Estimates of the efficiency of transfer of L ‐histidine from blood to milk when it is the first‐limiting amino acid for secretion of milk protein in the dairy cow

The efficiency of transfer of L ‐histidine into milk protein was measured in two experiments in which L ‐histidine was infused intravenously into dairy cows eating a basal diet of grass silage and a cereal‐based supplement containing feather meal. Both experiments used Latin square designs, and infusion periods lasted 10 days. In Experiment 1, histidine was infused alone at doses of 3, 6 and 9 g day −1 . The output of milk protein increased up to the 6 g day −1 dose but fell back to the basal level when 9 g day −1 was infused. The efficiency of transfer was highest for the 6 g day −1 dose, for which the value was 0.38. In Experiment 2 the same three histidine doses as in Experiment 1 were used, but this time the histidine was accompanied by 8 g L ‐methionine, 28 g L ‐lysine and 2.5 g L ‐tryptophan, to ensure that histidine remained first‐limiting over the whole dose range. The output of histidine in milk protein ( Y ) increased linearly with histidine dose ( X ) such that Y   = 0.431 X  + 0.070 r   = 0.998; n   = 4, indicating an efficiency of transfer of 0.43. © 2001 Society of Chemical Industry