This paper is devoted to a theoretical analysis of policies aiming at reducing the supply of agricultural produce through contractual or mandatory land set-aside. The model relies on contract theory under adverse selection: it allows us to evaluate the impact of asymmetric information about yields on the optimal price support policy and on land set-aside. Specifically, we analyze an optimal contractual policy consisting of a subsidy system which makes transfers dependent on the set-aside area and a policy of mandatory land set-aside applying uniformly to all farms and ignoring incentive constraints. Numerical simulations allow us to assess and compare the respective effects of these policies.