Integrative Microbiome–Metabolome Analysis Reveals Dose-Related Effects of Rumen-Derived Ligilactobacillus salivarius KS1018 on Rumen Function in Lambs

Rumen-derived probiotics may help regulate fermentation and microbial activity in lambs, but the dose-related effects remain unclear. This study evaluated the effects of rumen-derived Ligilactobacillus salivarius KS1018 on rumen function in Hu lambs using integrated assessments of growth performance, serum hormones, rumen fermentation, rumen microbiota and metabolomics profiling. Thirty-two male Hu lambs were allocated to four groups (n = 8/group): control (C) or KS1018 at 0.5 × 109 (L), 1.0 × 109 (M), or 1.5 × 109 CFU/lamb/day (H) for 56 days. Growth performance, including final BW and ADG, did not differ significantly among groups (p > 0.05). Serum T3 and T4 were significantly higher in M and H than in C (p < 0.01). Group H exhibited significantly lower NH3-N and higher total VFA concentration of rumen fluid compared with group C (p < 0.05). Significant increases in isobutyric acid concentration were observed in groups M and H (p < 0.05). Genus-level shifts and dose-related metabolite changes were detected; oxidative phosphorylation was enriched in group H and amino acid biosynthesis in group M. Procrustes analysis indicated significant concordance between the rumen microbiome and metabolome datasets (M2 = 0.56, p < 0.001). These results indicate that KS1018 supplementation was associated with dose-related changes in rumen fermentation and selected microbiome–metabolome associations in Hu lambs, although the optimal dose requires further validation.