Herbicide Mixes Key to Control

Tue. Apr 21, 2015 4:04 PM

By Emily Unglesbee
DTN Staff Reporter

ST. LOUIS (DTN) -- The best way to stop herbicide resistant weeds is with proper tank mixes each season, according to a study of field trials released Tuesday. The results are a warning that farmers will need to use new herbicide and seed trait packages to control resistant weeds they might already have and to keep weeds from developing resistance to these new packages.

The common, and easier, weed control tactic of rotating herbicide modes of action year-to-year failed, and actually increased the frequency of weeds becoming resistant to herbicides, according to the study of 105 Illinois grain fields, conducted jointly by USDA and the University of Illinois.

Controlling resistance requires exposing weeds to a multitude of herbicide modes of action in each sprayer pass, University of Illinois weed scientist Aaron Hager said.

"Exposing populations to multiple [modes of action] through tank mixtures greatly reduced the selection for glyphosate-resistant waterhemp," Hager noted in a university news release. "A field in which 2.5 [modes of action] per application were used was 83 times less likely to select glyphosate-resistant waterhemp within 4-6 years than a field in which only 1.5 [modes of action] per application were used."

Researchers set out to test just how effective are common Extension and university guidelines on preventing weed resistance through herbicide rotation and tank mixtures, Hager said. Up to a dozen cases of new herbicide-resistant weeds are documented each year, bringing the current global total to 450 unique cases, he pointed out.

With more than 500 site-years of herbicide application data, the study examined the effect of 66 variables on glyphosate-resistant waterhemp, ranging from farmer management practices to soil type, environment and weed populations.

Farmers can take heart knowing they have the most power to determine how weed resistance develops in their fields, Hager said. No other factors, such as field weed densities or a neighbor's weed populations, had as much of an effect on the control and spread of glyphosate-resistant waterhemp as farmers' management practices.

With that power comes the responsibility to use the right practices. Simply rotating herbicide modes of action annually won't cut it, the study showed.

"The occurrence of glyphosate-resistant waterhemp was greatest in fields where glyphosate had been used in over 75% of the seasons included in the analysis, where fewer [modes of action] were used each year, and where herbicide rotation occurred annually," Hager said. "Simply rotating herbicide [modes of action] actually increased the frequency of resistance."

Each herbicide pass needs to hit weeds with more than one effective mode of action, whether it be a residual soil application or post-emergence application, the study showed.

"But the researchers stressed that this strategy will work only if each component of the tank-mixture is effective against the target species," Hager added.

Farmers will need to know which types of resistant weeds populate their fields, as well as the number of modes of action within each tank-mix in order for tank mixtures to work.

Future herbicide-resistant traits coming to market are unlikely to be a long-term solution to weed resistance, Hager stressed. Both Monsanto's new Roundup Ready Xtend system and Dow's Enlist system rely partly on glyphosate, which has been widely compromised by weed resistance in most crop fields, he pointed out.

"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," he explained.

While tank-mixing herbicides for each application proved the most effective way to manage resistance in the study, it can't stand alone, Hager added.

"[The researchers] also emphasized that effective, long-term weed management will require even more diverse management practices," he said. "Long-term, sustainable solutions to herbicide resistance are unlikely to be 'simple' or 'convenient.'."

For more information on the different modes of action in herbicides and pre-mixes on the market, see this herbicide classification guide: http://bit.ly/….

Emily Unglesbee can be reached at emily.unglesbee@dtn.com

Follow Emily Unglesbee on Twitter @Emily_Unglesbee

(GH/SK)

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