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Farmers making plans to dodge glyphosate resistance

Jeff Caldwell 12/03/2013 @ 11:16am Multimedia Editor for Agriculture.com and Successful Farming magazine.

Glyphosate resistance among increasingly common weed pests has been a major issue for a lot of farmers in 2013. Now, a new study shows the majority of farmers are looking to change their weed-management systems in the coming year to combat this growing problem.

The number of glyphosate-resistant weed species around the country is increasing regularly, with states like Illinois and Kansas seeing as many as a half dozen resistant weeds.

"A key factor in the development of resistant weeds appears to be frequent and exclusive use of glyphosate for weed control," according to Kansas State University Extension weed management specialist Dallas Peterson. "Consequently, a key practice to prevent development of glyphosate resistance is to avoid exclusive use of glyphosate."

So, what are some of the things farmers say they're going to do in the next year to prevent future resistance development? Researchers with BASF recently conducted a survey of farmers on what they hope can prevent the problem in the future. More than two-thirds said they'd put down a preemerge application, while almost half said they'd use residual herbicides with overlapping action to knock down weeds.

"These results show that growers are beginning to understand the need for a comprehensive weed-management approach," says BASF technical market manager Greg Armel. "Growers are realizing the importance of using residual herbicides and multiple, overlapping herbicide sites of action."

K-State's Peterson recommends the following weed management strategies to minimize resistance risk:

  • Rotating competitive crops, including glyphosate-resistant and conventional crop varieties.

  • Using herbicides with different modes of action in sequence or in tank mixes where practical.

  • Using residual herbicides in the weed-control program, especially preemergence treatments.

  • Using tillage occasionally when it fits into the cropping system.

  • Applying glyphosate with appropriate adjuvants, at the proper rate and application stage, and under optimal conditions to maximize performance.

One thing to absolutely avoid, Peterson adds, is cutting application rates if you are relying solely on glyphosate. That's the quickest path to resistance.

"Evidence is building that use of lower glyphosate rates may increase the risk of weed shifts and selection for glyphosate-resistant weeds. With low glyphosate prices, it is not worth sacrificing performance by cutting the glyphosate rate," he says.

Another consideration for applying glyphosate and its ultimate efficacy: the environmental factors surrounding its application. Things like temperature and ambient moisture can be the difference between an effective treatment and one that winds up contributing to a resistant weed. So, what should you avoid when applying it?

"Environmental factors that may influence weed control with glyphosate in particular include drought, temperature, relative humidity, presence of dew, rainfall following application, and light," Peterson says. "Weed control and the speed of control with glyphosate generally increase as air temperatures increase, as long as the plant is still actively growing. However, high temperatures that place plants under stress, or when combined with dry conditions, likely will result in reduced control. Glyphosate applied during cool periods before, during, or after treatment will result in a slower plant response and may result in decreased control."

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