Milk is the only food of the young mammals during the first period of its life. The substances in milk provide both energy and the building material necessary for growth. Milk also contains antibodies, which protect the young mammals against infection.

There has been an enormous change since man took the cow into his service. Selective breeding has resulted in dairy cows, which yield an average of more than 6000 liters of milk per calf. Some cows can yield 14,000 liters or more.

Before a cow can start to produce milk she must have calved first. Heifers reach sexual maturity at the age of 7-8 months but are not usually bred until they are 15-18 months old.

The period of gestation is 265-300 days, varying according to the breed of cow, so a Heifer produces her first calf at the age of about 2-2.5 years.