Effects of intravenous infusion of amino acids and glucose on the yield and concentration of milk protein in dairy cows

TO test the hypothesis that the availability of glucose or its precursors c an influence the response of milk protein concentration to the intravenous infusion of amino acids, five cows were used in a 5 x 5 Latin square design with period lengths of 7 d. The five treatments were the basal diet of gra ss silage ad lib. plus 5 kg/d of a cereal-based supplement containing feath er meal (Basal); Basal plus 4 g/d histidine, 8 g/d methionine and 26 g/d ly sine (4H);Basal plus 8 g/d histidine, 8 g/d methionine and 26 g/d lysine (8 H); and these two amino acid mixtures together with 600 g/d of glucose (4HG and 8HG respectively). Earlier experiments with this basal diet had shown that histidine was first-limiting for secretion of milli protein, followed by methionine and lysine. The yield of milk protein was increased progressi vely with the amount of histidine infused. The efficiency of transfer of hi stidine into milk protein was 0.42 for the 4H and 4HG and 0.35 for the 8H a nd 8HG treatments, and the concentration of milk protein was increased over Basal by all infusion treatments. However, milk protein concentrations wer e higher, and lactose concentrations in the milk were lower. in the absence of added glucose. Concentrations of insulin in blood plasma were not affec ted by treatment. It is concluded that, with the treatments without added g lucose, a shortage of glucose prevented an increase in lactose secretion; a nd hence limited the increase in milk yield, leading to an increased concen tration of protein in the milk.