Another ALS-resistant weed identified

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COLUMBUS – 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.

Must understand. “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.”

Stachler said ALS-resistant common lambsquarters was found in Putnam County, and the potential exists for additional resistant lambsquarters populations throughout the state.

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.

Twice as good. Greenhouse studies have shown that the resistant population can survive two times the normal application rate of Harmony GT, one of the most common postemergence ALS-inhibiting herbicides for control of common lambsquarters.

The weed also shows some tolerance to Raptor, another commonly used ALS-inhibiting, postemergence 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 postemergence herbicides to control weeds is part of the problem with the development of resistant weeds,” said Stachler. “If growers would incorporate preemergence herbicides into their weed control program, then they might have fewer problems with resistant weeds.”

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

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

Joins the ranks. Common lambsquarters joins the ranks of other Ohio ALS-resistant weeds such as smooth pigweed, shattercane, giant ragweed, common ragweed, marestail (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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