Sodium selenite (Na₂SeO₃) is the most commonly used inorganic selenium source in the feed industry. As a trace element selenium supplement, it has a history of more than 60 years and is listed in the Catalogue of Feed Additives by China’s Ministry of Agriculture and Rural Affairs. Feed‑grade products comply with the national standard, requiring Na₂SeO₃ (on a dry basis) ≥98.0% and selenium (Se) content ≥44.7%. It appears as colourless crystalline powder, readily soluble in water, and is typically added to compound feed as a 1% or 2% premix. In animals, selenium is a core component of glutathione peroxidase (GSH‑Px), which protects cell membrane integrity by scavenging free radicals, thereby exerting antioxidant effects, enhancing immunity, maintaining normal function of cardiac and skeletal muscles, and improving reproductive performance. Deficiency of selenium leads to “white muscle disease” in pigs, cattle and sheep, “exudative diathesis” and liver necrosis in poultry, as well as growth retardation and reduced disease resistance. According to Announcement No. 1224 of the Ministry of Agriculture, the recommended supplemental level in compound feed is 0.1–0.3 mg/kg (calculated as elemental selenium), with an upper limit of 0.3 mg/kg in complete feed. Moreover, combined use with vitamin E significantly enhances the antioxidant effect.