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Selenium in horses: requirement, analysis and safety window
The trace element with the narrowest safety window. How much the GfE and NRC recommend, why the content in forage varies between zero and plenty depending on the soil, how small the gap between requirement and legal limit is – and why selenium is only supplemented on the basis of a lab value, never on suspicion.
Selenium is the nutrient where too little and too much lie dangerously close together. No other trace element has such a narrow range between requirement and poisoning in horses – and that is exactly why selenium does not belong in the feed bucket on suspicion.
Functionally, selenium is an essential component of the enzyme glutathione peroxidase and therefore part of the antioxidant protection system that keeps cell membranes from oxidative damage; it is also involved in thyroid hormone metabolism 1. Selenium and vitamin E work together in the same system – one cannot make up for a deficiency of the other 12. The vitamin E side is covered in a separate article.
How much does a horse need?
The NRC gives the selenium requirement as a concentration in the total ration: 0.1 mg selenium per kg feed dry matter 31. For a 500 kg horse eating around 10 kg dry matter a day, that is about 1.0 mg, and for 600 kg around 1.2 mg. The Gruber Table reproduces the GfE 2014 (German Society of Nutrition Physiology) requirement in tabulated form and likewise gives 1.2 mg selenium per day for a 600 kg warmblood at maintenance 45.
Unlike calcium, phosphorus and magnesium, the GfE and NRC do not diverge here – they meet at around 0.1 mg per kg dry matter.
| Reference | 500 kg | 600 kg |
|---|---|---|
| NRC 2007 (0.1 mg/kg DM) | ≈ 1.0 mg/day | ≈ 1.2 mg/day |
| GfE 2014 (Gruber Table 20) | – | 1.2 mg/day |
Selenium maintenance requirement. NRC as a concentration per kg feed dry matter, GfE as a tabulated daily value.
What forage provides – and what it does not
How much selenium is in hay depends directly on the selenium content of the soil and varies enormously by region. Analyses of mixed grass forage samples found an average of 0.194 ppm, with a range from 0 to 1.2 ppm 1. On selenium-poor soils, forage therefore provides practically nothing; on others, plenty.
In practice this means: whether the ration covers the selenium requirement cannot be said without an analysis – the content may be zero or sufficient. Guessing leads you astray in both directions, precisely because the window is so narrow.
The narrow window: requirement, law, tolerance
Selenium has the narrowest safety margin of all trace elements 35. Four values need to be kept clearly apart – they mean different things:
| Level | Value | Basis |
|---|---|---|
| Requirement (maintenance) | 0.1 mg/kg DM | per kg feed dry matter |
| EU legal maximum content | 0.5 mg/kg complete feed | all sources combined |
| Scientific tolerance (NRC) | 2 mg/kg DM | per kg feed dry matter |
| Minimum lethal oral single dose | about 3.3 mg Se/kg body weight | sodium selenite, historical study |
Four reference values, do not confuse them: requirement, legal limit, scientific tolerance and acutely lethal single dose.
When it becomes too much: name it, do not treat it
Chronic selenium excess leads to alkali disease: because of its similarity to sulphur, selenium displaces it in keratin and destabilises cornified tissue. Described signs include hair loss and breakage in the mane, tail and neck as well as damage to the hoof capsule – swelling of the coronary band, separation of the hoof wall, horizontal hoof cracks 9. The minimum lethal oral single dose of about 3.3 mg elemental selenium per kg body weight as sodium selenite, estimated in a historical equine study, is a separate matter: it is a poisoning situation, not a feeding error in the milligram range 10.
Both belong in your vet's hands, not in self-treatment. And one widespread attribution should be treated with caution: according to Merck, the neurological disease “blind staggers”, formerly blamed on selenium, is probably due more to a high sulphur intake than to selenium 9.
Organic or inorganic – and why only with a lab value
Selenium is supplemented in an organic form (selenium yeast, mainly selenomethionine) or in inorganic form (sodium selenite). In a controlled equine study, the organic form raised selenium in plasma (0.166 vs 0.144 ppm) and red blood cells (0.442 vs 0.299 ppm) more than the inorganic form – but the activity of glutathione peroxidase did not differ between the two 11. The honest statement is: the organic form is somewhat superior in building up levels in the blood, but this does not demonstrate any clear functional benefit.
The same study shows why a single blood value can be misleading: blood selenium and enzyme activity can diverge 11. What can be managed and documented is the intake through the ration – and that is shown by a feed or hay analysis, not a blood sample 67. Because of the narrow safety margin, the rule is therefore: no blanket topping up. We only release an automatic selenium dosage recommendation with a documented lab value – without an analysis, you risk oversupply up to the legal limit 8.
Requirement peaks and limits of the statement
The selenium requirement rises with breeding and growth: the Gruber Table gives around 1.8 mg per day each for pregnant and lactating mares and over 2 mg for growing horses 4. Persistent selenium and vitamin E deficiency is associated with neuromuscular disorders – interpretation and any dosing belong with the vet 21.
