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 b e expected on the basis of the individual activities of the herbicides . Under the assumption of similarity of response curves and by incorpo rating a function that can model the shape of isoboles, we can statist ically test whether divergence from the Additive Dose Model (ADM) is s ignificant. 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. Mixtur es of salt formulations of MCPA and mecoprop-P followed ADM, whilst es ter formulations of the same compounds showed synergism. To get reliab le estimates, the model requires mixture ratios covering the whole iso bole (C) 1998 SCI.