Effects of milk-delivered Saccharomyces cerevisiae fermentation products on growth, rumen fermentation, ruminal microbiota, and serum immune markers in pre-weaned Holstein calves

Introduction This study evaluated the effects of pre-weaning supplementation with Saccharomyces cerevisiae fermentation products (postbiotics) via whole milk on growth performance, rumen fermentation, ruminal microbiota, and immune responses in Holstein calves. Methods Thirty 7-day-old Holstein calves were randomly assigned to three treatments: POST0 (0 g/day), POST35 (3.5 g/day), and POST50 (5 g/day) of S. cerevisiae fermentation products administered via whole milk until weaning at 70 days of age. Growth performance, rumen fermentation characteristics, ruminal microbiota using 16S rRNA gene sequencing, and serum immune parameters were evaluated. Results and discussion Postbiotic supplementation increased withers height ( P < 0.01) and modulated rumen fermentation, with significant effects on straight-chain fatty acids (SCFA), total short-chain fatty acids (TSCFA), and ruminal NH 3 -N concentrations ( P < 0.05). The overall treatment effect on branched cahin fatty acids (BCFA) was not significant ( P = 0.098), although a significant treatment × age interaction was detected ( P < 0.001). Postbiotic supplementation influenced ruminal fermentation, with significant treatment × age interactions for propionic acid, SCFA, and TSCFA concentrations ( P < 0.001). Propionic acid concentrations were numerically higher in supplemented calves than in controls at both sampling ages, whereas the greatest increases in SCFA and TSCFA concentrations, particularly at 5 g/day, were observed on day 35. Calves receiving 5 g/day of postbiotics exhibited higher ruminal pH than those receiving 3.5 g/day ( P < 0.05), whereas neither supplemented group differed significantly from the control group ( P > 0.05). Postbiotic supplementation was associated with changes in ruminal bacterial composition, particularly in the relative abundances of Shuttleworthia, Saccharimonadales-related taxa, and the Eubacterium ruminantium group. Postbiotic dose significantly affected serum IgG and IgM concentrations, with significant treatment × age interactions for both immunoglobulins ( P < 0.001). The highest numerical concentrations were observed in calves receiving 5 g/day at weaning, whereas IgA and IgE were not significantly affected ( P > 0.05). Overall, pre-weaning supplementation with S. cerevisiae fermentation products modulated rumen fermentation and microbial composition and was associated with dose- and age-dependent changes in circulating immunoglobulin concentrations. These findings suggest that S. cerevisiae postbiotics may influence rumen microbial and fermentation characteristics and selected immune-related parameters during the pre-weaning period, although their functional significance requires further investigation.