Lactating dairy cows partition absorbed nutrients to milk and body components and catabolism. Nutrient catabolism results in O₂ consumption and CO₂ production which can be measured remotely and automatically in individual free-moving cows. To manipulate the partitioning of nutrients into milk vs. other sinks, a model that estimates each of the major nutrient flows would be useful. The objective of the present study was to modify the National Research Council (2001) model to estimate respiratory CO₂ output by lactating dairy cows. The predictions were compared against observations collected from 29 Holstein cows (22 to 444 DIM; 9 primiparous and 20 multiparous) housed in a freestall pen for a 29-d period, where daily DMI, milk yield and composition, BW, BCS, and respiratory O₂, CO₂ and CH₄ exchanges were monitored automatically. NRC (2001) equations were used to estimate digestible carbohydrate, fat and protein supplies from observed DMI and ration composition. Rates of incorporation of glucose into lactose, acetate into milk fat, fatty acids into milk fat, and amino acids into milk protein were estimated from observed milk and component yields. Incorporation of acetate into body fat and amino acids into body protein was estimated from the surplus of NEL between intake and production + maintenance. The balance of absorbed glucose, fat and amino acid was assumed to be catabolized, resulting in O₂ consumption and CO₂ production according to established stoichiometry. Net catabolism of glucose to acetate was assumed to occur in both the GI tract and the cow body, where observed CH₄ production determined the former rate. Average daily CO₂ production by cows on trial was 14567 ± 1700 g/d and predicted CO₂ output was 20619 ± 3234 g/d with a root mean square prediction error (MSPE) of 6729 g/d. Decomposition of MSPE indicated that 80.9% was due to mean bias indicating an overestimation of CO₂ production; 0.2% of the MSPE was due to slope bias and 18.9% was attributed to random disturbance. The modified NRC model offers a means to describe nutrient partitioning that can be applied to optimize nutrient supplementation on dairy farms.