Net Energy Value of Feeds for Lactation

Results of 543 energy balance trials with lactating cows were summarized to partition the energy required by lactating cows into maintenance and production components and to determine the irtfluence of energy source on the efficiency with which dietary energy is used for milk production.The-total energy requirement for lactating cows was expressed in net energy for milk production (NEmilk).The amount of energy required for maintenance of a uonpregnant, lactating cow in body energy equilibrium and ingesting a diet of optimum protein content under conditions of limited physical activity was 73 kcal NEmi~k per kg .r5body weight.The amount of net energy required for milk production was .74Mcal NEmilk/kg 4% fat-corrected milk.The NEr~ilk of individual diets was related to the concentration of digestible energy (DE) in the diet by the equation: NEm~1 k (Mcal/kg DM) = .68DE (Mcal/kg DM) --.36.Variation in energetic efficiency of metabolizable energy for milk production was much less than for digestible energy.Metabolizabflity of diets was not a constant proportion of digestible energy but was related to concentration of DE and to feed intake.