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Another Weed Standing Up to Herbicide Put-downs

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Written Tuesday, March 18, 2003  

Another weed standing up to herbicide put-downs

Judicious use of chemicals in Ohio crop fields is becoming more important as the list of herbicide-resistant weeds continues to grow.

Ohio State University Extension weed scientists have identified another ALS-resistant weed. Common lambsquarters makes the ninth confirmed weed species that has gotten the upper hand on acetolactate synthase-inhibitor herbicides, commonly referred to as ALS-inhibitors. This herbicide site-of-action group kills weeds by preventing the plants from producing essential amino acids that are needed for proper growth and development.

"Growers must understand their herbicide site-of-action and how it works, and change herbicides frequently enough to delay the onset of resistance," said Jeff Stachler, an Ohio State Extension weed scientist. "You can't stop resistance from happening, but the frequency of the selection pressure depends on how often and how quickly weed populations will change."

ALS-resistant common lambsquarters was found in Putnam County, Ohio, and the potential exists for additional resistant lambsquarters populations throughout the state, Stachler said. Common lambsquarters, a summer annual, can be a problematic weed, particularly in fields where manure has been applied in no-till corn and soybean fields.

Greenhouse studies have shown that the resistant population can survive two times the normal application rate of Harmony GT, one of the most common post-emergence ALS-inhibiting herbicides for control of common lambsquarters. The weed also shows some tolerance to Raptor, another commonly used ALS-inhibiting, post-emergence herbicide.

The resistant weed, however, does not exhibit cross-resistance. That is, it has not developed resistance to different herbicide chemistries within the same herbicide mode of action. This means that options may remain open to use other ALS-inhibiting herbicides to control common lambsquarters.

"The fact that growers continue to rely on post-emergence herbicides to control weeds is part of the problem with the development of resistant weeds," Stachler said. "If growers would incorporate pre-emergence herbicides into their weed control program, then they might have fewer problems with resistant weeds."

Pre-emergence herbicides make up nearly half of the herbicide site-of-action groups. Some of the pre-emergence herbicides for soybeans that control common lambsquarters include Prowl, Sencor, Valor, Authority and Command Xtra.

"With pre-emergence herbicides, there are so many options available that will work to get the most common lambsquarters under control," Stachler said. "But growers feel that it is less costly to apply a single application of post-emergence herbicides than to make a pre-emergence and a post-emergence application. Plus, they can identify which herbicides work the best with a post-emergence application."

Stachler emphasized, however, that if growers don't make changes to their weed control programs, the list of herbicide-resistant weeds will continue to expand and control options will become less abundant.

Common lambsquarters joins the ranks of other Ohio ALS-resistant weeds such as smooth pigweed, shattercane, giant ragweed, common ragweed, marestail or horseweed, Powell amaranth, common cocklebur and waterhemp. Kochia, in the same family as lambsquarters and found in limited areas in Ohio, is suspected to be ALS-resistant, although it is not yet confirmed.

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