Five-Way Resistant Weeds

One weed is threatening to overtake farms with its resistance to not one but five powerful herbicides.

Waterhemp is the first broadleaf weed to become resistant to all five herbicide classes. It all started in 1997 with ALS-inhibiting herbicides. The triazines were next, followed by PPO-inhibitors, then glyphosate. Now, HPPD-inhibiting herbicides have been added to the list of herbicides to which waterhemp (Amaranthus tuberculatus) has shown resistance. It's the first broadleaf weed to become re-sistant to all five herbicide classes, prompting University of Illinois weed scientist Patrick Tranel to describe waterhemp as "evolution in action."

A Looming Threat

Aaron Hager, a University of Illinois Extension weed specialist, sees multiple herbicide resistance as a rising trend among corn, soybean and cotton growers throughout the U.S. "Waterhemp has the potential to become an unmanageable problem with currently available post-emergence herbicides used in conventional or glyphosate-resistant soybean," he says.

"Waterhemp has the potential to become an unmanageable problem." University of Illinois Extension weed specialist Aaron Hager.

Look to Best Practices

Faced with waterhemp resistance, best practices can help. The Herbicide Resistance Action Committee (HRAC) has put together a list of scientifically sound guidelines for managing resistant weeds. 

  • Start with a clean field, and control weeds early by using a burndown treatment or tillage in combination with a pre-emergence residual herbicide as appropriate.
  • Apply Integrated Weed-Management practices. Use multiple herbicides with varying modes of actions.
  • Use the recommended herbicide rates and proper application timings.
  • Scout fields after herbicide application to ensure control has been achieved. Avoid allowing weeds to reproduce. 
  • Use cultural practices such as cultivation and crop rotation when appropriate.
  • Clean equipment between sites.
  • Use good agronomic practices that enhance crop competitiveness.

While sharing these fundamental recommendations, the Crop Science initiative, Respect the Rotation, places a special emphasis on herbicide diversity as a method of preserving the utility of herbicide technology. Specifically, Respect the Rotation recommends that growers:

  • Rotate crops. Crop rotation provides an opportunity to diversify herbicide use.
  • Rotate herbicide-tolerant traits. Alternate herbicide-tolerant traits or use herbicide-tolerant stacks for more efficient rotation of both nonselective and selective herbicides.
  • Rotate Modes of Action (MOA). Rotate MOA and use multiple MOA to reduce the selection pressure caused by overusing a single MOA.

Together, the LibertyLink® trait with Liberty® herbicide provides an effective tool to manage resistant weeds, enabling nonselective herbicide rotation in multiple crops.

Controlling Waterhemp

Respect the Rotation also incorporates the latest advances in trait and herbicide technology. Liberty herbicide, for instance, delivers nonselective postemergence control of broadleaf and grass weeds, including weeds resistant to glyphosate and multiple herbicide classes. The LibertyLink trait, when added to top-performing soybean, cotton or corn seed, gives crops a built-in tolerance to Liberty herbicide. Together, the LibertyLink trait with Liberty herbicide provides the most reliable weed management solution, enabling nonselective herbicide rotation in multiple crops.  Likewise, Corvus® pre-emergence herbicide offers an excellent option when using a burndown treatment or tillage with a pre-emergence herbicide, as the HRAC recommends.

For more information on managing resistant waterhemp, contact your local Crop Science US representative.

Works Cited

  • "HRAC Stewardship Guidelines for Herbicide Labels." HRAC website > Home. N.p., n.d. Web. 25 July 2011.
  • "Evolution in action: Weed scientist explains waterhemp resistance to herbicides | Science Codex."  Science Codex | Science news, science articles, all day, every day. University of Illinois at Urbana-Champaign, 5 Aug. 2009. Web. 25 July 2011.
  • AgroNews. "Attacking Waterhemp Resistance." N.p., 22 Mar. 2011. Web. 25 July 2011.
  • "Waterhemp Resistance to HPPD-Inhibiting Herbicides Confirmed | Issues content from Corn and Soybean Digest." Corn and Soybean Digest Home Page | . N.p., n.d. Web. 25 July 2011.

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