When breeding atleast 2 female bison cyropod any males you get at birth (unless you only get males, then leave 1) and raise all the females to adult. That way if no females can mate and no young bison around but you need the milk you can uncyro a male until to get them produce milk, its better to cryo males because if you raise the females you can produce more milk. Also you dont need to go kidnap wild bison kids so saves time too.