Selenium and vitamin E are essential antioxidant nutrients for all mammals, including horses. Together with vitamin C, they help minimize the damaging effects of oxidative stress in all cells and tissues of the body.

Although these nutrients are required in only low levels in the diet, they have profound effects on the horse’s immune system, muscle function, nervous system, and recovery from illness.

Deficiencies in selenium and vitamin E diminish protection against oxidation, which can contribute to cellular damage and premature aging. Prolonged deficiency in either of these nutrients can result in muscular and neuromuscular diseases.

Horses fed hay exclusively are susceptible to Vitamin E deficiency, as levels of this nutrient diminish when grass is harvested for hay. Likewise, selenium deficiency is common in horses from coastal regions of North America where forages have low concentrations of this trace mineral.

To prevent nutritional deficiencies, supplementation with these antioxidants is often necessary. Obtaining a forage analysis will help you determine nutrient levels in your hay or pasture so you can formulate a balanced diet.

Selenium for Horses

Selenium is an essential trace mineral for horses that plays several important roles in their health and well-being. It supports the immune system and is involved in the production and function of white blood cells, which help fight off infections.

This trace mineral is also a component of the antioxidant enzyme glutathione peroxidase, which helps protect the body from oxidative damage by neutralizing free radicals. Furthermore, selenium is important for thyroid hormone metabolism and plays a role in muscle function.

When selenium’s biological effects were first discovered in the 1930s, it was considered a toxic element because of its association with neuropathy of grazing horses and cattle. [1]

Despite these initial fears about selenium toxicity, it has since been observed that deficiency is a more common concern. In one study, over 37% of adult horses were considered to be selenium deficient. [26]

In the 1950s, scientists discovered the health benefits of selenium when it was found to prevent illnesses in animals and humans. [1]


Physiological Roles

Selenium is a critical component of several different proteins in the body that are collectively known as selenoproteins. These proteins include enzymes that are involved in various important bodily functions.

Some of the functional mechanisms of selenoproteins include the following effects: [1]

  • Antioxidant
  • Anti-inflammatory
  • Anti-carcinogenic
  • Anti-viral
  • Anti-bacterial
  • Anti-fungal
  • Anti-parasitic

The best understood and most important physiological role for selenium is as an antioxidant, as part of the enzyme glutathione peroxidase.

Antioxidant protection is particularly important when horses experience increased stress due to factors such as advanced age, exercise, infection or recovery from injury. [2]

As a component of deionidase enzymes, selenium is also necessary for regulation of thyroid hormone production. The thyroid hormones, triiodothyronine (T3) and thyroxine (T4), are key regulators of animal development, growth, and metabolism. [1]

Selenium in the Equine Diet

Selenium concentrations in forages (such as hay and pasture grass) can vary widely based on the selenium content in the soil where they are grown.

Some parts of North America (especially coastal regions) have soils with very low selenium levels, while central North America tends to have adequate selenium levels in soil. Soil concentrations range from 0.1 parts per million (ppm) in deficient areas to over 10 ppm in selenium-rich areas. [3]

Selenium Map of Canada the USA

Low Selenium Areas

Soils that contain between 0.1 to 0.6 ppm selenium are considered deficient, resulting in forages with inadequate selenium concentration. Areas with low selenium content in the soil include: [3]

  • The Pacific Northwest
  • The Great Lakes region
  • The Eastern seaboard of Canada and the US

Other areas known to have low concentrations of selenium in the soil include central Asia, Australia, Africa, and parts of South America. [4]

High Selenium Areas

Areas with high selenium concentrations typically include the midwestern states in the US. Horses consuming forages grown in soils from these regions may not need supplements to meet their nutritional requirement.

However, in rare cases, forages and other crops grown in these regions can accumulate selenium to the point of being toxic to animals.

In a U.S. survey of selenium toxicity and deficiency in a variety of animals, disease due to selenium deficiency were reported in 46 states. Naturally occurring selenium toxicity was rare, only reported in 7 states, whereas toxicity due to over supplementation was reported in 15 states. [27]

Other factors, including soil characteristics and weather conditions, also impact how much selenium accumulates in plants. For example, plants grown in alkaline soils (pH > 7.0) or during periods of low rainfall can accumulate higher levels of this trace mineral, potentially contributing to selenium toxicosis in animals. Such conditions are common in areas like South Dakota. [3]

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