Rations for dairy cattle are currently balanced to meet needs for energy, protein, vitamins, and minerals. While individual vitamins and minerals are considered, energy and protein terms are generally treated in aggregate, i.e. requirements have not been defined for the nutrients comprising those terms. A number of examples can be drawn from the literature demonstrating that milk yield and composition can be affected by varying the type of energy fed even when diets are isocaloric. While progress has been made towards the goal of balancing dairy rations to meet individual amino acid needs, we still have not achieved the level of sophistication realized for other commercial species. Although significant efforts have been undertaken to describe ruminal metabolism, ruminal output predictions have been largely channeled into aggregated descriptions of postabsorptive metabolism that assume constant fractional conversions of energy and protein to milk. If further progress is to be made, ruminal predictions must be matched with postabsorptive equations that track the key metabolites through that system to the tissues of interest. In that manner, the effects of individual metabolites on endocrine status and tissue metabolism can be more accurately reflected. To succeed in this latter effort, a quantitative understanding of the post-absorptive tissues is required. The splanchnic tissues are critical components of the postabsorptive system as they mediate absorption of nutrients and play a role in regulation of metabolite availability to the remaining postabsorptive tissues. The pertinent questions related to their metabolism are: 1) what proportion of each metabolite presented to the tissue is utilized by the tissue, and 2) is the proportion used constant across diets and physiological states?