Selenium is one of a horse’s important antioxidants. Antioxidants help prevent or limit cell damage caused by free radicals. Free radicals are known as harmful “reactive oxygen species,” which the horse produces during normal metabolic processes. These harmful atoms are believed to contribute to oxidative stress, which leads to cellular damage, aging, and disease. Selenium is an essential component of the antioxidant enzyme glutathione peroxidase, which plays a critical role in antioxidant defense and helps reduce cell damage.
The antioxidant system is the body’s protective and defense system against free-radical damage to cells and tissues. The body’s own antioxidants include SOD enzymes (which contain copper and zinc or manganese), glutathione peroxidase (which contains selenium), catalase (contains copper), and ceruloplasmin. Primary antioxidants are produced in the body to defend and support immunity. Production of these weakens with aging, which makes the importance of a healthy diet even greater.
Secondary antioxidants are obtained from the diet in the form of vitamins (vitamin C, vitamin E), trace minerals (selenium, zinc, copper), and plant phytonutrients. Both groups of antioxidants are absolutely essential for overall wellbeing.
Selenium is important for fertility, thyroid function, immune defense, and the body’s normal functioning. For sport horses, selenium and vitamin E are very important antioxidants that protect against cellular damage and support recovery.
A horse’s thyroid gland is crucial for hormone production and metabolic function. Selenium deficiency is believed to contribute to thyroid hyperfunction, meaning the thyroid does not function properly. Another concern related to selenium deficiency in horses is associated with hindquarter weakness and/or muscle weakness, respiratory difficulties, and heart failure. On the other hand, selenium can cause toxicity if fed in excessive amounts. This can lead to alkali disease, which damages tissues such as the heart, liver, and muscles. Selenium toxicity can also result in death. Signs of selenium toxicity in horses can include loss of mane and tail hair, lameness or joint degeneration, and cracking of the hooves.
How much selenium is appropriate for horses?
Selenium is a trace mineral that is required only in small amounts. Based on some studies, horses should receive at least 0.1 mg of selenium per kilogram of feed per day. However, this is the absolute minimum needed to avoid deficiency. The recommended dose for most adult horses to achieve optimal health is about 1–1.5 mg of selenium per day. Breeding horses and sport horses may benefit from a slightly higher dose to support performance, recovery, and fertility. The recommended safe upper limit for selenium is 2 mg per kilogram of feed per day. With selenium, it is important to follow the manufacturer’s instructions carefully when supplementing.
Selenium requirements are also influenced by vitamin E intake in the diet, because vitamin E can, as an antioxidant, partially compensate for selenium deficiency and vice versa.
Organic or inorganic selenium?
Most of the selenium a horse receives comes from its feed. In certain regions, the soil contains little selenium, which leads to selenium-deficient forage. In the Nordic countries, selenium levels in the soil are low, so selenium is often added to industrial concentrates and mineral feeds as well as to fertilizers. It has been estimated that up to 30% of horses may suffer from selenium deficiency.
The form in which the horse receives selenium also matters. Studies show that the types (sources) of selenium fed to horses affect their availability and biological utilization. Selenium can be provided to horses in inorganic form (sodium selenate/selenite) or in organic form (selenium yeast)—that is, in the same form in which it occurs in natural plants.
A horse’s selenium level in blood can be tested reliably. Data from horses show that horses absorb selenium significantly more effectively when it is fed in organic form. In addition, horses fed organic selenium have shown higher blood selenium levels compared to horses receiving inorganic selenium. For this reason, organic forms of selenium are considered superior for horses.
Selenium is important for broodmares and foals
Although a horse’s selenium requirement is small, selenium intake during pregnancy and lactation is crucial to avoid developmental abnormalities. In growing foals, selenium has its greatest effects on skeletal and muscular development. At its mildest, selenium deficiency causes slight growth retardation; at its worst, muscular degeneration and cardiac muscle development disorders can already be observed in newborn foals. Selenium is also important for the development and maintenance of immune defense.
In the milk of lactating mares receiving adequate selenium, there are significantly more IgG antibodies—vital for the foal’s immune protection—than in mares suffering from selenium deficiency. In mares, selenium deficiency can also cause reproductive disturbances, such as difficulty conceiving.
Organic selenium supports the mare’s wellbeing, and it is also transferred via the placenta to the fetus—the future foal. Foals from mares receiving organic selenium have been found to have higher blood selenium concentrations. Selenium is also transferred from the mare’s body into milk, and studies have shown that milk selenium concentrations have been higher in mares that received organic selenium. Adequate selenium intake is important for a newborn foal to ensure a strong start in life.
References:
- National Research Council. (2007). Nutrient Requirements of Horses, 6th rev. ed. National Academies Press.
- Pagan, J. et al. (1999). The effect of selenium source on selenium digestibility and retention in trained Thoroughbreds.
- Proc. Equine Nutr. Physiol. Soc. 2–5. *Deride, C., R. Chihuailaf, V. Arnés, G. Morán, and B. Uberti. 2023. Relationship between selenium, copper, zinc and their biomarkers in blood and skeletal muscle tissue in adult horses from Southern Chile. Journal of Equine Veterinary Science 128:104881
