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

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. The results showed that selenium (L-SeMet) supplementation at 0.4, 0.6, or 0.8 mg Se mare⁻¹ day⁻¹ significantly increased apparent crude protein digestibility and serum glutathione peroxidase (GSH-Px) activity in pregnant mares compared with controls. Compared with the control group, the 0.6 and 0.8 mg Se mare⁻¹ day⁻¹ 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 microbiota analysis further revealed an increased relative abundance of methanogens and Actinobacteriota, particularly in the 0.6 mg Se mare⁻¹ day⁻¹ group. Functional predictions indicated enrichment of microbial metabolic pathways related to carbohydrates and energy metabolism. Collectively, these findings indicate that selenium supplementation (provided as L-SeMet) enhances nutrient utilization, antioxidant defenses, and the endocrine milieu during pregnancy, with 0.6–0.8 mg Se mare⁻¹ day⁻¹ appearing to confer the broadest benefits; dose optimization and long term outcomes warrant further investigation.