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

Studies were conducted in growth chambers to characterize absorption and translocation of C-glyphosate applied alone or in mixture with MSMA in browntop millet and Palmer amaranth. MSMA antagonized activity of glyphosate in both weed species. Absorption of C-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 of C-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. Nomenclature: Glyphosate; MSMA; browntop millet, Brachiaria ramosa (L.) Stapf PANRA; Palmer amaranth, Amaranthus palmerii S. Wats. AMAPA.