Reduced Translocation Is the Cause of Antagonism of Glyphosate by MSMA in Browntop Millet (Brachiaria ramosa) and Palmer Amaranth (Amaranthus palmerii)

Abstract

Studies were conducted in growth chambers to characterize absorption and translocation of14C-glyphosate applied alone or in mixture with MSMA in browntop millet and Palmer amaranth. MSMA antagonized activity of glyphosate in both weed species. Absorption of14C-glyphosate in Palmer amaranth was rapid and increased with time from 11.1% at 0.5 h after treatment to 68.1% at 168 HAT. Absorption of14C-glyphosate in browntop millet ranged from 1.6% at 0.5 HAT to 39.1% at 168 HAT. MSMA in mixture with glyphosate did not affect the absorption of glyphosate. In browntop millet, only 2.8% of the applied radioactivity translocated out of the treated leaf to the rest of the plant when glyphosate was applied in mixture with MSMA compared to 10.8% when glyphosate was applied alone at 72 HAT. Similarly, in Palmer amaranth, 3.2% of the applied radioactivity had translocated out of the treated leaf when glyphosate was applied in mixture with MSMA compared to 10.6% when glyphosate was applied alone. Reduced translocation appears to be the cause of the previously observed antagonism of glyphosate by MSMA.