If a steer has been on feed already, then put him on as much feed as he'll eat. This fattens him up at a better rate. Although he will still take longer to finish out because he is on grass and at times will choose grass over grain. About the heifer, she can be fed full feed too until about 6-8 months of age. Then she should be gradually cut back so that she eats more grass. If you feed her ample grain, then she will deposit fat in her udder and therefore will not be able to produce as much milk.