Late August rains and soybean yield potential
The rain that fell over parts of Kansas on August 24-26 could help the soybean crops in those areas where it rained. The extent of any potential yield increase will depend on the stage of development the soybeans were in and the overall plant health of the crop at the time of the rain. The yield response to this rain could range from negligible to substantial. The crop condition was highly variable prior to the rain.
Situations in which soybeans are unlikely to respond to the rain
It is unlikely that plants in very poor condition with dead and yellow leaves throughout the canopy, or with limited vegetative development, will be able to respond to the rain. Plants that have quit flowering and have no pods, or very few pods, will not benefit from the rain. Soybeans that have reached the R7 stage (beginning maturity - leaves start to turn yellow and drop and at least one normal pod on the main stem has reached its mature pod color) will not show a yield response to the rain. The closer the seeds are to R7 -- the more mature the seeds, the less yield response there will be to the rain.
Situations in which soybeans should respond to the rain
On the positive side, some of the soybean crop had the potential to benefit from the rainfall. Plant heights and vegetative development varied, but in some areas canopies were still intact and functioning at the time of the rain. Even with functioning canopies, however, plants this year are generally shorter and have fewer nodes than normal. The latest state yield average is forecast at 22 bushels per acre. If you assume 3,000 seeds per pound, an average of 2.2 seeds per pod, and 100,000 plants per acre, 18 pods per plant will produce 22 bushels per acre. A relatively small plant is capable of filling 18 pods.
During reproductive development, the soybean plant can respond to favorable growing conditions by increasing seed size, increasing seed number, or both. By August 24, most of the soybeans in the state were nearly finished flowering, or had already finished flowering and were in the seed filling stage. The potential to increase seed number is either severely limited or not possible by this time. However, for beans in some stage of pod fill between R3 and R6 the rain can help the plant continue seed fill, minimize pod abortion, and increase seed size. Increasing seed size cannot totally compensate for severe losses in pod set and seed number, but the soybean plant is capable of producing slightly larger seeds (perhaps 10 to 20% larger than average). This can help compensate for some reduction in seed number.
Some soybeans were in the flowering and pod set stage at the time of the rain. For the most part, these are double-cropped or full-season maturity group V varieties. These beans have the potential to respond to the rain by retaining and filling pods, thereby increasing seed number. The question then becomes whether plants that just now set pods will have enough time to develop mature pods before the first hard freeze in the fall. In general, it takes a soybean plant about 50-62 days from pod set (R3) to full maturity (R8). At Parsons, in southeast Kansas, the average first frost date is October 23, if pod set began on August 24, this will provide for about 61 days of pod fill before the average first fall freeze.
Soybeans that did not receive rain on August 24-26
Unfortunately, some areas of Kansas got very little rain on August 24-26. Rain is falling in some of these areas of east central Kansas today, August 31. It may or may not be too late at this point. The potential for plants in these areas to respond to rainfall at this time or in the future will again depend upon the plant health and stage of plant development of the crop.