No. 20 Article 5/August 26, 2011

Lessons from Late-Season Stress in Corn

The condition of the corn crop continues to deteriorate as soils have continued to dry out. As of August 21, only about a third of the topsoil in Illinois was rated as having adequate moisture. There was some rain (even in Champaign-Urbana) this week, but much of the corn crop would have benefited a good deal more from rain a month ago than it will from rain now.

Whether rain now will benefit corn relates most closely to the amount of green leaf area still left on the plants. This varies considerably among and within fields. Growing degree-day accumulations since May 1 are some 150 to 200 GD ahead of normal, and corn planted in central Illinois in early April has accumulated about 2,700 GDD, enough to mature some hybrids. So some fields have lost their leaf area naturally. Kernels in this case should be of normal size.

More commonly this year, plants in fields and parts of fields have lost much, or even all, of their leaf area as a result of stress. Most of the stress is a lack of water, with contributions in many cases from lack of nitrogen related to inadequate water uptake. Nitrogen loss, or movement of N to beneath the root uptake zone, has also contributed in some cases. As the loss of leaf area moves up the plant past the ear leaf, the ability of the plant to intercept sunlight and so to photosynthesize diminishes quickly.

The other factor that affects the crop's ability to continue filling kernels is the state of the kernels themselves and of the ear on which the kernels reside. The reduction in sugar supply caused by leaf damage or loss eventually causes kernels to lose their ability to take in more sugars, so it's possible that some kernels that are still small won't be able to fill any more even if the leaf area revives some late in the season.

Kernels that stop filling prematurely are typically small, with some starch in the crown but liquid at the base of kernels. This liquid eventually dries, and with little starch deposited late, kernels will be shrunken at the base. Such kernels often are light in weight, they tend not to fit together very well, and their starch density may be lower than normal; all of this means low test weights. Protein levels may be higher than normal due to lower starch deposition. The kernels may also have some sugars still present that darken during high-temperature drying. Dockage can be substantial, and in some cases animal feed may be the best use for such grain.

Another factor from which we might take a lesson this year is the very high variability to be found, both among and within fields. It is not unusual to find fields that might yield less than 100 bushels across the road from fields that will yield twice that. Causes for this phenomenon are not always obvious and will have many people scratching their heads long after the season. Let me enumerate some possible reasons.

In a general sense, years like this, when lack and unevenness of rainfall define the season, we can expect more variability among and within fields. That's because even small differences in soil conditions can cause a little more or a little less water to be available and make a big difference in how much growth and yield result. We always say that our soils in Illinois are "forgiving"--that they are good enough to let us do things like compaction and still get good yields. But in years like this, the amount of reprieve we can get from good soil may simply not be enough.--Emerson Nafziger

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