EFFECT OF MCPA ON THE PHYTOTOXICITY OF IMAZAMETHABENZ-METHYL APPLIED TO WILD OATS (AVENA-FATUA L)

Greenhouse studies were conducted to determine the effect of both este r and amine formulations of MCPA on the phytotoxicity of imazamethaben z applied to wild oats (Avena fatua). The MCPA ester antagonized activ ity of the liquid concentrate (LC) formulation of imazamethabenz but n ot the suspension concentrate (SC) formulation of imazamethabenz when the combination was applied to wild oats at two- to three-leaf stage w ithout tiller. The MCPA amine antagonized the efficacy of both formula tions of imazamethabenz on wild oats at the two- to three-leaf stage. When the herbicides were applied at the three- to four-leaf stage with one or two tillers, the antagonism was found only with MCPA amine and imazamethabenz-LC. In general, antagonism is most likely to occur at an early stage of wild oats. MCPA amine was more antagonistic than MCP A ester on the phytotoxicity of imazamethabenz. Imazamethabenz-SC can be tank mixed with MCPA ester but should not be tank mixed with MCPA a mine. Imazamethabenz-LC should not be tank mixed with either ester or amine of MCPA because of antagonism, especially when herbicides are ap plied at early stage. When MCPA was applied at intervals of from 2 to 24 h following applications of imazamethabenz-LC, there was no antagon istic effect of MCPA and the same level of phytotoxicity was produced as with imazamethabenz-LC alone. In the reverse sequence of MCPA follo wed by imazamethabenz-LC, the greatest antagonism of phytotoxicity of imazamethabenz-LC occurred when 2 h separated the two applications. Th is effect was lessened when the elapsed time increased.