Active Dry Yeast (Saccharomyces Cerevisiae) Improves Rumen Fatty Acid Profile by Regulating Rumen Bacteria in Finishing Bulls

Active dry yeast (ADY) is frequently utilized as a probiotic to promote the growth and health of ruminants. However, it is not clearly established whether ADY influences and engages in the metabolism of fatty acids (FA) through interactions with rumen microbial communities. This study was to evaluate the effects of ADY on rumen FA and rumen bacterial community diversity in finishing bulls. Twenty Yanbian cattle were randomly divided into two groups (10 bulls in each). The control group (CON) received a basal diet, while the treatment group (ADY) received a basal diet supplemented with ADY (Levucell SC, <italic>Saccharomyces cerevisiae</italic> CNCM I-1077, 1.0 g/bull/day, viable count ≥ 8.0 × 10⁹ CFU/g). After the 100-day finishing trial, rumen fluid samples were collected to analyze rumen fermentation parameters, medium- and long-chain FA composition, and bacterial DNA sequencing. The results demonstrated that ADY noticeably increased the proportions of propionate, C18:1n9c, C18:2n6c, C20:1, and total monounsaturated fatty acids (MUFA) in rumen fluid (<italic>p</italic> &lt; 0.05). ADY supplementation tended to decrease the Simpson (<italic>p</italic> = 0.087) and Shannon (<italic>p</italic> = 0.052) indices. NMDS analysis revealed significant differences in beta diversity between the CON and ADY groups (PERMANOVA: R² = 0.104, <italic>p</italic> = 0.041). Furthermore, ADY supplementation effectively regulated lactate-utilizing and volatile fatty acid (VFA)-producing bacteria (<italic>p</italic> &lt; 0.05). Correlation analysis demonstrated that VFA-producing bacteria (<italic>Christensenellaceae</italic> <italic>R-7</italic> <italic>group</italic> and <italic>Schwartzia</italic>) were correlated with the proportion of propionate (<italic>p</italic> &lt; 0.05), and the members of the Lachnospiraceae and Ruminococcaceae (<italic>Lachnobacterium</italic>, <italic>Lachnospiraceae</italic> <italic>AC2044</italic> <italic>group</italic>, <italic>Lachnospiraceae</italic> <italic>UCG-006</italic>, <italic>Ruminococcaceae</italic> <italic>UCG</italic><italic>-</italic><italic>002</italic>, <italic>Ruminococcaceae</italic> <italic>UCG</italic><italic>-</italic><italic>010</italic>, and <italic>uncultured</italic> <italic>bacterium</italic> <italic>Ruminococcaceae</italic>) were noticeably correlated with C18:1n9c, C18:2n6c, C20:0, C20:1, and total MUFA (<italic>p</italic> &lt; 0.05). In conclusion, these findings suggest that ADY supplementation modulates the composition of rumen bacterial communities in finishing bulls, potentially contributing to a more favorable rumen FA profile characterized by increased propionate and MUFA.