by: Bonnie Coblentz
MSU Ag Communications

Mississippi State – Mississippi is one of six states participating in a study monitoring the problem of glyphosate-resistant weeds and trying to prevent any more from developing.

Roundup is the trade name for glyphosate, a powerful broad-spectrum herbicide that can kill a wide range of weeds in varying growth stages. Seed genetically modified to be resistant to glyphosate is known as Roundup Ready seed. With Roundup Ready cropping systems, producers can apply glyphosate across a field, killing weeds but leaving the crop undamaged.

But by this year's growing season, 19 weeds worldwide had become resistant to glyphosate, and five are found in Mississippi.

David Shaw, Mississippi State University vice president for research and economic development, is on a research team with five other universities performing a benchmark study of glyphosate resistance management.

The long-term study is assessing the sustainability of Roundup Ready technology for weed management in U.S. crops. The team is working to improve the sustainability of weed control systems and prevent the development of more herbicide-resistant weeds.

“The information gathered from this research study will provide university scientists with valuable data to develop and tailor effective strategies and outreach programs to improve sustained weed control in the Roundup Ready technology,” Shaw said.

Starting about five years ago, researchers gathered information from about 1,200 producers in Mississippi, North Carolina, Nebraska, Illinois, Indiana and Iowa.

“The growers from these states represent the major Roundup Ready crop growing regions of the United States and provide diversity in environments, cropping systems and weed populations,” Shaw said.

They established 25-acre test plots on about 150 of these growers' fields. One half of each plot, the grower continues the current weed management program. On the other side, the grower follows recommendations from the university weed specialist in that state.

“University scientists collect field data and soil samples each year to determine the impact of the two weed management programs on weed populations, weed species diversity, weed seed bank, crop yields and economic returns,” Shaw said. “The results of this study will provide valuable data to determine the sustainability and profitability of current growers' weed management programs compared to more diversified programs designed to lower the potential risk of selecting for weeds resistant to glyphosate.”

John Byrd, an MSU Extension weed scientist, said producers nationwide made a mistake when they started using glyphosate as the only herbicide in every major row crop.

“In the past, we used a variety of herbicides to produce a soybean crop, and then the next year rotated to a different crop and used different herbicides to control weeds,” Byrd said. “Now we're in a scenario where we use Roundup almost exclusively to provide weed control in soybeans, cotton and corn.”

Byrd said any time one herbicide or group of herbicides is used exclusively, all weeds but those with a natural resistance to the chemical will be eliminated. The remaining weeds will reproduce more weeds with the same resistance. When a variety of chemicals is used, one chemical will kill the weed that another missed, and resistant weeds will not develop.

“Weed control is a little bit like war,” Byrd said. “The more variety you have in your artillery, the more successful you're going to be at winning that war.”

The key to preventing herbicide resistance from developing in weeds is to rotate the use of herbicides with different modes of action. Mode of action is the process within the plant that the herbicide blocks to ultimately cause its death.

“Only by following that strategy will farmers be able to ensure that resistant populations don't develop on their property,” Byrd said.


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