The topic of green soybean stems at harvest was discussed at several of our post-harvest meetings. What makes this disorder especially frustrating is that the soybean pods and beans within those pods are fully mature and ready to harvest.
Unfortunately, all plants have a built-in mechanism that makes them want to shed their seeds on the ground when they’ve reached full maturity. This process is called dehiscence [di-’hi-s n(t)s] in soybeans and has erroneously been referred to as “shattering” by many. Regardless of what it is called, lost yield and lost income results.
There are several theories about what causes Green Stem Syndrome (GSS) but an exact diagnosis has not been found. Some researchers blame insects, diseases, foliar fungicides, nutrient deficiencies, low pH and even compaction. The bottom line, however, is that GSS has been proven to occur in the absence of all the above conditions. To make matters worse, the symptoms of GSS are not always the same. There are cases where all that is left on the plant are green stems and pods, but there are other cases where even some leaves remain on the plant. One thing that has been noticed is that it usually happens within a given geographic area or “neighborhood.” Also, GSS has never been associated with just one soybean variety from a single company.
One of the more practical explanations for this problem is something happened during the growing season, probably after flowering, to cause pods to abort or not to fill properly. This produces an abnormal build-up of carbohydrates within the plant. Once that early-season stress has gone away, the plant wants to funnel those carbohydrates to the pods for seed-fill. However, the pods are
either not there or they can’t finish the process. Carbohydrates are then trapped within the stem of the plant for a longer-than-normal period, so the stem remains green.
There really isn’t any clear way to manage or prevent this problem. One thing I highly recommend is NOT to wait to harvest soybeans that are showing GSS, but to slow the combine down as much as necessary and get them out of the field. Waiting until the stems are dry almost always costs you in lost bushels.