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About natural supplements · Mar 28, 2013

Most horse mineral supplements are either useless or actively dangerous

Selenium’s lethal margin is razor-thin, inorganic minerals resist absorption, and over-supplementing the wrong minerals can kill, here’s what the science actually says.

Dapple grey horse at a stable yard feed station with supplement tubs and powder scattered nearby

By Beryl Shuttleworth, BSc Hons Biochem

Last reviewed: 2026-06-12

Mineral supplementation in horses carries real risks that make casual, blanket supplementing a bad idea. Selenium is the clearest danger: the dietary requirement is just 0.1 mg/kg feed dry matter, yet chronic selenosis develops at intakes above 5 mg/kg, and acute overdose can kill within 36 hours. Inorganic mineral salts, the cheapest and most common supplement form, have poor bioavailability and increased excretory rates, raising both absorption and environmental concerns. Forage mineral content varies, and the only reliable way to know what your horse is actually getting is to test it.

More selenium is better, especially for hard-working horses

I understand why people believe this. Selenium deficiency is real, it causes muscle problems, and the instinct when you’re worried about your horse is to err on the side of more. The supplement industry doesn’t exactly discourage that thinking either.

But selenium is not like most minerals. It has one of the narrowest margins of safety of any nutrient your horse will ever consume. The dietary requirement is estimated at just 0.1 mg/kg feed dry matter, and chronic selenosis begins when intake exceeds 5 mg/kg [1]. In practical terms, that gap between “enough” and “too much” is uncomfortably small.

Acute overdose is not a slow, manageable process. Horses can die peracutely with no prior warning signs, or deteriorate rapidly, sweating, abdominal pain, cardiovascular collapse, pulmonary oedema, within hours or days of a single excessive dose [2]. Experimental work in ponies confirms this: animals given 6-8 mg selenium per kilogram of body weight developed colic, diarrhoea, tachycardia, and lethargy, and several died within 36 hours [3].

Does that mean you never supplement? No. Studies suggest that up to 3 mg daily may support immune function in horses in heavy work, and surveys do find that a meaningful minority of horses carry deficient or marginal blood selenium concentrations [11]. The point is that supplementing without knowing your horse’s baseline status, through blood testing and forage analysis, is genuinely dangerous, not just theoretically cautious. And if you do supplement, source matters: organic selenium raises plasma and red blood cell selenium concentrations more effectively than inorganic forms [5]. Getting the dose right from a well-mixed, quality-controlled product isn’t a luxury; with this particular mineral, it’s the whole ballgame.

How minerals work, and where supplementation goes wrong

Selenium sits at the sharp end of mineral safety. The dietary requirement is estimated at just 0.1 mg/kg feed dry matter, and chronic toxicosis begins when intake exceeds 5 mg/kg, a narrow gap that leaves almost no room for error. In acute overdose, horses develop abdominal pain, profuse sweating, tachycardia, and lethargy; cardiovascular collapse and pulmonary oedema can kill within hours or days. Pony trials make the danger concrete: animals given 6-8 mg selenium per kg of bodyweight without protective copper pretreatment died within 36 hours.

Organic selenium behaves differently from inorganic forms. Supplementation with organic selenium raises both plasma and red blood cell selenium concentrations more effectively than inorganic selenium, and it also increases relative lymphocyte expression of IL-5, pointing to a measurable effect on immune function, not just tissue loading.

Inorganic mineral salts, the cheap default in many supplements, show poor bioavailability because other co-elements in the gut compete and inhibit absorption. Feeding inorganic minerals also increases their excretory rates, which carries an environmental cost alongside the nutritional one.

Zinc excess creates its own downstream problem: intakes above 1,000 ppm have been shown to induce copper deficiency and developmental orthopaedic disease in young horses. Copper itself is required for enzymes involved in connective tissue formation and iron mobilisation.

Long-term magnesium oversupplementation carries a different risk, it may increase the likelihood of enterolith formation. Enteroliths are concretions of magnesium ammonium phosphate crystals, and large-colon contents from affected horses show elevated magnesium, calcium, and phosphorus concentrations compared with horses whose colic has another cause.

Finally, blood concentrations are a poor guide to dietary intake for macrominerals, particularly calcium. The only reliable way to know what your horse is actually consuming is to have the forage tested.

How to supplement minerals safely, what the evidence actually supports

Start with forage testing, not guesswork

The only reliable way to know what your horse is getting is to have the forage tested by a forage-testing laboratory. Blood concentrations do not reflect dietary intake adequately for macrominerals, so a blood result alone will not tell you whether the ration is balanced, you need to look at the total mineral contribution from every part of the diet before adding anything.

