No. 22 Article 6/September 2, 2004

Toward the Finish Line

With the return of seasonably warm temperatures this week, following rainfall over most of the state last week, both corn and soybean are heading down the home stretch, even though the maturity of both crops has been delayed somewhat by an unusually cool August. As of August 29, 14% of the corn crop was rated as mature, and 25% of the soybean crop was starting to turn yellow. Both of these figures are within normal ranges, which is remarkable given the fact that growing degree-days in August barely exceeded GDD totals for May and were actually less than June GDD totals in much of Illinois. GDD totals since May 1 are running 200 to 300 below average in much of Illinois, but adding the 200 or so that much of the corn received in April brings seasonal totals received by the crop to about average. Still, some of the crop in northern Illinois will need almost all of a mild September to reach maturity.

One recurring question we are getting is whether or not the cool temperatures in August will reduce crop yields. Because we know that cool night temperatures are positive for corn, and because the corn crop was planted early and was ahead of normal development during most of the season, there is less concern for corn than for soybean. In fact, the delay in corn development as it "sat" during cool, dry weather during much of August has almost certainly been positive for the crop. With adequate soil moisture now, there has been rapid resumption of photosynthesis in corn that still has green leaf area, and I think we can even expect to get some of the late-season "bonus" grain fill that was so important at the end of the 2003 season. Bright sunshine, highs in the lower 80s, and cool nights that we are getting this week are exactly what this crop needs as it finishes out.

Effects of the unusual August weather on the soybean crop are more difficult to predict. Cool nights are considered detrimental to soybean, but I believe this is more of a concern during early pod setting than when the crop is well into the seed-filling stage. When temperatures dropped around August 5, pod setting was about 80% complete, so loss of pods or failure to set pods as a result of cool temperatures should have been rare. While cool temperatures certainly decreased photosynthetic rates, they probably did not decrease seed numbers, except perhaps in late-planted fields, including double-cropped. Good soil moisture during establishment helped double-cropped soybean, though, and its condition is good in most fields.

One encouraging sign is that, while pod setting reached 50% around July 25 in 2004, which was about 2 weeks earlier than in 2003, percentage of yellowing is only slightly ahead of normal. This suggests that the cool weather delayed development, rather than causing deterioration and early maturity. As with corn, soybean fields that retain green color should be filling seed very rapidly under the great weather conditions this week. I wouldn't try to predict yields before harvest; some of the early soybeans seem rather short, but pod numbers and seed size so far don't point to the disasters that many of us found when leaves dropped in soybean fields in 2003.--Emerson Nafziger

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