GDD Deficits Decreasing. but Some Can't be Made Up - UNL CropWatch, August 19, 2011

GDD Deficits Decreasing. but Some Can't be Made Up - UNL CropWatch, August 19, 2011

August 19, 2011

As we enter the final two weeks of August, this is the perfect time to look at how growing degree day (GDD) accumulations are faring for the state’s corn crop.

More on GDDs for Crops

Growing degree day units (GDDs) are based on the understanding that there are low and high temperature points for each crop beyond which a plant doesn't grow.  For corn and grain sorghum, the low point is 50°F and the high point is 86°F.

Hybrids require different GDD accumulations to reach maturity.  In Nebraska growers generally plant corn hybrids needing 2400 to 2700 GDDs to reach maturity.

Total accumulated GDDs are used to estimate growth stages, maturity, and whether the crop is likely to mature before the first hard frost.

To view accumulated GDDs for a number of crops and Nebraska sites, visit the GDD and ET data page on CropWatch.

Temperatures were well below normal for the northern three-fourths of the state from May through mid-June, leading to GDD totals averaging 50 to 170 units below normal. The greatest departures occurred across extreme northern Nebraska and the Panhandle.

Temperatures during the mid-June through early August period averaged 2-4 degrees above normal, with intense heat reported during the last three weeks of July. Unfortunately, these warmer conditions did little to reduce the GDD deficits that had already developed.

The standard GDD formula uses a lower base of 50°F and an upper base of 86°F. That means that GDDs are not accumulated for temperatures below the minimum or above the maximum. When high temperatures exceed 86°F, additional GDDs are not accumulated. In order for accumulated GDD deficits to be reduced during the heart of the growing season when highs regularly exceed 86°F, the minimum temperature would need to be above normal.

For this article, the GDD units that have accumulated for the 2011 corn crop are based on the mean emergence date of May 20, the date used by the Nebraska office of the National Agricultural Statistics Service. In general, the largest accumulated deficits continue to be across the Panhandle and northern quarter of the state, while surplus accumulations have fallen south of the I-80 corridor in south central and southeast Nebraska.

Following are the ranges of accumulated GDDs for each of the Nebraska agricultural districts as of August 16:

Panhandle – 1560 to 1700 GDDs
North Central – 1600 to 1760 GDDs
Northeast – 1790 to 1850 GDDs
Central – 1740 to 1960 GDDs
East Central – 2020 to 2120 GDDs
Southwest – 1740 to 1910 GDDs
South Central – 1850 to 2070 GDDs
Southeast – 2070 to 2210 GDDs

Accumulated GDD unit departures from average by district:

Panhandle – deficits of 30 to 240 units with largest deficits across the north
North Central – deficits of 140 to 260 units with largest deficits in the northwest
Northeast – deficits of 125 to 170 units with the largest deficits in the north
Central – deficits of 150 units to surplus of 35 units with the largest deficits in the northwest
East Central – deficits of 90 units to surplus of 45 units with the largest deficits in the north
Southwest – deficits of 135 units to surplus of 25 units with the largest deficits in the northwest
South Central – deficits of 60 units to a surplus of 130 units with the largest deficits in the north
Southeast – deficits of 40 units to a surplus of 100 units with largest deficits in the northeast

Will Accumulated GDDs Catch Up Before Harvest or the First Freeze?

Daily normal high temperatures are now cooling from their late July peak and will drop below 86°F for all locations by August 20. Since normal high temperatures are dropping below the upper threshold of the GDD equation, it becomes increasingly easy to cut into accumulated GDD deficits as we progress toward the end of September. Based on normal temperatures, we should average 22-25 units per day on August 20, 18-21 units on September 1, 13-16 units on September 15, and 10-11 units on September 30.

How many units would be eliminated from accumulated deficits if temperatures on every day through the end of September replicated temperatures typical of August 16?  (On August 16 the low was near 62 and the high was near 87.) On average, deficits would be reduced by 30 units by September 1, 95 units by September 15, and 200 units by September 30.

Of course we know that this is an unlikely scenario, but it does tell us that the large deficits accumulated over northern and western Nebraska can be cut, but not completely eliminated, with above normal temperatures. It also suggests that the freeze risk before maturity will be elevated in these regions. I will put together a freeze risk susceptibility analysis for CropWatch during the first week of September.

Allen Dutcher
Extension State Climatologist