Glufosinate resistance was previously confirmed in three Palmer amaranth accessions from Arkansas (MSR1, MSR2, and CCR). Greenhouse screening results suggested the presence of multiple herbicide resistance. Therefore, this study aimed to determine the postemergence resistance profile of three glufosinate-resistant Palmer amaranth accessions from Arkansas. Field experiments were also conducted to assess preemergence (PRE) and postemergence (POST) herbicide options to control the accession with the highest glufosinate resistance level (MSR2). A dose-response assay with the three resistant accessions and two susceptible standards was conducted with the herbicides 2,4-D, atrazine, dicamba, diuron, fomesafen, glyphosate, imazethapyr, and mesotrione. The PRE and POST field experiments with MSR2 evaluated fifteen and sixteen single active ingredients, respectively. The previously described Palmer amaranth accessions carrying glufosinate resistance were also confirmed resistant to six other POST herbicides: 2,4-D, diuron, fomesafen, glyphosate, imazethapyr, and mesotrione. CCR is also resistant to dicamba. Therefore, accessions MSR1, MSR2, and CCR have evolved resistance to POST herbicides pertaining to seven sites of action. A shift towards increased tolerance to atrazine has been observed among all resistant accessions as well. Overall, field PRE treatments with atrazine, pyroxasulfone, or trifludimoxazin obtained the highest MSR2 control levels at all evaluation times and the lowest number of seedlings emerging at 3 and 6 weeks after treatment. In the POST experiment, only paraquat obtained MSR2 control levels above 90% at all ratings. The lowest number of alive MSR2 plants was obtained in POST treatments with paraquat or trifludimoxazin. Fields near where glufosinate resistance has been confirmed in Palmer amaranth will likely demand a more diverse and proactive management strategy relying on combinations of chemical, cultural, and mechanical control tactics. Future efforts should focus on sequential applications and mixture, the elucidation of all resistance mechanisms in the evaluated accessions, and soil applied dose-response.
