A heifer is a mature female cow that has not calved, or given birth to a calf, yet. As soon as a heifer gives birth, she becomes a cow. A cow refers to a female of any age that has given birth.
Heifers are considered valuable for milk production and for breeding purposes. They can be bred when they reach 65 percent of their adult body weight, which usually happens around their first birthday. The average heifer gains 1 pound a day during the nine-month gestation period to reach 85 percent of her adult weight by the time she calves.