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Issue No. 18, Article 8/July 22, 2005

Corn and Soybean Crop Conditions--Time to Go to the Field

It was no surprise that the Illinois corn crop rating slipped a little more in recent weeks, with more than half of the crop rated as poor or very poor as of July 17. Still, large areas of the state received some rainfall from the remains of Hurricane Dennis and from the weak cold front that moved through after that. The rainfall deficit measured from May 1 continues to be large in most crop-reporting districts, especially in the northern part of the state. Are such low ratings justified, and do they mean, at this stage of the season, that hope is more or less gone for at least half of the crop?

To help answer this question, I traveled on July 19 through some 20 Illinois counties, in an oval including Champaign, Princeton, Macomb, and Vandalia. Some areas had received little if any rain in recent weeks, while others had received a considerable amount; fields north of Champaign were wet. This tour was not designed to assess the whole Illinois crop, of course, in that I certainly missed some of the driest areas and some of the wetter areas of southern Illinois. I will use suitable caution as I report what I saw.

While the corn crop in most areas shows effects of lack of adequate water during much of its development, recent rains in most areas have improved crop color and have helped move the crop past the pollination process. Pollination success varies a great deal across the state, from little seed set in areas where the corn is most shortened and the soils are driest, to normal-sized ears in other places. Corn rootworm adult numbers vary tremendously, with high numbers in the northeastern quadrant and low numbers in western Illinois. In one very good field in southern LaSalle County, corn rootworm numbers were high even though silks were brown, while in a droughted field in western Livingston County, numbers were not extremely high even though the crop was still pollinating. As an observation, the short, pale green plants in the most affected fields don't seem much more attractive to western corn rootworm adults than they do to people. I saw a few fields where silks were clipped but the "shake test" showed that the pollination process had been successful.

While it would be foolish to declare the state of the corn crop based on such a quick trip, the crop I observed was in somewhat better condition than the dismal ratings would suggest. Ears have set some kernels even in some of the driest fields, where plants are 5 feet tall or less. In other places, especially the Macomb area through Jacksonville to Hillsboro, crop height is normal, color is good, and ear size seems normal--one would have difficulty saying that the crop there is in trouble. In the lighter soils south of Route 16, crop height is less than normal to much less than normal, but ears are present in most fields and leaf color and health seem to be good, even if canopy cover is incomplete.

Kernel number is still unknown in some fields, both because some pollination may still be going on (mostly north of I-80) and because we expect a great deal of kernel abortion if soils stay dry and temperatures stay high. On the positive side, leaf color was recharged by water and nutrients following rainfall in many areas, and such a crop should be able to fill the kernels it has, providing more rain falls before present soil moisture runs out again. Soil moisture was not fully recharged in most areas, so deficits will appear again soon if rain doesn't occur. While low wind speed this week will reduce water loss some, water loss can be estimated to be on the order of 1/4 inch per day at this time of year, so inches of rainfall times 4 will roughly calculate how many days it will take for deficits to reappear. For many of us, this period will probably end before the next rainfall arrives. The crop will do its best to fill grain in the meantime, but there's no guarantee that the crop is now safe from further losses due to dry weather.

In areas that received rain, the soybean crop responded nicely by increasing plant height and setting a flush of pods. In fields that continue to stay dry, pod setting is reduced, to very low numbers in some cases. We can expect flowers to continue to abort and pod numbers to stay low as long as dry soils persist, and in fields that received some rain but where soil moisture becomes depleted again quickly. As with corn, the effect of lack of water is most easily seen as short plants; the crop is less than a foot tall in some of the driest areas. Compared to the disastrous soybean problems in some places in 2003, plants in most fields have produced good root systems that have not been compromised by excessive rainfall up to now. However, plants that have lost nearly all of their flowers to abortion over the past 2 weeks are now starting to lose the ability to produce enough pods to compensate for such losses, and so are losing yield potential for each day they go without rain.

For both corn and soybean, it is important, at least for those who want to understand what's happening, to get into fields soon to judge how plants have responded. We cannot depend on the rain gauge or rainfall record to tell us this -- the response to weather, especially when the weather is poor, will be somewhat different in every field. Only the crop can tell us how it's doing, and that happens only when we actually look at it. Expect some surprises, but both negative and positive.

One piece of good news in 2005 is that the wheat yield estimate for Illinois has been raised to 64 bushels per acre, only a bushel less than the record set in 2003. Weather dry enough to hurt the spring-planted crops can often help boost winter wheat yields in this way. Data from the University of Illinois variety trials have been posted at the variety testing Web site.--Emerson Nafziger

Author:
Emerson Nafziger

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