It is a dubious honor, says Darrin Dodds, that Mississippi leads the nation in the number of glyphosate-resistant weeds — and pigweed (Palmer amaranth) is now at the top of that list, he said at the annual joint meeting of the Mississippi Boll Weevil Management Corporation and the Mississippi Farm Bureau Federation Cotton Policy Committee.

Mississippi’s weed resistance problems began developing more than 10 years ago, he says, with glyphosate-resistant horseweed.

“It was a grave concern — a lot of effort was devoted to developing burndown and other programs to address the issue, and we got the problem under control.”

Then resistance developed in other species, says Dodds, who is associate Extension professor of agronomy at Mississippi State University. They included johnsongrass, some ragweed species, and others.

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But Palmer amaranth is the one that is causing nightmares for production agriculture.

“It grows very aggressively, has a very deep root system that allows it to thrive in hot, dry weather, and it produces a tremendous amount of seeds — a single mature plant can set several hundred thousand seeds,” he says. “All this makes it an extremely competitive weed. With glyphosate being used on the majority of our crop acres, the development of resistant pigweed and the rate at which it has spread has created complex issues for agriculture.”

Further complicating the situation is that several weed species have developed resistance to ALS chemistry, which in cotton includes products such as Staple and Envoke.

“We’re seeing more and more cotton acres in Mississippi with Liberty being applied over the top, and there is concern that we may be relying too heavily on this chemistry, particularly north of Highway 6 in the Delta,” Dodds says.