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Heat trimming corn yields

Jeff Caldwell 07/19/2011 @ 2:36pm Multimedia Editor for Agriculture.com and Successful Farming magazine.

It's hot out there. And, your corn doesn't like it any more than you do. But, is the current hot snap doing anything to your yield potential yet?

Data from the last 35 years in Illinois illustrate just how much yield potential could be trimmed by hotter-than-normal temperatures in July and August. From 1975 to 2010, the average corn yield in Illinois was 168.7 bushels/acre. But, in most years when the average temperature for July exceeded 77 degrees, the corn ended up yielding below the trendline yield.

"The state average trend-adjusted corn yield in the selected 10 years varied from 116.7 to 182.5 bushels and averaged 157.8 bushels, compared to the 36-year average of 168.7 bushels," according to a recent report from University of Illinois crop marketing specialist Scott Irwin and ag economist Darrel Good. "A substantial part of the variation is associated with the large range in July precipitation and varying temperature and precipitation conditions in August."

For example, in 1983, the average temperature in Illinois was 79 degrees and rainfall was less than 2 inches. That year, the state's corn crop averaged 132.2 bushels/acre. In 1988, the temperature averaged 77.5 degrees and the state received an average of 2.6 inches of rainfall. That year, the crop made 116.7 bushels/acre on average.

But, those are a couple of the more eye-catching numbers; Irwin and Good say there are other years in that timeframe that, to this point, look a lot like this year. So, how did yields end up in those years?

"The other years since 1975 that most resemble 2011 July weather conditions (hot and dry) are 1977, 1980, 1983, 1999, and 2002. What happened in those years?" according to Irwin and Good. "Crop condition ratings (percent rated good or excellent) declined sharply in each year, except 1977, as the growing season progressed from late June through early September. To date, 2011 crop condition ratings are generally tracking the average of the 5 analog years."

Doug Martin's crop is starting to show the signs of heat stress on his farm near Mount Pulaski in central Illinois. But despite the triple-digit daytime temperatures, he's more concerned with the mercury reading once the sun goes down.

"Once the plant starts stressing it will start to abort some of the kernels on the ear. Night time temperatures are more important than day time ones. When night time temps fall below 70 degrees it allows the plant to slow down and rest," he says in his blog. "When temps stay above 70 at night then the plant is forced to keep working which diverts energy being used by the plant from filling the ear to survival mode."

That doesn't mean all hope is lost for this year's crop. Though the current heat wave is far from ideal, the weather over the next 6 weeks will be more important to the crop's final yield in the long run, Irwin and Good say.

"Corn yield prospects in Illinois this year are still very uncertain. However, the history of corn yields in years with hot, dry conditions in July clearly points to the potential for a below average yield. Much will depend on actual weather conditions in the last week of July and in August," they say. "Without favorable conditions during that period, a state average corn yield in the mid- to low-150s might be expected. That compares to a trend yield for 2011 of 168.7 bushels.

"Some other Corn Belt states have experienced hot, dry conditions in July. We have not done a similar analysis for these states, but comparable results would be expected," Irwin and Good add. "The implications for the U.S. average corn yield are less certain because parts of the Corn Belt have fared better than Illinois this summer."


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