No. 22 Article 4/September 2, 2005

A Drought-Stressed Corn Crop Nears Harvest in Illinois

As we move into September of a tough year for corn in Illinois, the wish of many is to get harvest done and fields readied for 2006, putting this year behind us. Very early yield reports have been coming in, ranging from "about as expected" to "less than hoped for." Whether or not current yield estimates change, it is clear that very few farms in Illinois will have yields as high in 2005 as they were in 2004. Some areas within crop reporting districts received one or two more rains than did other places, but some also received less. As a result, we expect yields to be perhaps more variable than ever before in Illinois, with highest-yielding fields producing twoto four times the yields of lower-producing fields, sometimes in the same general area.

While mid-August rains helped improve soybean prospects (or at least slowed the decline), much of the corn crop had low kernel number or poor canopy conditions (yellow or dead leaves), or both, at the time of the rain. The rain likely did little good in these fields, while fields that still had good canopies benefited. The main factors that will be affected by this year's weather are kernel number, which was fixed by late July, and kernel size, which will increase as long as the leaves are still photosynthesizing, up to a maximum. The "bonus" fill that occurs under moderate temperatures and high sunlight, when plants are well watered and have healthy leaves, can result in kernels getting larger than normal. The chances of this happening in 2005 are small, given the ongoing lack of soil water and the periods of dryness that affected both kernels and canopies.

Smaller-than-normal corn kernels will likely be common this year, reflecting canopy conditions that deteriorated before kernels were fully filled. In such cases, kernel protein tends to be higher due to less "dilution" of protein by starch. Test weight might or might not be affected. If kernel fill gradually slows, kernels may stay small but with normal shape and normal endosperm density, in which case test weight might be normal. If grain filling stops abruptly, endosperm density is often decreased and test weight lowered. If kernel number is low, yields will be low, but kernel size and test weight may not be reduced. The relationships among yield, test weight, and kernel size and shape are complex, but in general, test weight is not closely tied to yield, except in cases where kernel fill stops very early and kernels end up light and shrunken.

When grain fill ends early due to canopy deterioration, stalk quality is often compromised. This happens whenever there is not enough photosynthesis to both fill kernels and maintain stalk integrity. Fields most at risk will be those with kernel numbers above 400 or so, but where canopy decline started by mid-August and kernel size has been clearly limited by lack of photosynthesis over the past 2 weeks. Fields with low kernel number had less demand for sugars, so they might have better stalk quality. Grain may not dry down very well in some of these fields due to higher-than-normal sugar content of kernels caused by inability of kernels to convert sugar to starch. However, drydown started early in many of these fields, so harvest will not necessarily be delayed.

We estimate when kernels have stopped filling by the appearance of the "black layer" at the base of kernels. The darkening of the "transport cell layer," where the kernel attaches to the cob, indicates that this tissue is no longer actively pumping sugars into the kernel. When the crop canopy stays healthy, the black layer forms following some sort of signal indicating that the kernel can't accept more sugar. This signal might involve increasing sugar concentration in the kernel after the kernel has made all of the starch that it can make. We don't know if that's the same signal when the canopy dies and the supply of sugar coming from the stalk to the cob is cut off. In any case, when sugar movement stops and the black layer forms, kernels stop getting heavier, and yield is as high as it will get.

Many fields in central Illinois are at black layer or are approaching it quickly. This is to be expected, given the early planting (which added about 200 GDDs from early April) and the fact that GDD accumulations since May 1 are 2,600 to 2,800, or 200 to 300 GDDs ahead of average for the end of August. Most hybrids grown in central Illinois are listed as maturing in 2,600 to 2,800 GDDs from planting, so maturity may be arriving even a bit later than GDD accumulations would suggest. This is probably due to cold weather setbacks in May and to delays in development under very dry conditions.

Harvest concerns will also include high drying costs and drydown rates. Corn reaches black layer at 30% to 32% grain moisture and can be harvested at any time after that. The concern that Dr. Bob Nielsen at Purdue University has expressed about the loss of kernel dry weight as grain loses moisture is not shared by many agronomists, due to the failure of other research to support this finding. It is certain, however, that harvest losses increase as corn dries down from the low 20s, due mostly to increases in shelling loss at the header.

