The link between serum metabolism and milk production in Sanhe and Holstein cows has been explored previously, showing that metabolic responses vary with breed and parity. However, the role of the rumen microbiome in these differences remains unclear, despite its central function in nutrient absorption and metabolic processing. In this study, we investigated rumen fermentation characteristics and nutrient digestibility in Sanhe (S1–S4) and Holstein (H1–H4) cows across four parities, alongside a high-resolution analysis of their rumen microbial communities. Among Sanhe cows, dry matter digestibility and ammonia nitrogen levels showed significant variation, with S1 tending to have higher volatile fatty acid concentrations. Holstein cows exhibited notable differences in crude protein digestibility, higher isovaleric acid in H1, and a lower acetate-to-propionate ratio in H3. Microbiome profiling revealed patterns consistent with these metabolic changes: S1 diverged from S2–S4, while H1 and H2 were distinct from H3 and H4. While the overall microbial composition was broadly similar between the two breeds, relative abundances differed. For example, Rhizophagus (Glomeromycetes), Neocallimastix, and Piromyces were more prevalent in early-parity cows, and pathways such as autophagy, plant-pathogen interactions, and endocytosis were enriched. In older Sanhe cows, ATP-binding cassette transporter pathways were more prominent. Furthermore, specific carbohydrate-active enzymes (GH84, GH37) were closely associated with physiological traits and milk production indicators. Overall, these results highlight the intricate interactions between the rumen microbiome and metabolism, suggesting that microbial composition shifts may underlie parity-dependent differences in lactation performance.
