When a soybean plant is progressing normally at the beginning of the season, it produces high levels of carbohydrates to feed itself, but when exposed to drought or other stress, pods may fall off, says Shaun Casteel, a Purdue University Extension agronomist.
After the pods fall off, the plant has a pool of reserved carbohydrates and will stay green longer - known as green stem syndrome. While the condition does not harm the seed, oftentimes when farmers look into the field and see green stems, they will delay harvest beyond the optimal harvest moisture of 13-15%.
"I've seen farmers not harvest until the stems are brown, but the beans are at 9 to 10% moisture," Casteel says. "With the lower moisture content, you lose yield in the form of water."
If growers want to harvest earlier, they should combine at a slower pace because the stems will be tougher and could damage equipment, he said.
Green stem syndrome is one problem that could further decrease yields following a planting season inundated with heavy rains and a growing season hindered by heat and drought. The U.S. Department of Agriculture projected in its September crop report that soybean yields nationally could average 41.8 bushels per acre, down 1.5 to 2 bushels from trend. Indiana yields were projected at 42 bushels per acre, down 6 from what they might have been with a normal growing season.
One of the main reasons behind expected low yields has been loss of pods. With heat stress, some plants have dropped high numbers of pods; others have underdeveloped seeds within the pod. If there is rain during early seed fill stages, they might recover; however, if the plants are already dropping leaves, it is unlikely that they will benefit.
"We still have the story of two crops; one was planted on time and still has good yield potential, while the other started out late and has been further compromised by dry weather," Casteel says. "At this point, rain might be too late for much of the Indiana crop."
Insects could be a challenge for soybean growers as bean leaf beetles and stinkbugs can attack the more mature crop and move onto nearby younger beans. Farmers can scout fields to identify pests and better predict yield potential, he said.
"This is a hard year," Casteel says. "I know farmers are really hoping for a good harvest with $14 a bushel beans, but this year's yields probably won't meet that expectation."