Cattle in the U.S. start life on a pasture, nursing from their mothers and eating grass. When the calves are about 12-18 months old, they are transferred to a feedlot where they eat mostly grain. Grain is an unnatural diet for cows, but raising cows in feedlots is cheaper than raising them on large pastures, where they can roam and graze on grass. Because the cows in feedlots are crowded, they are more likely to become sick, and are more likely to be given routine antibiotics as a preventive measure. Cows raised in this way are also typically given growth hormones so that they can reach slaughter weight faster. Because grain-fed cows grow faster, farmers can produce more meat in a shorter amount of time. After approximately six months in a feedlot, the cattle are sent to slaughter.