Current feed evaluation systems for ruminants include very simple models of intermediary metabolism -either single, static coeffi cients or simple equations to describe the effi ciency with which the energy and nitrogen available for absorption are converted into milk and meat. For example, the revised UK system 'Feed into milk' defi nes a fi xed effi ciency of utilization of metabolizable protein (MP) for milk production of 0·68 (Thomas, 2004) . Such a simple 'effi ciency' coeffi cient is the

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... ip of the tip of a rather large iceberg, representing the metabolism of the end-products of digestion between the point of their absorption into the animal and their use for the synthesis of milk and meat. The complex biochemistry of metabolism in the tissues of the gut (the portal-drained viscera, PDV), the liver and other internal organs, the adipose tissue, muscle and mammary gland: all reduced (a term used nonpejoratively) to single coeffi cients or simple equations. Simple and very useful -up to a point.