Selenium in a horse’s diet – how much to give, which form is best, and what are the symptoms of deficiency and excess? | Equibiotic.com

Although horses need selenium only in trace amounts, it plays a crucial role in their health. So how can it be supplemented wisely and safely?

Selenium is one of those trace elements for which the saying “less is more” truly applies. Even though horses require it in very small quantities, its role in the body is invaluable. It supports muscle health, fertility, and immune function, and protects cells from oxidative stress. The problem is that the line between deficiency and toxicity is very narrow. That is why it is so important to know how much selenium to provide, how to test selenium levels, and how to supplement selenium in horses in a safe manner.

Selenium in a horse’s diet

• why selenium is crucial for horses’ health,
• what the symptoms of deficiency and excess are,
• how much selenium a horse actually needs in its diet,
• how to choose the best form of selenium to ensure it is effective and safe.

Why is selenium important for horses?

Although selenium is present in a horse’s body only in trace amounts, it is of great importance for its health and performance. It is one of the key trace elements whose deficiency very quickly affects muscle condition, immune function, and the animal’s overall fitness.

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1. Protection of muscles against damage
During physical exertion, a horse’s body produces large amounts of free radicals that damage muscle fibers and slow down regeneration. Selenium, as a component of the enzyme glutathione peroxidase, neutralizes these harmful particles and protects muscles from oxidative stress. As a result, sport and working horses recover faster after training and are less prone to injuries.

2. Synergy with vitamin E
Selenium and vitamin E are an invaluable duo—both act as antioxidants whose effects complement each other. Vitamin E protects cell membranes, while selenium works deeper, inside the cells. Together, they support post-exercise muscle recovery, immune function, and the horse’s overall vitality. A deficiency of one intensifies the effects of a deficiency of the other.

3. Support for fertility
In stallions, selenium plays an important role in testosterone production and semen quality. Deficiencies of this trace element can lead to reduced fertility, and in mares—to reproductive problems. For this reason, selenium is particularly important in the diet of breeding horses.

4. Stronger immunity
A horse’s immune system also benefits from selenium’s antioxidant protection. Thanks to selenium, immune cells are less susceptible to damage and can fight infections more effectively. In practice, this means a lower susceptibility to illness and a faster recovery after infections or injuries.

5. Metabolic balance and a healthy thyroid
Selenium is involved in the metabolism of thyroid hormones, which regulate metabolism and the rate of energy turnover. Disturbances in this area can lead to weight problems, lethargy, or reduced performance in horses. An adequate amount of selenium helps maintain metabolic balance and stable energy levels during work.

Where does a horse get selenium from?

The main source of selenium in a horse’s diet is forage—grasses and hay. However, the selenium content of these feeds largely depends on soil type. In many regions of Europe, soils are low in selenium, which automatically increases the risk of deficiencies in horses fed exclusively on hay and pasture.

Selenium is also found in plant products and grains, although its level can vary significantly depending on where they are grown. Good sources may include cereals, wheat and rice bran, and certain plants such as garlic. However, if these feeds come from selenium-poor areas, their value as a selenium source will be limited.

Other sources of selenium in a horse’s diet include:

• Concentrates and complete feeds – often fortified with added selenium.
• Mineral salt licks – they may help support selenium intake, but the exact amount consumed is difficult to control.
• Supplements – the most precise way to adjust selenium intake, especially when there is a risk of deficiency or when a horse has special needs (e.g. sport, breeding, recovery).

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Selenium deficiency and excess – how to recognize them and why are they dangerous?

Selenium plays extremely important roles in a horse’s body—above all, it acts as an antioxidant, protecting cells from oxidative damage. Both deficiency and excess of selenium can have a serious impact on a horse’s health and condition. What’s more, the symptoms are often non-specific and easy to overlook, especially in the early stages.

Symptoms of selenium deficiency:

• muscle stiffness and soreness, tremors, and rapid fatigue,
• reduced immunity and increased susceptibility to infections,
• reproductive problems in stallions, retained placenta in mares,
• poor coat quality, exercise intolerance, premature signs of aging,
• in foals: so-called white muscle disease (muscle degeneration, difficulty moving),
• in severe cases: cardiomyopathy, myositis, growth retardation, or cataracts.

