To test the hypothesis that the availability of glucose or its precursors can
influence the response of milk protein concentration to the intravenous infusion of
amino acids, five cows were used in a 5 × 5 Latin square design with period lengths
of 7 d. The five treatments were the basal diet of grass silage ad lib. plus 5 kg/d of a
cereal-based supplement containing feather meal (Basal); Basal plus 4 g/d histidine,
8 g/d methionine and 26 g/d lysine (4H); Basal plus 8 g/d histidine, 8 g/d methionine
and 26 g/d lysine (8H); 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 milk protein, followed by
methionine and lysine. The yield of milk protein was increased progressively with the
amount of histidine infused. The efficiency of transfer of histidine into milk protein
was 0·42 for the 4H and 4HG and 0·35 for the 8H and 8HG treatments, and the
concentration of milk protein was increased over Basal by all infusion treatments.
However, milk protein concentrations were higher, and lactose concentrations in the
milk were lower, in the absence of added glucose. Concentrations of insulin in blood
plasma were not affected by treatment. It is concluded that, with the treatments
without added glucose, a shortage of glucose prevented an increase in lactose
secretion, and hence limited the increase in milk yield, leading to an increased
concentration of protein in the milk.