Effects of rumen-protected methionine, lysine, and histidine on lactation performance of dairy cows.

The objective of this study was to evaluate the effects of supplementing a metabolizable protein (MP)-deficient diet with rumen-protected (RP) Met, Lys, and His, individually or combined, on the performance of lactating dairy cows. The experiment was a 9-wk randomized complete block design with 72 Holstein cows. Following a 2-wk covariate period, cows were blocked by days in milk, milk yield, and parity, and randomly assigned to 1 of the following 6 treatments: (1) MP-adequate diet [MPA; +243g/d MP balance, according to the National Research Council (2001) requirements]; (2) MP-deficient diet (MPD; -54g/d MP balance); (3) MPD supplemented with RPMet (MPDM); (4) MPD supplemented with RPLys (MPDL); (5) MPD supplemented with RPHis (MPDH); and (6) MPD supplemented with RPMet, RPLys, and RPHis (MPDMLH). Dry matter intake (DMI), yields of milk and milk components (fat, protein, lactose) and energy-corrected milk (ECM), feed and ECM feed efficiencies, and milk and plasma urea N were decreased by MPD, compared with MPA. Supplementation of the MPD diet with RPLys increased milk protein content and plasma glucose concentration and tended to increase milk urea N. Addition of RPHis tended to increase DMI, increased milk protein concentration, and numerically increased yields of milk fat, protein, and ECM. In addition to the trends for increased DMI and milk fat content, and higher milk protein concentration, supplementation of the 3 RP AA also increased yields of milk fat, protein, and ECM and ECM feed efficiency. Relative to MPA, milk N efficiency tended to be increased by MPD. Concentrations of plasma essential AA (except Met and Thr) were decreased by MPD compared with MPA. Supplementation of RPMet, RPLys, and RPHis increased plasma Met (except for MPDM), Lys, and His concentrations, respectively. Cows fed MPD had lower blood hemoglobin concentration and numerically higher plasma ghrelin than cows fed MPA. Concentration of total saturated fatty acids in milk fat were or tended to be higher for MPD compared with MPA and MPDMLH, respectively. Concentration of total polyunsaturated and yield of milk odd- and branched-chain fatty acids were or tended to be decreased by MPD compared with MPA. Overall, the results of this study confirm our previous data and suggest that His stimulates DMI and the combination of the 3 RP AA (Met, Lys, and His) has the potential to improve milk and milk component yields in dairy cows fed MP-deficient diets.