Many cattle ranches and stations run bulls with cows, and most dairy or beef farms traditionally had at least one, if not several, bulls for purposes of herd maintenance.
However, the problems associated with handling a bull (particularly where cows must be removed from its presence to be worked) has prompted many dairy farmers to restrict themselves to artificial insemination (AI) of the cows.
Semen is removed from the bulls and stored in canisters of liquid nitrogen, where it is kept until it can be sold, at which time it can be very profitable, in fact, many ranchers keep bulls specifically for this purpose. AI is also used to increase the quality of a herd, or to introduce an outcross of bloodlines. Some ranchers prefer to use AI to allow them to breed to several different bulls in a season or to breed their best stock to a higher quality bull than they could afford to purchase outright. AI may also be used in conjunction with embryo transfer to allow cattle producers to add new breeding to their herds