Stewardship of Synthetic Auxins in 2,4-D and Dicamba–resistant Crops and Mitigation of Off-target Movement

Abstract

A series of greenhouse, field, and laboratory experiments were conducted to evaluate preemergence (PRE) and postemergence (POST) herbicides for Palmer amaranth (Amaranthus palmeri L.) control in addition to assess the potential for dicamba to move off-target to sensitive soybean (Glycine max L.) through tank contamination and volatility routes. Cotton (Gossypium hirsutum L.) residual herbicides acetochlor, diuron, fomesafen, fluridone, and pendimethalin were found to provide the greatest Palmer amaranth control when applied at highest use rates and activated with 1.91, 0, 0.64, and 1.27 cm of water, respectively. Furthermore, fomesafen combinations with acetochlor, diuron, fluridone, and prometryn were not shown to significantly impact cotton yield when applied up to 2x use rates as compared to the nontreated control. Field experiments in a non-crop setting indicate sequential applications of dicamba + glyphosate followed by (fb) glufosinate and 2,4-D + glufosinate fb glufosinate at seven day intervals have potential to effectively control Palmer amaranth escapes that have exceeded heights recommended for chemical control. However, control was variable among years and timely applications of POST herbicides remain the best approach. Greenhouse experiments suggest glufosinate severely impacted Palmer amaranth photosynthesis with up to 90% reductions in CO2 assimilation. Furthermore, applications of dicamba + glyphosate seven days before glufosinate could reduce regrowth as compared to the reverse sequence or tank mixture. A replicated field study and sprayer survey indicate triple rinse with water was is sufficient for dicamba removal from equipment following applications and remaining contaminants (concentrations <1.25 mg L-1) were not shown to reduce soybean yields. A field study where dicamba was applied at 0.56, 5.59, 56.42, 559.17, 5591.75, and 11183.51 g ae ha-1 to soil pans placed under sealed low tunnels covering two rows of soybeans for 48 hours resulted in visual injury for all dosages, ranging from 1 to 45% across all dosages and site-years. However, no significant soybean yield reductions were observed over three site-years as compared to the nontreated control. These results can provide guidelines for row crop producers to practice proper stewardship of synthetic auxins and aid to preserve technology for future use.