The paper examines the possibility of lock-in on the area contracted under an agri-environmental contract in Dutch dairy farming, using a mathematical programming model, and the interaction of these contracts with Dutch national manure policy. Stricter manure policies increase contract participation, since more restrictive N application standards lower the opportunity cost of contracting. If contract payments are halved in a later period, 95 per cent of the contracting farms in the model would like to alter their contracting decision but they do not because of the cost of grassland renewal (switching cost). These farms are locked-in. The model incorporates time, transaction cost and technical and institutional constraints.