1Department of Animal Science, University of Tennessee, Knoxville, TN 37996. 2Balchem Corp., New Hampton, NY.

Methionine (Met) is essential to support milk and milk protein synthesis. A study was conducted to evaluate the effects of a rumen-protected Met (RP-Met) prototype on milk production and plasma metabolite concentrations in dairy cows. Ten multiparous Holstein cows (76 ± 17 days in milk) were used in a 5 × 5 duplicated Latin Square with 18 d treatment periods. Cows were randomly assigned to either a Met deficient control diet (-16.4 g/d of MP-Met; -CTL), -CTL supplemented with Smartamine-M (28.7 g/d of RP-Met; SA), or one of three levels of a RP-Met prototype (Balchem Corporation, New Hampton, NY) to provide 1.0 (31.6 g/d of RP-Met; 1.0Met), 1.25 (38.9 g/d of RP-Met; 1.25Met), or 1.5 (47.3 g/d of RP-Met; 1.5Met) times the amount of DL-Met supplied by Smartamine-M. The prototype contained 69% Met. Feed ingredients and milk and blood samples were collected and analyzed. Data were analyzed using the Mixed procedure of SAS 9.4 and reported as least square means ± SEM. Compared with -CTL, SA, 1.25Met, and 1.5Met decreased (P = 0.02) lactose percent by 1.3%. Relative to -CTL, SA tended to increase milk protein content (P = 0.07). Relative to -CTL and SA, 1.0M, 1.25M, and 1.5M treatments tended to maintain milk protein content. Relative to -CTL, SA and Met1.5 increased plasma Met concentrations (27.9 vs. 38.1 and 32.6 µM, respectively; P < 0.01). Compared with SA, 1M, 1.25M, and 1.5M treatments support similar plasma methionine concentrations. In summary, SA and 1.5M treatments increased plasma methionine concentrations. Smartamine-M tended to promote milk protein content, however protein yield was not affected.
Keywords: dairy cows, milk protein, rumen-protected methionine