Before any person, firm, association or corporation turns upon a range area in this state any female cattle of breeding age of more than fifteen in number, they shall procure and turn with said female breeding cattle one registered bull of recognized beef breed for every forty females or fraction thereof of twenty-five or over. All persons running cattle in common on any range area may, however, agree to any other proportion of bulls to female cattle of breeding age as they may deem appropriate for their area.