Effects of rumen-protected methionine and other essential amino acid supplementation on milk and milk component yields in lactating Holstein cows

摘要

Objectives of this study were to investigate the effectsof supplementing rumen-protected methionine(RP-Met), threonine (RP-Thr), isoleucine (RP-Ile),and leucine (RP-Leu) individually or jointly to a lowproteindiet, on the performance of lactating dairy cows,as well as to determine the effects of these amino acids(AA) on the mammalian target of rapamycin (mTOR)in vivo. Ten lactating Holstein cows were randomly allocatedto a repeated 5 × 5 Latin square experimentwith five 19-d periods. Treatments were high-proteindiet (16% crude protein, positive control; HP), lowproteindiet (12% crude protein, negative control; LP),LP plus RP-Met (LPM), LP plus RP-Met and RP-Thr(LPMT), and LP plus RP-Met, RP-Thr, RP-Ile, andRP-Leu (LPMTIL). The dry matter intakes (DMI) ofthe LP, LPM, and LPMT diets were lower than thatof the HP diet, whereas the DMI of the LPMTIL dietwas intermediate between the HP diet and the otherLP diets. Supplementing RP-Met to the LP diet increasedthe yields of milk and milk protein, increasedthe content of milk urea N, and tended to increase milkN efficiency. Co-supplementation of RP-Thr with RPMetresulted in no further milk production increase.Co-supplementation of all 4 rumen-protected aminoacids (RP-AA) increased milk and lactose yields tothe level of the HP diet and tended to increase milkprotein yield compared with the LPMT diet. We foundno significant differences in the contents and yields ofmilk components between the LPMTIL and HP dietsexcept for a lower milk urea N content in the LPMTILdiet. Venous concentrations of the measured AA weresimilar across the LP and LP diets supplemented withRP-AA. Relative to levels of the HP diet, LP dietshad higher venous concentrations of Met and Gly andtended to have higher Phe concentration and lower concentrationsof Val and BCAA. The LPMTIL diet hadhigher venous concentrations of Arg, Lys, Met, Phe,and Glu, and a lower Val concentration. Phosphorylationstatus of the measured mTOR components inLPM and LPMT treatments were similar to those inthe LP treatment but phosphorylation status of mTORand eIF4E-binding protein 1 (4eBP1) in LPMTILtreatment were higher. The phosphorylation rates ofeukaryotic elongation factor 2 (eEF_2) in the 4 LP andLP plus RP-AA diets were higher than that of the HPdiet. Overall, results of the present study supportedthe concept that under the relatively short time of thisexperiment, supplementing RP-AA, which are believedto stimulate the mTOR signal pathway, can lead toincreased milk protein yield. This increase appears tobe due to increased DMI, greater mTOR signaling, andgreater eEF_2 activity.