Simple modifications of a crow's environment or habitat (cultural control) can be effective ways to disperse winter roosts.
Crows like to make their roost sites in large trees with many branches (perches) because they offer protection from the elements. You can make these roosts undesirable by thinning branches from individual trees, or by thinning thick stands of trees.
To protect your agricultural crops, you may want to provide an alternative food source that is more palatable and easier to get than the crop that is being damaged. One example is whole corn (softened by soaking in water) spread throughout a field to protect newly planted corn seedlings.