I don't know if it is a distinction that matters, however, I was under the impression that Tyson et al., SELL the chicks at a set price to the farmer; then, offer to buy back all the chickens that are X days old after that, that meet their standards, whatever they are.

This is Tyson's overview of their contracts: http://www.growwithtyson.com/overview-of-contract-poultry-fa...