Good question that I honestly don't know the answer to. I assume it would be the responsibility of the home club, but I really am not sure. MLB Rule 3.01© Before the game begins the umpire shall: © Receive from the home club a supply of regulation baseballs, the number and make to be certified to the home club by the league president. The umpire shall inspect the baseballs and ensure they are regulation baseballs and that they are properly rubbed so that the gloss is removed. The umpire shall be the sole judge of the fitness of the balls to be used in the game; Sounds like the home team. I feel like if my mom were an owner, the balls would never get tossed into the stands or discarded by the catcher. "That ball's still good, no reason to throw it out!"