A bio-economic linear program based on data from 20 commercial extensive sheep farms was used to predict the effects on aspects of farm management of response to CAP reform in Great Britain. The objective function of the LP was to maximise gross margin while meeting ewe energy requirements from farm grown or purchased feeds on a monthly basis throughout the farming year. Three farms were constrained by availability of home-grown grass and thus contracted under a subsidy free scenario. Just one farm justified expansion using extra labour purchased at £5/hour. The other farms remained at current flock sizes using existing unpaid labour. However, all farms adjusted their grazing regime according to the balance of land types available. These adjustments varied from month to month, reflecting the complex and dynamic pattern of interactions between resource demand and supply throughout the farming year. The implied shifts in land use have implications for the environment and for animal welfare.