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"I’ve been told that supplementing selenium can be a good thing but I’ve also read that horses have died from selenium toxicity. What is selenium used for in the horse and can I supplement it safely?"
Selenium (Se) is a component of around 25 proteins, including glutathione peroxidise. These selenoproteins have major roles in metabolism and rely on the availability of Se in the body and/or diet. Selenium also plays a role in the immune system, gene expression, thyroid metabolism and male fertility. Therefore, adequate Se status in the horse is vital for optimum health and performance. Many areas of Europe have soils that are classed as Se-deficient (especially Scandinavia) and it is common place to fortify feed with inorganic Se, usually as sodium selenite (NaSe). However, inorganic Se, as well as other minerals is poorly absorbed by the digestive tract and not readily stored in the body, resulting in very low, if any, Se reserves. In contrast, Se absorbed from organic sources, such as selenium yeast, can be incorporated into either selenoenzymes or general proteins in the body as storage for future use. This enables the animal to build Se reserves for times of stress, such as foaling, illness and high intensity exercise. Exercise may be of particular importance with regards to Exertional Rhabdomyolysis (Tying-up, Azoturia).
Many studies have shown greater incorporation of Se from an organic source into body tissues compared with NaSe. This is because organic Se is the form that the mammalian body has evolved to use. The body recognises this form and it is absorbed and incorporated into the body much more effectively. With regards to the issue of toxicity, NaSe is actually toxic to mammals over certain levels and, as such, European law states maximum supplementation of 0.5mg/kg in complete feeds. The toxicity of NaSe is in contrast to organic selenium as selenium yeast, for example no toxic level (as measured by LD50) has been found for Sel-Plex® (Alltech Inc., Nicholasville, KY). Horses are very sensitive to Se levels and develop signs of toxicity more quickly than other animals, such as cows, for example, therefore, organic selenium seems to offer a safer way of supplementing optimum levels of selenium.
Expert Answer by: Helen Warren