One of the most disturbing late-season issues can be pod splitting and/or seed sprouting in the pod. Pod splitting is most common when pods develop and seed begin to grown (R4 to R6 stages) during dry conditions and seed finish filling under wet conditions. Sound familiar. This is more-or-less what we experienced this year. Seed sprouting is usually caused by extremely wet conditions after the crop is mature and seed moisture has dropped below 50%.
What causes pod splitting? The reason is not clear, but here are my observations. Generally pod splitting happens when the crop is under severe stress, usually drought conditions up until the full-seed stage (R6). Pods are generally small due to the drought. Then rains set in between R6 (full seed) and R7 (physiological maturity). The seed grow and grow, and seem to outgrow the pods, causing them to split. Obviously, this splitting can then lead to seed quality issues. It can also open the plant up to seed sprouting,
Even if the crop does not experience the above conditions and pods do not split due to rapid seed enlargement, wet conditions after the crop is mature can lead to sprouting seed. Sprouting seed is not always directly related to the pod splitting; pods may not split until seed sprout. I’ve seen up to 30% of pods with sprouted seed when conditions are perfect for this. Although an unusual occurrence, seed sprouting can occur if soybean seed drop below 50% moisture, then increase to 50% or more moisture.
In addition, I have seen more sprouting in pods showing Cercospora blight (very dark pods). I do not understand why and could not definitively relate the sprouting to this disease. But, there appeared to be a relationship. Sprouting occurred primarily at the top of the plant where the dark pods were located. In contrast when pods were not dark, I have observed most sprouting at the bottom of the plant where the relative humidity is greater.
Usually, the number of split pods and sprouting seed is low and yield and seed quality effects are minimal. After a week of drying conditions, the sprouted seed will dry up and may fall out of the pods. At the worst, there could be some lower test weight and seed could contain more foreign material (from the dried up sprouts). However, the light seed will likely be blown out the back of the combine. If you do observe the problem and it is severe, I suggest that the air on the combine be adjusted to remove those light, sprouted seed at harvest. Too many sprouted seed in the bin could lead to rejection by the buyer.