Water extracts of leaves and stems of wild radish tested separately and jointly affected the germination and early growth of wheat and oats. Interaction of effects, antagonistic and synergic, between leaves and stems extracts were generalized and established using the Loewe model without knowing the isoeffective concentrations. The subsequent use of mechanism-dependent models evidenced multiplicative (in wheat) and additive (in wheat and oats) interactions suggesting that different phytoactive compounds of wild radish are involved in these responses and may act on wheat and oats either at distinct targets by different processes or have similar modes of action.