Bill Johnson does not mince words when he talks about the impact of weeds on this year's soybean crop in Indiana. "We have already lost yield potential on our soybean crop in the state due to weed issues encountered with the cool, wet season," Johnson says.
Johnson, Purdue University Extension weed control specialist, says yield loss can happen in two ways. First, if you put a residual herbicide down, planning to come back with a post treatment, and the post treatment was delayed or never made due to wet soils, the residual, while a big help, won't likely last all season.
"We're seeing escapes or some weeds break through in those fields," he observes. "There will likely be an impact on yield in some cases."
Perhaps the more costly way that weeds will knock bushels off the state average yield will be from fields where no residual herbicides went down before or just after planting. "Some people were planning to go all post-emergence and then the rains set in," he says. "They couldn't get applications made in time. If they did, weeds were often much larger than specified on the label and even at high rates they likely didn't get great control."
Whether they got control or not when they finally made an application, if they did, some damage was already done, Johnson says.
"We have good data to show that weeds growing with soybeans early in the season causes lost yield potential," he notes.
How much yield potential is lost will depend upon the species of weed and thickness of the stand. The Purdue University Corn & Soybean Field Guide contains tables for certain weeds that show how much yield loss to expect at various populations of weeds per foot of row in the field.
Some of the more prevalent weeds as of late aren't in the tables, but they cause yield losses as well.