Life Cycle of a Dairy Cow After a dairy cow is born, it is weaned from milk at about 7 to 8 days old. A 6 month old cow weighs about 400 pounds and gains about 1.6 to 1.8 pounds per day. A dairy cow yearling weighs about 700 pounds and grows for a few more years. A 2 year old cow weighs about 1,200 pounds, produces milk, and recently had a calf. A 5 year old cow is fully mature. It produces about 12 gallons of milk per day and eats over 100 pounds of food per day. After 5 years, a cow is removed from the heard and sent to slaughter to make room for other cows.