Calves are fed milk until they are 8-9 weeks old. A young female cow is called a heifer. Before milking machines were invented in 1894, farmers could only milk about 6 cows per hour. Today, farmers use machines to milk more than 100 cows per hour. Cows are milked for an average of 3-4 years.
Cows are ruminants, which are cud chewing mammals.
Sheep and camels also are ruminants. The best cows give over 25 gallons of milk each day. U.S. cows give an average of 2,000 gallons of milk per year. A cow's heart weighs about 5 pounds and must pump 400 gallons of blood through the udder to produce 1 gallon of milk. Cows have stomachs with four compartments.