Maybe some herbicide labels should come with the warning, "Don't go back to the field for three weeks after spraying."
For some soybean growers, the burned, or what they call "fried" bean syndrome, is more than they care to look at.
Postemergent herbicide damage, particularly from the diphenylether (DPE) chemical group, looks worse than it is, according to research at the University of Missouri's Hundley-Whaley Research Farm.
"The DPE herbicides are trade-named Blazer, Cobra, Flexstar, etc., with some products like Galaxy and Stellar that are combinations that contain a DPE herbicide," says extension agronomist Don Null, who studied DPE damage last summer.
Null's study included drilled (10") and row (36") beans planted May 13 and June 19. The plots were sprayed with Cobra or Flexstar at the V3 (second trifoliate) stage or at the R1 stage as soybeans began to bloom. Null controlled weeds with Roundup, using Roundup Ready beans so yield would be the only factor affected by DPE applications.
When the plots were harvested, there was no statistical difference in yield between untreated checks and all other treatments, except late-planted row beans.
There, Null saw yields drop in plots treated with Cobra at the R1 stage.
"The untreated, late-planted, wide-row soybeans yielded 45.6 bushels per acre," he reports. "But plots in that same group that were sprayed with one-half and full rates of Cobra yielded 40.98 and 40.15 bushels per acre, respectively."
With just one year's data, Null is quick to not jump to any conclusions. But he does offer several educated observations.
"It appears that, with the early planted beans and the early applications, even though the beans were burned by the DPE herbicides, they had enough time to recover so yields weren't affected," he says. "In Missouri, we have a longer growing season than the northern Corn Belt, where the results might be different.
"In theory, it makes sense that wide-row beans receive more damage than drilled soybeans because a larger percentage of their total foliage is covered by herbicide spray. When you spray drilled beans, you're really only hitting the top of the canopy."
The fact that Cobra seemed to damage soybeans more than Flexstar wasn't a total surprise to Null.
"I really hadn't anticipated seeing a difference between the products," says Null. "But this year we did see a difference. With only one year's data, I don't want to draw too many conclusions about Cobra. Next year could be different."
The data does lead Null to recommend that growers avoid late applications of DPE herbicides to soybeans.
"When it's late in the season, we tend to go with full rates to try and control waterhemp or other members of the pigweed family," he says. "The odds are you're going to reduce yields and you're not going to control the weeds, anyway, at that stage. We need to continue to get better at catching weeds early."