Current ration formulation schemes for calculating the feeding requirements of dairy cattle are primarily based upon empirical relationships between nutrient availability and milk production. Although these schemes can provide reasonable estimates of nutrient requirements, they are inadequate at predicting the responses in milk yield and composition to changes in nutrient availability. Clearly, a more integrated approach is required where the predictions are based on the metabolic fate of the absorbed nutrients. An ability to achieve this requires knowledge of the precursors used for milk protein synthesis and how nutrient intake influences their partition to the mammary gland for utilisation in milk protein biosynthesis.