No. 21 Article 5/August 18, 2006

Corn Marches On, Crunch Time for Soybean

According to official August 1 estimates, Illinois corn will yield an average of 172 bushels per acre in 2006, and soybean will yield 45 bushels per acre. If these yields are realized, 2006 will go into the books as a very good year. Can anything go wrong to make these estimates too high, or can things go so well over the next month that we harvest even higher yields than the estimates?

The answers are probably yes for both crops, though the probabilities differ for the two. Corn was planted relatively early and has had warm or hot growing weather most of the season, with growing degree-day (GDD) accumulations since May 1 above normal in most places in Illinois. Corn planted in early April in central Illinois has experienced about 2,500 GDDs since planting. Most hybrids grown in this region require 2,600 to 2,800 GDDs from planting to black layer (physiological maturity), meaning that the earliest-planted corn is going to reach black layer by late August or early September. The recent cooldown will delay this some, but that is favorable, especially since nights are cooler than they were several weeks ago, which means less respiration loss and more sugars to fill kernels.

Another favorable sign is that crop progress reports show corn reaching the dent stage on a normal schedule, meaning that late July's high temperatures did not bring on premature ripening in most of the corn crop. Exceptions are found in parts of western and southwestern Illinois, where rainfall has been very skimpy and where some fields now have dropped ears and dried leaves. Yields will be disappointing after such a promising start to the season in these areas. Plants that were able to fill grain for only a month or so after the end of pollination did not have time to fill kernels very well. But plant and root sizes were good in many of these fields, and it's possible that plants filled grain a little longer than we thought possible and that they might yield a little more than we expect.

Daily water use drops in corn during the last half of grain filling, and in the more advanced (but still green) fields, it is now only about half of what it was during mid-July. The crop needs 2 to 3 inches of water to go from dough stage to maturity. In fields where rainfall in July and August has been average or above, this requirement for water will be easily met. The color and health of the canopy remain good in most fields where there is enough water, and except for wind strong enough to knock the crop flat or an unprecedented attack on grain by insects or diseases, few things can wreck yield prospects in these fields now. Be sure to check stalk integrity, especially in those fields where dryness is bringing the crop to an end quickly. In such cases, the ear will normally draw most of the available sugar from the stalk, and the stalk cells will die off early, making stalks vulnerable to disease moving up from the roots.

In fields that are still green and where there is enough moisture, the threat of stalk weakness is minimal. Leaf disease can shorten grain fill, but each day that passes with sunshine and good temperatures is enabling high rates of photosynthesis to fill grain and making the crop less vulnerable to disease or insects that can limit yield. As maturity approaches, days like we're experiencing this week--highs in the 80s and lows in the low 60s or upper 50s--are nearly ideal. We expect the crop to add 6 to 8 bushels of yield each day like this, as long as the canopy is intact and there are enough kernels to fill. The rate of fill will decrease in the last week before the crop reaches maturity.

If good conditions persist up to physiological maturity, kernel weight--and therefore yields--may well be higher than we estimate when we count kernels and guess at the eventual weight per kernel. For those inclined to make yield estimates by counting kernels per ear and multiplying that by the number of ears in (a thousandth of) an acre, the typical estimate of kernels per bushel that is divided into kernel number per acre is 90 (per thousandth of an acre), or 90,000 kernels per bushel. If grain size is already visibly large and conditions are good, this can be dropped to as low as 80,000 or even fewer.

While the yield estimate for soybean reflects average pod and seed numbers, it will be a few weeks until we can be confident that the seeds filling now will reach their normal size. Signs are favorable, though soybean would maintain a slightly higher rate of seed filling if night temperatures stayed at about 65 rather than dropping into the 50s. While we have been looking at a fairly good canopy for the past month, recognize that the real work of getting the seeds filled is only now fully under way in most fields, and what happens over the next three weeks will largely determine weather the crop is average, better than average, or disappointing. The chances for the crop to disappoint by not filling its seeds are diminishing quickly where conditions are good now and where there is an adequate supply of soil water.

Unlike the 2005 soybean crop, the 2006 crop did not set pods and start to fill seeds earlier than normal. The concern remains that seed numbers per acre might not be as high as needed for top yields. At a final seed size of 3,000 seeds per pound, each 4 seeds per square foot means 1 bushel per acre. So if there are three plants per square foot, each plant needs to fill 80 seeds for the crop to produce 60 bushels per acre. Given the current conditions, seed size could turn out a little larger than normal, so if seed numbers are high enough, yields could be above the current estimate. Insects that eat leaves, insects that pierce pods and seeds, diseases that destroy green leaf tissue, and diseases that restrict water flow in stems will all mean less sugar moving to seeds and, hence, lower yields. Chances of these problems developing are diminishing with time, and so we have reason to be optimistic about soybean. But we'll need to keep our fingers crossed a little longer.--Emerson Nafziger

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