The current Northern American feeding system for dairy cows suggests that new feeding systems calculate milk energy requirements as individual requirements for milk fat, protein and lactose. Extant models from the literature provide efficiencies that are unrealistically larger than the partial efficiency of utilizing dietary metabolizable energy (ME) for milk production (k_L) or require inputs that are often not available to nutritionists when formulating diets. The objective of this study was to develop a model to estimate partial efficiencies of simultaneously synthesizing milk fat, protein and lactose from dietary ME and body stores. The model was developed as a multivariate mixed effects model and it was structured such that the partial efficiencies of synthesizing individual milk components additively determined k_L and the partial efficiency of utilizing body stores for milk production (k_T). The new energy balance function described the trivariate response vector comprising milk fat, protein and lactose energy outputs as functions of the animal’s ME intake and tissue energy balance. The model was fitted with 930 energy balance records from 244 Holstein lactating cows. Measurements were conducted through indirect calorimetry at the former USDA Energy Metabolism Unit at Beltsville, Maryland. The partial efficiencies of synthesizing milk fat, protein and lactose additively determined a k_L of 0.62 (0.009) and a k_T of 0.80 (0.020) which are in good agreement with the partial efficiencies adopted by the NRC (2001). The net energy requirement for maintenance (NE_M) estimated with this model was 0.34 (0.015) MJ/kg BW^0.75 which is virtually identical to the NE_M adopted by the NRC (2001). The determination of the ME required to synthesize each milk component may be further evaluated with the estimation of the fraction of the ME intake directed to each milk component and through the estimation of the partial efficiencies of individually synthesizing each milk component. Therefore, the estimated partial efficiencies can be directly implemented in the current Northern American net energy system.