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Hot summer nights test newest genetics

USDA market estimates through the remainder of the 2010 corn marketing year will be interesting to watch. The August report projected high yields. However, the first 10 days of August generated sweltering heat across most of the Corn Belt. Rainfall was plentiful in some areas during August, but scarce in others.

Key Points

• Agronomists say hot, humid nights cause corn to mature faster.

• Weather data from past several decades points to lower final yield estimates.

• The wild card could be improved genetics that offset warm nights.


One factor that often stays under the radar is nighttime temperature and its effect on corn yield. A careful study of the past 40-plus years indicates that high nightly temperatures push corn to maturity faster and aren’t conducive to high yield. In that case, USDA may decide earlier projections were too high. What’s impossible to know is whether today’s genetics can handle warm, humid nights better than before.

Circumstantial evidence

When I was a county agent in southwest Kansas, I noted farmers produced top yields under irrigation every year. After I moved 100 miles east to Wichita, I discovered that farmers there usually produced 30 bushels less corn per acre, even under irrigation.

The air in southwest Kansas is dry, and nights are almost always cool. Nighttime summer temperatures at Wichita are generally in the low 70s. They were even higher this year since the humidity was also high.

The theory is that warmer night temperatures keep plants respiring at night, speeding them to maturity. It’s a fact that yields are higher when the grain-fill period is longer.

What history shows

Commodity Weather Group recently searched past records for trends. It discovered that six analog warm, humid summers out of the past 60 years each produced below-trend corn yields. Soybean yields were below trend in only four of the six years.

CropCast examined data in Illinois and Iowa over the past 40 years. The magic number in those states was 64 degrees F for average nighttime temperature from June 1 through Aug. 31. In years where the temperature was below 64 degrees, yields tended to be above trend. At 64 degrees or higher, yields were below trend.

Elwynn Taylor, an Iowa State University climatologist, pulled out 1995 as a year for comparison when he addressed the Purdue University Top Farmer Crop Workshop. That crop had everything going for it except a warm, humid August. USDA eventually pulled 12.1 bushels per acre off its August estimate. It was USDA’s biggest miscalculation ever.

As of the first week of August, Des Moines, Iowa, showed the second highest nighttime reading in 40 years. At Springfield, Ill., nighttime numbers were the highest ever. In Indiana, assistant state climatologist Ken Scheeringa says the June to August daily minimum averaged 65.1 degrees F, 4 degrees above normal.

Under the radar

Looking over earlier years similar to 2010, the highest yield estimate usually comes early, followed by disappointing numbers later. Damage from hot nights can stay under the radar because damage doesn’t show up in weekly crop condition reports.

That brings us to 2010. Did today’s newer genetics help the crop survive hot nights better than before? We’ll know once the crop is in the bin.

Tom J. Bechman contributed to this story.

This article published in the October, 2010 edition of INDIANA PRAIRIE FARMER.

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