I would be a little afraid of using a 3100++ pounder; but most mature bulls can cover heifers just fine. The important thing is their birth wt, birth wt EPD, and Calving Ease EPD (if the breed has one). With cattle he is not up there long enough to do much damage to a heifer who is 65% of her mature weight or better. In hogs on the other hand, where "the act" can take ten minutes or more, we try to match younger boars with gilts.