Milk you feed it for imprint will go towards its food. You really only HAVE to feed it when its food is at zero or you will be leaving it long enough for its food to become zero or if you have milk thats about to spoil you might as well feed it. Generally at every imprint we check all the babies and make sure theyre all pretty well fed and we try to kill every alpha wyvern we see and then we'll max out all their food. You can also use deadons passive healing and just feed the daedons meat and not worry about milk