Few people complained about soybean yields last fall. Many acknowledged privately that they had their highest-yielding soybeans ever. If your yields were more inconsistent, odds are you didn’t get as many favorable breaks with the weather.

However, those who harvested the highest yields complained more than anyone else about waiting for stems to dry down, or else combining anyway and letting the machine handle the stems as best it could. More than one farmer reported that "it was like trying to harvest rope."

Green stem phenomenon
Shaun Casteel, Purdue University Extension soybean specialist, notes that the green stem phenomenon at harvest is not new. In fact, some farmers have encountered green stems at harvest going back more than three decades. Before the condition became more prevalent, some were fooled by green stems and held off harvesting. They didn’t realize the soybeans themselves were dry enough to harvest. In some cases, beans became too dry before the farmer realized it.

The green stem phenomenon has been more widespread and more of an issue in some years than others, Casteel says. Often, he and others have traced the cause back to feeding by insects or disease issues that affect how the plant matures. The beans inside the pods may dry down while the stems are still green.

That’s not likely what was going on in most fields in 2016, Casteel says. The specialist offers another explanation.

Big yields
Casteel believes what many farmers saw in 2016 in terms of green stems at harvest was actually because soybean plants were still healthy and producing sugars through photosynthesis. The same weather conditions and favorable growing environment that set up many fields for very high yields also set the stage for stems to stay green longer than normal. 

There was plenty of rain in late summer in most areas of Indiana — too much rain in some places, Casteel notes. And temperatures averaged above normal, with warmer-than-normal nights. Favorable conditions signaled plants to continue the photosynthesis process and continue producing. As a result, many plants continued pumping nutrients into soybeans within the pod. The result was more dry matter in the pods and higher yields.

There was also a record-late first frost and killing freeze in most areas, Casteel observes. There was nothing to shut off photosynthesis in the plants. The same conditions that filled pods and packed in dry matter kept stems green once pod fill finished. The products of photosynthesis had to go somewhere, and they typically accumulated in the stems.

The ability for plants to stay healthy longer in late summer is normally a good thing, Casteel says. It’s a trait you would want to select for, not against. The green stems many saw in ’16 were simply an unintended consequence of a very favorable growing season and high yields, he concludes.