By Jennifer Shike, University of Illinois
Soybeans are entering the critical pod-filling phase in most parts of Illinois. Emerson Nafziger, University of Illinois Extension agronomist, said although soybean crop condition ratings are not very high, there have been few serious problems in the soybean crop so far.
Flowering in the 2011 crop started at about the normal time, and crop progress has been close to normal up to now, he said. High temperatures in July and limited rainfall in some areas have produced afternoon stress symptoms, leading to concerns about the ability of the crop to set pods.
"The normal to above-normal height of the crop indicates that the soil moisture supply has been less limiting than we might have expected in the drier areas," Nafziger said. "The crop canopy appears healthy, and in general the crop looks fairly good in many fields."
However, it takes more than a good canopy to produce good soybean yields. Wet weather and high temperatures in July can result in "overgrowth" in which plants and leaves get large, but pod numbers suffer. Nafziger attributes this in part to high levels of internal shading that can reduce the sugar supply to individual nodes and the pods at those nodes.
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 stem that may or may not develop, but pod numbers may be considered as close to final. He said an exception to this might occur in fields where it's been dry. Rainfall within the next week might still 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 seedfilling is getting underway," Nafziger said. "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."
Research specialist Val Clingerman made some estimates in the planting date study at Urbana on August 9 and found good pod numbers, in the range of 40 per plant, in the April-planted soybeans, and about half that number in June-planted beans. The late-planted crop still has some flowers and is probably not yet done setting pods. In both cases, there were about 120,000 plants per acre.
Complete the yield estimate by assuming 2.5 seeds per pod, so about 100 seeds per plant, or about 12 million seeds per acre, he explained. 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," he said. "It's too early to complete the estimate for the late-planted crop, since pod numbers are not yet final."