233
B9.~
89.~
98.4
104.6
90.1
235
92.1
95.9
98.8
97.1
239
87.4
93.5
103.4
101.8
242
91.0
101.7
243
92.1
100.5
249
90.8
95.5
255
87.1
98.4
1-'
w
. .~
'J:)
.~
""
'0

---

Table 57. Number oÎ
of kerne1s
kerue1s fi11ed/doub1e rowon the topmost ear samp1ed at various dates for 2 corn
com
hybrids (C281
(C2S1 and P3780)
P37S0) p1anted in the spring of 1979 at 4 dates.
P1anting Date
C281
C2S1
P3780
Julian
P37S0
Sampling
Samp1ing Date
12 May
30 May
08
OS June
22 June
12 May
30 May
08
OS June
22 June
------------------------------# kerne1
keme1 sites/double
sites/ double row--------------------------------
235
80.0
86.9
239
82.8
66.8
242
73.3
61.5
22.6
248
70.3
61.5
69.9
19.0
249
68.8
77 .1
253
61.4
60.5
59.9
255
67.1
32.3
78.4
13.8
260
66.5
57.1.
57.1
69.5
28.0
263
63.1
39.4
76.6
19.6
267
61.5
56.5
26.2
270
43.6
16.0
277
57.3
46.0
44.3
70.0
57.8
23.9
28.0
283
47.8
43.0
54.3
24.7
22.6
t-'
.1>-
o

---

Table 58.
Ear 1ength (cm) measured at various periods of growth for 2 corn hybrids (C281 and P3780)
p1anted in the spring of 1979 at 4 dates.
P1anting Date
C281
P3780
Julian
Harvest Date
12 May
30 May
08 June
22 June
12 May
30 May
08 June
22 June
------------------------------------ear 1ength (cm)------------------------------------
197
1.3
0.6
201
2.2
0.3
1.3
0.2
204
4.5
2.8
205
1.0
0.5
208
7.6
2.2
4.1
0.8
212
11. 9
7.1
213
4.7
4. 7
2.8
215
7.4
3.7
218
4.9
2.8
219
20.2
12.3
15.5
8.4
222
13.1
9.2
11.2
5.4
225
22.0
21.7
226
17.3
10.8
2.1
12.2
6.6
1.1
229
22.0
20.5
12.9
21.9
17.3
8.6
230
3.3
1.5
232
21.0
17.3
233
14.2
4.6
10.4
2.3
235
22.4
17.3
6.5
22.0
13.6
3.9
239
22.4
20.7
10.1
22.8
17.8
7.3
242
22.4
21. 7
21.7
22.3
21.1
243
11.6
9.7
248
22.9
22.2
23.5
23.6
249
22.6
20.0
22.4
16.6
t-'
.,.
f-'
""'
f-'

---

Table 58 (Continued).
P1anting Date
C281
P3780
Julian
Harvest Date
12 May
30 May
08 June
22 June
12 May
30 May
08 June
22 June
------------------------------------ear 1ength (cm)------------------------------------
253
21. 9
22.6
23.4
24.1
255
22.4
21.1
22.8
21.3
260
22.1
22.3
24.0
24.0
263
21.3
22.7
267
22.7
24.0
270
20.8
22.2
277
22.0
24.2
22.8
283
22.8
f-'
.l:-
N

---

143
APPENDIX F
Kernel and Ear Dry Weight Data

---

Table 59.
Dry (104°C) weight per kerne1 samp1ed
sampled at various periods during grain fi11ing for 2 corn
hybrids (C281 and P3780) p1anted in the spring of 1979 at 4 dates.
P1anting Date
C281
P3780
Julian
Sampling Date
12 May
30 May
08 June
22 June
12 May
30 May
08 June
22 June
-----------------------------------------grams-----------------------------------------
239
0.037
242
0.155
0.026
0.105
248
0.126
0.062
0.082
0.032
249
0.206
0.165
253
0.157
0.101
0.112
0.063
255
0.240
0.215
260
0.198
0.152
0.171
0.121
263
0.279
0.052
0.240
0.029
267
0.274
0.228
0.187
0.201
0.152
270
0.084
0.053
277
0.266
0.239
0.130
0.287
0.259
0.207
0.110
281
0.290
283
0.268
0.251
0.151
0.278
0.223
0.145
288
0.261
291
0.254
0.154
0.240
0.150
1-'
.&:-
.&:-

