A cow & bull mate to start their life cycle again. A cow must first calve that is give birth, before she start producing milk. When a calf is born if it is a boy it is called bull and a girl it is called a heifer. The lifespan of a cow is around 12 years but it can sometimes be longer as well. A heifer is bred at around fifteen months in age. About nine months later, she has a calf weighing around 40 to 50 kg.