Effects of Yeast Cell Walls on Rumen Fermentation, Microbial Composition, and Metabolite Profiles in Early-Weaned Simmental Calves

This study aimed to evaluate how dietary yeast cell wall (YCW) supplementation in the starter feed affects ruminal fermentation parameters, microbial community composition, and metabolite profiles in early-weaned Simmental calves. Twenty-four newborn Simmental heifer calves (initial body weight: 37.53 ± 2.50 kg) were randomly assigned based on birth date sequence into the experimental group and the control group (12 calves per group). Calves in the experimental group (YCW) received a daily supplement of 5 g/head/day of yeast cell wall in the starter diet, whereas those in the control group (CON) received no supplementation. The experimental period lasted for 100 days, with weaning conducted at 70 days of age. On day 70, rumen fluid samples were randomly collected from six calves per group for analysis of rumen fermentation parameters, microbial community composition, and metabolomic profiles. (1) YCW supplementation significantly increased ruminal butyrate concentration and the relative abundance of the genus Ruminococcus (p < 0.05); (2) Metabolomic analysis identified 43 differential metabolites (20 upregulated and 23 downregulated), with nucleotide metabolism–related compounds such as guanylic acid and deoxycytidine monophosphate being prominently enriched (p < 0.05); (3) Spearman correlation analysis further revealed positive associations between Ruminococcus and both butyrate levels and selected upregulated metabolites, including guanylic acid (p < 0.05). Dietary yeast cell wall supplementation enhanced ruminal fermentation in early-weaned Simmental calves by increasing butyrate concentration and altering the ruminal microbiota and metabolome. Enrichment of Ruminococcus and nucleotide-associated metabolites, with positive correlations to butyrate, indicates a coordinated shift in the microbiota–metabolite axis. These findings support YCW as an effective nutritional strategy to promote rumen development and health during the early weaning period.