Rain needed in September and August to boost corn and soybean yields this year didn’t happen, worsening a growing season fraught with weather woes.

Four weather stations in Missouri’s soybean-producing areas report less than a third of the normal September precipitation, says Bill Wiebold, University of Missouri Extension agronomist. A fifth station, in Gentry County, showed a single 3-inch rain before rain shut off, leaving northwestern Missouri as dry as the rest of the state.

Normal rainfall might have repaired some damage to late-planted soybean. Usually, September rainfall has minimal effect on soybean yield, except for double-cropped.

One million acres of soybean were not planted this year. A significant amount of acres were planted after July 1 because of a historically wet spring, Wiebold says.

Delayed planting shifted the most critical stage for soybean water need from the first two weeks of August to the first two weeks of September, Wiebold says.

“Farmers who planted soybean in July understood their crop was at risk,” he says. “But the degree of drought in August and September was highly unusual and impossible to predict.”

For good to excellent soybean yields, plants need about 1.2 inches of rain each week during grain fill. In most of Missouri, monthly average rainfall in August and September is 4 inches, which would have helped greatly this year, Wiebold says.

Claypan fields in northeastern Missouri, which were drenched by heavy rains this spring, are especially hard hit. Claypan soils restrict water drainage and reduce root depth during the growing season, making the region especially vulnerable to drought stress.

Producers report reasonably high soybean yields so far, Wiebold says, but these yields come from fields planted in a timely manner.

Soybean harvest season continues for longer than normal this year because of the wide range of planting dates. Soybean harvested in mid-to-late October will be from fields most affected by the lack of rain in late summer and fall.

For more information, go to missouri.edu.