In parts of Kansas, there are many fields of soybeans in which the pods are brown and the seed is at 13 percent to 14 percent moisture, but the leaves and stems are still green. A hard freeze will kill the leaves and stems, but it still may take a while for the leaves to drop.
Producers can either harvest these soybeans now if the seed moisture is dry enough, or wait until the leaves have dropped. In most cases, it would be best to harvest sooner rather than later to reduce losses from shattering and lower seed quality. Harvesting beans before the leaves have dropped can be messy and gum up the combine, but at least the yield level will be maintained. Make sure harvesting equipment is sharp and in top condition, and take it slow in the field.
What caused this unusual situation this year? It’s most likely due to a combination of stress, low pod counts, and late rains.
In a normal situation, soybeans will accumulate carbohydrates and proteins in the leaves and stems up until seeds begin to form (R5). The leaves provide the photosynthates needed by the newly formed seeds as they begin filling. As the seeds continue to get bigger, their need for photosynthates will eventually become greater than what the leaves can provide through normal photosynthesis. As this happens, the plants will move carbohydrates and proteins from the leaves and stems into the seeds. This can be referred to as “cannibalization” of the vegetative tissue, but it’s a normal process. This eventually causes leaves to turn yellow and drop, and the stems to turn brown and die.
This year, however, some fields had far fewer pods than normal. Stress conditions around flowering caused flower drop or pod abortion. Corn earworms and other insects can also cause low pod counts. When pod counts are unusually low, the demand for carbohydrates and proteins by developing seeds is low enough that plants may not need to cannibalize the leaves and stems as extensively as normal. As a result, the leaves and stems retain their photosynthates longer, and can remain green even up through physiological maturity of the beans. Late-season rainfall can make the problem worse by keeping the plants alive as the seeds have dried down. It will take either a frost or a desiccant to kill the leaves and stems in this situation.
If the leaves are still green and intact when pods have turned brown and have reached 13-14% moisture, it’s almost always an indication of mid-season stress around flowering/pod set and low yield potential – at least relative to the amount of foliage produced.