Jody begins feeding Flag with his own rations, willing to sacrifice and deal with some hunger in order to feed the helpless creature he has taken in. The first real incident where Jody has to make a decision about what to do with Flag comes when Flag has eaten most of the seedlings of their corn crop.
Jody knows that corn is the most important crop on the farm but to placate Ma Baxter (who wants to kill Flag at once) he works extremely hard to build a high fence around the corn crop and keep Flag out. He is demonstrating that he understands the responsibility he bears and willing to work to mitigate the damage.
But when Flag leaps the fence with ease and destroys another round of corn plantings, Ma Baxter has had enough and shoots the fawn. Unfortunately she only wounds the poor creature and Jody has to bear the responsibility of finding and killing his closest companion. This is when Jody begins to truly bear the responsibility of his love of nature and natural things versus the reality of providing for his family.