---

Table 60.
Grain dry (104 oC) weight per ear samp1ed at various dates during
durillg kerne1 fi11ing
filling for 2 corn
hybrids (C281 and P3780) p1anted in the spring of 1979 at 4 dates.
P1anting Date
C281
P3780
Julian
Sampling Date
12 May
30 May
08 June
22 June
12 May
30 May
08 June
22 June
-----------------------------------------grams-----------------------------------------
235
23.8
14.1
239
12.4.
12.4
242
59.9
7.0
44.4
248
50.4
19.4
30.9
3.1
249
83.5
75.5
253
55.6
32.0
39.1
4.8
255
92.5
93.3
260
79.5
52.0
72 .1
19.6
263
100.7
7.9
113.4
2.4
267
105.0
86.4
53.3
74.4
21. 7
270
18.2
4.4
277
92.3
65.2
30.l<
129.3
96.5
26.3
14.8
281
124.1
283
68.6
33.3
32.4
17.0
288
107.1
291
67.8
31.7
31. 7
35.3
23.3
f-'
.::-
-i
lJl
-j

---

146
Table 61.
Final grain dry (104°C) weight (g/6 foot row) of two corn
hybrids (Corne11 281 and Pioneer 3780) p1anted in spring
of 1979 at successive dates.
Hybrid
1979
C281
_ _~ P-"-3-'--78c..;0'--
_
P1anting
Date
Set l
Set II
Set l
Set II
----------------------g/6 ft. of row------------------
12 May
940
835
1157
1226
30 May
957
848
995
900
08 June
670
597
244
110
22 June
272
307
155
177

---

147
LITERATURE CITED
1.
Agronomy Guide.
1980.
The Pennsylvania State University.
pp. 5-23.
2.
Benoit, G. R., A. L. Hatfield, and J. L. Ragland.
1966.
The
growth and yield of corn.
III. Soil moisture and temperature
effects.
Agron. J. 58:223-226.
3.
Blacklow, W. M.
1972a.
Mathematica1 description of the influence
of temperature and seed qua1ity on inhibition by seeds of corn
(Zea mays L.).
Crop Sei. 12:643-646.
4.
Blacklow, W. M.
1972b.
Influence of temperature on germination
and e10ngation of the radicle and shoot of corn (Zea mays L.).
Crop Sei. 12:647-650.
5.
Blacklow, W. M.
1973.
Simulation model to predict germination
and emergence of corn (Zea mays L.) in an environment of
changing temperature.
Crop Sei. 13:604-608.
6.
Bonaparte, E. E. N. A.
1975.
The effects of temperature, day-
length, soil fertility, and soi1 moi sture on 1eaf number and
duration to tasse1 emergence in Zea mays L.
Ann. Bot. 39:
853-861.
7.
Bonnett, O. T.
1966.
Inflorescences of maize, wheat, rye, barley,
and oats:
their initiation and development.
Univ. of Ill. Agr.
Exp. Sta. Bull. 721.
8.
Bouyoucos, G. J.
1950.
A practical soil moisture meter as a
scientific guide to irrigation practices.
Agron. J. 42:104-107.
9.
Bouyoucos, G. J., and A. H. Mick.
1940.
An electrical resistance
method for the continuous measurement of soil moisture under
field conditions.
Mich. State Col1ege of Agri. Exp. Sta. Bull.
172.
10.
Boyle, M.
1980.
Kerne1 growth rate, yield, and pedice1 acid
invertase activity in maize.
Master's Thesis (The Pennsylvania
State University).
Il.
Breuer, C. M., R. B. Hunter, and L. W. Kannenberg.
1976.
Effects
of 10 and 20-hour period treatrnents at 20 and 300
30 C on rate of
developmen t of a single-cross maize (Zea mays L.) hybrid.
Cano J. Plant Sei. 56:795-798.
12.
Brouwer, R., A. Kleinendorst, and J. Th. Locher.
1973.
Growth
responses of maize plants to temperature.
Unesco:169-174.
13.
Brown, D. M.
1977.
Response of maize to environmenta1 temperature:
a review.
Proc. Symp. Agromet. Maize Crop. WMO No. 481:15-25.

