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Are You Actually Helping Herbicide-Resistant Weeds?

Herbicide resistance has been a big issue for corn and soybean farmers for the last few years, and those concerns have grown more recently with the increasing discoveries of glyphosate-resistant waterhemp. Rotating herbicide mechanisms or modes of action (MOA) is typically seen as the best way to stave off resistance. However, doing so may be hurting your crop more than it's helping if you're not doing it right, new data show.

To date, there are 450 documented cases of herbicide resistance in 245 different plant species around the world, with about a dozen added to that list each year. The science of resistance is a difficult study, making it tough for research to yield reliable strategies to resistance control down the road. A study completed this month encompassing 500 different field sites under a wide range of conditions sought to find out how glyphosate-resistant waterhemp would relate to to "66 variables related to environment, soil, landscape, weed community, and weed management," according to University of Illinois Extension agronomist Aaron Hager.

What did it find? Not surprisingly, resistance showed up most in the fields where glyphosate had been used three-fourths of the testing time. The same was true in fields where the same MOA were used. That one was also not much of a surprise to researchers. A third source of higher-frequency resistance came from a simple two-year herbicide rotation strategy.

"Simply rotating herbicide MOA actually increased the frequency of resistance," Hager says. "On the other hand, exposing populations to multiple MOA through tank-mixtures greatly reduced the selection for glyphosate-resistant waterhemp. A field in which 2.5 MOA per application were used was 83 times less likely to select glyphosate-resistant waterhemp within four to six years than a field in which only 1.5 MOA per application were used. Researchers stressed, though, that this strategy will work only if each component of the tank-mixture is effective against the target species. They also emphasized that effective, long-term weed management will require even more diverse management practices."

The study shows it's not the products used that fall short in their efficacy, but the overall process of using different combinations in the wrong frequency that can add to the growing resistant-weed problem. That will make it critical to examine MOA closely in whatever product you base your herbicide program on as you work to keep resistance at bay.

"The data indicate that lack of effective tank-mixtures, not lack of residual herbicides, was the best predictor of glyphosate-resistant waterhemp. In other words, each application made to a population, whether that be before emergence or after, should expose the population to more than one effective MOA. A soil-residual herbicide applied before or after planting followed by a single postemergence herbicide is unlikely to be a sustainable resistance management approach," Hager says. "Simply switching from a Roundup Ready system to Liberty Link, Enlist, or Extend is unlikely to provide a long-term solution. If glyphosate-resistant weeds are already present in fields planted with varieties containing these other traits, we will begin selection for resistance to glufosinate, 2,4-D, and dicamba, respectively, if we don’t carefully consider how to best steward these traits."

There is a bright side; factors like neighboring fields infected with resistant weeds don't contribute to your fields' herbicide resistance. Just stick to your strategy of rotating herbicide MOA, and you should be able to stay free of resistant weeds.

"One of the encouraging findings of the study was that, as previously stated, management practices were the most important predictors of glyphosate-resistant waterhemp. Proximity to infected fields, or weed population densities within a given field, were not important predictors of resistance," Hager says. "In other words, even if you have large populations of waterhemp in your field, or a neighbor with glyphosate-resistant waterhemp in their field, you can keep glyphosate-resistant waterhemp at bay if you implement appropriate weed-management strategies."

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