University of Illinois

No. 14/June 25, 1998

Is the Corn Crop Prepared for Dry Weather in July?

Though excessive rainfall in May and June has resulted in planting delays, replanting, N loss, and other problems, it has meant that the crop in the field has not been stressed due to lack of moisture. That, coupled with warm temperatures, has the early planted crop farther along in its development than we normally see in late June. Corn planted in late April at Urbana is at the 12- or 13-leaf stage (V12-3) and was about 5-ft tall on June 24. With warm temperatures expected to continue, the crop should need only another 10 days or so (about 200 growing-degree days, GDD) to tassel emergence and about 10 more days (another 200 GDD) after that to reach midsilk. Warmer than average temperatures can boost daily GDD accumulations to 25 or even 30, so these time periods could be even shorter. We thus could see silks about July 5 to 10 in some fields that are more advanced or where earlier hybrids are planted in the central part of the state.

There has been increasing concern about the ability of the corn crop this year to withstand dry periods during July and August. In general, we are probably more concerned about the size and status of the corn root system than we would normally be, at least in those parts of the state that have received above normal rainfall after planting. The winter was mild, which meant that the normal amount of frost-loosening did not take place. Planting was often done in soils that were not very well dried out, and a lot of compaction took place. Then, it has been so wet since planting in many fields that the root system was unable to grow very deep due to lack of oxygen, compounded by compacted soil conditions.

As a result of all this, root systems are very likely shallower than normal, though we don't have a good way to measure this. If the soils dry out relatively slowly, the root system should be able to grow some more, reaching deeper in the soil. If it turns hot and dry quickly, I expect the crop will come under stress very quickly, after which root recovery growth will be more difficult, due to limited water uptake and less photosynthesis.

On the positive side, the crop is advanced where it was planted early, and earlier pollination will tend to favor crop prospects. Where the crop was planted late or replanted, however, a quick turn to hot, dry conditions could severely restrict the ability of the plant to produce a good root system and so could greatly limit crop prospects.

What should we expect from late-planted corn? First, we would expect that corn planted the end of June (and there are still some fields to be planted where corn was the planned-for crop) will emerge and develop quickly, albeit at considerable cost to the plant in terms of size and weight, and thus yield potential. There is some evidence that photoperiod (decreasing daylength) may help speed development during July; and as a result of this, and possibly one or two fewer leaves, we expect the crop to reach silking in 150 to 200 GDD fewer than it would have taken if planted early. That still means pollination in early to mid-August, however, which likely will mean a shorter period with good grainfilling conditions. Rainfall during pollination and grainfill will help the crop, but decreased plant size will likely mean substantially reduced yields.

Emerson Nafziger (, Crop Sciences, (217)333-4424