Weed Wreck

March 22, 2013 09:45 PM
Weed Wreck

50 species now exhibit multiple herbicide resistance

Roughly a decade has passed since the first alarm was sounded to alert farmers to the development of resistant weeds. Today, resistant weeds continue to proliferate—and some researchers are finding cases of multiple herbicide resistance within individual weed plants and weed populations in a field.

"That’s the direction we’re headed in Illinois and probably in other states as well," says Aaron Hager, University of Illinois Extension weed scientist.

It will take crop diversity, tillage and herbicide trait diversity to stop resistance

Hager says that of the 100 Illinois fields the university tested in 2011, roughly one-third contained plants that demonstrated resistance to three different active ingredients. Some demonstrate four-way resistance.

"I’ve worked with one Illinois waterhemp population in which the individual plants can no longer be controlled with glyphosate, atrazine, or ALS and PPO inhibitors," he adds.

Hager defines field-level multiple resistance as a situation in which individual weeds within a population resist control from two or more herbicides with different sites of action. Multiple-herbicide resistance at the plant level means individual plants resist control from multiple herbicides with more than one site of action.

The Weed Science Society of America reports that 50 weed species with multiple forms of herbicide resistance have been confirmed in the U.S. "Simplistic programs got us to this point, and the long-term solution won’t be poured out of a jug," Hager says.

"Even residual herbicides are not a complete answer to control resistant populations because they do not last long enough in the season to take out resistant waterhemp," he notes.

Assume the problem exists. Mike Owen, Iowa State University Extension weed scientist, also sees cases of multiple herbicide resistance and increased weed resistance in general. He tells farmers to develop control strategies on a field-by-field basis, much like planning cropping strategies.

"You can’t tell you have a resistant weed problem until 30% of the population in the field is resistant to the herbicide. By then, you’ve gone through a couple years of seeding the field with resistant plants and you’ve got a big problem," Owen says.

Farmers need crop diversity, tillage diversity and herbicide trait diversity to stop resistance, says Ford Baldwin, owner of Practical Weed Consultants LLC and former University of Arkansas Extension weed specialist.

New herbicide trait systems are in development to address weeds. While these systems will offer more options, all rely on existing herbicide active ingredients with known resistance cases, cautions Mark Jeschke, DuPont Pioneer agronomy research manager.

Dow AgroSciences anticipates that the Enlist weed control system will be available for corn in 2014, soybeans in 2015 and cotton in 2016.

BASF and Monsanto Company are collaborating on dicamba-resistant soybeans and new dicamba herbicide formulations, which could debut in farmers’ fields as early as 2014.

Baldwin encourages farmers to adopt good stewardship practices now before such new products are commercialized.

"We’ll use up these products if we don’t rotate them around," he says. "If we use them all in a diverse manner, then we keep them all alive."

You can e-mail Rhonda Brooks at rbrooks@farmjournal.com.

Weed Species Chart

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