Symptoms of excess (toxicity):

• loss of mane and tail hair, general hair loss,
• brittle, cracking hooves, painful lameness, reluctance to move,
• loss of appetite and weight loss,
• damage to the liver, heart, and skeletal muscles, degeneration of bones and joints,
• in cases of chronic poisoning: so-called alkali disease, which can be fatal.

Why do imbalances occur so easily?

Deficiency occurs in regions with selenium-poor soils—this is relatively common in Europe. Forage feeds (grasses and hay) from such areas contain little selenium, which automatically increases the risk of deficiency.

Toxicity threatens horses grazing in areas with excessive selenium levels in the soil or when consuming plants that “accumulate” this element (e.g. certain species of vetch or plants from the aster family).

Subclinical deficiencies – a silent problem

Many horse owners may not realize that their animal is suffering from selenium deficiency, as the initial symptoms are difficult to detect. These include fatigue, a dull coat, or poorer recovery after exercise. Only prolonged deficiency leads to more serious health consequences.

Horses’ selenium requirements – how much is really needed?

The National Research Council (NRC, 1989) defines the minimum selenium requirement for horses at 0.1 mg/kg of diet (ppm). In practice, this means that an adult horse weighing approximately 450 kg and consuming 10 kg of feed per day needs about 1 mg of selenium daily to prevent clear deficiency symptoms. The FDA has established the same value as the legal limit for selenium supplementation in feeds.

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It should be emphasized, however, that this is a minimum value, sufficient only to prevent deficiency—not necessarily to maintain optimal health and performance. In practice:

• Leisure horses usually require about 1–2 mg of selenium per day,
• Sport horses—due to the higher oxidative load associated with exercise—may need as much as 2.5–3.5 mg per day,
• Breeding horses (stallions and pregnant mares) also show increased requirements, often above 2 mg per day.

On average, it is assumed that horses require 1–3 mg of selenium per day, depending on body weight, activity level, and physiological status.

Which form of selenium is best?

Selenium in feeds and supplements can occur in different forms, and this significantly affects its bioavailability and safety:

• Inorganic forms (sodium selenite, sodium selenate) – these are the cheapest and most commonly used sources. They can be absorbed effectively, but are less stable, less bioavailable, and carry a higher risk of overdosing.
• Organic forms (e.g. selenium-enriched yeast, selenocysteine, selenomethionine) – these mimic the natural forms of selenium found in plants. Studies show they are significantly more bioavailable than inorganic forms, raise blood selenium levels in horses more efficiently, and are safer to use.

Particular attention should be paid to selenomethionine—a natural, organic form that the horse’s body can easily utilize, store in tissues, and gradually release when needed. As a result, it supports not only cellular protection against oxidative stress, but also muscle health and reproductive function.

Selenium deficiency? Here is our Equi Selen™

If you are looking for a safe and effective way to support your horse’s muscles and immune system, choose Equi Selen™– a hypoallergenic supplement based on selenomethionine.

See Equi Selen in the Equibiotic.com store 

What makes Equi Selen™ stand out?

✅ Highly bioavailable selenomethionine – the best form of selenium, supporting muscle health and stallion fertility.
✅ Vitamins E, C, and K2 (MK-7) – powerful antioxidant, circulatory, and immune support.
✅ Lysine and methionine – essential for muscle tissue building and regeneration.
✅ Turmeric, rosemary, nettle – natural ingredients supporting metabolism, joint health, and immunity.
✅ Apple pomace – improves palatability and supports gut health.

Who is it especially recommended for?

• sport horses in intensive training,
• stallions of breeding age,
• horses after muscle injuries and during recovery,
• horses exposed to oxidative stress and rapid fatigue.

Hypoallergenic formula:

Free from molasses, grains, alfalfa, and yeast – safe also for horses with allergies and sensitive digestive systems.