Transcript:

[0:00]

Hi everyone, welcome back to Mad Barn Academy, and if you’re new here, welcome — we hope to earn your subscription today. A big thank you to everyone who’s liked and subscribed so far, we greatly appreciate the continued support. I’m Dr. Fran Row, one of the veterinary nutritionists here at Mad Barn. Today we’ll be following up on our vitamin E video by talking about selenium. Like vitamin E, our team gets many questions about selenium — mainly “Am I giving too much?” and “Is my horse at risk for toxicity?” While many horse owners are cautious about selenium due to toxicity risk, we also find that many don’t realize their horses might actually be deficient. So today we’ll dive into selenium, why ensuring appropriate intake matters, and some tips for supplementation.

[1:06]

Selenium is an essential trace mineral with numerous critical roles in neuromuscular, immune, and reproductive health. The main biological form, selenocysteine, is an analog of the amino acid cysteine where sulfur is replaced by selenium. Nearly all known selenium-containing proteins and enzymes use selenocysteine at their active catalytic site. Selenium can be acquired from the diet in two main forms: organic and inorganic. Organic forms are more bioavailable and preferred.

[2:12]

One of selenium’s best-understood functions is as an antioxidant, protecting cell membranes from oxidative damage. Selenium and vitamin E often work together in this role. Examples of selenoproteins include glutathione peroxidase (GPx), which converts harmful peroxides into inert molecules, and thioredoxin reductase (TrxR), which regenerates oxidized vitamin E so it can keep working. Other important functions include supporting thyroid function, reproductive health (especially male fertility), muscle health, and immune function. Some mechanisms remain under active research.

[3:46]

The NRC baseline requirement is 0.1–0.125 mg/kg of dry matter per day, depending on physiological status. These are minimums to prevent deficiency; optimal health and performance may require double or triple this amount. For a 500 kg horse consuming 2% of body weight in dry matter (10 kg/day), the target would be 1–1.25 mg/day at baseline, with optimal intake closer to 2–3 mg/day. The upper tolerable limit is 2 mg/kg dry matter — about 20 mg/day for an average horse — meaning there’s a significant gap between optimal and toxic intakes.

[5:09]

Horses get selenium from forages, but content depends on soil selenium. Low-selenium soils are common in the Pacific Northwest, Great Lakes, and much of the Eastern Seaboard. Adequate soils are found mainly in the Midwest. Accumulation is also influenced by soil pH, moisture, and weather — alkaline soils or drought conditions can increase selenium content, while acidic or wet conditions can decrease it. Forage testing is the best way to determine selenium content in your pasture or hay.

[6:25]

Selenium deficiency is most common in low-selenium regions and leads to nutritional myodegeneration (NMD), also called white muscle disease. Highly metabolically active muscles — heart, diaphragm, gluteals, tongue — are most affected. While it can occur in adults, it’s most common in foals. Signs include progressive weakness, increased lying down or inability to stand, difficulty chewing or swallowing, muscle tremors, elevated heart and respiratory rates, shallow breathing, and in severe cases, sudden death from heart failure. Early detection in foals can improve recovery chances, but advanced cardiac involvement is usually fatal. Prevention starts in utero — pregnant mares need adequate selenium during gestation to protect the foal.

[8:37]

Selenium toxicity, on the other hand, occurs when intake exceeds the tolerable limit (about 20 mg/day for an average horse). This is much higher than the 2–3 mg/day optimal target, but it’s possible with over-supplementation, formulation errors, or ingestion of selenium-accumulating plants (common in the American mid- and southwest). Acute toxicity usually causes sudden death, often from a massive overdose. Chronic toxicity develops from moderately high intake over time and is known as “bobtail disease” or “alkali disease,” characterized by tail hair loss and other signs.

[11:07]

Selenium status is best assessed by measuring whole blood selenium (more accurate than serum for current status). For deficiency, vets may use a series of intramuscular selenium injections to quickly raise levels. For toxicity, treatment is supportive and focuses on lowering dietary selenium.

[12:01]

For supplementation, choose organic selenium (e.g., selenium yeast or selenomethionine) for better bioavailability and lower toxicity risk. Inorganic forms are listed as selenate, selenide, or selenite. If you’re in a deficient region, supplementation is almost always necessary; in marginal or adequate regions, forage testing is recommended to guide decisions. Local variations mean testing is the only way to be certain. When in doubt, consult a nutritionist — links to Mad Barn’s forage analysis and diet evaluation services are provided in the description.

[13:30]

Thanks for listening! Don’t forget to like and subscribe, and check out my recent vitamin E video, which pairs well with today’s discussion. Additional links are in the description. Until next time!