Synergistic joint action of MCPA ester and metsulfuron‐methyl

Summary MCPA is antagonistic to a range of herbicides but there is limited research on its compatibility with other broad‐leaved weed herbicides. In this study we assessed the joint action of MCPA ester and metsulfuron‐methyl on Brassica napus L. using the additive dose model (ADM). The fresh weight responses for selected ratios of the mixture were plotted on the ADM and compared to the predicted mixture response at three assessment levels ‐ 50, 75 and 90% control. The mixture demonstrated increasing synergism as the response level increased from 50 to 90%. This result for MCPA ester is in marked contrast to that of the MCPA amine formulation, which was shown in previous studies to be antagonistic towards sulfonylurea herbicides. Formulation is the major deciding factor in determining the compatability of phenoxy herbicides with other herbicides.