A general joint action model for herbicide mixtures

The assessment of mixture effects is usually done with isoboles which illustrate whether mixture effects are greater or smaller than would be expected on the basis of the individual activities of the herbicides. Under the assumption of similarity of response curves and by incorporating a function that can model the shape of isoboles, we can statistically test whether divergence from the Additive Dose Model (ADM) is significant. Two dose-response experiments with mixtures of either salt or ester formulations of MCPA and mecoprop-P and one experiment with tribenuron-methyl and mecoprop-P were analysed. Mixtures of tribenuron-methyl and a salt formulation of mecoprop-P showed antagonism. Mixtures of salt formulations of MCPA and mecoprop-P followed ADM, whilst ester formulations of the same compounds showed synergism. To get reliable estimates, the model requires mixture ratios covering the whole isobole © 1998 SCI