Corn plants for most of Minnesota have been short of moisture or under high temperature stress for the past four weeks. Since most fields are past tasseling and silking, one can now determine the success of the pollination and fertilization process. The objective here is to describe a simple method to determine the fertilization success and to discuss the effect of stress on plant recovery and grain yield.

Pollination and Fertilization
A quick review of pollination and fertilization is in order. Under normal conditions, a corn plant tassels followed shortly by ear and silk emergence. When the tassel is fully developed, pollen is shed from the tassel and falls onto the silks. The release of pollen from the tassel is called pollen shedding. Pollen landing on silks is called "pollination." After landing on a silk, an individual pollen grain germinates and grows down the silk to the ovule on the cob where the pollen merges with the ovule to form the start of a kernel. The successful merging of the pollen with the ovule is called "fertilization." Fertilization is necessary for each kernel that develops on a cob.

Determining if Fertilization Has Occurred
One can easily determine whether a kernel is developing by removing the ear and then carefully removing the husks to expose the silks and the cob. Shaking the cob carefully will let the silks drop from the cob from those ovules that have been fertilized. The silk will remain attached to the ovules on those that are not fertilized. Leaving the cob lay in the sun to dry for a few hours shows very visibly those ovules that were fertilized and those that were not. There will be some small “blisters” where fertilization has occurred and some developing kernels may show yellow color. The ovules not fertilized will be a clear blank on the cob.
For fields that have not shown visible wilting, fertilization appears to be good. But the continued high temperatures are using the available moisture faster than normal and the yield potential of these plants will be affected if rainfall doesn't occur soon.

Low Moisture and High Temperature
The low moisture and high temperature stresses have affected the pollination and fertilization process for some plants in many Minnesota cornfields. Tassel emergence was delayed on some of the stunted plants and delayed for several days on other more stressed plants. Some of the stunted plants have responded to the rainfall that has occurred during the past two week by shooting out a tassel, but there is no pollen in the tassel. For some of the extremely stunted plants, the tassel will never emerge. Plants can set ears and develop some grain without a tassel if there is pollen in the field when ears and silks emerge on the severely stunted plants. But this scenario is unlikely because there has been too much delay in the development of ears and silks on these stressed plants and the taller less stressed plants that did produce pollen are now through shedding pollen. The result is plants without grain (barren plants). This is the case for those places in fields with stunted plants.

Other less stressed plants with normal or near normal height have passed through pollination and fertilization with varying degrees of fertilization success.

Optimum Temperatures
The ideal temperature for corn growth at this time would be low 80's for the day and low 70's for the night. This combination of temperatures maximizes photosynthesis during the day and minimizes respiration during the night. The net effect is maximum kernel growth rate.
With daytime temperatures substantially above 85 degrees, the photosynthetic machinery of the plant shuts down and there is a lag for it to crank back up when the temperature drops down to the 80's. The exceptionally high nighttime temperatures are also not good for grain filling because high respiration rates reduce the amount of daytime accumulated photosynthates. Early in grain filling this would not be of concern if there were sufficient moisture to sustain reasonably normal growth. That is the case in some fields, but for many fields with a shortage of moisture the high nighttime temperatures add to the deterioration of corn plants.

Stress Effect On Yield Where Pollination
and Fertilization is Good
The plant is most sensitive to stress at the early kernel filling stage, which is where corn is today. One can use Figure 1 to estimate the effect on grain yield for the stress that has occurred in any particular field. The outer lines show the range of experimental results and the middle line shows the average effect. For current growth stages (about 10 days after pollination) one day of wilting reduces yield from 3 to 5 percent. Many fields have been wilted for several days so the impact on yield will be severe in some areas even though pollination and fertilization have been good.

What Happens If It Rains?
There are areas with a reasonable level of stored soil moisture that would be used to fill grain for a few days, but ultimately rainfall is necessary to produce good corn yields. Figure 2 summarizes some of the rainfall the state received this past week. The stored soil moisture would be preserved if temperatures return to normal.

For areas with stunted corn, stored moisture doesn't exist. Without rainfall those areas will continue to deteriorate. With rainfall (best case scenario would be weekly rainfall of one inch or more), kernels would fill that are now fertilized.

Dale R. Hicks, Professor of Agronomy
University of Minnesota