Soybeans were forecast low in the August crop estimate compared to corn, both in Indiana and nationally. As far as Indiana goes, some areas received yield-saving rains in mid-to-late August, but the rainfall was very spotty, even in areas where the storm tracks were more inclined to follow a pattern across the state. Much of the state remained in a vise-grip of drought into early September.
Can rain and relief from heat now help soybean yields? One point Emerson Nafziger, University of Illinois Extension agronomist, makes is that if the plant has brown leaves, the photosynthetic factory is closed for the season. Those plants won't make more sugars because the sugar-making process that turns light into plant food can't happen without green chlorophyll, that makes plants green.
Soybeans started to turn yellow in some fields about two weeks ago in parts of Indiana. This response was likely triggered by cooler night temperatures. Nafziger acknowledged that specialists don't understand soybean signals every well. That includes the signal that tells leaves to move nitrogen to seeds and turn yellow, then drop. Still, fields that have yellowed so far, and not just in spots, have pods that tend to be relatively well filled.
In the driest areas leaves may be turning yellow or brown because plants simply are running out of water. Soybeans will be small, and in some cases, leaves may just dry up instead of turning yellow/. Don't expect good yields from such fields.
For the soybean crop that remains green or still has some green leaves, the crop is still capable of producing sugars that will help fill seeds. Ironically, though, daytime highs in the low 70s and nighttime lows as low as the upper 40s, while a relief from the heat and less stressful on water demand, are not favorable for filling seeds.
The result is yield is probably being added slowly in most of these fields. As temperatures warmed back, seed fill processes may have sped up again. The kicker is that senescence and the end of the season may not be far behind.The other factor that makes it difficult to determine soybean yields is the number of seeds per pod factor. Some healthy fields planted in early June that have received enough moisture to stay green, or at least only start to yellow in spots, may have more two-weeds-per-pod pods than normal. Only time will tell what this means when the crops are finally harvested.