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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 50° is used because little or no growth occurs when temperature is less than 50° F. Extreme temperatures are not used. When the maximum temperature is greater than 86°F, 86 is used in the formula and when the minimum daily air temperature is less than 50° 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 70°, which has been common, and day maximum of 86° gives a GDD accumulation for the day of 28. The same GDD accumulation of 28 occurs for a day when the night minimum is 70° and the day maximum is 95°, 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 86°; the rate of photosynthesis was lower when temperature was less than 86° and lower when temperature was higher than 86°. When the temperature was less than 86°, the rate of photosynthesis could be changed by simply changing the temperature. However, when the plant leaf was subjected to temperatures higher than 86° and then lowered below 86°, 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 GDDs are not as efficient compared with GDDs 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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