Glyphosate-Based Herbicide Programs for a Mixed Population of Glyphosate-Resistant and Glyphosate-Susceptible Palmer Amaranth

Abstract

Glyphosate‐resistant (GR) Palmer amaranth (Amaranthus palmeriS. Wats.) is an increasing problem in the southern United States. Experiments were established in 2009 and 2010 to evaluate the use of glyphosate‐based herbicide programs in a soybean [Glycine max (L.) Merr.] field heavily infested with a mixture of GR and glyphosate‐susceptible (GS) Palmer amaranth. In 2009, glyphosate applied at 0.77 followed by (fb) 0.77 fb 1.54 lb ae/acre at 1, 3, and 5 weeks after planting (WAP) provided 82 to 85% control at 8 WAP. In 2010, a hot and dry year, control with that same treatment was reduced to 23‐30%. Glyphosate tank mixed with imazethapyr at 0.06 lb ai/acre or imazethapyr at 0.06 lb ai/acre alone applied 2 WAP provided greater than 90% control in 2009, but no postemergence (POST) treatment gave greater than 90% control in 2010. Glyphosate applied 3 WAP following preemergence (PRE) applications of flumioxazin or fomesafen provided greater than 90% control in 2009, but the only PRE fb POST treatment to exceed 90% control in 2010 was s‐metolachlor + metribuzin applied PRE followed by glyphosate tank mixed with fomesafen 4 WAP. Although using PRE herbicides before glyphosate application or tank mixing POST herbicides with glyphosate can improve control in fields with a mixture of GR and GS Palmer amaranth, hot and dry conditions greatly reduces the effectiveness of such programs.