Palmer amaranth in two southern U.S. states now resistant to S-metolachlor

Palmer amaranth is one of the most problematic weeds in agriculture – known for its resistance to multiple herbicides of varying sites of action. An article featured in the journal Weed Science shows the tools available to control this weedy invader are narrowing yet again.

After reports that Palmer amaranth populations in Arkansas had exhibited resistance to the herbicide S-metolachlor, a team of university researchers set out to document the problem and to learn more. They conducted greenhouse studies to assess the occurrence of S-metolachlor resistance among Palmer amaranth populations in Arkansas and two other Southern states –Mississippi and Tennessee. They also evaluated the level of resistance and the susceptibility of resistant populations to other soil-applied herbicides.

Four populations of Palmer amaranth from Arkansas and two from Mississippi were found to be resistant to S-metolachlor, a Group 15 herbicide that inhibits absorption of very long-chain fatty acids (VLCFA). The level and frequency of resistance generally increased from parent plants to first-generation progeny. Resistant populations, for example, required 2.2 to 7.0 times more S-metolachlor to reduce seedling emergence by 50 percent than nonresistant populations. Up to 9.2 times more herbicide was needed to achieve the same result when treating the progeny of resistant plants.

What are the implications for weed control? Researchers say that for now, the Palmer amaranth populations resistant to S-metolachlor remain susceptible to dimethenamid-P and to pyroxasulfone, both of which are also VLCFA inhibitors. Tests show resistant populations can also be controlled by atrazine, a photosystem II inhibitor, and by flumioxazin and fomesafen, which are PPO enzyme inhibitors.   

To learn more, read the article, ″Resistance of Palmer amaranth (Amaranthus palmeri) to S-metolachlor in the Mid-southern United States″ free for one month.