Lactation and metabolic responses to graded abomasal doses of methionine and lysine in cows fed grass silage diets

Abstract

Five rumen-cannulated Finnish Ayrshire cows were used in two 5 × 5 Latin square experiments designed to study the lactation and metabolic responses to increasing doses of DL-Met or L-Lys infused into the abomasum. The cows were fed grass silage ensiled with a formic acid additive for ad libitum intake. A supplement with barley and oats was given at a rate of 9 kg/d (Experiment 1) or 7 kg/d (Experiment 2). The experimental treatments were 0, 10, 20, 30, or 40 g of Met/ d (Experiment 1) and 0, 15, 30, 45, or 60 g of Lys/d (Experiment 2). The infusion of Met did not significantly affect feed intake or daily milk yield, but increased milk fat content, ECM yield, and C4 to C14 and C18 to C20 fatty acid production in milk. The infusion of Met caused an increase in arterial plasma Met concentration and a decline in branched-chain amino acids (AA). Mammary gland uptake of Met was not related to plasma AA concentration. The infusion of Lys did not affect feed intake, milk yield, or milk composition, except for increases in milk urea and NPN contents. The infusion of Lys increased plasma Lys, BCAA, EAA, and the EAA to TAA ratio. Uptake of plasma BCAA and NEAA by the mammary gland decreased, which suggests that Lys was used as a substrate for milk NEAA synthesis. These data demonstrate that Met is important in the milk fat synthesis, and Lys is important in mammary gland AA metabolism. However, neither Met nor Lys is the first-limiting AA in the milk protein yield of cows fed a grass silage and cereal diet.