Tri-State Neighbor: South Dakota Farm Newspaper

BROOKINGS, S.D. - The discovery of South Dakota's first Roundup-resistant weed emphasizes the need for producers around the state to manage herbicides with care.

Glyphosate-resistant common ragweed was found in a soybean field in Charles Mix County in spring 2008, and South Dakota State University Cooperative Extension weed specialist Mike Moechnig said the discovery has implications for producers across the state.

“We see resistance to Roundup, or glyphosate, becoming widespread in states such as Ohio, Indiana and Illinois. In the worst situation, a common ragweed biotype has evolved resistance to all soybean post-emergence herbicides,” he said. “We may see that in South Dakota some day, because it's already happening in other states.”

Roundup-resistant weeds will pose the greatest challenge for soybean farmers, he said.

“It's something we want to look out for, especially in soybeans, because we just don't have very many alternative post-emergence broadleaf herbicide options,” Moechnig said. “If glyphosate-resistant weed species become more common, it's the soybean crop that's going to be most problematic for controlling these biotypes.”

The soybean producer in Charles Mix County worked with his Monsanto representative, who sent seed samples to a company lab where the confirmation of glyphosate resistance was made.

Moechnig said SDSU Extension Weeds program also provides some testing for producers who suspect they face resistant weeds.

“It's something people need to look for because weed populations that escape a Roundup application are not necessarily resistant. For example, weeds like wild buckwheat and common lambsquarters occasionally escape glyphosate applications, and weeds like scouring rush (or horsetail) and primrose have a natural, almost complete, tolerance to glyphosate,” he said. “Resistant weed biotypes often start out as a patch in a field, but in some cases, resistance may not be noticed until it has spread across the field. Weed species that pose the greatest risk for evolving resistance include marestail (horseweed), common ragweed, waterhemp, common lambsquarters and kochia.”

Scouting, crop rotation, and herbicide application rotation are the best ways to combat resistant biotypes, said Moechnig.

“There are some simple things a person can do to minimize the chance of selecting for resistant biotypes, such as diversification in crop rotations and weed management programs,” he said. “In rotations of Roundup Ready corn followed by Roundup Ready soybeans, using residual herbicides not only prevents early season competition, it may minimize selection for glyphosate resistance.

“In the long run, herbicide resistance will be one of the most significant factors contributing to greater weed control costs. Preventing herbicide resistance by utilizing different crop varieties and herbicide chemistries will help keep weed management simple and affordable for several years,” Moechnig said.

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