Herbicide doses used by farmers are often lower than registration doses. One of the reasons put forward is that field assays for registration are performed at relatively high application volumes (typically 300 L ha(-1)), whereas farmers use lower volumes (down to 75 L ha(-1) or even lower). The resulting concentration of the active ingredient(s) and the formulants is supposed to enhance efficacy. To test this hypothesis in the case of specific graminicides, we compared the efficacy of clodinafop-propargyl and fenoxaprop-ethyl on Avena sativa at two application volumes. Fenoxaprop-ethyl was more efficacious when applied in 75 L ha(-1) as compared to 300 L ha(-1); ED95 were 20.0 and 26.0 g ha(-1), respectively. By contrast, clodinafop-propargyl exhibited the same efficacy under both conditions. Studies of dynamic surface tension showed differences in behaviour between spray dilutions of the two herbicides, especially at concentrations corresponding to ED95s. After 100 ms, surface tension decrease was lower than 2 mN m(-1) for clodinafop-propargyl at both application volumes. By contrast, surface tension decrease was 7 mN m(-1) for fenoxaprop-ethyl at 300 L ha(-1), and 18.5 mN m(-1) at 75 L ha(-1). In the case of fenoxaprop-ethyl sprayed at doses used in the practice, the retention of active ingredient by Avena sativa was lower at high application volume. This study shows that in some cases, concentration of the spray dilution may increase efficacy, formulation being probably involved.