Nine Holstein Friesian cows, with an average live weight of 520.8 kgm. 1146 lbs., and an average milk production, for one period of lactation, of 5.356 kgm. 11,783 Ibs., transformed 20.96 per cent of their feed energy into milk energy during a period of lactation averaging 313 days in length; and transformed 18.68 per cent of their feed energy into milk energy during a calendar year. The most efficient cow converted 23.35 per cent of her feed energy, and the least efficient cow 18.04 per cent of her feed energy, into milk energy during the period of lactation.