Approximately 72 billion liters of milk are produced each year in the united states, from 8 million cows. on average, a single cow consumes 13500 kg of corn feed each year. it required 40 mj to produce a kilogram of corn feed, which contains 20 mj of energy. there are 15 mj of energy in a single liter of milk. calculate the energy efficiency of growing corn and converting it into milk.