Corn in mid September in Illinois

Variable maturity in crops is noticeable in mid-September in central Illinois.

Editor’s note: The following was written by Emerson Nafziger, University of Illinois Extension crop production specialist, for the university’s crop development Bulletin Sept. 5.


The Sept. 1 crop production report, released on Sept. 12 by USDA-NASS, dropped the Illinois corn yield by 1 bushel, from 181 to 180 bushels per acre.

But the Illinois crop does look a little better than we thought it would by now, given that more than half of it was planted after June 1.

Canopy cover and color in early July were mostly good, but lack of rainfall and a less vigorous root system on late-planted corn meant that water stress began to show up in parts of Illinois. In areas where the dryness continued through August and into September, some later-planted fields have little green leaf area left, and ear tips have dropped in drier parts of fields, signaling an end to grainfilling.

The driest part of the state has shifted south a little with some rain in September in the area near the Quad Cities; the driest band is a few counties wide on both sides of an east-west line through about Peoria, all across the state.

The northern few tiers of Illinois counties are mostly wet, and southern Illinois ranges from good moisture (from August rainfall) in the region east of St. Louis, to areas that are drying out farther south.

Although late planting has gotten most of the attention, rain amounts, including lack of rain in some areas and more than needed in others, will be a big part of the 2019 cropping story. Late planting made the lack of adequate water a bigger problem.

Many late-planted fields showed signs of water stress before soil moisture was depleted, and some ended up with shorter-than-normal plants. These both point to soil compaction as an issue. The smaller root systems and drying surface soils showed access to water deeper in the soil was restricted.

Temperatures this season have tracked very close to normal. From May 1 through Sept.15, the statewide GDD accumulation is about 2,880, which is 45 GDDs above normal.

Had the crop been planted by early May, nearly all fields in central and southern Illinois would be mature by now. According to the Sept. 15 NASS report, only 14% of the state’s crop is mature, and 1% has been harvested.

Corn planted on June 1 instead of May 1 in northern and central Illinois lost about 350 and 450 GDDs, respectively, from late planting, but above-normal temperatures most days since early September have helped to recoup some of that.

If we assume for simplicity that hybrids grown in northern and central Illinois require 2,550 and 2,700 GDDs from planting to maturity, corn planted on June 1 stills needs about 325 GDD after Sept. 15 to reach maturity. With near-normal temperatures for the last week of this month, GDD accumulation during the last half of September should be close to 250 GDDs — about 50 more than normal — bringing June 1 to Sept. 30 totals to about 2,500 in northern Illinois and about 2,625 in central Illinois.

That doesn’t sound much different than normal, but with GDD accumulations slowing to a crawl in October, getting to within only 100-150 GDDs needed to reach maturity by the end of September is a positive for the late-planted crop this year.

Corn planted on or after June 10 is still likely to need more GDDs to mature than it will get by mid-October, but the extra GDDs in September will help.

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