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

yThe efficiency of transfer Of L-histidine into milk protein was measured i n two experiments in which L-histidine was infused intravenously into dairy cows eating a basal diet of grass silage and a cereal-based supplement con taining feather meal. Both experiments used Latin square designs, and infus ion 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, f or 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 h istidine remained first-limiting over the whole dose range. The output of h istidine in milk protein (Y) increased linearly with histidine dose (X) suc h that Y = 0.431 X+0.070 r = 0.998; n = 4, indicating an efficiency of tran sfer of 0.43. (C) 2001 Society of Chemical Industry.