SELENIUM: ON THE FINE LINE BETWEEN NECESSITY AND RISK – PART I

Ing. Lucie Humeni

Selenium is a trace mineral that horses require only in very small amounts, yet it plays a crucial role in numerous metabolic processes. In the body, selenium primarily functions as a component of selenoproteins and enzymes with antioxidant activity, particularly glutathione peroxidase. This enzyme protects cell membranes from damage caused by free radicals generated during normal metabolism as well as during physical exercise. However, the importance of selenium extends beyond the protection of muscle tissue. It also contributes to the proper functioning of the thyroid gland, immune system, reproductive system, and metabolism.

Selenium clearly illustrates how narrow the boundary can be between an optimal intake and a health problem. A deficiency can lead to muscle disorders and a weakened organism, while excessive intake poses a risk of toxicity. In practice, the issue is therefore not simply “how much selenium to add,” but primarily knowing the horse’s total selenium intake from forage, concentrate feeds, and mineral supplements.

How Much Selenium Does a Horse Actually Need?

According to the NRC (National Research Council), the recommended daily selenium requirement for an adult horse weighing 500 kg is approximately 1 mg of selenium per day under maintenance conditions and during light work. Higher physical workload does not automatically mean that selenium requirements increase dramatically. Although the body produces more free radicals during intense exercise, the situation also depends on adequate vitamin E intake, the quality of the basic diet, and the horse’s overall health status.

An increased selenium intake may be warranted particularly when:

• the diet contains forage from areas with low selenium concentrations in the soil,
• predominantly home-produced hay is used without regular laboratory analysis,
• the horse is involved in intensive athletic training,
• mares are in the last trimester of pregnancy or are lactating,
• the horse shows muscular signs such as reluctance to move after exercise, muscle soreness, or stiffness,
• laboratory blood tests confirm low selenium status and changes in muscle enzymes (CK, AST).

Even in these situations, however, supplementation should not be based on guesswork. Selenium has a relatively narrow safety margin, and prolonged excessive intake can be highly dangerous for horses.

A total intake of approximately 3 mg per day for a 500 kg horse is often considered the upper safe limit. However, toxic doses also depend on the chemical form of selenium, the duration of supplementation, and the individual sensitivity of the animal.

What Happens in Selenium Deficiency?

Clinical signs of selenium deficiency are closely associated with muscle metabolism and the body’s antioxidant defense. The best-known condition is nutritional myodegeneration, also known as “white muscle disease.” It most commonly affects foals, but problems can also occur in adult horses.

Typical signs include:

• muscle weakness,
• stiffness,
• reluctance to move,
• impaired recovery after exercise,
• muscle soreness,
• reduced performance,
• impaired immune function,
• reproductive problems.

In foals, severe deficiency can lead to damage to the cardiac or respiratory muscles. Selenium deficiency is also often associated with inadequate vitamin E intake, as these two antioxidants are closely linked in their functions.

In everyday practice, subclinical deficiency may manifest very subtly. A horse may be less willing to work, recover more slowly, or experience recurrent muscle problems without an obvious cause. This is why laboratory blood testing may be useful, along with monitoring glutathione peroxidase activity.

References

Ferrari, L. et al., 2023. Advances in Selenium Supplementation and Selenium Status in Horses. Animals. Online. ScienceDirect. Accessed: 25 May 2026.

Mendoza-Núñez, V. et al., 2023. Influence of Dietary Selenium on Oxidative Stress in Horses. Biological Trace Element Research. Online. SpringerLink. Accessed: 25 May 2026.

Araya, O. et al., 2023. Selenium Biomarkers in Horses: Relationship Between Blood and Muscle Selenium Concentrations. Journal of Equine Veterinary Science. Online. ScienceDirect. Accessed: 25 May 2026.

Zakova, N. et al., 2016. The Soil-Plant-Feed Transport of Selenium and Other Essential Micronutrients in Diet of Sport and Recreational Horses at Two Different Locations. Journal of Animal and Feed Sciences. Online. Journal of Animal and Feed Sciences. Accessed: 25 May 2026.

Valberg, S.J., 2024. Nutritional Myopathies in Horses. MSD Veterinary Manual. Online. MSD Veterinary Manual. Accessed: 25 May 2026.

National Research Council, 2007. Nutrient Requirements of Horses. Sixth Revised Edition. Washington, DC: National Academies Press. Online. National Academies Press. Accessed: 25 May 2026.