Cover crops and residual herbicides reduce selection pressure for Palmer amaranth resistance to dicamba‐applied postemergence in cotton

Abstract

AbstractThe most widely planted cotton (Gossypium hirsutum L.) and soybean [Glycine max (L.) Merr.] varieties in the United States are resistant to the herbicide dicamba. Measures to reduce selection pressure for dicamba‐resistant weeds is paramount. Four studies evaluated strategies to reduce selection pressure for Palmer amaranth (Amaranthus palmeri S. Watson) resistance to dicamba applied postemergence (POST) in cotton. A split‐plot arrangement consisted of conventional tillage or cereal rye (Secale cereale L.) cover crop as the whole plot. The subplot included four herbicide systems: no herbicide, 3 POSTs, fomesafen plus diuron applied pre‐emergence (PRE) followed by (fb) 3 POSTs, and PRE fb 2 POSTs fb diuron plus monosodium methyl arsenate applied lay‐by post‐directed (LPD). The POST herbicide applications were glyphosate plus dicamba. The cover crop reduced Palmer amaranth density 75, 70, and 54% at the time of POST 1, POST 2, and POST 3 application, respectively. Pre‐emergence herbicides fb POST applications reduced Palmer amaranth densities 99, 99, and 96% at the aforementioned application timings, respectively. Cover crop plus PRE herbicides provided similar reductions. Cumulative for the season, Palmer amaranth exposure to dicamba was reduced 65% by the cover crop, 98% by both PRE herbicides and cover crop plus PRE herbicides compared to 3 POST applications. An LPD application reduced Palmer amaranth exposure to dicamba by 38,319 plants ha–1 compared to a third POST. Cotton stand was higher in conventional systems when soil temperatures were 30–37 °C during emergence; however, stand in the cover crop was greater when soil temperatures were 40–43 °C. Yields were reduced 14% in a total POST herbicide program, and were improved 14% in a cover crop. A cereal rye cover crop and residual PRE herbicides had the biggest impact on reducing selection pressure for Palmer amaranth resistance to dicamba while also improving yield.