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Heat, humidity starting to spawn corn issues

Jeff Caldwell 08/05/2010 @ 11:00pm Multimedia Editor for Agriculture.com and Successful Farming magazine.

Driving around the countryside on those hot, humid days of June and July, you can almost hear the corn growing.

But, now that summer's starting to wind down, those conditions that are great for a young corn crop are turning painful for this year's crop that's ahead of the normal development pace, according to a Purdue University agronomist.

"Leaf diseases are more likely to spread in high humidity situations," says Purdue's Tony Vyn. "On susceptible hybrids and in fields not sprayed with effective fungicides, the leaf area available for photosynthesis during the critical grain filling period can decline too rapidly. Corn plants also are more susceptible to stalk rot infection with high humidity and wet surface soils."

According to a Purdue report, high daytime temperatures can result in corn plants having lower net photosynthetic energy and fewer sugars available for corn kernel development. When nighttime temperatures also are high, corn plants expend more sugars gained during daylight hours on nighttime maintenance respiration. Ideal nighttime temperatures during grainfill range between 60 to 65 degrees Fahrenheit.

Recent observations of diplodia ear rot in corn ears that are only halfway through the grain filling period are a direct consequence of excessive and prolonged humidity. Problems with ear rot will limit corn yields most on susceptible hybrids already under stress, Vyn says.

At a time of year when early summer rains have usually dried up, many parts of the Corn Belt have seen continued rain to this point. And that's kept the soils damp enough to cause roots to develop shallower than normal as well as affect the crop's ability to utilize nitrogen.

"Excessive rain in many parts of Indiana in June already compromised nitrogen availability to corn plants, and heavy rains in July and early August have further limited the plant availability of mineral nitrogen," Vyn says in a Purdue report. "The expectation of corn being able to take up 40% of the total season nitrogen during grain fill is less likely this year than last. That means there will be more mobilization of leaf nitrogen and stalk nitrogen to the ear during grain fill. Too much nitrogen mobilization will impair late season photosynthesis rates."

But it's the current heat, not the moisture, that's got Vyn more concerned about the corn crop moving forward despite the fact today's hybrids are better suited to such stressors.

"I am more concerned about the negative consequences of excessive heat and humidity this season than I would normally be because of the other stress factors corn has experienced in 2010," Vyn says. "Overall, corn hybrids are much more stress tolerant today than those of 30 years ago, but compounding stresses, especially during the grain fill period, can impose significant restrictions to final grain yield."

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