Selenium is an essential trace element with several important roles in the horse’s body. Selenium is involved in approximately 20–30 different enzymes that are crucial for bodily function. One of its most important tasks is to participate in the body’s defence reactions and to function as an antioxidant. In other words, it helps protect cells from oxidative stress and supports the normal function of the immune system. Selenium is also essential for thyroid function, immune resilience, and fertility in horses.
Selenium is especially important for sport horses, because hard work increases the amount of oxidative reactions occurring in the body. It has also been shown to influence both performance and recovery. Horses that receive more selenium from their diet have been observed to recover faster after exertion, both in terms of heart rate and breathing rate*.
Selenium levels in plants and grains vary depending on soil content. In the Nordic countries, soils are low in selenium, which is why selenium is often added to industrial concentrates and mineral feeds, as well as to fertilisers. Generally speaking, garlic and bran contain fairly good amounts of selenium. Selenium deficiency is a risk for animals in regions where soil selenium levels are low.
The antioxidant pair: selenium and vitamin E
Vitamin E is a fat-soluble vitamin that the horse must obtain from the diet daily. Selenium is a water-soluble trace element that is needed only in very small amounts, and in excessive intake it is toxic. Vitamin E and selenium work together by protecting the body’s cells from oxidative reactions.
In the horse’s body, selenium functions, among other roles, as part of the enzyme glutathione peroxidase in fat metabolism. Glutathione peroxidase protects cells from harmful oxidation. Selenium requirements are also influenced by the amount of vitamin E obtained from the diet. Too low a selenium intake can be partially compensated by vitamin E intake above recommended dosing, and vice versa.
Selenium is also important for fertility and reproductive functions. In addition, it binds toxic heavy metals such as mercury, lead, and cadmium. Selenium is also important for normal thyroid function.
Selenium in feeds: inorganic vs. organic
Selenium is low in Nordic soils, so it is often added to industrial concentrates and mineral feeds, and also to fertilisers. Traditionally, selenium has been added to feeds in an inorganic form, as sodium selenium. A more effective approach is to provide selenium in an organic form. That is, in the form in which it naturally occurs in plants.
In plants, selenium occurs as selenomethionine or selenocysteine. It is digested and absorbed significantly more efficiently than inorganic selenium. This helps ensure an adequate selenium intake that the body can actually utilise. Organic selenium has many benefits for horses. It can be stored in muscle tissue and support performance and recovery in sport horses*.
In pregnant mares, organic selenium can also pass through the placenta during pregnancy for use by the foetus. Foals born to mares receiving organic selenium have been shown to have higher blood selenium concentrations. In addition, organic selenium supports a higher selenium concentration in the milk of a lactating mare. Through this, it supports selenium intake for the foal. Organic selenium raises horses’ serum selenium levels more easily and more quickly to an adequate level compared to inorganic selenium.
Selenium deficiency and its importance for breeding horses
Selenium deficiency is fairly rare, except in very young foals under one month of age. In those cases, the deficiency usually results from too low selenium and/or vitamin E intake in the mare during lactation. Selenium is therefore particularly important for breeding horses, as it has been shown to influence the mare’s wellbeing and ability to conceive. In foals, selenium and vitamin E deficiency can cause muscle degeneration. It can also predispose the foal to different inflammatory diseases and slow growth. In addition, horses suffering from certain muscle disorders, such as PSSM, are often recommended higher-than-normal vitamin E and selenium dosing.
Selenium deficiency can predispose horses to illness, hypothyroidism, poor performance, muscle problems, a poor coat, or premature ageing. Excess selenium, on the other hand, can damage organs such as the liver and heart. It can also cause degeneration of bones and joints. The presence of certain minerals, such as sulfur and copper, can reduce selenium toxicity. They can do this either by interfering with selenium absorption or by increasing selenium excretion.
In general, during the grazing season horses do not suffer from vitamin E or selenium deficiency, because green pasture grass contains high amounts of vitamin E.
Whether your horse is getting enough selenium can be easily checked with a blood test. Selenium status can be measured from blood.
Sources:
*Hornic et al. 2008
NRC 2007. Nutrient Requirements of Horses, sixth revised edition. National Academic Press, Washington, D.C., USA.
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