Cows may produce milk for two, even three years when nursing their calves. In the absence of the calf, they dry earlier. To keep the milk flowing cows (and buffaloes) are artificially inseminated within 2-3 months of giving birth. The animal has to produce milk while pregnant. As a result, she is spent after about 4 pregnancies and must be retired (slaughtered). The average natural life span of a cow is 26-27 years yet today most live no more than 6-8 years.