Exclusive contracts (often called "captive supplies") between processors and farmers are in increasingly important feature of modern agriculture. We study an interesting empirical regulatory occurring in markets that feature both contract and spot exchange: the spot price is inversely related to the incidence of contract use in the market. We use a spatial model and a noncooperative game approach to show that processors can use exclusive contracts to manipulate the spot price in certain situations. Captive supplies in these settings represent geographic buffers that reduce competition among processors. However, in markets where the spatial dimension is less important, captive supplies are ineffective as barriers to competition because firms have incentive to "jump" across a captive supply region to procure the farm product.