As the end of the growing season approaches, concern about whether the corn and soybean crops will reach maturity and harvest moisture in time this fall is mounting due to the late planting date for many crops along with recent unseasonably cool temps. University of Illinois crop sciences professor Emerson Nafziger provides some insight on this topic.
Lagging Corn Crop Development
Nafziger notes, "Corn is 10 days to two weeks behind normal, and soybeans are two to three weeks behind normal. The number of days behind will 'stretch' as the weather cools, so late crops get even later. Ten days behind in mid-August will become 15 or 20 days behind in mid-September even if temperatures are normal."
Late-planted corn typically requires fewer growing degree days (GDDs) to reach maturity than corn planted at a normal date, and the lower GDDs typically translates to lower yields because the crop experiences stress during high temps and dry weather in mid-summer.
But Nafziger says accelerated development is not expected this year due to recent below-normal temps. "Instead, we are seeing that corn development is following closely the normal number of GDDs required to reach each stage. This means less chance of premature death and a better chance to fill grain completely. But for late-planted corn it also means late maturity," Nafziger explains.
Given normal GDD accumulations this month and in September, Nafziger says corn planted in the Champaign area on May 1, May 15 and May 31 will accumulate about 3,020, 2,850, and 2,530 GDD, respectively, by the end of September. If a mid-season (111-day relative maturity) hybrid needs 2,700 GDD from planting to maturity regardless of planting date, corn planted on these dates should reach maturity (black layer, about 32% grain moisture) on about Sept. 5 to 10, Sept. 15 to 20, and mid-October, respectively. Nafziger says the crop seems on course to do so.
A 50% frost date is estimated for Oct. 20, which signals a mid-season corn hybrid planted in early June should mature before that date.
"But drydown slows quickly as we move into October, and even early-planted corn will dry slowly after maturity unless September is unusually warm," Nafziger cautions. He goes on to explain that corn planted in mid-June is not likely to mature before the normal frost date if temps are normal.
Cool Temps Are Not Ideal for Beans
Considering the soybean crop, Nafziger says that while the Illinois crop looks healthy, podsetting is later than normal and beans planted the second half of May haven't reached or moved past the start of podfill. Therefore, "If temperatures continue to be cooler than normal, we can expect the crop to reach maturity only by late September or early October," Nafziger reports.
Cool temps cause other issues for the bean crop as well. While Nafziger says the crop "has good photosynthetic ability due to its healthy, complete canopy" and below-normal soil water use rates extend the water supply, "below-normal daytime temperatures (and clouds) mean less photosynthesis, and cool nights can physiologically limit growth rates and photosynthetic rates the next day." He says this may result in below-normal pod numbers or pods initiated in late August that may not fill.
Nafziger says the best situation would be for both night and day temps to return to normal of just above normal with enough rain to enable the crop to photosynthesize fully as seeds fill. Even then, how the crop finishes this fall will be key.
"Good soybean yields are still possible if the weather remains good into September, but seed-filling rates will remain slow as long as temperatures remain low," Nafziger explains.