Effects of intraperitoneal L-leucine, L-isoleucine, L-valine, and L-arginine on milk fat depression in early lactation cows.

Eight Holstein cows were assigned following calving to two groups, balanced for parity, using a continuous completely randomized block design. Cows were fed a diet with 13.5% CP and 22.4% ADF from 35 to 55 DIM and then 13.8% CP and 15% ADF from 56 to 92 DIM. Alfalfa grass hay was the forage source, and concentrate mixtures contained primarily corn and soybean meal. Cows were given daily intraperitoneal infusions of a solution of L-Leu (46.1 g, 84.2 mM), L-Ile (31.4 g, 57.3 mM), L-Val (38.3 g, 78.2 mM), and L-Arg (25.0 g, 34.4 mM) or physiological saline following the a.m. milking from 42 through 84 DIM. Infusion of AA significantly increased plasma concentrations of Leu, Ile, Val, and Arg. Effects of AA infusion occurred during the low fiber period. Cows receiving AA maintained daily milk fat yield, increased p.m. milk fat yield, decreased less in p.m. milk fat percentage, and increased daily and p.m. yields of C16 fatty acids in milk. During the posttreatment period, cows previously receiving AA declined in daily milk fat yield, milk fat percentage, and total daily C4 to C16 milk fatty acid yield. Results suggest that the infused AA may have increased de novo synthesis of C16 milk fatty acids.