Hardly anyone has thousand pound cows anymore.  If a thousand pound cow goes through the sale barn, they are usually emaciated from sickness or malnutrition.  Plugging in more realistic numbers using an 800 pound yearling heifer, a 1200 pound dry cow, or a 1200 pound cow with a 400 pound calf at side, more realistic figures would indicate that a yearling heifer would eat approximately half as much as a pair, or two thirds as much as a dry cow.