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Issue No. 18, Article 9/July 25, 2008

What the Crop Needs Now

The condition of the Illinois corn crop has improved slowly following the slow start, and the latest numbers from the National Agricultural Statistics Service show that 68% of the crop is now rated good or excellent. At the same time, only 55% of the crop was silked by July 20, compared to the 5-year average of 85% by that date, and 94% in 2007. The warm temperatures of mid-July have helped to accelerate development, but growing degree day accumulations since May 1 remain about 100 behind normal.

The fact that GDD numbers have not caught up reflects the fact that temperatures remain at about average levels, which is good news. At this point in the season, 100 GDD is only about 4 days' worth of GDD. Talk that the crop is two to three weeks behind normal is true only where corn was planted very late; corn planted in late April or early May is only about a week behind normal. As long as temperatures remain average or a little below average, we don't expect the crop to catch up. We also don't expect that to be a problem as long as moisture supplies remain good. A week behind today will stretch into two weeks as we get into September, but the crop planted in early May has accumulated about 1,500 GDD since planting, and so it needs only about 1,200 more to reach maturity. It takes roughly 50 days to accumulate 1,200 GDD starting at this time of the season.

Most of the late-planted crop should reach the silking stage by the end of July, except in central and northern Illinois, where the crop might not pollinate until early August. Late-planted corn usually has a decreased total requirement for GDD compared to the same hybrid planted early. This is partly due to some photoperiod response, with corn flowering in response to shortening days. High temperatures also play a part in this, and the lack of high temperatures this year may mean less reduction in GDD than in some years. Much of this reduction applies to the vegetative stage and so results in earlier pollination.

So far, we have to consider the summer a good one in helping overcome the problems of late planting and relatively poor soil, weather, and crop conditions in May. Rainfall has been above average in many areas so far during July, but there have been enough dry days to get soils dried out and to allow some root recovery and growth. One widely observed problem that will not go away is uneven growth, due in most fields to unevenly wet soil conditions early in the season. Root systems there are not likely to be in good shape, and these areas will show water and nutrient stress more quickly than better-drained areas. There's not much we can do about this. If there are questions related to staging of the crop in terms of fungicide application and other canopy protection measures, it may be necessary to do some "triage" to determine where in the field the yield potential is highest, hence where to apply protection first.

The story for soybean is not yet as favorable, though the weather has helped the crop to start growing much better in recent weeks. As of July 20, 59% of the soybean crop is rated good or excellent, and only 39% of the crop had flowered, well behind the 5-year average of 73%. The percentage setting pods was reported at only 6%. This delay in development is some cause for concern; as I have noted before, early podding followed by normal or even delayed maturity in soybean is usually associated with high yield. The late onset of flowering and podding may not be a serious problem as long as pod filling can be maintained over a relatively long period and maturity also ends up being late. This will take unusually good September conditions.

Both crops would benefit from continued favorable rainfall and abundant sunshine, with dry days outnumbering wet or cloudy days by a considerable margin. Corn will benefit from daytime temperatures in the upper 80s or (if there's good soil moisture) lower 90s, and night temperatures in the lower 60s. Soybean plants will set more pods and fill seeds faster if daytime temperatures are in the 80s and nighttime temperatures in the upper 60s to low 70s. Water-use rates are at a peak for corn and will reach peak levels for soybean as soon as canopy formation is complete.

Corn that is silking now will, if temperatures remain normal, require about 55 to 60 days to reach physiological maturity. This means that September will be important this year, for both earlier- and later-planted corn. We have measured yield accumulation rates as high as 10 bushels per acre per day during a week in the middle of the grain-filling period. Most crops do not sustain such rates for a whole week, but stress-free crops with a good canopy can form yield very quickly. It's critical that canopy health be maintained, and even short periods of water stress will mean loss of yield.

Soybeans have much farther to go, and we'll watch the crop carefully over the next month to see how many pods it sets and how well they stay on the plant. The highest rate we've found for soybean yield accumulation is about 4 bushels per acre per day, and this rapid seed growth rate usually lasts for about 4 weeks for the majority of the pods on the plant. Because of late planting and slow growth before July, wet weather this month has not caused the excessive vegetative growth that we have seen other years. This is positive in terms of producing less internal shading, but there is still some concern--diminishing as we experience moderate temperatures now--that flowering and node-setting might not last as long as normal and that seed numbers per plant and per acre could suffer. For soybean, that almost always means lower yields.

Wheat variety trial results from 2008 are now available at vt.cropsci.uiuc.edu/wheat.html.--Emerson Nafziger

Emerson Nafziger

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