Residual Herbicide Options to Control Glufosinate-Resistant Palmer Amaranth.

Resistance to glufosinate in Palmer amaranth (Amaranthus palmeri S. Wats.) was first reported in 2021 in Arkansas. Alternative chemical control methods are highly sought to avoid the spread of this problematic herbicide-resistant weed across the southern region of the United States. This study aimed to determine the effectiveness of herbicides labeled for preplant or preemergence applications in cotton (Gossypium hirsutum L.) and soybean (Glycine max L.) to control glufosinate-resistant Palmer amaranth. The preemergence (PRE) treatments were imazaquin (0.12 lb ai/ac), pendimethalin (1 lb ai/ac), diuron (1 lb ai/ac), metribuzin (0.67 lb ai/ac), flumioxazin (0.063 lb ai/ac), saflufenacil (0.045 lb ai/ac), fomesafen (0.25 lb ai/ac), trifludimoxazin (0.045 lb ai/ac), acetochlor (1.124 lb ai/ac), S-metolachlor (1.24 lb ai/ac), pyroxasulfone (0.129 lb ai/ac), and fluridone (0.15 lb ai/ac). Irrigation in the amount of 1 inch was applied after application to ensure the activation of herbicides. A postemergence (POST) treatment with glufosinate at 0.585 lb ai/ac was applied twenty days after preemergence treatments to confirm the presence of a glufosinate-resistant accession in the trial site. A randomized complete block design with 4 replicates was used with a nontreated control for comparison. Visual control (%) was rated, and aboveground biomass was collected 6 weeks after PRE treatments. Biomass reduction (%) was calculated in comparison to the nontreated control. The lowest visual control and biomass reduction were observed in treatments that received only glufosinate POST or imazaquin preemergence due to resistance to these herbicides. Low control and biomass reduction were also detected following the treatment with pendimethalin. Besides the herbicides mentioned above, visual control and biomass reduction rates were similar for all other herbicides. Although most of the tested herbicides provided acceptable Palmer amaranth control, overlapping multiple effective residual herbicides and POST-applied options are crucial to controlling glufosinate-resistant populations. [ABSTRACT FROM AUTHOR]