Effects of dietary protein level and rumen-protected methionine on lactation performance and blood metabolites in postpartum cows.

Lactation responses, such as milk and milk protein yield, to rumen-protected Met (RP-Met) have been inconsistent in studies especially with transition cows and may depend on the MP supply. The objective of this study was to evaluate the interactive effects of RP-Met supplementation with dietary CP levels on lactation performance and blood metabolites in postpartum cows. Sixty-eight pre-fresh Holstein cows, including 36 multiparous and 32 primiparous cows, were blocked by parity and expected calving date in a randomized complete block design. Immediately after parturition, cows within each block were assigned to one of the 4 diets in a 2 × 2 factorial arrangement and remained until 28 DIM: LPD, a low-CP diet (16.5% on a DM basis); HPD, a high-CP diet (18.5%); LPDM, LPD supplemented with RP-Met (25 g/d of Smartamine M); HPDM, HPD supplemented with RP-Met (15 g/d). For LPDM and HPDM, the RP-Met was mixed with TMR. The MP and EAA supplies relative to the requirements were estimated for the diets according to NASEM (2021) using actual DMI, BW, and milk yield and components. In multiparous cows, LPD was deficient in MP (87%), and the MP deficiency was moderate for HPD (99%). In primiparous cows, LPD was moderately deficient in MP (94%), whereas HPD exceeded the MP requirement (110%). The LPD and HPD diets were deficient in Met, while LPDM and HPDM were adequate or nearly adequate in Met. Three-way interactions of CP × RP-Met × parity were detected for DMI, milk yield per unit of DMI, ECM, and milk fat and protein yields. In multiparous cows, DMI tended to be greater for HPDM compared with HPD (21.2 vs. 19.7 kg/d) but tended to be lower for LPDM compared with LPD (20.2 vs. 21.8 kg/d). In contrast, DMI was greater (16.9 vs. 15.0 kg/d) for LPDM compared with LPD in primiparous cows. Consistent with the response pattern of DMI, HPDM increased ECM in multiparous cows compared with HPD (50.2 vs. 45.3 kg/d) and LPDM in primiparous cows increased ECM compared with LPD (42.2 vs. 37.5 kg/d). Plasma AA profiles were not affected by dietary CP, but RP-Met supplementation increased plasma Met concentration regardless of dietary CP or parity. Postpartum BW change and plasma NEFA concentrations also showed 3-way interactions, where RP-Met supplementation decreased both BW loss (-4.10 vs. -4.96 kg/d) and NEFA concentration (233 vs. 357 μEq/L) under HPD, but increased both responses for LPD in multiparous cows (-5.46 vs. -3.96 kg/d; 298 vs. 214 μEq/L). In primiparous cows, RP-Met only increased NEFA concentration under HPD (306 vs. 219 μEq/L) with no effect on BW change. In conclusion, increases in DMI and production responses to RP-Met supplementation occurred under marginally deficient MP supply during early lactation, suggesting the need to group postpartum cows by parity. Furthermore, avoiding deficient or excessive MP supply from the basal diet may result in more consistent positive responses to supplemental RP-Met. Further studies with a larger number of animals are needed to confirm the results.