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Issue No. 21, Article 4/August 17, 2007

Crop Prospects as the Season Winds Down

While it is a little early to talk about the season winding down, this year the relatively early planting coupled with above-average temperatures during parts of the season have the crops a little more advanced than usual. This is particularly true in the southern half of Illinois, where dry soils have also been taking a toll, in some cases bringing the seed-filling period to a premature end.

While we would have much preferred to receive more rainfall from mid-July to mid-August, those whose corn crop is losing color and starting to dry down might find more and larger kernels than they normally expect this early. Growing degree day (GDD) accumulations have been steady since pollination, with some acceleration due to warm night temperatures over most of the state during most of August. Here at Urbana, corn planted on April 20 has accumulated about 2,500 GDD, nearly 300 GDD above average for this period. Totals in most southern Illinois locations are similar to this, while in the northern counties in Illinois, only about 2,000 GDD have accumulated.

So while dry weather might bring a premature end to grain filling in the dry southern half of the state, grain has had a considerable amount of time to fill, and kernels might not be as small as expected. Exceptions will be in fields that were planted late. May GDD totals were above average, so late planting "cost" more GDD than usual. Thus late-planted corn might have accumulated only 2,000 to 2,200 GDD by now, and grain in these fields may have accumulated only about half their normal amount of weight. If leaf color is being lost in these fields when kernels are only in the dough stage, kernels will end up smaller than normal, and yields will suffer. One way to check this, besides calculating GDD totals, is to twist ears. If filling ended prematurely and kernels are starting to dry, ears will twist easily, and gaps will appear between kernels as the kernels dry and shrink.

Many people, continuing to associate test weight with corn yield, worry that yield will follow test weight, both dropping as grain filling ends early. Because the end of grain filling involves starch "packing" into the kernel, endosperm density may indeed be low when grain filling ends early, and test weight can be lowered as a result. In some hybrids, though, premature death of the plant might result in small kernels but test weights that aren't too much less than normal. The real problem with early death of leaf tissue is kernels that did not fill as much as they could have and so are lighter in weight. The weight per kernel does not correspond perfectly with test weight, since test weight involves shape and density, not just weight per kernel.

Corn that dies early due to stress or disease usually moves most of its reserve sugars to the ear, leaving the stalk and root without much protection against diseases and deterioration. Where photosynthesis ends very early, stalks do not have enough time to develop the lignin they need for good strength, and loss of living tissue in the stalk can leave the stalk very weak. Thus premature death often means weak stalks.

How fast ears on plants that stop filling prematurely will dry down depends on several factors, including how tightly husks adhere to the kernels and how much sugar remains in kernels. When sugars that move into kernels can't be converted to starch, they add to the "osmoticum" in the kernels, and this can slow the loss of water. In addition, such kernels are prone to browning if they are heated during drying. Finally, immature kernels on plants killed by dry weather can be more prone to infection of Aspergillus fungus, the green mold that can produce aflatoxin. Aflatoxin is usually a much larger problem in corn that develops under hot, dry conditions.

The USDA estimates released on August 10 have corn yields in Illinois averaging 178 bushels per acre in 2007. For many in the southern half of the state that have watched their crops suffer in the heat and dry soils in recent weeks, this seems implausible. The corn crop in the northern half of the state, with a few exceptions, is in very good shape, and some areas have had above-average rainfall during the past month. On average, the crop in Illinois will be good, but the variability among different parts of the state will be higher than normal.

Soybeans are suffering in dry areas of the state as much as corn, though if there is some rainfall in the next week, the crop's prospects will revive to some extent. Except in fields with light soils where there has been little rainfall during the past month, the soybean crop canopy has more or less survived and is still green, at least in the morning. The view from inside the canopy might be less favorable, though. We are starting to get reports of pods dropping off plants, even in fields where the leaves have not yet started to dry up. Mike Vose at the Orr Agricultural Research and Demonstration Center in Pike County reported pods dropping, and that those dropping seemed to be mostly ones with only one bean developing.

It is likely that the first pods to drop off are those that formed last at each raceme. The raceme is the tiny branch, attached at the node, where flowers form and develop into pods. Pods form nearest the stem first, and then continue over a few days to form farther up the raceme. Under stress, flowers near the end of the raceme will drop off and not form pods, while the later-forming pods may be able to form fewer seeds. If stress intensifies, signaled perhaps by the lack of sugars coming from the leaves, then pods can begin to drop off, starting usually with the latest-formed, youngest pods. It is thus possible under extreme stress to end up with few pods or seeds on a plant, hence little yield potential. At some point pods will tend to remain attached even as stress continues and intensifies. If stress results in loss of green leaf area, seeds will stop filling early and will end up as small seeds. In extreme cases, seed weight can be as little as half of normal.

Soybeans often retain their ability to put on more flowers and to set more seed after severe stress is relieved, even if this relief comes as late as growth stage R6 (seed filling the cavity in most pods on the plant). In general, the later this happens the less chance there is for the new pods to fill seed, and pods and seeds that form after plants have reached R6 often drop off the plant or stay very small and are lost at harvest. In part this is because such pods are at nodes with only small leaves, and these leaves are not able to produce enough to fill the pods. Older pods have much better ability to draw nutrients than the new seeds, and they have much larger leaves from which to draw. In contrast, new seeds at the top of the plant are not very well nourished after they form, so their chance of forming full-sized seed is low. Double-crop soybeans are much better able to add larger leaves and pods to fill, so they could benefit much more from rainfall at this time than could full-season soybeans planted early. Still, rainfall would help pods and seeds to stay on the plant and to fill more completely, regardless of when the crop was planted. Compared to corn, soybeans have farther to go before maturity, and so are more vulnerable to current problems and also have more potential to benefit from improving conditions now.--Emerson Nafziger

Emerson Nafziger

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