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Agronomists confirm glyphosate-resistant giant ragweed in Indiana, Ohio

Researchers have confirmed glyphosate-resistant giant ragweed populations in Indiana and Ohio.

The three Ohio fields with glyphosate-resistant giant ragweed are in central and southwest counties. Purdue Extension weed scientist Bill Johnson and Ohio State University Extension weed scientist Mark Loux urge farmers to alter their weed control strategies in 2007 to slow the development of glyphosate-resistant weed populations.

First, they recommend starting with a weed-free field at planting and using a program of pre-emergence herbicides, followed by a series of timely postemergence herbicide treatments.

Alongside, officials with Monsanto say field representatives will be available to aid field diagnosis and treatment efforts.

"We have provided in-depth training for our sales force as well as our local field agronomists on how to assist growers in making their weed management decisions next year," says Jennifer Ralston, a Roundup Technical manager with Monsanto. "We want to make sure farmers are choosing the weed control options that will minimize the risk for glyphosate resistance."

Ralston says Roundup Ready systems remain viable options for pre-emergence herbicide protection.

While giant ragweed can complicate corn production, it is a bigger problem in soybeans because there are few alternative herbicides that provide effective control. About 90% of Indiana's soybean acreage is planted to Roundup Ready varieties.

"The reason this is a problem in soybeans is because we have only four effective post-emerge herbicides for giant ragweed," Johnson says. "Those are glyphosate, Flexstar, Cobra and FirstRate. If the giant ragweed population is resistant to ALS inhibitors, we are left with only glyphosate, Flexstar or Cobra. If the populations are resistant to glyphosate and FirstRate, then we're left with either Flexstar or Cobra as a post-treatment."

"Our on-farm field research in 2006 demonstrated that resistant populations were not adequately controlled by glyphosate-based programs that have been effective in other populations," Loux adds.

Johnson and Loux expect glyphosate resistance to show up in more giant ragweed, although it might not spread as easily as it has in marestail, another problem weed.

"The wind can blow marestail seeds longer distances than giant ragweed," Johnson says. "Giant ragweed seeds are large and heavy, so we don't think seed movement is going to be a huge issue. It is unknown whether the resistance trait might be able to spread in giant ragweed pollen."

Producers have a big role to play in managing weeds to avoid glyphosate resistance, Johnson said. They should start before planting their 2007 crop, he says.

"If growers have fields with a history of poor control of giant ragweed with glyphosate, they need to change their management tactics," Johnson says. "One big key is to start out with a clean field, with tillage or an effective burndown, which includes 2,4-D."

Researchers have confirmed glyphosate-resistant giant ragweed populations in Indiana and Ohio.

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