---

148
14.
Bryant, H. T., and R. E. Blaser.
1968.
Plant constituents of an
ear1y and 1ate corn hybrid as affected by row spacing and plant
population.
Agron. J. 60:557-559.
15.
Buren, L. L., J. J. Mock, and 1. C. Anderson.
1974.
Morpho1ogica1
and physio1ogica1 traits in maize associated with to1erance to
high plant density.
Crop Sei. 14:426-429.
16.
Carr, M. K. V., and G. M. Mi1bourn.
1976.
Maize:
C1imatic and
physio1ogica1 factors 1imiting yie1d.
Span. 19, 2, 1976:65-67.
17.
Carr, M. K. V.
1977.
The influence of temperature on the develop-
ment and yield of maize in Eng1and.
J. of Agri. Sei.
Cambridge
71 :117-125.
18.
Carter, M. W., and C. G. Poneleit.
1973.
Blacklayer maturity
and filling period variation among inbred lines of corn (Zea
mays L.).
Crop Sc!. 13:436-439.
19.
Cerning, J., and A. Guilbot.
1971.
Die entwickelung hoch mole
kularer kohlenhydrate im maiskorn wahrendder reifeprozesses.
Die St~rke 23: 238-244.
(English summary)
20.
Chabot, B. F., T. W. Jurik, and J. F. Chabot.
1979.
Influence of
instantaneous and integrated light flux density on leaf anatomy
and photosynthesis.
Amer. J. Bot. 66(8):940-945.
21.
Coe1ho, D. T., and R. F. Dale.
1980.
An energy-crop growth
variable and temperature function for predicting corn growth
and development:
planting to si1king.
Agron. J. 72:503-510.
22.
Coligado, M. C., and D. M. Brown.
1975.
Response of corn (Zea
mays L.) in the pre-tassel initiation period to temperature and
photoperiod.
Agr. Met. 14:357-367.
23.
Cooper, P. J. M.
1979.
The association between altitude, environ-
mental variables, maize growth, and yields in Kenya.
J. Agric.
Sei. Cambo 93:635-649.
24.
Cooper, P. J. M., and R. Law.
1977.
The effect and importance of
soil temperature in determining the early growth vigor and final
grain yie1ds of hybrid maize in the highlands of Kenya.
Proc.
Symp. Agromet. Maize (Corn) Crop.
WMO No. 481:265-281.
25.
Cross, H. Z., and M. S. Zuber.
1972.
Prediction of flowering
dates in maize based on different methods of estimating thermal
units.
Agron. J. 64:351-355.
26.
Daynard, T. B., and W. G. Duncan.
1969.
The black1ayer and grain
maturity in corn.
Crop Sei. 9:473-476.
27.
Daynard, T. B., J. W. Tanner, and W. G. Duncan.
1971.
Duration of
the grain filling period and its relation to grain yield in corn,
Zea mays L.
Crop Sei. Il:45-48.

---

---------------....-----------
149
28.
Daynard, T. B., and L. W. Kannenberg.
1976.
Re1ationships between
length of the actua1 and effective grain fi11ing periods and
the grain yie1d of corn.
Cano J. Plant Sei. 56:237-242.
29.
Duncan, W. G.
1972.
Plant spacing, density, orientation, and
1ight re1ationships as re1ated to different corn genotypes.
Proc. 27th Ann. Corn and Sorghum Res. Conf. 159-167.
30.
Duncan, W. G., and A. L. Hatfie1d.
1964.
A method for mensuring
the dai1y growth of corn kerne1s.
Crop Sei. 4:550-551.
31.
Eastin, J. A.
1969.
Leaf position and 1eaf function in corn--
Carbon-14 1abe11ed photosynthate distribution in corn in relation
to 1eaf function.
24th Ann. Corn and Sorghtm Res. Conf. 81-89.
32.
Eik, K., and J. J. lillnway.
1966.
Leaf area in relation to yie1d
of corn grain.
Agron. J. 58:16-18.
33.
Elias, J. E., A. A. Gagianas, and P. A. Gerakis.
1979.
Inter-
relationships and p1asticity parameters in Zea mays L. populations
as influenced by density and nitrogen.
Oeco1. Plant 14(2):159-168.
34.
Evans, L. T.
1963.
Environmental control of plant growth.
Academie Press, New York and London.
35.
Gi1more, E. C., and J. S. Rogers.
1958.
Heat units as a method
of measuring ma turity in corn.
Agron. J. 50:611-615.
36.
lillnway, J. J.
1962.
Corn growth and composition in relation to
soil fertility:
1. Growth of different parts and relation
between 1eaf weight and grain yie1d.
Agron. J. 54:145-148.
37.
Hanway, J. J.
1966.
Howa corn plant develops.
Special report
No. 48.
Iowa State Univ. Coop. Ext. Ser.
38.
Hanway, J. J., and W. A. Russell.
1969.
Dry matter accumulation
in corn (Zea mays L.) plants:
comparisons among single-cross
hybrids.
Agron. J. 64:351-355.
39.
Hollinger, S. E., and H. F. Reetz, Jr.
1979.
~ea mays canopy vs.
climatological station air temperature differences.
Agronomy
Abstracts, p. Il.
40.
Hoyt, P., and R. Bradfie1d.
1962.
Effect of varying leaf area by
partial defoliation and plant density on dry matter production
in corn.
Agron. J. 54:523-525.
41.
Humphries, E. C., and A. W. Wheeler.
1963.
The physiology of leaf
growth.
Ann. Rev. Pl. Physiol. 14:385-409.
42.
Hunter, R. B., T. B. Daynard, D. J. Hume, J. W. Tanner, J. B. Curtis,
and L. Kannenberg.
1969.
Effect of tassel removal on grain yield
of corn (Zea mays L.).
Crop Sei. 9:405-409.

