Still Behind on Growing Degree Days

Still Behind on Growing Degree Days

The late start may have knocked off some top-end yield potential, says U of I's Emerson Nafziger.

Illinois farmers doubled the amount of corn planted last week, moving from 34% planted on May 8 to 69% planted on May 15.

However, the temperature roller-coaster continues, says Emerson Nafziger, University of Illinois Extension agronomist.

"With highs in the 80s or 90s for a few days during the second week of May, but with highs only in the 50s in much of the first and third weeks of May, daily growing degree day accumulations to date are lower than normal," he adds. "Hopefully, warm days in the next two weeks will get us back to normal by the end of the month."

During the week of May 7 to May 13, Urbana accumulated 130 GDD – enough to get a planted crop up. Nafziger says most of the corn planted the first week of May is now up, and corn planted by May 8 or 9 should also be up.

"Previous studies indicate that corn planted on May 20 will have lost about 15 bushels of yield potential, and that delays past that date will cause yield losses at the rate of about 1 ½ bushels per day," he notes. "Because GDD accumulations have been below normal over the last six weeks, it's possible that this rate of loss might be somewhat lower than average this year. But, if that's the case, it's mostly because top-end yield levels might have been compromised by the slow start."

In other words, when it's cool in April and May all corn tends to act as if it had been planted somewhat late. And if GDD accumulations are average for the rest of the growing season, crop development will continue to lag some. This shouldn't be a problem as long as there is enough rainfall to keep the crop from being stressed.

With only about 400 GDD since April 1, the most advanced corn in central Illinois is only at the V4 stage.

"Lack of consistently warm temperatures, along with recent cool temperatures, have resulted in limited growth and plants that are small for their physiological age," he says. "We still do not know if this by itself might limit yield, but it is likely that corn planted in early May will almost catch up in size with corn planted a month earlier."

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