Ruminal bacterial and protozoal features associated with milk yield in Chinese Holstein cows.

BACKGROUND: Milk yield is a key economic trait in dairy cows and is closely linked to rumen microbial function. However, the associations between rumen microbial communities and milk yield remain incompletely understood. This study compared the rumen microbial composition and function of Chinese Holstein cows with high and low milk yields to clarify the relationships between differential rumen microbes and lactation performance.

RESULTS: Results showed that HY cows had higher milk yield, milk fat yield, milk protein yield, lactose yield, and total milk solids (TMS) yield (P < 0.05), but lower milk fat concentration (P < 0.05). Regarding rumen fermentation, the yields of propionic acid and microbial crude protein (MCP) were increased in the HY group (P < 0.05), whereas the concentration of acetic acid and the acetic acid/propionic acid (A/P) ratio were significantly reduced (P < 0.05). Additionally, neutral detergent fiber (NDF) digestibility was higher in the HY group compared to the LY group (P < 0.05). 16 S rRNA gene sequencing revealed that the relative abundance of the bacterial genera such as Succiniclasticum, Prevotella, and Ruminococcus was increased in the HY group, while 18 S rRNA gene sequencing showed that the relative abundance of the protozoan genus such as Entodinium was higher in the HY group (P < 0.05). Furthermore, the Phylogenetic Investigation of Communities by Reconstruction of Unobserved States (PICRUSt2) analysis revealed that the pyruvate fermentation to propanoate I pathway and the synthesis-related enzymes within this pathway were upregulated in the HY group. Notably, we found that Prevotella and Succiniclasticum were positively correlated with milk yield and propionic acid concentration, whereas Entodinium was positively correlated with Prevotella, Succiniclasticum, and Ruminococcus, and the partial least square-structural equation model (PLS-SEM) revealed that rumen microbes influence milk yield via enhanced rumen fermentation and nutrient digestibility.

CONCLUSIONS: Collectively, HY cows exhibited higher NDF digestibility, propionic acid concentration, and MCP production than LY cows, and also showed higher abundances of Prevotella, Succiniclasticum, Ruminococcus, and Entodinium, as well as enrichment of the pyruvate fermentation to propanoate I pathway. These microbial characteristics were associated with differences in rumen function and milk yield between HY and LY dairy cows.