University of Illinois

No. 7/May 8, 1998

Wet Again: Corn Planting Delays

The official statistics indicate that 30 percent of the Illinois corn crop was planted by May 3, 1998, which is not much different than the average for that date. Planting of much of the crop in the state, however, continues to be delayed by wet soils. Following is a list of considerations as we face delays in getting the corn crop planted:

How fast are we losing yield? Estimates of yield loss are based mostly on planting-date studies that have been done periodically. Planting-date responses, like the weather, vary a great deal among years and locations. As a general summary of planting-date response in Illinois, we use the following:

  • From May 1 to May 10, we lose about one-half bushel per day of delay.

  • From May 10 to May 20, we lose about one bushel per day of delay.

  • From May 20 to May 30, we lose about 1.5 bushels per day of delay.

  • After May 30, yield losses accelerate sharply, from 2 bushels per day for the first week of June, and at least 3 bushels per day by June 10.

According to these estimates, about 15 bushels of potential yield have been lost by May 20, and about 30 bushels by May 30.

When do I switch to an earlier hybrid? In southern and central Illinois, most first-choice hybrids are not extremely full-season: The supply of accumulated growing-season temperatures (measured as growing-degree units, or GDU) is morethan adequate to meet the needs of these hybrids. Most popularly grown hybridscan be planted into early June in the southern half of the state and still beexpected to mature before frost.

In the northern part of the state, seasonal GDU accumulations are lower, butmany of the first-choice hybrids are the same as those in Central Illinois,meaning that they are less likely to receive adequate GDUs if they are plantedlate. In the northernmost counties, where full-season hybrids would generallyrequire about 2,600 GDU, such hybrids should be replaced with earlier hybridsif planting is delayed beyond May 20 to 25 or so. Such a switch is notnecessary in the central part of the state (where "full-season" hybrids tendto require 2,700 to 2,800 GDUs) until after May 31, and perhaps not even then.

In actual practice, there is evidence from both research and late-plantingexperience that GDU requirements for late-plated corn are usually less thanfor corn planted early. In general, this decrease in GDU requirements wasabout 5 GDU for each day of planting delay from late April to mid-June. Thisdecrease in GDU requirement probably stems mostly from the fact that astemperatures decline late in the season, the plant simply stops filling and soreaches black layer early, filling the kernels less and yielding less in theprocess. This acts as an additional safety factor for late-planted corn andsuggests that we not rush into changing to an earlier hybrid too quickly.

How late can we plant corn? According to the loss estimates given above, we are likely to reach loss levels of about one-third of the potential yield bythe end of the first week of June, and losses approach 50 percent by themiddle of June. Anticipated losses on such a scale would likely cause a shiftto another crop (possibly grain sorghum in the southern half of the state,probably soybeans in much of the state). For those who can harvest corn as aforage, it can be planted even later and still compete with other cropalternatives.

Do we change any other practices as planting is delayed? Our research showsthat harvested plant population should probably not be changed very much asplanting is delayed. With warmer soils at planting, we may want to reducedropped-seed populations slightly in anticipation of a higher percent of theseed-producing plants. Herbicide adjustments depend on time of application andtype of herbicide. Unless planting is very much delayed and soils stay extremely wet, N applied in April should be adequate, particularly if expected yields are reduced. If N is to be side-dressed, slightly lower rates may be appropriate to match slightly lower expected yields, and more rapid uptake(less time for loss) as plants grow faster with warmer temperatures. Lateplanting also changes the expected weather at each stage of growth, and wewill need to watch for insects and diseases that might take advantage of such"ecological shifts" during the season.

Is there a bright side to planting delays? Not an obvious one. While thecorrelation between average planting date and average yield in Illinois is notparticularly strong, delays usually cut yields, and often decrease quality. Good yields are still possible, but they become statistically less likely asplanting is delayed. One possible benefit might be an increase in the price ofcorn due to lower yields, but that requires lowered production in much of thecorn-growing area. That could happen, but it will not be a result of delayedplanting in much of the northern and western part of the Corn Belt, whereplanting progress has been good.
Emerson Nafziger (, Department of Crop Sciences,(217)333-4424