xt73j9606z63 https://exploreuk.uky.edu/dips/xt73j9606z63/data/mets.xml   Kentucky Agricultural Experiment Station. 1971 journals 197 English Lexington : Agricultural Experiment Station, University of Kentucky Contact the Special Collections Research Center for information regarding rights and use of this collection. Kentucky Agricultural Experiment Station Progress report (Kentucky Agricultural Experiment Station) n.197 text Progress report (Kentucky Agricultural Experiment Station) n.197 1971 2014 true xt73j9606z63 section xt73j9606z63 C ` ·
Grewnng Degree Days
for Corn 1n Kentucky
By D. B. Egli, Doyle Cook, Allen B. Elam and C. G. Poneleit l
Progress Report `l€‘7
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 Growing Degree Doys for Corn in Kentucky
D. B. Egli,y Doyle Cook,g/ Allen B. Elam, jr. ,-3/ and C. G. Poneleit·4/
Selection of a corn hybrid is based on many factors. One of the factors of
particular concern to the farmer is the maturity rating of the hybrid. Maturity
rating systems in the past have included the use of the number of calendar days
from planting to maturity or were systems in which hybrids were compared with a
standard hybrid and assigned relative maturity ratings. The 1967 growing season,
which was unusually cool throughout much of the corn belt, pointed up the short-
comings of the older systems of labeling corn maturities. Primarily as a result of
the cool weather, many corn hybrids did not mature when they were supposed to,
and farmers sent millions of bushels of high—moisture corn to market.
A The growth and maturity of a corn hybrid are closely related to daily and
seasonal temperature levels. Consequently, a maturity rating system based on
temperature should provide a better index of maturity than one not related to
temperature .
Since corn growth is largely controlled by seasonal temperature levels, it
has long been postulated by many agronomists and corn breeders that a more accurate
_ scheme for labeling corn hybrid maturity is possible by applying the concept of 4
growing degree days. The growing degree day approach is a method of predicting
corn maturities by summing the mean daily temperatures above an assumed base or
· threshold temperature. It is assumed that little or no plant growth occurs below
the selected threshold. A base temperature of 50 degrees (F) is usually assumed
for corn. A particular hybrid has a rather specific growing degree requirement
sometimes called the varietal constant. Thus, regardless of the planting date or
location, the hybrid will mature when the total number of accumulated growing degree
days is equal to the varietal constant, assuming relatively normal growing conditions.
By knowing the varietal constant for various hybrids, it is possible to use
the information presented in this publication to choose a hybrid that will mature at
the desired time—or for late plantings or double cropping management systems,
to select a hybrid that will have a reasonable chance of maturing before frost.
Methods of Calculation of Degree Days
Growing degree days for Kentucky were calculated by averaging the maximum
and minimum air temperatures for a particular day and subtracting the base
temperature (50 degrees F,). This gives the growing degree days for that day.
L/Assistant Professor of Agronomy, Agronomy Department, University of Kentucky.
E/Meteorologist, NOAA National Weather Service, Louisville, Ky. 40221.
2/Climatologist for Kentucky, NOAA National Weather Service, University of Kentucky,
Lexington, Ky. 40506.
i/Assistant Professor of Agronomy, Agronomy Department, University of Kentucky.

 For example, if the maximum air temperature was 80 and the minimum was 60,
the average for that day would be 80 + 60 3 70 Subtracting the base of 50 gives 20
growing degree days for that day. 2 '
The following modification was applied to the basic daily growing degree
equation. If the maximum air temperature exceeded 86OF, 860F was used as the
( maximum to calculate the mean. This largely eliminates the excessive accumu-
lation of growing degree days in periods of dry, hot weather when corn is usually
under moisture stress during the hot portion of the day. Similarly, if the minimum
air temperature was less than 500F, 500F was used as the minimum air temperature
to calculate the mean. This corrects for a deficient accumulation of growing degree ‘
days when the maximum air temperature may be in the 60‘s while the night
temperature drops quickly to a value below 50. The growing degree days were
than accumulated across the growing season. l
It has been shown (1)5 that the time of day at which temperature readings ( '
are made at a particular weather station can influence the mean temperature
observed and the growing degree day accumulation for that station. Consequently,
all stations used in this summary were adjusted to a common observation time
(midnight), following the method developed by Weaver and Miller (2). .
The summation of growing degree days began on March 15 in each year and
stopped with the first frost (minimum air temperature less than 28OF) in the fall.
Thirty—nine National Weather Service climatological stations in Kentucky were the .
source of the data used to calculate the growing degree data presented in the
accompanying charts. The stations used and their locations are presented in .
Fig. 1. Temperature data for the period 1949-68 were utilized for most stations;
however, at some stations shorter periods of record were used as a result of
missing or irregular data.
Figure 2 represents the mean number of growing degree days occurring
from March 15 to the first frost in the fall. Differences in terrain and exposure
may cause local variations from the values in Figure 2, particularly in the
mountainous areas of eastern Kentucky.
Figures 3 through 6 represent the mean date that totals of 2400, 2800, and
3200 growing degree days were reached when the accumulation is started on
various dates, for the various climatological divisions of Kentucky (see Fig. 1
for the climatological divisions). These figures make it possible to determine the
approximate date of maturity for a range of planting dates and varietal constants.
Corn planted on these dates and requiring less than 3200 growing degree days to
reach maturity will mature before frost at most locations in Kentucky.
For example, in the western part of the state (Western Division), a hybrid
that requires 2800 growing degree days to reach maturity, if planted on April 5
would reach maturity, on the average, approximately August 24. A
Q/Numbers in parenthesis refer to the references listed at the end of this

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Fig. 2. —Mean number of growing degree days, March 15 to frost. _
20 .
_ Degree Ddy Totals
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Fig. 3. —Mean date that various totals are reached for a given planting date,
Western Division. For example, a hybrid requiring 2800 growing
degree days for maturity, if planted on April 5 would reach maturity,
on the average, on August 25 (dotted line).

 20 Degree Day Totals
5 20
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15 20 25 30 1 5 12
Fig. 4. ——Mean date that various totals are reached for a given planting date,
Central Division. 4
Degree Day Totals
_ 3200
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15 20 25 30 1 5 l2
Fig. 5. —Mean date that various totals are reached for a given planting date,
Bluegrass Division.

 Degree Day Totals
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Fig. 6. ——Mean date that various totals are reached for a given planting date, V
Eastern Division.
Figures 7 and 8 represent the mean number of growing degree days
remaining until frost from two starting dates. As corn planting is delayed into
May and June a hybrid must be selected that will mature before the first frost.
The values presented in these figures are average values calculated from
long term temperature records. The growing degree accumulation will deviate
from these values as the daily temperatures are warmer or cooler than normal.
Differences in terrain and exposure may also cause differences in the accumu-
lation in local situations. These factors should be taken into consideration when
utilizing the data presented in this publication.

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Fig. 7. —Mean number of growing degree days remaining until frost after May 10.
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Fig. 8. —Mean number of growing degree days remaining until frost after June 7.

(1) Mitchell, J. Murray, Jr. 1958. Effect of Changing Observation Time on
Mean Temperature. Bull. Amer. Meteor. Soc. 39(2):83-89.
(2) Weaver, C. R. and M. E. Miller. 1970. Temperature Adjustments for
Discrepancies Due to Time of Observation (unpublished manuscript).