Cattle are bred either naturally through bull mating or artificially through artificial insemination. For artificial insemination, top quality bulls are selected and their semen is collected and tested on various farms to evaluate the quality of the resultant calves. If the calves meet standards, the bull's semen can be frozen in straws and sold to farmers. When a cow shows signs of being ready to mate, an artificial insemination technician will be called and use frozen semen from a selected bull, transported in liquid nitrogen, to efficiently inseminate the cow. The cow's pregnancy can then be tested after one month.