The effect of soybean meal (SBM) replacement with fermented SBM (FSBM) on ruminal fermentation and bacterial abundance in Holstein calves was investigated in this study. Thirty nine calves were randomized to: (1) control: 27% SBM + 0% FSBM (FSBM0, n = 13); (2) 18% SBM + 9% FSBM (FSBM9, n = 13); and (3) 13.5% SBM + 13.5% FSBM (FSBM13, n = 13). SBM contained a greater amount of large peptides containing 3 to 10 amino acids (AAs), while FSBM had a greater amount of ammonia nitrogen (NH 3-N), free AAs, and small peptides containing 2 to 3 AAs. The calves fed FSBM13 had the lowest acetic acid, NH 3-N, and the ratio of acetate to propionate, with the greatest concentration of caproic acid, valeric acid and isovaleric acid in ruminal fluid. Compared to those fed FSBM9 or FSBM13, the calves fed FSBM0 had the greatest proportion of Butyrivibrio fibrisolvens and Ruminococcus albus in rumen fluid. However, the ruminal abundance of Prevotella ruminicola in calves fed FSBM13 was greater than in calves fed FSBM0. Network analysis showed that the abundance of the Ruminococcus albus was associated with large peptides, and butyric acid was correlated with small peptide. Taken together, our findings suggest that FSBM may have the potential to boost calf performance by changing the fermentation products and the relative abundance of some members of the bacterial community in the rumen. The calf starts its life with a physically and metabolically undeveloped rumen. Calves generally experience significant changes not only in the development of their rumen, but also in the rumen bacterial composition and community 1. This, in turn, enables calves to digest dry feeds for their growth needs, impacting productivity and future milk production capacity 1,2. Fibrolytic bacteria are important for the rapid anatomical and physiological growth of rumen, as the newborn calf moves from a liquid diet (i.e., milk) to a solid diet, such as grass and hay 3. Rumen microbes are classified into various functional groups, for example cellulolytic, amylolytic and proteolytic bacteria species 4. These organisms metabolize feed components and other microbial products 5. The rumen microbial community and the functional development of the stomach are affected by diets 4. For example, when forages are fed to ruminants, Bacteroidales and Ruminococcaceae become dominant 4. On the other hand, it is known that animals fed high-concentrate diets have more Prevotella and Succinivibrionaceae in their rumen fluid 4,6-8. Therefore, diets fed to animals can alter the relative abundance of bacteria and the profile of volatile fatty acids (VFAs) in the rumen 1,4. This, in turn, affects the evolution of the early ruminal development and microbiota 9 .
