Response of Glyphosate Resistant Palmer Amaranth (<i>Amaranthus palmeri</i>) to Protoporphyrinogen Oxidase Inhibiting Herbicides in Tennessee

In many agronomic cropping systems across the United States, Palmer amaranth (Amaranthus palmeri) is the most economic and troublesome weed for producers. The introduction of glyphosate resistant (GR) crops gave producers the benefit of controlling Palmer amaranth as well as other weeds, a broad window of application, and reduced tillage practices. With the confirmation of GR Palmer amaranth, producers implemented protoporphyrinogen oxidase (PPO or Protox)-inhibiting herbicides to control these populations in crops such as soybean [Glycine max (L.) Merr.] and cotton [Gossypium hirsutum (L.)]. However the continuous use of PPO herbicides has caused a shift in Palmer amaranth populations for PPO resistance. Therefore the scope of this study was to observe fomesafen response to four Palmer amaranth populations, determine the fomesafen resistance level, evaluate the effect of Palmer amaranth size on fomesafen efficacy, determine susceptibility to other foliar-applied herbicides, and evaluate the efficacy of four soil-applied PPO-inhibiting herbicides on PPO-resistant (PPO-R) and PPO-susceptible (PPO-S) Palmer amaranth populations. The PPO-S population was observed with 98% control however, fomesafen efficacy was reduced in SPA, LPA, and WPA populations with 24%, 4%, and 2% control, respectively at 14 days after treatment (DAT). The level of resistance for the PPO-R population SPA was 4-fold relative to the PPO-S population KPA. When determining the height of Palmer amaranth on fomesafen efficacy, control of SPA Sm, Md, and Lg sized plants was 62%, 49%, and 18%, respectively. Atrazine, glufosinate, and mesotrione were observed to have the greatest control (>70%) of the SPA population but resistant to glyphosate and chlorimuron. When subjected to soil-applied PPO herbicides, SPA showed reduced control with fomesafen and saflufenacil however greater control was observed with flumioxazin and sulfentrazone at 35 DAT.