The Palmer amaranth, a problematic weed species in the U.S., is pictured. Photo provided by the University of Arkansas, Weed Science Program.

Palmer amaranth has been found to be resistant to herbicides, according to University of Arkansas researchers.

Additional information and resources can be found from the University of Arkansas as well as the International Herbicide-Resistant Weed Database managed by Dr. Ian Heap.

Researchers from the University of Arkansas have identified Palmer amaranth populations that survived several applications of glufosinate, the active ingredient in Liberty herbicide. Seed collected from these Arkansas populations and tested in the greenhouse showed resistance of 16 times the typical field use rate of 32 fluid ounces per acre. Glufosinate resistance has been documented in grass species, however, these findings represent the first confirmed case of broadleaf resistance to Liberty herbicide in the world.

Palmer amaranth is now present in three counties across New York state, with the most severe case located in the southwestern part of the state. Seed collected from the population in the fall of 2020 and is now being tested for resistance to commonly used herbicides like Roundup, atrazine, dicamba and Liberty. Dr. Lynn Sosnoskie, weed scientist at Cornell University, will test for resistance over the next few weeks. Field research is planned for 2021 to evaluate Palmer amaranth’s emergence, growth, competitive features, and in-field herbicide control options.

Farms should continue to monitor this new resistance and monitor their fields for Palmer amaranth during the upcoming growing season. The weed species can significantly reduce crop yields and spreads rapidly. For more information about Palmer amaranth identification, control, and prevention connect with Cornell Cooperative Extension Field Crop Specialists.