Selenium: the mineral where the margin for error is genuinely small

The dietary requirement is estimated at 0.1 mg/kg feed dry matter in the total ration. The National Research Council recommends 1 mg of selenium daily for a 400 kg horse in heavy or very heavy work, and evidence suggests that up to 3 mg daily might support immune function, but this is not a figure to apply casually. Chronic selenosis occurs when animals consume more than 5 mg/kg feed dry matter for weeks or months. Acute toxicosis can cause cardiovascular collapse, pulmonary oedema, profuse sweating, and death within hours or days. In experimental ponies, doses of 6-8 mg selenium per kg of body weight caused sweating, diarrhoea, tachycardia, lethargy, colic, and death within 36 hours. The gap between what a horse needs and what causes harm is narrow, if you are considering selenium supplementation beyond trace levels, blood selenium status should be assessed first.

When you do supplement selenium, source matters. Organic selenium raises plasma and red blood cell selenium concentrations more effectively than inorganic selenium, and shows a measurable effect on immune function (relative lymphocyte expression of IL-5) that inorganic forms do not. Inorganic mineral salts generally show poor bioavailability and higher excretory rates, relevant both for efficacy and for the environmental load they create.

Zinc: requirement and upper limit

The estimated requirement is 40 mg/kg feed dry matter. Intakes above 1,000 ppm have been shown to induce copper deficiency and developmental orthopedic disease in young horses, so there is a meaningful ceiling, particularly if you are supplementing both minerals simultaneously or feeding a fortified hard feed.

Copper: a daily target for maintenance

The estimated requirement for a 500 kg adult horse at maintenance is 100 mg per day in the total ration. Copper plays a direct role in enzymes affecting connective tissue and iron mobilisation, which is why it appears repeatedly in discussions of hoof and joint support.

Magnesium: genuine deficiency is uncommon

Dietary magnesium deficiency in horses is described as very rare under normal management conditions, it tends to arise only when extreme circumstances combine reduced consumption with increased demand (e.g., prolonged transport of unfed lactating mares). If you are supplementing magnesium, note that excessive long-term supplementation may increase the risk of enterolith formation; enteroliths are concretions of magnesium ammonium phosphate crystals, and large-colon contents from affected horses show higher magnesium, calcium, and phosphorus concentrations than controls.

When not to supplement without proper assessment

– Do not add selenium on top of a diet that has not been analysed. Given that deficient whole-blood selenium was found in only 3.3% of horses in one study (with a further 13.6% marginal), many horses being supplemented do not need it.
– Do not assume blood results alone confirm macomineral status, they don’t.
– Do not stack high-zinc supplementation in young horses without monitoring copper status concurrently.

What we’d recommend


Herbal Horse Chelated Minerals

Bioavailable chelated minerals your horse can actually absorb, without mineral overload risk.

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Frequently asked questions

How much selenium does a horse actually need, and how easy is it to overdose?

The dietary requirement is 0.1 mg/kg feed dry matter, roughly 1 mg/day for a horse in heavy work. The margin between enough and too much is genuinely narrow: chronic selenosis can develop at intakes above 5 mg/kg feed, and acute toxicosis, which can be fatal within hours, involves cardiovascular collapse and pulmonary oedema. I treat selenium as the mineral that demands the most respect in any formulation.

What are the signs of selenium poisoning in horses?

In acute cases, horses show abdominal pain, profuse sweating, diarrhoea, tachycardia, and lethargy, and can die from cardiovascular collapse and pulmonary oedema within 36 hours of overdose. Chronic oversupply at more moderate levels causes a slower, insidious deterioration over weeks or months. Both scenarios are well documented; neither is rare enough to dismiss.

Is organic selenium genuinely better absorbed than inorganic selenium in horses?

Yes, measurably so. Research shows organic selenium supplementation raises both plasma and red blood cell selenium concentrations more effectively than inorganic forms, and also improves relative lymphocyte immune markers. Inorganic mineral salts in general show poor bioavailability because other co-elements compete for absorption and increase excretory losses, including an environmental cost from what passes straight through.

Can too much zinc cause problems even if it’s within ‘supplement’ doses?

Zinc itself requires 40 mg/kg feed dry matter to prevent deficiency, but intakes above 1,000 ppm have been shown to induce copper deficiency and developmental orthopaedic disease in young horses. Because zinc and copper compete for absorption, getting the ratio wrong matters as much as the absolute amounts, which is exactly why I always consider the whole ration, not just the supplement.

Do horses really need magnesium supplements, or is deficiency rare?

Dietary magnesium deficiency in horses is genuinely uncommon under normal management. It tends to occur only when extreme conditions combine, prolonged transport of unfed lactating mares, or deficient parenteral fluids. That said, excessive long-term magnesium supplementation carries its own risk: it may increase the likelihood of enterolith formation, which is a serious colic cause involving magnesium ammonium phosphate crystal concretions.

How do I know whether my horse is actually mineral-deficient before supplementing?

Blood concentrations are an unreliable guide for macrominerals, particularly calcium, because they don’t reflect dietary intake accurately. The only reliable way to know what minerals your forage provides is laboratory forage testing. Once you have that, you can calculate what the total ration delivers and supplement only genuine gaps. Guessing and stacking supplements is how horses end up with excess.

Sources

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