Precisely these groups – foals, young horses during rearing, pregnant and lactating mares – belong in specialist and veterinary advice and sit behind the safety and advice gate. Selenium is dosed tightly; a visible but deliberately uncovered gap is better than oversupply on the off chance.
Sources & studies
The content is based on the references listed below. It does not replace individual veterinary or feeding advice.
Show list of sources (11)
- Merck Veterinary Manual: Nutritional Requirements of Horses and Other EquidsSummary of the NRC recommendations: water requirement 5 litres per 100 kg body mass per day at maintenance, sweat losses of around 3.1 g sodium, 5.3 g chloride and 1.6 g potassium per litre, salt recommendation 1.6–1.8 g per kg feed dry matter at a maximum of 6% of the ration; from a 50% forage share potassium is adequately covered. Also covers the selenium requirement (0.1 mg/kg feed dry matter or at least 1 mg/day for a 500 kg horse, selenium as a component of glutathione peroxidase together with vitamin E) and the strong location-dependent variation of selenium in forage (grass forage on average 0.194 ppm, range 0–1.2 ppm); some regions are markedly low in selenium and may need supplementation. Also gives the iodine requirement (0.35 mg/kg feed dry matter, mares in late pregnancy 0.4), iodine poisoning of pregnant mares from around 40 mg/day, documented goitre cases in mares and their foals associated with large amounts of dried seaweed (kelp), that iodised salt easily covers the requirement, and that iodine supplementation is usually unnecessary except in severely iodine-deficient regions
- Finno CJ, Valberg SJ (2012): A Comparative Review of Vitamin E and Associated Equine Disorders. Journal of Veterinary Internal Medicine 26(6):1251–1266Peer-reviewed review (PMID 22925200): structure, absorption and metabolism of vitamin E in horses, higher bioavailability of the natural (RRR) form, antioxidant function during exercise, need for supplementation with little access to pasture, and the neuromuscular diseases associated with persistent vitamin E deficiency (incl. eNAD/EDM, EMND, vitamin E-responsive myopathy).
- NRC – Nutrient Requirements of Horses, 6th ed.International reference for the nutrient requirements of horses (National Research Council)
- Bayerische Landesanstalt für Landwirtschaft (LfL): Gruber Tabelle zur PferdefütterungPractical reference table with requirement and feed values based on the GfE recommendations; the basis of many ration calculations in German-speaking countries. Chapter 9.3, Table 39: most common poisonous plants for horses with habitat, toxicity and risk level (after Kamphues et al. 2014)
- GfE (2014) – Empfehlungen zur Energie- und Nährstoffversorgung von PferdenSpecialist reference in German-speaking countries (GfE, German Society of Nutrition Physiology), Committee for Requirement Standards
- Weender Futtermittelanalyse (Weender Analysensystem)Standard method for determining crude nutrients (crude protein, crude fibre, crude fat, crude ash) as the basis of feed and hay analyses
- VDLUFA – Methodenbuch III (Futtermittel)Reference methods of agricultural testing laboratories (LUFA) for the analysis of feeds and forages
- Durchführungsverordnung (EU) 2020/2117Renewal of the authorisation of selenium yeast from Saccharomyces cerevisiae CNCM I-3399 (identification number 3b812) for all animal species; total maximum content of selenium 0.50 mg/kg complete feed (12% moisture) and at most 0.2 mg/kg of added organic selenium. Authorisation expires according to the Annex on 6 January 2031. Other selenium sources with the same total value: Implementing Regulations (EU) 2019/49, 2019/804 and 2024/3168.
- Merck Veterinary Manual: Selenium Toxicosis in AnimalsStandard veterinary reference work (toxicology): chronic selenosis/alkali disease with hair loss and breakage in the mane, tail and neck, plus coronary band swelling, separation of the hoof wall and horizontal hoof cracks; notes that the neurological disease “blind staggers” historically attributed to selenium is more likely polioencephalomalacia due to high sulphur intake. The horse-specific acute single dose is documented by the primary study Miller and Williams (1940).
- Miller WT, Williams KT (1940): Minimum lethal dose of selenium, as sodium selenite, for horses, mules, cattle, and swine. Journal of Agricultural Research 60(3):163–173Primary study on acute oral sodium selenite toxicity: in five horses and three mules, the authors estimated the minimum lethal single dose at about 1.5 mg elemental selenium per lb body weight, equivalent to around 3.3 mg elemental selenium per kg body weight. Small historical sample; no LD50 and not transferable to other selenium compounds or routes of administration.
- Montgomery JB, Wichtel JJ, Wichtel MG, McNiven MA, Horohov DW (2012): Effects of selenium source on measures of selenium status and immune function in horses. Canadian Journal of Veterinary Research 76(4):281–291Feeding trial in horses at 0.3 ppm Se (dry matter basis): organic selenium (selenium yeast/selenomethionine) raised total selenium in plasma (0.166 vs 0.144 ppm) and erythrocytes (0.442 vs 0.299 ppm) more than inorganic sodium selenite; glutathione peroxidase activity did not differ between the two supplemented groups. Evidence of differences in bioavailability between selenium forms and that blood selenium and enzyme activity can diverge.