Energy metabolism of dairy cattle

Energy metabolism research during the past 25 yr has resolved many uncertainties of energy use by lactating cows. Use of metabolizable energy for milk production essentially is unaffected by milk yield but is slightly influenced by its source. Estimates of efficiency of use for milk production (60 to 64%) are lower than earlier estimates (69 to 70%) primarily because of lower maintenance costs. Efficiency of metabolizable energy for body gain is higher in lactating (75%) than nonlactating (60%) cows. Use of body tissue energy for milk production is about 82% efficient. End products of digestion contribute to variation in efficiency of fattening and in partition of energy between milk and body gain in the lactating animal. Energy use in the growing animal is influenced by composition of tissue gained and composition of the diet. Energetic efficiency of protein deposition is apparently much lower than that of fat deposition. A substantial part of the lower efficiency of protein deposition is related to energy costs of protein turnover. Incomplete digestion of mixed diets at high intake by lactating cows and methods to predict energy partition are serious practical problems. In the short term, improved methods to predict intake effects on metabolizable energy of mixed diets will increase accuracy of diet formulations. In the longer term, methods to predict quantities of nutrients absorbed from the gut will permit a more flexible and accurate method of evaluating diets and predicting animal performance.