How can cows make so much milk?

A cow, like all mammals, produces milk to feed its young. If its calf nurses regularly, the mother cow's mammary glands will produce enough milk to give the baby animal all the food it needs. Gradually a calf will nurse less as grass and other feed makes up more of its diet. A mother cow, in turn, will produce less milk until it is no longer needed.

But by milking the cows regularlytwo or three times a daydairy farmers can cause the cows to continue producing milk. Certain breeds of cows are particularly good at milk-making, producing 18-27 pints (around 2-3 gallons, or 10-15 liters) each day. A cow's large, round udder, located on its underside, has four nipples, or teats, that are squeezed to release stored milk. While once done by hand, milking is done on modern dairy farms by machines with suction hoses, which do the job more quickly and cheaply. Tank trucks collect milk from farms daily and take it to processing plants where it is pasteurized (made germ-free) and used to make dairy products like cheese, butter, and ice cream.