Effects of rumen-protected methionine and lysine supplementation on milk yields and components, rumen fermentation, and the rumen microbiome in lactating yaks (Bos grunniens)

The objectives of this study were to investigate the effects of supplementing rumen-protected methionine (RPM) or lysine (RPL) alone or in combination with RPM and RPL (RPML) on milk yield and milk components, rumen fermentation characteristics, and the rumen microbiome in lactating yaks. Thirty-two multiparous lactating yaks with parity (2.13 ± 1.08 lactations), body weight (169 ± 15.0 kg), and milk yield (1.02 ± 0.13 kg/d) were blocked by milk yield and randomly assigned into one of the four treatments: control (CON; basal diet without adding rumen-protected amino acid) and the basal diet supplemented with RPM (50.0 g/d), RPL (66.7 g/d), or RPML (12.5 g/d RPM and 50.0 g/d RPL). The basal diet consisted of 600 g/kg rye hay and 400 g/kg corn-based concentrate. The dry matter intake, lactoferrin concentration, and lactose yield were greater (P < 0.05) in the RPM and RPML groups than those in the CON and RPL groups. Compared to the other three groups, the RPM supplementation increased (P < 0.01) the milk and protein yield. The milk protein content in the RPM group was greater (P < 0.01) than that in the CON and RPML groups. The RPM group had a higher milk content of alanine (P < 0.01) and aspartic acid (P = 0.01) than that in the other three treatments. The RPM group had a lower (P < 0.01) rumen pH and a higher (P = 0.02) valerate proportion than those in the other three treatments, whereas the RPML group had a higher (P = 0.03) rumen ammonia nitrogen concentration than that in the other three treatments. The relative abundances of Proteobacteria (P = 0.01) and Prevotella_1 (P = 0.02) were increased in the RPM and RPML groups, respectively. Operational taxonomic units (OTU) unique to the yaks of RPL and RPML groups belonged mainly to the phylum Bacteroidetes, whereas the majority of OTUs unique to the yaks of the RPM group belonged mainly to the phylum Firmicutes. The relative abundances of Fibrobacter and Selenomonas_1 were positively correlated with milk yield and concentration of propionate, valerate, butyrate, and total volatile fatty acids. The relative abundances of Candidatus_Saccharimonas, Ruminococcaceae_UCG-014, Rikenellaceae_RC9_gut_group, and Erysipelotrichaceae_UCG-004 were positively correlated with the content of milk protein. These results suggest that supplementation of RPM and RPL can affect rumen microbial populations, which in turn affect rumen fermentation and thus, the metabolites available for milk synthesis.