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.

This study aimed to determine the effects of dietary supplementation with a compound additive containing guanidinoacetic acid (GAA) and cysteamine on rumen metabolism, fermentation, and the microbiota of fattening Holstein bulls. Sixty Holstein dairy bulls (294.33 ± 3.42 kg) were divided into three groups (n = 20). Each treatment comprised three independently fed pens (n = 3 pens per treatment), and one bull was sampled from each pen for the rumen measurements. Group A served as the control group, whereas Groups B and C received a concentrated supplement containing 1 kg/t and 2 kg/t of a compound additive (GAA combined with cysteamine), respectively, over 90 days. Kyoto Encyclopedia of Genes and Genomes identified five key pathways. Specifically, indole-3-acetaldehyde, 2-amino-3-methoxybenzoic acid, 2-succinyl-5-enolpyruvyl-6-hydroxy-3-cyclohexene-1-carboxylate, and cholesterol glucuronide were upregulated, whereas 4-fumarylacetoacetic acid and pseudouridine 5′-phosphate were downregulated. Compared with those in the control group, acetic acid, butyric acid, total volatile fatty acids (VFAs), propionic acid, and microbial crude protein (MCP) increased, whereas pH, acetic acid/propionic acid (A/P) ratio, and NH₃-N did not differ. At the genus level, Weissella abundance increased, Methanobrevibacter abundance decreased, and Xylanibacter and Ruminococcus abundances increased in the low-supplementation group (1 kg/t). Methanobrevibacter correlated negatively with VFAs, MCP, and NH₃-N but correlated positively with A/P. Weissella and Ruminococcus showed the opposite trend. Increasing the inclusion level of the compound additive improved rumen metabolism, fermentation, and the microbial environment. This ultimately reduced stress and enhanced rumen health in fattening Holstein bulls.