A range of plant and environmental variables is known to influence the efficacy of herbicides. This paper explores whether environmental factors influencing efficacy of a herbicide can be quantified by analysing a set of industry data involving 59 experiments conducted throughout Australia in the years 1986'1995 for clodinafop-propargyl on Avena spp. A spline method was used to analyse the combined data set of observed and interpolated covariates. In addition to dose, it was found that efficacy was significantly influenced by maximum temperature on the day of application, spray water volume, the interaction of maximum temperature and spray volume, the sum of minimum temperatures experienced in the 7 days prior to application, and the soil moisture deficit estimated for day 10 prior to application. The findings are discussed in relation to testing of new products for providing commercial factor-adjustment information as an additional, marketable outcome of existing product testing procedures. Advantages of the spline model over the commonly used log'logistic model for evaluating dose'response and factor-adjustment relationships are presented.