No. 15 Article 8/July 16, 2010

Is Drought Next for the 2010 Illinois Corn Crop?

As of July 11, 72% of the Illinois corn crop was showing silks, well ahead of normal and on pace with the 2004 and 2007 crops. The good-to-excellent rating has dropped slowly, from 77% on May 24 to 65% on July 11. The same rating was above 80% at this point in the season in both 2004 and 2007.

At this point in 2008 and 2009, silking was well behind what we see this year, and good-to-excellent ratings were similar. We had good corn yields both years, following temperatures cooler than normal after pollination. Temperatures in Illinois in 2010 have been above average in June and about average so far in July. A change to below-normal temperatures is not in the forecast. This of course doesn't mean this can't happen, but it will not happen soon.

After a wet June in nearly all of Illinois, rainfall so far in July ranges from above normal in western Illinois to about normal in much of the state to below normal in parts of eastern Illinois. In some areas that show normal or below-normal rainfall, much has come in the form of thunderstorms, which do not drop rain uniformly. So some places remain dry and others have too much water.

Elwynn Taylor of Iowa State University recently suggested that the weather pattern this year resembles that in 1983, with cool, wet conditions into early July followed by hot, dry conditions into September. For those who remember 1983 as one of the worst corn years in Illinois in the past 40 years, this comparison rings some alarm bells. The Illinois corn yield in 1983 was only 79 bushels per acre. The only year since 1971 with a lower yield was 1988, at 73 bushels per acre.

Will we see another 1983 in 2010? One of the largest differences between the two years is the fact that the crop in 2010 is much ahead of the crop in 1983. By July 11, 1983, the Illinois crop was only 6% silked, and only 84% was silked by August 1. Except for some replanted corn, we can expect nearly all of the 2010 crop to be silked by July 20. In addition, the weather during the silking period in 1983 was dry and unusually hot, with temperatures above normal every week for about two months beginning in mid-July. The combination of late silking and high temperatures was one of the main factors in the low yields of 1983. We already know that most of the 2010 crop will not experience this same combination, though some of the late-planted corn still could.

Hybrids have also changed a great deal over the past 30 years, with today's hybrids generally considered more tolerant of hot, dry conditions, especially during silking. We saw again this year the aggressive, early silking--often before tassels fully emerged--that we have been seeing in recent years. This is not an accident; breeders long ago identified "silk delay"--the failure of silks to emerge during some or all of the period over which tassels shed pollen--as a major problem, and they have selected against hybrids that show this tendency. As a result, we are almost (but not quite) ready to take silk delay off our list of common problems of corn crop development. It can still happen, but it has become rare.

Though supporting evidence is less clear, we also think that today's hybrids tend to grow more "competent" root systems, which should help plants take up water better under marginally dry conditions. This does not necessarily mean much deeper root systems, but it may mean more fine roots, and thus better connections with soil particles to help move water through the soil. As I wrote before, the wet weather in June, along with demands for resources to support pollination, may well have compromised the root system to some extent and may have partly neutralized the genetic tendency toward better roots. But compared to the late-planted crop in 1983, the root system in 2010 should be better able to carry this crop through dry periods.

If we do enter a period without significant rainfall this month, how might the Illinois corn crop respond? Topsoil moisture is currently rated as adequate or excessive in 90% of Illinois. In most such areas, there is enough moisture to carry the crop through at least the next two weeks, though if temperatures rise to much above average, the demand for water could outstrip the ability of the plants to take it up and move it up the plant. The 10% of acres with below-normal soil moisture could struggle to take up enough water to keep plants hydrated and to keep stomata open so photosynthesis can proceed at a rapid rate. The plants will tell us how they're doing; as long as leaves retain a healthy green color and stay unrolled in mid-afternoon, they are doing fine.

Another critical difference between 1983 and 2010 is the rapid pace of development of the crop this year. Every part of the state is ahead of average in growing degree-day accumulations since May 1, from about 100 GDD in northern Illinois to about 180 GDD ahead in southern Illinois. This, along with the 100 to 200 GDD that the early-planted crop accumulated by May 1, means that the crop will have accumulated 1,800 to 2,000 GDD by the end of July in much of Illinois. If August temperatures are normal, much of the early-planted crop will be at, or approaching, physiological maturity by the end of August. This will be an advantage in the event that the weather turns hot and dry. By the time this crop runs out of water if the rain stops now, it will have produced half or more of the potential yield.

So we hope that it continues to rain enough to keep the crop watered until late August, and for the late-planted crop even longer. If it doesn't, then we will be able to track the potential for yield (or yield loss) by watching the crop canopy. The health (color) and duration of the canopy are directly related to yield after kernels are set. Drying soils, especially where nitrogen loss occurred earlier under wet conditions, may result in loss of leaf color. This may appear first as firing, or loss of green leaf area starting from lower leaves and moving up. If temperatures go higher, upper leaves may also lose their color, perhaps in part due to inability to move N-carrying water to the upper canopy. Rainfall can sometimes restore leaf color if tissue is still alive, especially in the upper canopy; dead tissue on fired lower leaves can't recover.

Even fields where supplemental N was applied might show loss of canopy color, since N applied to the surface may not have reached the roots, especially if soils have dried since then. In fields where additional N was not applied, N from the soil organic matter has been helping to supply the crop. But N gets to the roots primarily in water that moves to the roots, so dry soils can result in N deficiency symptoms even when enough fertilizer was applied. This is one of the reasons that the "rain makes grain" slogan so often rings true. We don't expect 2010 to be an exception. And for most fields, so far so good.--Emerson Nafziger

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