Dairy cattle are kept and raised specially for milking. Herds of cows are kept and are regularly mated with a bull, so that they produce calves. This keeps the milk supply going. However, most commercial dairy farms do not keep bulls because of the concern that such bulls are very dangerous when being handled. Instead, cows are artificially inseminated with bull semen that is stored kept frozen in liquid nitrogen, and is "bred" by a person who artificially inseminates cows for a living.