---

.-
";."
149
28.
Daynard, T. B., and L. W. Kannenberg.
1976.
Re1ationships between
1ength of the actua1 and effective grain fi11ing periods and
the grain yie1d of corn.
Cano J. Plant Sei. 56:237-242.
29.
Duncan, W. G.
1972.
Plant spacing, density, orientation, and
1ight re1ationships as re1ated to different corn genotypes.
Proc. 27th Ann. Corn and Sorghum Res. Conf. 159-167.
30.
Duncan, W. G., and A. L. Hatfie1d.
1964.
A method for measuring
the dai1y growth of corn kerne1s.
Crop Sei. 4:550-551.
31.
Eastin, J. A.
1969.
Leaf position and 1eaf function in corn--
Carbon-14 1abe11ed photosynthate distribution in corn in relation
to 1eaf function.
24th Ann. Corn and Sorghum Res. Conf. 81-89.
32.
Eik, K., and J. J. Hanway.
1966.
Leaf area in relation to yie1d
of corn grain.
Agron. J. 58:16-18.
33.
Elias, J. E., A. A. Gagianas, and P. A. Gerakis.
1979.
Inter-
re1ationships and p1asticity parameters in Zea mays L. populations
as inf1uenced by density and nitrogen.
Oeco1. Plant 14(2):159-168.
34.
Evans, L. T.
1963.
Environmenta1 control of plant growth.
Academie Press, New York and London.
35.
Gi1more, E. C., and J. S. Rogers.
1958.
Heat units as a method
of measuring maturity in corn.
Agron. J. 50:611-615.
36.
Hanway, J. J.
1962.
Corn growth and composition in relation to
soi1 ferti1ity:
1. Growth of different parts and relation
between 1eaf weight and grain yie1d.
Agron. J. 54:145-148.
37.
Hanway, J. J.
1966.
Howa corn plant deve1ops.
Special report
No. 48.
Iowa State Univ. Coop. Ext. Ser.
38.
Hanway, J. J., and W. A. Russell.
1969.
Dry matter accumulation
in corn (Zea mays L.) plants:
comparisons among single-cross
hybrids.
Agron. J. 64:351-355.
39.
Ho11inger, S. E., and H. F. Reetz, Jr.
1979.
~ea mays canopy vs.
c1imato1ogica1 station air temperature differences.
Agronomy
Abstracts, p. Il.
40.
Hoyt, P., and R. Bradfie1d.
1962.
Effect of varying 1eaf area by
partial defo1iation and plant density on dry matter production
in corn.
Agron. J. 54:523-525.
41.
Humphries, E. C., and A. W. Whee1er.
1963.
The physio1ogy of 1eaf
growth.
Ann. Rev. Pl. Physio1. 14:385-409.
42.
Hunter, R. B., T. B. Daynard, D. J. Hume, J. W. Tanner, J. B. Curtis,
and L. Kannenberg.
1969.
Effect of tasse1 remova1 on grain yie1d
of corn (Zea mays 1.).
Crop Sei. 9:405-409.

