Corn is extremely sensitive to soil moisture conditions now. For most of the state corn plants are under some level of low moisture stress. The effect this has on yield cannot be accurately assessed now, but the conditions are certainly not good for the crop.

The corn crop can best be described as extremely variable, even within fields. The parts of the fields with coarser textured soils (sandy) have low water holding capacity and plants on those field areas are extremely stunted, leaves tightly rolled, dying, and or dead while other areas in the same filed have corn that is under stress but will fully recover if rainfall occurs soon. The very high temperatures predicted for the next few days will cause plant condition on all parts of these fields to deteriorate quickly.

There are some areas of the state with a normal amount of stored soil moisture and with sufficient topsoil moisture such that plants are not showing any stress or only mild stress on field margins. Plants in some of these fields are wilting during the hottest portions of the day. Root systems cannot meet the heat load of temperatures higher than 85 degrees F even though there is good topsoil moisture. We can expect to see leaf wilting over a wide geographic area of the state the next few days if the predicted temperatures occur. Except for a small area in the southwest corner of Minnesota, the drought map shows moderate to severe drought for the rest of Minnesota’s corn growing area.

Corn is beginning to tassel now and ear shoots are also due to emerge in the next few days. This low moisture stress will delay both tassel and ear shoot development for a few days (extreme low moisture conditions will delay these events even more than a few days). Tassels will eventually emerge (even with continued low moisture stress) and begin to shed pollen (tassels will never emerge on those plants that are extremely stunted, i.e. they are less than a foot tall now with other plants in the field ready to tassel). The amount of pollen will be reduced and the percentage of pollen that is viable will be reduced. Silks will be under stress also and less receptive to pollen tube growth. The net effect is that fewer florets will be successfully fertilized and ear size (kernel number per ear) will be lower. This sets a yield limit by reducing the number of kernels per ear. Growing conditions after pollination will determine the number of these potential kernels that will fill and how well they fill which affects grain yield.

It's too early to make an assessment of the effect these weather conditions will have on corn yields in any one field, but if the weather predictors are correct, corn yields in Minnesota will be significantly lowered from the potential yield that was developing. It's also too early to think about any action other than to hope for rainfall soon.

Dale R. Hicks, Extension Agronomist
University of Minnesota