Milk output has been manipulated by dietary methods for many years, however the underlying mechanisms for changes in milk composition are as yet unclear. In an attempt to further understand these mechanisms we have compared the mammary uptake of metabolites on two isoenergetic diets with different protein levels expected to provide different amounts of amino acid to the mammary gland.

Early- to mid-lactation Friesian cows were used to examine the effect of increased dietary protein supply on the supply of metabolites to and uptake by the mammary gland in relation to milk protein synthesis. Two barley based concentrates were formulated to contain 20.8 (Cl) or 29.1 (C2) g N/kg DM, using fishmeal (Provimi 66, high UDP) as the supplementary protein and fed with grass silage (28.3 g N/kg DM) at a total dry matter intake of 17 kg/day. In Experiment 1 these diets were fed at 50:50 (concentrate:silage) to four lactating cows fitted with rumen and duodenal cannulae in a simple crossover design.