Aaron,I think they contract with farmers to grow the wheat varieties they want. My father in law was in the tomato business, he owned and operated a greenhouse. He had a contract with the tomato processor and they would tell him what varieties they wanted. They would visit the greenhouse to make sure certain things were up to standard. But that didn't guarantee they would buy his product, they would inspect and cherry pick the tomatoes and there was one year they refused the tomatoes grown in that locality from all the growers.
