Multiple mode-of-action resistance

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Weeds resistant to individual families of chemistry are commonplace and have been for more than 20 years, but how can farmers best manage the challenge when a weed is resistant to more than one herbicide?

Multiple-mode-of-action resistance is a challenge farmers could be facing when it comes to tough weeds like waterhemp and ragweed. Whether it is ALS or glyphosate, farmers are finding weeds that are resistant to multiple herbicide modes of action, making management a bigger challenge.

When a herbicide is used on any given weed population, there may be a few plants that have a natural resistance to it. When a herbicide is used in a repetitive manner without other herbicides or management tactics, farmers may be selecting for the weeds carrying the resistance, even if they don't know it at the time. Once this has developed, you have to manage around the issue as if it were a "new" weed requiring different control practices.

"Farmers throughout the Midwest have been dealing with weeds that have resistance to products like ALS herbicides and atrazine," says Dr. Rick Cole, weed management technical manager at Monsanto Company. "In many cases, they have been either reducing the use of the herbicides or using other tools, but the resistance still can be found."

The fact that resistance issues typically don't go away puts even more importance on the proactive management of emerging resistance issues today.

Fortunately, the answer both in theory and in practice is fairly straightforward - using integrated management practices, including multiple modes of action, is the most effective way to manage current resistant weed populations and reduce the risk of development of additional challenges.

That approach had been more easily achieved in the past when new chemistry classes were being introduced to the marketplace on a regular basis. But, as the discovery and introduction of new chemistry has slowed, farmers are going to have to find new ways to use the tools they already have.

"By using multiple herbicide modes of action to control problem weeds growers are being both proactive and reactive," says Dr. Aaron Hager, University of Illinois associate professor of weed science. "This herbicide rotation is just one of the control tactics that we are recommending."

Research conducted by weed scientists in the 1990's shows that the combination of herbicide application - soil applied and post emergence - and mechanical cultivation where feasible, was the best approach for managing weed biotypes with multiple resistance profiles. Universities, like the University of Illinois, are seeing that this approach still holds true and are recommending it to farmers.

"There isn't a silver bullet for growers," says Hager. "Growers have to track their herbicide applications, scout fields, plan for several years down the road and integrate some more traditional cultural and mechanical practices to stay ahead of resistance."

Hager, like weed scientists across the country, is encouraging farmers to plan for the long term on their weed control, focusing on the three-to-five year target. This long-term planning will help farmers not only better manage the issues that they are facing, but track how different weed management approaches work.

For farmers facing the challenge of multiple herbicide - resistant weeds, taking the next step of making that plan can be daunting. Should plans only be created for "problem" fields or for every acre? With everything else a farmer has to do between harvest and planting, is it really worth the effort?

The answer is a resounding yes.

"Growers should work with their local extension experts, crop consultants or ag retailers," says Hager. "The return on managing resistance issues goes far beyond the crop in the field to future harvests for a grower and his neighbors."

Monsanto agrees and created Roundup Ready PLUS™ Weed Management Solutions to help farmers stay on top of the resistance issues and make developing long-term resistance management plans a little easier.

"Managing resistance to a single mode of action can be a challenge," says Cole. "And managing for multiple resistance issues can be achieved; Roundup Ready PLUS is designed to make it easier."

A combination of agronomic information and recommendations, Roundup Ready PLUS incentivizes farmers for diversifying their weed management programs and thinking long term.

"We completely support the message that our university research and extension collaborators in the field are delivering," added Cole. "Successful resistance management is about long-term planning and diversifying your approach to a particular field and issue. Roundup Ready PLUS makes that easier for farmers."

Farmers who are looking for more information or assistance in developing long-term weed management plans are encouraged to contact their local extension expert. Additionally, there are online tools at with agronomic and product information designed to help farmers develop a comprehensive weed management program.

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