Screening Palmer Amaranth (Ameranthus palmeri) to Confirm Resistance to Multiple Herbicides.

Palmer amaranth (Ameranthus palmeri) is commonly found in the southern states and is continuously working its way north. It is an aggressive summer annual, C4 broadleaf weed. Overuse of herbicides without rotation of modes of actions has led to an increased resistance to herbicides in this weed species. In this experiment we tested for multiple resistance to glyphosate, atrazine, ALS, mesotrione, and 2,4-D in Palmer amaranth seeds collected from Pawnee County. We compared our findings to how susceptible Palmer amaranth plants (SUS)  responded to the same rates and chemicals used on the Pawnee County population (PC-R). The objective of this study was to determine if this Palmer amaranth population is exhibiting resistance to glyphosate (EPSPS inhibitor), chlorosulfuron (ALS inhibitor), atrazine (PS II inhibitor), mesotrione (HPPD inhibitor), and 2,4-D (Growth regulator) herbicides. An experiment was conducted in the greenhouses to determine if the seeds collected from Pawnee County were truly resistance to five modes of action. At approximately 10-12 cm tall, 24 PC-R plants and 10 SUS plants were treated with a 1x dose (field recommended rate) of each of the herbicides using a sprayer chamber. Each set of 24 PC-R plants and 10 SUS plants were screened for resistance to glyphosate (EPSP, Roundup WeatherMax ®), atrazine (PS II, Aatrex 4L ®), chlorsulfuron (ALS, Glean ® ), mesotrione (HPPD, Callisto ® ), and 2,4-D (Growth Regulator, 2,4-D LV4 ® ). Percent survival data was recorded one, two, three, and four weeks after treatment (WAT). Two experimental runs of the study were completed. The results showed PC-R exhibiting resistance to glyphosate, atrazine, ALS inhibitors and mesotrione. However, this population only showed an elevated tolerance to 2,4-D and did not have high enough survival level to prove resistance.