“Those soybeans won’t run today. The stems are still green.”

“Are you sure, Dad? Have you checked the beans, because the pods look brown.”

“Go check them, son, but that field is a good three days away from being ready to run.”

The son checked. Guess what? The soybeans inside turned out to be 13% moisture. In fact, the first load shipped to the elevator later that afternoon tested 12.8% moisture. Dockage for excess moisture starts above 13%, but loss of dry weight of beans sold begins below 13%. Shattering and loss of individual beans at the combine head increases below 13% as well.

“Green stems at harvest with brown pods and dry beans on the plants is a condition that has been reported for a long time,” says Seth Naeve, Extension soybean agronomist at the University of Minnesota. “We can’t give growers good guidelines on how to prevent it because we still don’t know what causes it. However, through research, we are gaining information that helps us begin to understand it better.

“It’s not necessarily a drag on yield, but it can make harvest more difficult because stems are tougher. And if growers don’t recognize that soybeans inside the pods are dry enough to harvest, then when they do harvest, low moisture content could lead to more shattering and higher losses during combining.”

Related:How soybeans pack in yield late in the season

Managing green stem

Naeve says anything a grower can do through management to prevent or lessen stress, particularly during reproductive phases, appears to increase the odds of minimizing green stem syndrome as a problem. “Any factor that reduces pod development and seed set in developing soybean plants can lead to green stem issues,” he says.

Here is the leading theory for why that occurs. As soybean plants mature naturally, Naeve notes, there is a strong draw of nutrients from leaves and stems to developing pods and the beans inside those pods. The vegetative portions are the source, and the pods, or destination, are the “sink,” or reservoir where these nutrients should end up.

When there are fewer pods or fewer beans inside pods, proteins and sugars are retained inside leaves and stems, especially stems. That likely allows stems to remain healthy and green even though the rest of the soybean plants mature.

Dean Malvick, Extension plant pathologist at the University of Minnesota, notes that green stem syndrome has been attributed to many causes over the years. His list of possibilities include viruses, low soil moisture, potassium deficiency, soybean population density, genetic mutations and insect damage.

He adds that while viruses, especially bean pod mottle virus, were thought to be associated with green stem syndrome, recent research showed no cause-and-effect relationship. Specific viruses are often not detected in plants with green stem, and plants with specific viruses often don’t get green stem.

“Many times, when we explore a possible cause, we end up back at the ‘source and sink’ theory, but without conclusive proof,” Naeve says. “Our current work is looking at stresses on soybeans and seeing how it relates to green stem syndrome.”

Naeve and co-workers devised an experiment to limit sunlight reaching soybeans at key times through artificial shading. So far, when soybeans are shaded and light is limited at key times, more green stems show up at harvest.

“We’re seeing it for shading even at R3, which is earlier than we might have anticipated. Right now, it appears that any stress which occurs from R2 through R7 could increase the percentage of green stems at harvest,” he says.

REDUCED LIGHT: This experiment looking at the impact of light on green stem was done in southern Indiana, but Extension agronomist Seth Naeve and co-workers have conducted similar trials in Minnesota.

Fungicides and green stem

Some believe applying fungicides during the season can result in green stems at harvest. However, Naeve draws a sharp distinction between what’s become known as the “stay green effect” and green stem syndrome.

“Is it truly green stem, or are plants just staying alive longer than perhaps in the past?” Naeve asks. “That is the first question to ask.

“Second, is it true that fungicides keep plants alive longer? From our work, that is not always the case. Fungicides can contribute to plants living longer if there is disease present, but that isn’t always the case. ‘Stay green’ and ‘green stem’ are simply not the same thing.”