Responses of cows to abomasal infusion of lysine and methionine at two levels of dietary protein

The response in dairy cows fed high-concentrate diets to abomasal infusion of lysine (Lys) and methionine (Met) at two levels of dietary CP was examined. Four multiparous Israeli Holstein cows (day in milk=180±30, means±SE) were utilized in a 4x4 Latin square design experiment that included a 2x2 factorial arrangement with 18-d periods. Cows were surgically prepared with abomasal cannulae and catheters implanted in the costoabdominal artery. Two diets were composed to contain high and low crude protein (CP) content (152 vs 132 g/kg dry matter). Abomasal infusion of either water or Lys (38 g/d) plus Met (14 g/d) was performed with each diet. On the last day of the experimental period the metabolism of amino acids (AA) across the mammary gland was monitored. Dry matter intakes and milk and protein yields were not affected by either dietary CP level or postruminal infusion of Lys plus Met and averaged 15.9, 21.4, and 0.694 kg/d, respectively. Milk fat content and yield were not affected by dietary CP concentration, but did increase with abomasal infusion of Lys plus Met (33.3 vs 37.2 g/kg; P<0.04, and 0.703 vs 0.762 kg/d; P<0.05, respectively). Arterial plasma concentration of Lys and Met increased by 2.4- and 3.5-fold, respectively, when these AA were infused abomasally. Net mammary gland uptake of both AA increased during the abomasal infusion in the low CP diets but not in the high CP diets. The ratio of uptake across the udder of these AA to their corresponding output value in milk, suggested that there was no shortage of supplying Lys and Met from dietary intake. It was also apparent that increased uptake of Met across the udder was accompanied with higher fat secretion in milk.