Nan Su

Intramuscular fat (IMF) significantly influences meat quality, particularly flavor. The gastrointestinal microbiota can regulate lipid metabolism. The relationship between intramuscular fat metabolism, rumen microbiota, and beef quality remains unclear. This study enrolled 22 30-month-old Xinjiang Brown Beef cattle, which were randomly allocated to two groups: an intact bull group (n = 15) and a castrated bull group (n = 7). All experimental animals were housed and maintained under consistent feeding and management conditions throughout the entire experimental period. By combining in vivo ultrasonography, slaughter trials, rumen microbiome diversity analysis, and metabolomics techniques, and after adjusting for covariates including intramuscular fat (IMF) content, body weight, and backfat thickness, the present study demonstrated that castration regulates muscle lipid metabolism by reshaping the composition of the rumen microbial community, thereby exerting a cascading effect on key beef quality traits. (1) Production and meat quality: Live weight, carcass weight, eye muscle area, backfat thickness, and intramuscular fat (IMF) content were significantly higher in the YN group than in the GN group (p < 0.01). Conversely, dressing percentage, shear force value, and muscle protein content were significantly lower in the YN group than in the GN group (p < 0.01 or p < 0.05). (2) Rumen microbiota–metabolite correlation: Significant differences existed in microbial composition and community structure between groups (with significant differences in both α and β diversity). Core microbes regulated by castration exhibited distinct co-variation patterns with metabolites: genera such as Anaeroplasma showed significant positive correlations with hydroxy fatty acids, while Sharpea and others showed significant negative correlations with saturated fatty acids. (3) Microbial–metabolite axis and host phenotype correlation: Axes composed of Eubacterium uniforme and others showed significant positive correlations with IMF, while Docosapentaenoic acid (22n-3) exhibited significant negative correlations with IMF. Anaeroplasma and others showed significant positive correlations with oleic acid and others, as well as BFT, while saturated fatty acids showed significant negative correlations with BFT. (4) Covariate validation: After adjusting for covariates including body weight, backfat thickness, and IMF, castration was confirmed to significantly regulate the abundance/content of core genera such as Anaeroplasma, Eubacterium uniforme, as well as key metabolites such as hydroxy fatty acids and docosapentaenoic acid (22n-3) (p < 0.05 after adjustment), making it a core driver regulating rumen microbial composition and muscle lipid metabolism. After adjustment, the regulatory effects of IMF, body weight, and backfat thickness on the aforementioned microorganisms and metabolites were no longer significant (adjusted p > 0.05). Intramuscular fat (IMF), body weight, and backfat thickness are not independent drivers but rather indirect effects resulting from castration-induced physiological state remodeling. This study did not include feeding rate measurements, which represents a limitation. Future research should incorporate this data to further validate the conclusions. This study elucidates the interactive mechanisms between rumen microbiota and their metabolites, identifies the key pathways governing intramuscular fat (IMF) deposition, pinpoints potential regulatory targets for beef quality optimization, and clarifies the intermediate regulatory mechanisms underlying the modulation of meat quality traits by castration.