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Leaning Corn and Suffering Soybeans

July 24, 2003

On July 22, the governor declared several Illinois counties disaster areas, in part because of crop damage. This probably surprised some people, including some who have heard glowing reports on crop conditions from this and other news sources. But there are places where wind, excessive rain, and/or hail have caused crop damage, and questions remain about how much effect on yield such damage will cause. Even though the areas of damage tend to be smaller than a county in most cases, being in such areas is especially painful in a year when most fields in the state are in good shape.

Heavy rains and winds in early July caused extensive "leaning" in many cornfields in different parts of central Illinois and some in northern Illinois. Rains softened the soil, and roots, which we thought were probably deeper than normal, proved inadequate to keep the plants from leaning over. The "degree of down" ranged from almost flat to barely off the vertical. Most of the corn at that time was about to tassel, which is the prime stage for greensnap, or lower stalk breakage at a node. Corn with greensnap can no longer produce yield, or only very little yield (lower nodes can shoot an ear, but there's not enough leaf area to fill much). It is not clear why more corn did not snap instead of leaning over, but I expect that windspeed may have been lower than in the major greensnap events we have had in the past, and the drier weather in June might also have resulted in slightly smaller plants and slightly less brittleness in the nodes. In any case, while flattened corn looked bad, it was much better that it blew over instead of snapping off.

Before plants tassel, the lower stalk has enough flexibility to bend upward and to bring the upper stem and most of the leaves back to a more normal orientation. The main problem in such fields is that the root system was partly pulled out of the ground, and plants had to use energy to regrow some roots. Energy is in short supply anyway as the plant approaches pollination, and so we expect slightly smaller ears in such fields. In some cases the root system is compromised, such that the plant may suffer more quickly from drying soils--if we ever get drying soils this year.

The prognosis for corn flattened during the past week, at or after pollination, is less favorable than for corn flattened in early July. Stalks develop lignin (become woody) as they get older, which is helpful for standability later but also means that plants are less able to grow back up toward the vertical when they are blown over this late. If stalks broke (stalk-lodged) rather than pulled out roots (root-lodged) from the wind, then the "plumbing" of the lower stalks is compromised, water and nutrient flow is reduced, and the plants have little chance to grow back much toward upright. If this happens at or just after pollination, we expect such plants to yield little. Most stalks did not break, but root systems that pulled out of the ground now will regenerate active roots more slowly than if the plants were younger, and doing so will detract from grainfilling ability. There just aren't enough sugars to go around.

A decrease in the sugar supply of lodged plants is probably a more serious problem than too much demand. Leaves lying on the ground or under the plant do not photosynthesize (produce sugars) very well, partly because they are more shaded and also because the air movement around them is restricted. Mud on the leaves is another problem, and diseases usually have an easier time infecting leaves that are near the ground, especially when those leaves are already beat up. Root systems of root-lodged plants are usually compromised so that water supply to the plant can become limiting, especially if the soils start to dry out. With frequent rainfall, I would expect plants that root-lodged to 30° or less from horizontal at about the end of pollination to yield perhaps half to two-thirds of normal. If it dries out or diseases take over, then this figure will drop quickly.

In fields that have lodged and in most fields that haven't, this will be a year to watch stalk quality closely. When there is not enough sugar to go around, whether that's because of low supply from lodging, disease or other problems, or high demand by a lot of kernels on the ear, then the stalk is the first to suffer. As sugar content of the stalk drops, disease organisms come in, and stalk weakness results. There's not much to do about this if it happens, but earlier harvest and special equipment might be called for if you know the problem exists.

Besides the disease problems outlined elsewhere in the Bulletin, soybean plants that are standing in water or in saturated soils are struggling greatly. On a trip north on July 19, I saw large portions of fields with such soggy soils, especially in Livingston, Iroquois, and Ford counties. Crop color is less than ideal in many of these fields, even in places where the soils weren't visibly saturated. Other fields in the same vicinity had good color and better growth. I think the differences are mostly related to drainage, perhaps some to planting date and some to variety. In any case, the prognosis for some of the fields I saw cannot be very positive. Some drying weather will help, but with what are probably compromised root systems on affected plants, they will suffer from water shortage about as soon as soils start to dry out.

Soybean plants are mostly in R2 (full flower) to R3 (beginning pod) stages, with some earlier varieties planted early perhaps in full pod (R4) by now. The crop continues to lag a bit in its development because of cool temperatures. This should not be a problem if water supply remains adequate through August, but unless the crop dries up with large yield losses, we cannot expect harvest to start early. Normal August temperatures and relatively warm weather into mid-September should help yields. --Emerson D. Nafziger

Author: Emerson Nafziger

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