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The Curtain Falls on the 2003 Crop

September 5, 2003

I doubt that anyone needed the 6 to 8 inches of rain that fell in some places in Illinois over the past week, but rain was very welcome in areas that had received little rain in August and none since the first week of the month. Here are some questions and answers as we approach the final weeks of the crop year.

Will the rain help increase yields? The answer to this depends, in both corn and soybean, on how much healthy green leaf color remains on the crop. Many early-planted or drought-stressed cornfields have little green color left, and husks are starting to dry even if there is some green leaf area. The fact that this crop will probably reach (or has reached) maturity in fewer GDDs than predicted will be reflected in lower yields, though many yields will still be good. Even at the end of grain filling, though, a "charge" of water (and nitrogen) will help the crop pack in some more bushels, as long as it has some green leaf area to make this happen. Once the sun comes out and warm temperatures return, the crop will be back up and running if it still has the ability to do so. Most soy-bean fields are in better shape to take advantage, though maximum temperatures in the low 70s will delay maturity and retard filling in both crops. Where canopy color was lost, it won't come back, and those fields or areas will likely have smallish seeds, with yields to match. Overall, though, the rain came at a good time for the soybean crop, and I expect every day with sun and good temperatures to add around 2 bushels per acre, as long as the canopy stays green, up to physiological maturity.

Any guesses on yields? We're approaching the time when we can harvest and won't have to guess about yields. In the meantime, the longer we wait, the better our estimates ought to be. For both corn and soybean, I suggest counting plants and seeds per plant in order to estimate seed number per 1/1000 of an acre, then dividing this number by expected number of kernels in (a thousandth of) a bushel to give bushels per acre. Doing this late should give better estimates of seeds that are actually filling and should also allow us to better guess at eventual seed size, thus fine-tuning our estimate. A useful calculator to do this for corn is located at the Web site This site suggests how to make seed size adjustments, and it will do the calculations for you from counts you make. The ear weight method takes more time, and many people do not have the scales to weigh ears accurately, so it is less desirable in most cases than the kernel counting method.

We do not have an equivalent site for soybean, but the numerical yield estimation technique takes longer and is less accurate for soybean than for corn. It is possible that simply guessing at yields based on a subjective look at plant numbers, pods per plant, and whether most pods have 2, 3, or 4 seeds filling might be as accurate as counting. For soybean, count plants per 20 square feet (8 row feet of 30-inch rows, 16 row feet of 15-inch rows, 32 row feet of 7.5-inch rows), and count seeds per plant on 10 randomly selected plants. You might exclude from both the count and the sampling plants that are spindly and have few if any pods. Average the counts to get seeds per plant, then multiply the number of plants per 20 square feet times seeds per plant times the factor 2.18 to give number of seeds per 1/1000 of an acre.

Now the tricky part: Soybean seed size varies a lot depending on the number of seeds filling and the conditions. We can start with an estimated 180 seeds per 1/1000 of a bushel (3,000 seeds per pound), and move that number down to perhaps 120 for very large seeds and up to perhaps as high as 250 if the seeds look like they will end up very small. Divide the number of seeds per 1/1000 of an acre by this number. As an example, if we count 65 plants in 20 square feet and get an average of 78 seeds per plant, and we guess that seeds will be of average size, we would calculate a yield of 65 x 78 x 2.18/180 = 61.3 bushels per acre. If it looks like the seeds are small and the canopy is starting to lose its color, then we might divide by 210 instead of 180 and get a yield estimate of only 52.6.

Is there anything we should watch for? There have been a lot of warnings about possible stalk quality problems in corn, and it would be useful to walk into fields as they reach black layer and push on stalks to see if they still have strength. Without strong winds, even weak stalks will probably stand reasonably well until harvest, but most people would prefer to harvest at 25% or higher moisture and pay to dry than to face a down and tangled mess at harvest. Given the way the last month has gone, we might have fields that reach black layer and still have green stalks and leaves. Stalk strength will remain high in such fields, but grain will probably dry relatively slowly due to restricted air movement through the canopy. In soybean, continued cool weather might delay maturity considerably, and the possibility of green stem is usually (but not always) higher when maturity is late. Be sure to check grain moisture on soybean once pods have lost their color; it may be preferable to harvest with tough stems instead of waiting for stems to dry, in which case grain moisture could fall to 10% or less and harvest losses could increase greatly.

How will the 2003 season be remembered? As is usually the case, some will probably prefer not to remember it much at all, especially in those areas where planting was delayed, it turned dry, and the canopy deteriorated before it could be revived by rain. We expect that a very good start to the season in many parts of Illinois will be tempered by yield loss from dryness in August, but for many people the season will go down as a good one, even if few yield records get broken. There were a lot of "good days" for crops throughout the season, and we will benefit from those.--Emerson D. Nafziger

Author: Emerson Nafziger

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