In a normal July, corn farmers don't even think about the date of the first killing frost in the fall. But this is not a normal year. When July mornings feel chilly, farmers begin to worry about corn growth. Below-normal temperatures dominated the July weather reports.
In general, cool summer weather is good for corn planted early, says Bill Wiebold, University of Missouri Extension crops specialist. Unfortunately, the wet spring forced many farmers to plant late.
"Some planted extraordinary late,” he says. "Those fields planted very late would be of concern about an early frost, even in a normal year.”
Corn plant maturity depends on accumulated exposure to heat, unlike the soybean plant, which depends on day length to trigger maturity.
Heat units in corn are called "growing degree days” or GDDs, Wiebold says. "Each corn variety has its own requirement of accumulated degree days.”
In a normal July, when daily average temperatures range from 70 degrees to 90 degrees, a corn plant accumulates 30 GDDs each day.
"On some days this July we've gained only 12.5 GDDs, less than half of normal,” Wiebold says. "That isn't bad for a few days, because corn plants can make up the difference with hot August days. But if a big deficit accumulates, it would take a very hot August to average out.”
Corn hit by freezing weather before it matures will have reduced yields. The plant dies and the kernels on the cob stop filling.
Late-planted corn that has not yet pollinated this year is at greatest risk, he says. "Some of my late-planted corn research plots have not set tassels and silked. They are close, and should be done by Aug. 1.”
As a rule, it takes 60 days from silk set to harvest, Wiebold says. Corn silking on Aug. 1 would mature near Oct. 1. Average first frost in mid-Missouri is Oct. 15. In the counties along the Iowa state line, that average is a week earlier.
However, those are just averages, Wiebold says. Actual first frost can vary considerably around those dates. In addition, it's not possible to know what the weather will do for the rest of the growing season. "A cool July doesn't mean an early freeze. A cool July doesn't even mean a cool August.”
Late-planted corn will be of concern all across the Corn Belt, particularly further north where cooler temperatures and earlier first-frost dates prevail.