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Issue No. 19, Article 3/August 12, 2011

Soybeans at Pod-filling Time

While we've been focusing on the problems of high temperatures and lack of rainfall with the corn crop in some areas, most soybeans have progressed most of the way through pod set and are entering the critical phase of filling pods. Crop condition ratings are not very high for soybeans, but the canopy has closed on all but the late-planted crop, and there have not been many serious, widespread problems in soybeans so far.

Flowering in the 2011 crop started at about the normal time, and crop progress has been close to normal up to now. The high temperatures during July and limited rainfall in some areas have produced afternoon stress symptoms, leading to concerns about the ability of the crop to set pods. But the normal to above-normal height of the crop indicates that the soil moisture supply has been less limiting than we might have expected. The canopy appears healthy, and in general the crop looks fairly good in many fields.

As we know, however, it takes more than a good canopy to produce good soybean yields; wet weather and high temperatures in July can in fact result in "overgrowth," in which plants and leaves get large, but pod numbers suffer. We think this is due in part to high levels of internal shading, which can reduce the sugar supply to individual nodes and the pods at those nodes. We've had some reports of plants as tall as 5 feet this year (to the top of the canopy, not the tip of the stem), so there is some concern about pod numbers in some fields.

Most May-planted soybeans are at or near stage R4, which is full podding. They may still have some small pods at the tip of the stetm that may or may not develop, but pod numbers can be considered close to final. An exception might occur in fields where it's been dry; rainfall in the next week might stimulate a flush of pods that could increase this number.

Though final seed size will vary with conditions, seed number per unit of ground area is closely related to yield potential as seed-filling is getting underway. Anyone who has tried to count seeds per plant in soybean knows that pod and seed numbers vary widely among plants. But it is usually possible to get an estimate, or at least to know if pod and seed numbers are high or low.

Val Clingerman made some estimates in the Urbana planting date study on August 9 and found good pod numbers--about 40 per plant--in the April-planted soybeans and about half that in late-planted beans. The late-planted crop still has some flowers and so is probably not done setting pods. In both cases there were about 120,000 plants per acre.

We can complete the yield estimate by assuming 2.5 seeds per pod, so 100 seeds per plant, or about 12 million seeds per acre. Dividing by 168,000 seeds per bushel (2,800 per lb) gives about 71 bushels per acre. While it may well take a rain event or two to keep plants functioning well enough to fill that many seeds, this may not be an unreasonable estimate of yield in these plots. It's too early to complete the estimate for the late-planted crop because pod numbers are not final.

The hope is that we're seeing a repeat of the 2010 soybean season, when warm, dry weather in August did not seem to slow seed-filling, the crop filled well into September, and we ended up with the highest state average yield ever recorded in Illinois. It's premature to assume a repeat, but I would not rule out the possibility.

Positives include the facts that the flowering and seed-setting periods have not been shortened, pod numbers appear to be reasonable, and the crop shows few signs of premature drying, which can signal an end to seed-filling under stress conditions. I even think that warm nights, which have contributed some to decreased yield potential in corn, may have helped soybean come through pod-setting in good shape.--Emerson Nafziger

Emerson Nafziger

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