While soybean harvest is about a week ahead of the 5-year average, Purdue University agronomist Shaun Casteel says many fields appear to be 7 to 10 days away from harvest due to the greenness of the stems.

"But grain moisture is as low as 10% in many of those same fields," Casteel says. "This scenario is a one-two punch to soybean producers."

Harvesting and selling soybeans at 10% moisture means that growers are losing out on the opportunity to sell 3% water weight, Casteel says. The recommendation is to harvest soybeans at or slightly above 13% moisture to maximize yield; however, green stems are tough to harvest.

"Two options exist for this scenario, he says.

"You can harvest at optimal grain moisture to capture water weight even with green stems that will likely slow harvest and potentially increase fuel expense, or harvest plants with brown stems for easier threshing at the loss of yield via water weight and potential shattering in the coming weeks," Casteel says. "There is no simple answer."

Casteel says green-stem syndrome is where pods and seeds mature, turning a harvest color and drying down while the stems remain green. He says it's not the same as “stay-green,” which is where a plant retains the green color (chlorophyll) in the stems, leaves and pods longer.

Stay-green is often associated with varietal differences, fungicide applications or growth regulator applications, Casteel says. However, viral diseases, insect infestations and environmental stresses during seed fill have been suggested as the sources of green-stem syndrome in soybean.

He says a recent study proposes that the common link among these culprits is the effects on pod retention and seed fill.¹ Researchers removed 25% and 50% of the pods at R6 (full seed) among varieties within maturity groups III, IV and V in Kentucky. The pod removal minimally affected the rate of pod maturation, but stem maturation was delayed significantly (10 to 20 days and sometimes greater) with the greatest delay in the 50% pod-removal treatments.

In some cases, the stems did not mature before the first frost, Casteel says.

Similarly, he adds that stressed-soybean plants “decide” which pods to retain and seeds to fill. A loss of pods and/or seeds reduces the demand for photoassimilates (sugars and nutrients) that are transported from the leaves. The redistribution of photoassimilates is rapid during R5 (beginning of seed fill) and continues at a slower pace through R6 (full seed).

"High temperatures and limited rain in August stressed many soybeans during the critical seed-fill period — and pod development with late-planted soybeans," Casteel says." Many pods and seeds were aborted during this photoassimilates in the leaves and the stems.

"Late-season bean leaf beetle feeding could also be stressing some soybeans. Low humidity and relatively warm temperatures in September have also provided a situation for fast grain drydown in the field. However, the stem tissue is maintained with the “extra” supply of photoassimilates and retains the green color."

Casteel says green-stem syndrome yield losses are usually related to the stresses that caused a reduction in demand (pod or seed loss). Delayed harvest often results in yield loss via grain moisture and shattering.

"Growers need to be aware that this phenomenon is occurring, so they can make informed decisions about optimizing harvest and reducing losses in yield and profit," Casteel says.

¹Egli, D.B. and W.P. Bruening. 2006. Depodding causes green-stem syndrome in soybean.