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Tips to back off herbicide resistance

A herbicide-resistant weed or 2 here and there is inevitable in today's crop production systems. It's going to happen. Now, whether those 1 or 2 weeds turn into widespread resistance that threatens your crop's yield potential is an entirely different matter, one that poses one of the greatest challenges in raising corn and soybeans today.

There are things to do, though, to keep herbicide resistance from causing economic damage in your fields. And, it starts before you even crack open the next jug of weed-killer. Rather, it starts before the point of sale, according to a new set of guidelines from the Weed Science Society of America (WSSA). The group's leaders recently signed off on a new set of best practices for preventing and managing resistant weed.

"Today it is common to rely on repeated use of a single class of herbicides. It is clear we need a different approach if we want to protect the future effectiveness of these products, which are important tools for farmers," says Rod Lym, WSSA president in a report from the organization this week. “Many in agriculture are in denial. They seem convinced they can ignore the threat of resistance and wait for new herbicides to come along and solve the problem. Yet the discovery of new herbicide chemistries is very rare. A solutions-based approach that incorporates all the tools at hand is essential."

Key steps in the process of preventing the propagation of herbicide-resistant weeds, according to WSSA, include:

  • Requiring that product labels show each herbicide’s mechanism of action – helping growers more readily identify suitable products for a diversified weed management program.

  • Developing government and industry incentives to encourage adoption of best practices.

  • Using federal, state and industry funding to support education programs and to pursue research that will help everyone learn more about resistance.

"Using herbicides in an appropriate way as part of an integrated weed management program can mitigate resistance and preserve herbicide effectiveness for future generations," Lym says.

That all may be easier said than done if you're facing a lot of weed pressures and you want to knock them down quickly and efficiently. But, reaching for a quick fix instead of considering a more integrated approach can make a mild problem this year a monster one in the future, says University of Illinois Extension weed scientist Aaron Hager.

"Our greatest concern is when we start selecting plants with stacked resistance. That's unfortunately going to be the kind that's most common," Hager says. "If we had a weed that's resistant to glyphosate, for example, I can fix that. If I've got one resistant to 2 [herbicides], then I'm in trouble."

Such a worst-case scenario can be avoided by planning ahead, Hager says. It all starts from the seat of the combine each fall.

"Planning for resistance management has to take place after last year's harvest when you can survey fields from the combine and know where you had problems," he says. "You should also take the time to begin thinking if you're going to be more proactive from the standpoint of trying to better manage weed seeds so the selection intensity is reduced. Ask yourself 'If I don't have a trainwreck yet, what do I have to do to avoid it?'"

One key thing to keep in mind with whatever strategy you employ, Hager says, is not every product or system you choose is going to work 100% of the time. But, one will likely work better than others in your area depending on what's historically been used to control weeds and herbicide resistance.

"If you want to rotate away from a post-only program and use residual herbicides, for example, that's a good thing. Just don't assume it's going to fix the problem, because it won't," he says. "In the central Corn Belt, you're not going to have a product that's going to last the whole season. It's not the herbicide's fault."

And, even if you're not in an area where herbicide resistance in a particular weed species isn't an issue, don't just sit back and relax. Resistant weeds, Hager says, can cost you up to 30% of your corn yield, making the expenses to control weeds well worth it.

"The money you invest now is not wasted. It will return to you at some point in the future. If you never have resistance because you do something different, that's great. The most expensive year you have resistance is not 2 years after it shows up. It's the first year when you don't do anything," he says. "You can run whatever yield scenario you want to, but we know that waterhemp, for example, has probably a worst-case scenario of a 40% yield loss. It's not an expenditure, but it's money you could have, but you don't if you don't act."

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