Corn grain drydown rates vary widely, depending on husk thickness and tightness, grain characteristics, and weather. Grain can lose as much as 1 percentage point on a warm, sunny, breezy day but might lose little or none under cool, rainy conditions. Grain has even been known to take on moisture during a rainy period. Drydown tends to be more rapid the earlier black layer is reached, due to warmer temperatures. As a rough guideline, figure on grain losing moisture at a rate of 0.6 to 0.7 points per day for the first half of September, and 0.4 to 0.5 points per day for the last half of the month. Keep in mind that actual rates can differ considerably from these estimates, especially if the weather is much different from average.


As we have observed in the past, corn following corn tends to be more affected by stress than corn following soybean or another legume. That certainly seems to be the case in most areas in 2005. We have heard of yield differences between corn following corn and corn following soybean of up to 100 bushels per acre under stress in past years, and we expect to hear of numerous such cases this year. Besides the obvious effects of lack of water, indications are that it is more difficult to maintain high yields when corn follows high-yielding corn than when the previous corn crop did not yield very well. High yields in 2004 and low rainfall in 2005 are thus both contributors to the loss of yield this year where corn followed corn.

On the positive side, low yields in 2005 might provide better prospects in fields where corn in 2006 follows corn in 2005. However, given the fact that the ratio of corn yield to soybean yield is likely to be lower in many areas in 2005 than it has been for several years, some producers might want to rethink the strategy of moving aggressively to more corn and less soybean, which usually means more corn following corn. We will be reminded often this year of the stabilizing effect of soybean in the rotation on corn yields.

As a final comment on rotation, winter wheat produced good yields in most areas in Illinois in 2005, even though acreage was down. For some, especially where winter wheat yielded more than corn (this won't be widespread, but it will occur for some producers), the value of spreading weather risk by having a crop that is unaffected by summertime drought will be clear and might stimulate more thinking about adding winter wheat acres this fall. Early harvest in some areas should make wheat planting easier unless wet weather sets in before wheat can be planted. Even if it's possible, do not plant wheat much before the fly-free date, which ranges from mid-September at the Wisconsin border to mid-October at the southern tip of Illinois.

Soil Fertility

There have been many questions about crediting nitrogen left after a short corn crop in 2005 to the (corn) crop in 2006. In areas of short rainfall this summer, especially where tile lines have not run since spring, most of the N that was applied but not taken up by the crop is still present in the soil, mostly in the nitrate form. The nitrate made available by mineralization of soil organic matter and not taken up by plants is also present. It is possible to measure nitrate content of soil samples (usually taken to a depth of 3 feet) and to estimate total N supply in the soil this fall. The nitrate in the soil is very water soluble, however, and so it will move downward with rainfall between now and next spring. If rainfall is high enough to get tiles to run, then some of the nitrates will leave the field in tile water. Nitrates can also leach to below the rooting depth in soils with fair to good internal drainage.

Because loss of nitrate-N from the soil during the next 7 to 8 months is impossible to predict, estimates of the amount that will be available for next year's crop cannot be very precise. As a way to estimate, we suggest that the yield of corn (bushels per acre) be subtracted from the amount of N (pounds per acre) applied to the 2005 crop and that half of this difference be credited to next year's crop. Thus, if 180 lb N was applied to a crop that yielded 80 bushels per acre, the amount credited to next year's crop would be 180 - 80 = 100 / 2 = 50 pounds N. If rainfall is less than normal over the October to April period, then more could be credited, especially in soils where organic matter is above 3.5% or so. If excessive rainfall occurs, it might be appropriate to reduce this credit. Such adjustments should take into account soil texture and drainage, both of which have a large effect on how much nitrate-bearing water leaves the soil.

Estimates of crop removal of P and K should be adjusted according to crop yield. Maintenance requirements, according to the Illinois Agronomy Handbook, are 43 lb of P2O5 and 28 lb of K2O per acre for a 100-bushel corn crop; raise or lower by the same percentage by which yield is higher or lower than 100. P and K removal might be slightly higher per bushel for very low-yielding corn, but replacement amounts do not need to be adjusted when actual removal amounts are so low in low-yielding fields.--Emerson Nafziger

Close this window