The grower who operates the Soybean Watch ’21 field read an article on IndianaPrairieFarmer.com about how the five- to six-week dry stretch likely caused some soybean plants to cut back pods from four beans to three and went to the field to look. That’s exactly what he found. He could still find some four-bean pods in the no-till field, planted May 15, but they weren’t as numerous as he had hoped they’d be back in mid-July, when conditions looked ideal.
“There was mud on the road in mid-July there, and then the rains stopped,” says Steve Gauck, a regional agronomy manager for Beck’s, based near Greensburg, Ind. Beck’s sponsors Soybean Watch ’21.
The rains didn’t return until the end of August. “When we visited the field in late September, there were still some leaves, and even a few green leaves,” Gauck says. “The latest-maturing of the mid-Group 3 varieties seemed to still be adding size to beans inside the pod.
“However, there is no doubt that the dry weather likely took the top end off soybean yield potential in that field and many like it. With the way the season was going this year, early to mid-August was a critical time for soybean reproduction, and the rains just didn’t come.”
Where yield went
Since this field is a non-tiled silty clay loam soil, with some parts of it on the wet side, it held moisture better than some other fields, Gauck observes. The corn residue left from last year likely helped hold in moisture too. Still, at the height of the dry period — right when soybeans were continuing to fill pods in the top of the plant and fill beans in pods throughout the plant — soil moisture at the 6-inch depth reached 11% to 12% in this field, and below 10% in an adjoining conventionally tilled field. Crops begin to show signs of dry weather stress at soil moisture percentages below 20%.
There was likely some pod abortion at the top of plants, which were still trying to add pods in late July and early August, Gauck says. “When you lose the potential for more pods, you know you’re losing yield potential,” he adds.
After inspecting the field in late September, he realized that several individual pods throughout each plant aborted some individual beans. He found pods with spots for three or four beans that only contained two or three instead. One good rain sometime in early to mid-August likely would have helped hold on to many of those upper pods and individual beans in pods.
“One of the biggest factors for good soybean yields is individual bean size,” Gauck says. “Some say that size can contribute up to one-third of the yield. After opening several pods, it was evident that while soybean seed size was still average for a couple of the varieties in the field, and perhaps above average for the later variety, beans would have been bigger if more moisture was available when needed.”
Some early soybeans on nonirrigated ground with gravel at 3 feet yielded in the mid-30-bushel-per-acre range. The grower says in some dry years, yields have been in the 20s. He believes the table was set for good yields until mid-July, so there were lots of beans, but size turned out very small, hurting yield.