Rumen-protected methionine and lysine: effects on animal performance, milk protein yield, and physiological measures.

Eight midlactation Holstein cows were used in a three-period (28 d) switchback design to evaluate addition of a combination of ruminally protected methionine (15 g/d) and lysine (40 g/d) on animal performance, milk protein yield, and physiological measures. Cows were fed for ad libitum intake a total mixed diet consisting of 50% corn silage and 50% concentrate (DM basis) or a similar diet to include the addition of the amino acids. Plasma methionine and lysine were elevated with the addition of amino acids. There were no differences between treatments in DM intake, milk yield, fat yield, milk fat percentage, or 4% FCM yield. Addition of amino acids increased milk protein percent (3.15 vs. 3.25). The quantities of alpha- caseins and beta-casein were increased while the quantity of kappa-caseins was decreased. Analysis of the major whey proteins revealed a decrease in serum albumin. Nutrient digestibility and nitrogen balance did not differ. Addition of methionine and lysine to a corn-based diet increased milk protein percentage and yield, plasma methionine and lysine concentrations, and increased yield of the casein proteins in milk.