Both selenium and vitamin E play an important role in mitigating free radicals and oxidative damage in horses. Every day functions, including metabolism and the process of converting fuel consumed as fats, carbohydrates and proteins into energy, creates free radicals and other unstable compounds. These molecules are highly reactive as they have an unpaired electron, so they will steal an electron from another molecule to stabilise themselves. This, in turn, creates another free radical, and a chain reaction starts, which can cause major damage to a previously healthy cell. While free radicals are a normal part of many bodily processes, at high levels they can damage major components of cells, including DNA. The body protects itself against these free radicals using antioxidants. These are compounds that ‘neutralise’ free radicals, making them stable, and preventing them from causing damage. Infection, inflammation and high levels of exercise all increase the horse’s requirements of antioxidants. These are compounds that ‘neutralise’ free radicals, making them stable, and preventing them from causing damage. Certain antioxidants can be produced in the body, and others need to be obtained from the diet. Some that can be made by the horse, but in certain situations, such as high workload, injury or illness that increase requirements, additional levels provided through the diet may be helpful.
Counteracting extra free radicals from higher workloads
When the horse’s workload increases, more fuel is converted to energy and more free radicals are produced, so more antioxidants are needed. In addition to this, diets high in oil require additional antioxidants to deal with the extra free radicals produced when oils are metabolised. Vitamin E plays an important role in this respect and has also been demonstrated to help maintain muscle health in horses with myopathies such as Polysaccharide Storage Myopathy (PSSM).
While vitamin E is an antioxidant, selenium is a vital co-factor for the antioxidant family of enzymes, the glutathione peroxidase group. Selenium and vitamin E are often discussed together, as a deficiency in one can result in an increased requirement of the other. Glutathione peroxidase enzymes are produced by the horse, with production capacity increasing in response to exercise training. However, as selenium from the diet is required for several of these enzymes to function, production of them can only increase when adequate selenium is available. While a horse that has a selenium intake below requirements will produce more glutathione in response to supplementation, feeding selenium more than required does not stimulate extra glutathione peroxidase production, and indeed, high levels of selenium are toxic.
Selenium content in forage and feed
Levels of selenium in cereals, grains, grass and forage are dependent on levels in the soil. Various areas of the UK have very low soil selenium content, which translates to low levels in any crops grown in those areas. While there are maps depicting areas that are likely to be deficient, the levels can change from field to field, and according to weather conditions affecting the depth of minerals in the soil, as well as what type of fertilisers have been used and when.
Daily recommended intake levels for horses according to the National Research Council are 1-1.25mg per day for a 500kg horse, rising to 2-3mg for horses in hard work and pregnant mares in the last trimester.
Antioxidants and fertility
Antioxidants have been shown to support fertility and reproductive health in both mares and stallions. In stallions, deficiencies of selenium have been shown to lead to lower levels of fertility, and some sperm may form abnormally. Adequate levels of dietary antioxidants can maintain normal sperm activity. Selenium is also involved in many aspects that affect mares’ fertility, and it is known that mares with selenium deficiencies are more likely to have reproductive problems.
Foal health
Adequate provision of selenium and vitamin E is essential to produce a healthy foal, as these nutrients affect immunity, both while being carried by the mare and, after birth, through lactation. Mares consuming adequate selenium (3mg per day) produced milk with higher levels of IgG antibodies, which are an important part of the immune system.
Summary
In summary, selenium and vitamin E are fundamental to the horse’s ability to manage oxidative stress, supporting everything from cellular health and muscle function to fertility and immune status. While these nutrients play distinct roles, their close interaction means that intakes of one can influence requirements of the other, and an increased intake of both can be supportive during periods of increased demand, such as intense exercise, reproduction, or illness.
Given the variability of selenium levels in forage and the increased requirements seen in modern equine management, careful attention to dietary intake is essential. However, balance is key—while insufficient levels can impair performance, fertility, and immunity, excessive selenium intake carries a real risk of toxicity and offers no additional benefit.
Ultimately, providing appropriate levels of selenium and vitamin E, tailored to the individual horse’s workload, diet, and physiological status, is a practical and effective way to support overall health, resilience, and long-term wellbeing.