---

150
43.
Hunter, R. B., L. A. Hunt, and L. W. Kannenberg.
1974.
Photoperiod
and temperature effects on corn.
Cano J. Plant Sci. 54:71-78.
44.
Hunter, R. B., M. To11enaar, and C. M. Breuer.
1977.
Effects of
photoperiod and temperature on vegetative and reproductive
growth of a maize (Zea mays L.) hybrid.
Can. J. Plant Sci. 57:
1127-1133.
45.
Iremiren, G. O., and G. M. Mi1bourn.
1979.
The influence of soi1
temperature as contro11ed by mu1ching on growth and deve10pment
in maize.
Ann. App1. Biol. 91:397-401.
46.
Johnson, D. R., and J. W. Tanner.
1972.
Ca1cu1ation of the rate
and duration of grain filling in corn (Zea mays L.).
Crop Sci.
12:485-486.
47.
Kauffman, N. M., and R. G. Butler.
1961.
Late spring and ear1y
fa11 freezes in Pennsylvania.
Bulletin of Pa. Crop Reporting
Service.
48.
Kiesse1bach, T. A.
1949.
The structure and reproduction of corn.
Nebr. Agr. Exp. Sta. Res. Bull. 161.
49.
Kleinendorst, A., and R. Brouwer.
1970.
The effect of temperature
of root medium and of the growing point of the shoot on growth,
water content, and sugar content of maize 1eaves.
Neth. J. Agric.
Sci. 18:140-148.
50.
Lee, R.
Forest Microc1imato1ogy.
Columbia University Press.
New
York and Guildford Surrey.
1978.
pp. 217-249.
51.
McGahen, J. H., and M. W. Johnson.
1978.
Pennsylvania commercial
hybrid corn tests report:
Short-season hybrids (Maturity zone 1).
The Penna. State Univ. Coll. Agr. Ext. Ser.
52.
McKee, G. W.
1964.
A coefficient for computing 1eaf area in hybrid
corn.
Agron. J. 56:240-241.
53.
McKee, G. W., H. J. Lee, D. P. Knieve1, and L. D. Hoffman.
1979.
Rate of fi11 and 1ength of the grain fi11 period for nine culti-
vars of spring oats.
Agron. J. 71:1029-1034.
54.
Mederski, H. J., M. E. Miller, and C. R. Weaver.
1973.
Accumu1ated
heat units for c1assifying corn hybrid maturity.
Agron. J. 65:
743-747.
55.
Mi1thorpe, F. L., and J. Moorby.
1974.
An introduction to crop
physio1ogy.
Cambridge University Press.
Cambridge, London,
New York, Melbourne.
pp. 110-139.
56.
Nishikawa, H., and M. Kudo.
1973.
Exp1icationa1 studies on the
sterile ear as appeared on mechanized cu1tivation of the corn
plant (Zea mays L.).
Tohoku Agri. Exp. Sta. Res. Rep. 44:51-95.

---

151
57.
Paddick, M. E.
1944.
Vegetative deve10pment of inbred and hybrid
maize.
Iowa Agri. Exp. Sta. Res. Bull. 331.
58.
Pone1eit, C. G., and R. E. Blaser.
1968.
Plant constituents of
an ear1y and 1ate corn hybrid as affected by plant density and
genotype.
Crop Sei. 19:385-388.
59.
Pone1eit, C. G., and D. B. Eg1i.
1979.
Kerne1 growth rate and
duration in maize as affected by plant density and genotype.
Crop Sei. 19:385-388.
60.
Prine, G. M.
1971.
A critica1 period for ear deve10pment in
maize.
Crop Sei. Il:369-386.
61.
Rag1and, J. L., A. L. Hatfie1d, and G. R. Benoit.
1966.
Photo-
period effects on the ear components of corn (Zea mays L.).
Agron. J. 58:455-456.
62.
Sass, J. E., and F. A. Loeffe1.
1959.
Deve10pment of auxi1iary
buds in maize in relation to barrenness.
Agron. J. 51:484-486.
63.
Shaw, R. H.
1975.
Growing degree units for corn in the north
central region.
Iowa Agr. and Home Ec. Exp. Sta. Res. Bull. 581.
64.
Shaw, R. H., and H. C. S. Thom.
1951.
On the pheno10gy of field
corn, si1king to maturity.
Agron. J. 43:541-546.
65.
Sp1inter, W. E.
1973.
Corn growth mode1.
ASES Winter Mee~ing,
Paper no. 73-4535.
66.
To11enaar, M.
1977.
Sink-source re1ationships during reproducnive
deve1cipment rn',maize.
A Review.
Maydica XXII:29-75.
67.
To11enaar, M., and T. B. Daynard.
1977b.
Effect of defo1iation on
kerne1 deve10pment in maize.
Cano J. Plant Sei. 58:207-212.
68.
Van Wijk, W. R.
1965.
Soi1 microc1imate, its creation, observation,
and modification.
Meteoro10gica1 monographs 6(28):59-73.
69.
Watson, D. J.
1947.
Comœarative physio10gica1 studies on the
growth of field crops.
II. The effect of varying nutrient supp1y
on net assimilation rate and 1eaf area.
Ann. Bot. N.S. Il:375-407.
70.
Watson, D. Jo
1952.
The physio10gica1 basis of variation in yie1d.
Adv. Agron. 4:101-145.
71.
Watts, W. R.
1971.
Ro1e of temperature in the regu1ation of 1eaf
extension in ~ea ma~ L.
Nature 229(5279):46-47.
72.
Watts, W. R.
1972.
Leaf extension in Zea mays.
II. Leaf extension
in response to independent variation of the temperature of the
apical meristem, of the air around the 1eaves, and of the root
zone.
J. Exp. Bot. 23:713-721.

---