SELENIUM: ON THE FINE LINE BETWEEN NECESSITY AND RISK | PART II

Ing Lucie Humeni

Underestimated Selenium Excess

While in the past most attention was focused primarily on deficiency, current practice increasingly encounters the opposite extreme – excessive selenium supplementation.

This is often caused by the accumulation of several sources at the same time. A horse may receive a mineral feed, a vitamin-mineral supplement, a compound feed containing a premix, and possibly additional products intended to support muscles or hoof health. The owner often does not calculate the horse’s total selenium intake.

Chronic selenium toxicity in horses is primarily manifested by:

• cracking and deformation of the hoof horn,
• hoof cracks,
• loss of mane and tail hair,
• deterioration in coat quality,
• weight loss,
• movement disorders.

Acute selenium toxicity may cause neurological signs, colic, diarrhea, or respiratory disturbances. Horses are generally more sensitive to excess selenium than, for example, cattle. In addition, the difference between the recommended and potentially toxic dose is not as wide as it is for many other minerals. For this reason, it should not automatically be assumed that “more is better.”

Which Is More Dangerous: Selenium Deficiency or Excess?

Both situations pose a health risk, but in terms of acute risk, excess selenium is generally more dangerous. A horse’s body can tolerate a mild deficiency for a certain period, particularly if the horse is not subjected to high physical demands. Chronic deficiency, however, gradually leads to impaired muscle metabolism, reduced immune function, and decreased overall vitality.

In contrast, toxicity can damage tissues relatively quickly, and some changes, such as damage to the hoof horn, may have long-term consequences. In practice, it is therefore important to avoid both extremes.

The most reliable approach is not preventive administration of high doses, but knowing the actual selenium intake from the entire diet.

Why Do Some Areas of Europe Have More Selenium and Others Less?

The selenium content of feed is closely related to the geological and climatic conditions of a given region. Plants take up selenium from the soil, and when the soil is deficient in this element, low selenium concentrations subsequently occur in hay, pasture, and cereals as well.

European studies have consistently confirmed substantial differences between coastal and inland regions. Higher selenium concentrations are often found in countries closer to the ocean, such as the Netherlands and Denmark. In contrast, Central Europe, including the Czech Republic, Austria, and some regions of Germany, has traditionally been considered an area with lower selenium concentrations in the soil.

One possible explanation is the cycling of selenium between the ocean and the atmosphere. Some selenium is released from the marine environment into the atmosphere and subsequently returned to the soil through precipitation. Coastal regions may therefore naturally receive greater selenium deposition. However, bedrock type, soil pH, organic matter content, and the plants’ ability to take up selenium also play important roles.

Low selenium concentrations are particularly common in acidic soils and areas with high precipitation, where selenium may be leached from the soil. In the Czech Republic, it is also important that a large proportion of soils developed on geological substrates that are naturally low in selenium.

This helps explain why hay or pasture can contain substantially different levels of this element, even between individual regions.

Is the Czech Republic Still a Selenium-Deficient Region?

Until several years ago, it was commonly stated in scientific and professional circles that the Czech Republic was a selenium-deficient region. Research did indeed confirm low selenium concentrations in both soil and forage. Studies investigating selenium transfer through the soil–plant–feed system in horses in the Czech Republic demonstrated low selenium concentrations in both soil and forage. Forage alone would therefore often be unable to fully meet the horse’s selenium requirements.

At the same time, modern feeding practices present a somewhat different picture than they did twenty years ago. Today, most sport and recreational horses routinely receive mineral supplements, commercial muesli feeds, or pelleted compound feeds containing supplemental selenium. As a result, widespread selenium deficiency is probably less common than it was in the past.

European studies evaluating selenium concentrations in horse blood confirm that selenium status is strongly influenced by feeding practices. Even in regions with low selenium concentrations in the soil, horses may have adequate selenium levels as a result of supplementation. This does not mean that the problem has disappeared. Rather, its nature has changed.

In horses fed only hay and home-grown cereals without mineral supplementation, deficiency can still represent a real risk. Conversely, in horses receiving several supplements simultaneously, the opposite problem – excessive selenium intake – is increasingly encountered.

For this reason, it is no longer sufficient to simply state that “there is little selenium in the Czech Republic.” What matters is the specific diet, the origin of the forage, and the actual selenium content of all products being fed.

Laboratory Analysis Is More Valuable Than Estimation

When selenium deficiency or excess is suspected, laboratory testing provides the most useful information. In practice, selenium concentration can be measured in whole blood or serum, together with glutathione peroxidase activity.

Regular monitoring may be particularly appropriate for sport horses, breeding mares, foals, or horses with long-term muscle problems.

Conclusion

The Czech Republic is still considered an area with relatively low selenium concentrations in the soil, but thanks to improved knowledge of equine nutrition and feeding practices, selenium deficiency is probably less common than it was in the past. Current feeding practices also demonstrate that, alongside deficiency, attention should be paid to the risk of excessive supplementation. A balanced intake, regular evaluation of the overall diet, and laboratory monitoring are more important in selenium nutrition than preventive administration of high doses.

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.