Effects of L-selenomethionine supplementation on nutrient digestibility and metabolism, antioxidant capacity, hormone levels, and fecal microbiota diversity in pregnant Yili mares during mid- to late gestation

Background L-selenomethionine (L-SeMet), a highly bioavailable organic form of selenium, plays a critical role in maintaining antioxidant homeostasis, regulating reproductive hormone secretion, and improving intestinal microbial ecology. Previous studies have demonstrated that appropriate supplementation with L-SeMet can significantly enhance the production performance and health status of ruminants. However, the nutritional regulatory mechanisms and physiological effects of L-SeMet in monogastric herbivores, particularly horses during mid- to late gestation, remain inadequately understood. Therefore, this study investigated the effects of different levels of L-SeMet supplementation on nutrient digestibility and metabolism, antioxidant capacity, reproductive hormone profiles, and fecal microbiota diversity in pregnant Yili mares, under standardized dietary composition and nutritional levels. Results The results showed that L-SeMet supplementation significantly improved crude protein digestibility and serum glutathione peroxidase (GSH-Px) activity. The 0.3 and 0.4 g/d groups exhibited significantly higher neutral detergent fiber (NDF) digestibility, nitrogen metabolism rate, total antioxidant capacity (T-AOC), catalase (CAT) activity, progesterone, and estradiol levels, while malondialdehyde (MDA) and urinary estrone levels were reduced. Fecal microbial analysis revealed that, particularly in the 0.3 g/d group, the relative abundance of methanogens and Actinobacteriota increased. Functional predictions indicated enrichment of microbial metabolic pathways related to carbohydrates and energy metabolism. Conclusions In conclusion, moderate supplementation with L-SeMet can enhance nutrient utilization, antioxidant function, endocrine stability, and gut microbiota composition in pregnant mares.