|Koger Iii, Clifford|
Submitted to: Weed Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/24/2006
Publication Date: 1/1/2007
Citation: Burke, I.C., Koger Iii, C.H., Reddy, K.N., Wilcut, J.W. 2007. Reduced translocation is the cause of antagonism of glyphosate by msma in browntop millet (brachiaria ramose) and palmer amaranth (amaranthus palmer). Weed Technology 21:166-170. Interpretive Summary: Cotton growers across the southeastern United States routinely mix glyphosate and MSMA as a directed application in cotton, and sometimes observe reduced control of weeds as a consequence. Scientists at the Southern Weed Science Research Unit, Crop Genetics and Production Unit, Stoneville, MS, and North Carolina State University, Raleigh, NC examined the reason for the reduction in control by using a radioactive formulation of glyphosate applied with and without MSMA to browntop millet and Palmer amaranth. Glyphosate has to be translocated within the weed to work, and when glyphosate was applied with MSMA to browntop millet or Palmer amaranth, glyphosate did not translocate. An understanding of the basis of the antagonism allows scientists to both more effectively relate to growers the cause of the lack of weed control when using MSMA and glyphosate in mixture also provides a better understanding of how MSMA and glyphosate control weeds.
Technical Abstract: Studies were conducted in growth chambers to characterize absorption and translocation of 14C-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 14C-glyphosate in Palmer amaranth was rapid and increased with time from 11.1% at 0.5 hours after treatment (HAT) to 68.1% at 168 HAT. Absorption of 14C-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.