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Horseweed resistant to certain herbicides

A University of Missouri plant science research assistant says glyphosate and cloransulam, known as Roundup and First Rate, are not effective killers of horseweed.

Eric Oseland tells Brownfield horseweed has built up a resistance to certain herbicides.

“We did not find any populations that were able to be controlled with glyphosate,” Oseland says. “We’ve really lost that herbicide for controlling horseweed and even the ALS inhibiting herbicide is almost eighty-percent of the populations we screened were resistant to that as well.”

He says the most effective herbicides against horseweed are dicamba and 2,4-D.

“We have zero known resistance cases to dicamba.” Oseland says. “We do have a couple cases of some decreased control with 2, 4-D but overall ninety-eight percent of out horseweed populations in Missouri are able to be controlled with 2, 4-D.”

Two cases found resistant against 2, 4-D were in the north west corner of Missouri. Oseland says it might be a coincidence or horseweed developing a resistance to the herbicide.