Greenhouse studies were conducted to determine the effect of both ester and amine formulations of MCPA on the phytotoxicity of imazamethabenz applied to wild oats (Avena fatua). The MCPA ester antagonized activity of the liquid concentrate (LC) formulation of imazamethabenz but not the suspension concentrate (SC) formulation of imazamethabenz when the combination was applied to wild oats at twoto three-leaf stage without tiller. The MCPA amine antagonized the efficacy of both formulations of imazamethabenz on wild oats at the twoto 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 MCPA ester on the phytotoxicity of imazamethabenz, Imazamethabenz-SCcan be tank mixed with MCPA ester but should not be tank mixed with MCPA amine. Imazamethabenz-LC should not be tank mixed with either ester or amine of MCPA because of antagonism, especially when herbicides are applied at early stage. When MCPA was applied at intervals of from 2 to 24 h following applications of imazamethabenz-LC, there was no antagonistic effect of MCPA and the same level of phytotoxicity was produced as with imazamethabenz-LC alone. In the reverse sequence of MCPA followed by imazamethabenz-LC, the greatest antagonism of phytotoxicity of imazamethabenz-LC occurred when 2 h separated the two applications. This effect was lessened when the elapsed time increased.Effet du MCPA sur I'efficacite de I'imazamethabenz-methyle sur la folle avoine fAvena fatua L.)

Abstract. Experiments were conducted in greenhouse, growth chamber, and laboratory conditions to determine the effect of ammonium sulfate [(NH4)2504] on the phytotoxicity, foliar uptake, and translocation of imazamethabenz on wild oat. Rates of (NH4)2SO 4 up to 5% (w/v) applied with a greenhouse sprayer did not affect the phytotoxicity of the herbicide when the mix was applied at the one-to two-leaf stage. However, inclusion of 1 and 2% (NH4)2SO 4 increased the phytotoxicity of the herbicide when the mix was sprayed at the two-to three-leaf, or the three-to four-leaf stage. At 10%, (NH4)2SO 4 decreased the phytotoxicity of the sublethal dosage of the herbicide. When the herbicide was applied as individual drops to the growth chamber-grown plants, inclusion of (NH4)2SO 4 at 1% did not affect phytotoxicity as measured by shoot growth. The presence of (NH4)2SO 4 did not affect the amount of imazamethabenz retained by wild oat foliage, but it decreased [14C]imazamethabenz absorption, slightly antagonized acropetal translocation, and increased the basipetal translocation of [14C]imazamethabenz. It was concluded that application methods greatly modify the effect of (NH4)2SO 4 on imazamethabenz phytotoxicity. Herbicide absorption and translocation as determined by one method do not necessarily represent the absorption and translocation patterns when different application methods are used. Absorption and translocation were not the factors that were responsible for the observed effect of (NH4)2SO 4 on the herbicide phytotoxicity.