Testicular and Rumen Microbial Responses to Selenium-Enriched Malt in Heat-Exposed Simmental Bulls

This study evaluated testicular and rumen microbial responses to selenium-enriched malt in heat-exposed Simmental bulls. Eighteen bulls received a basal diet alone or supplemented to provide target supplemental Se levels of 0.3 or 0.5 mg/kg dietary DM. Metagenomics included six bulls per group, host assays four per group, and RNA sequencing three per group. Compared with controls, the high-dose group had higher serum antioxidant indices and reproductive hormone concentrations, lower inflammatory markers, higher Johnsen scores, and lower TUNEL positivity. Gene-level FDR control identified 23, 1, and 4 differentially expressed genes in the low-dose versus control, high-dose versus control, and high-dose versus low-dose contrasts, respectively. Provider-reported GSEA showed negative normalized enrichment scores for steroid biosynthesis in the control-versus-low-dose and control-versus-high-dose contrasts, indicating enrichment toward the supplemented side under the provider’s contrast convention. qPCR showed lower CYP11A1, CYP17A1, SCARB1, and LDLR expression at the low dose, whereas Bax, Bcl-2, and CYP17A1 protein abundance did not differ. Rumen microbial diversity and community composition differed among groups, and two post hoc selected cofactor-related pathways had lower abundance at the low dose. None of the 160 microbiome–testis or 720 microbiome–serum associations survived FDR correction. Supplementation was associated with favorable systemic and testicular histological responses and dose-specific molecular and microbial changes under heat exposure.