RESPONSES IN MILK CONSTITUENTS TO INTRAVASCULAR ADMINISTRATION OF 2 MIXTURES OF AMINO-ACIDS TO DAIRY-COWS

Four Holstein-Friesian cows were used to investigate the effects of in travascular infusions of AA mixtures on milk constituents. Cows were i n wk 11 to 28 of lactation and were fed a basal concentrate (142 g of CP/kg of DM) and grass silage (149 g of CP/kg of DM) in a 60:40 ratio (percentage of DM). Cows were fed hourly, and feed intake was fixed at 95% of ad libitum intake for each experimental period. Each cow recei ved a 4-d jugular saline infusion, followed by a 5-d jugular infusion of a mixture of AA. Two mixtures of AA were used in a crossover design . The first mixture contained both the essential AA and nonessential A A found in milk protein (total AA); this mixture was infused at 400 g of AA/d. The other mixture represented the essential AA fraction only and was infused at 208 g/d. Infusion of total AA increased milk protei n concentration from 32.4 to 35.0 g/kg, and essential AA increased mil k protein concentration from 32.5 to 36.9 g/kg; milk protein yield inc reased by 87 g/d (total AA) and 143 g/d (essential AA). Intravascular administration of AA specifically stimulated milk protein concentratio n, and the efficiency with which the AA were used was higher than had been previously reported when AA supply was increased either by dietar y supplementation or by abomasal infusion.