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Minnesota Crop News > 2001-2008 Archives
August
7, 2001
When
is a Growing Degree Day Not a Growing Degree Day?
D.R. Hicks
Professor of Agronomy and Plant Genetics
Heat
accumulation during the growing season can be used as a predictor
of plant development. The
very high temperatures we’ve had, especially the high
night temperatures, have caused the Growing Degree Day accumulation
to be slightly ahead of normal for most of the state. But
corn development is still lagging so there is the question
of how effective this temperature accumulation really is
for corn development. GDD's
are calculated using the maximum and minimum daily air temperature
to determine the average daily temperature. From
the average temperature, a base is subtracted. The
result is the GDD's accumulated for the day.
Daily
GDD = (Maximum temperature + Minimum temperature)/2 -
Base temperature
For
corn, a base of 50º is used because little or
no growth occurs when temperature is less than 50º F.
Extreme temperatures are not used. When the maximum temperature is greater
than 86ºF, 86 is used in the formula and when
the minimum daily air temperature is less than 50º F,
50 is used in the formula.
Both
night minimum and day maximum temperatures have been high,
so the rule for substituting 86 for the maximum temperature
has been used often during July. For
example, a day with a night minimum of 70º, which
has been common, and day maximum of 86º gives
a GDD accumulation for the day of 28. The same GDD accumulation of 28 occurs
for a day when the night minimum is 70º and the
day maximum is 95º, which has also been common. Are
these 28 GDD’s equal in efficiency of corn growth?
For
sure the answer is “No” if the plant is moisture
deficient. Except for isolated areas, most of Minnesota
has adequate available soil moisture. However
plants may be moisture stressed because of the extreme heat
load due to the high temperatures and the shallow root systems
that occur in most cornfields. Shallow roots and a restricted root volume
limits water uptake that is necessary to cool leaves, especially
the uppermost leaves.
Without
moisture stress, these GDD’s we’ve had are probably
not as efficient at promoting corn growth and development. The
effect of temperature on the rate at which the plant carries
on photosynthesis has been studied by putting a single corn
leaf in a sealed chamber where temperature can be varied
and the rate of photosynthesis measured. The
temperature that resulted in the maximum rate of photosynthesis
was 86º; the rate of photosynthesis was lower
when temperature was less than 86º and lower when
temperature was higher than 860. When
the temperature was less than 86º, the rate of
photosynthesis could be changed by simply changing the temperature. However,
when the plant leaf was subjected to temperatures higher
than 86º and then lowered below 86º,
the photosynthetic activity lagged for a time. The conclusion was that the “photosynthetic
machinery” needed to be repaired before the plant could “get
back to normal activity”.
This
suggests that these high temperatures are in fact detrimental
to normal corn development and that these GDD’s are
not as efficient compared with GDD’s that accumulate
at lower maximum daytime temperatures. What can’t be determined now is how this will affect
yield. The
two factors that will minimize the impact on yield are
1) there
is good available soil moisture in most areas, and 2)
it’s
early in the grain filling period. Certainly these high temperatures would
have a much greater negative impact at later grain filling
stages.
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