Soybean fields with Palmer amaranth must start clean with tillage or an effective burndown and receive a pre-emerge residual herbicide. This is the most critical part of a Palmer amaranth soybean control program as post-emergence options are limited and variable. There are a large number of pre-emerge soybean herbicides available for Palmer amaranth control, although flumioxazin and sulfentrazone-containing products will provide the most residual activity.
Chloroacetamide herbicides, dinitroanilines, and higher rates of Sharpen-based herbicides can help as well. Post-emergence products are limited to the PPO inhibitors and Liberty (Liberty Link beans only) and the effectiveness of these products can be variable. PPO inhibiting herbicides often referred to as burners and blazers, include Cobra, Reflex, Flexstar, and Ultra Blazer and must be applied to small plants to achieve full control. Liberty in Liberty Link beans can be an effective management technique if one doesn’t want to grow continuous corn. However, you should also use residual herbicides and remember that Liberty is effective on small (2-3”) plants; the efficacy of Liberty on large plants and heavy infestations will be variable. In heavy infestations producers should overlap residuals by also applying a residual herbicide at the post-emergence application. The fomesafen products (Reflex and Flexstar), have residual properties as well as post-emerge efficacy. Other post-emerge residuals include Dual II Magnum, Outlook, and Warrant. These products will not control emerge weeds and thus should be tank mixed with a post-emerge herbicide.
Specific herbicide programs and rates will vary between soil types and production practices. Purdue Weed Science will be working to produce an in-depth Palmer amaranth identification, control and management publication in the next couple of months.