Dietary Hydroxy-Selenomethionine Improves Antioxidant Status and Reduces Somatic Cell Count in Dairy Cows: Multi-Omics Insights into Rumen Microbiota and Metabolic Profiles

High-yielding dairy cows are highly susceptible to lactational oxidative stress, which compromises mammary barrier integrity and elevates mastitis risk. This study investigated the potential biological mechanisms by which dietary hydroxy-selenomethionine (HMSeBA) alleviates oxidative stress and improves health in dairy cows. Forty Holstein cows were assigned to a basal control group (0.32 mg Se/kg DM) or an HMSeBA-supplemented group (0.64 mg Se/kg DM) for 105 days. HMSeBA significantly enhanced selenium bioavailability in both milk and blood, comprehensively strengthening antioxidant defenses (increased glutathione peroxidase activity, decreased malondialdehyde) and elevated serum immunoglobulins (IgA, IgM, IgG), accompanied by a reduction in milk somatic cell count, without significantly affecting milk yield, feed intake, or milk production efficiency. Multi-omics analysis revealed that HMSeBA supplementation altered the rumen microenvironment by enriching fiber-degrading genera (Prevotellaceae_Ga6A1_group, Xylanibacter, Segatella) and shifting metabolites, including feed flavonoids, peptides, 1-deoxy-D-xylulose-5-phosphate, and 3-OH-C6-HSL. The positive correlation of ruminal 3-OH-C6-HSL with both blood selenium and these enriched taxa suggests a potential link between microbial activity and host selenium status. These findings indicate that HMSeBA supplementation improves the antioxidant and immune status of dairy cows, accompanied by exploratory, hypothesis-generating shifts in the ruminal microbiome and metabolome. Collectively, these findings highlight HMSeBA as a promising nutritional strategy to produce selenium-enriched milk while safeguarding udder health.