Supplementing safely6 min read
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This article has not yet been professionally approved. The information is based on the sources listed below.
When trace elements become too much
Deficiency is not the only problem: an excess of selenium, cobalt, iodine or iron can cause serious harm. Selenium illustrates how narrow the window is.
If mineral feed, muesli and special supplements are combined without control, the risk is not deficiencies but oversupply. Trace elements in particular sometimes have very narrow safety margins 12.
Maintenance requirement about 0.1 mg/kg DMNRC tolerance limit from 2 mg/kg DM
Selenium: the narrow window
Selenium has an extremely narrow therapeutic range. With chronic excess, selenium – owing to its similarity to sulphur – takes over sulphur's role in keratin, which destabilises keratinised tissue (“alkali disease” with damage to the hoof capsule and coat) 2.
Other critical excesses
| Element | Consequence of oversupply |
|---|---|
| Iodine | Can block thyroid hormone synthesis (goitre formation) 2 |
| Iron | Hardly excretable – accumulates chronically in the liver 1 |
| Calcium/phosphorus | An imbalanced ratio puts strain on bone metabolism and promotes urinary/intestinal stones 2 |
| Cobalt | Misused as a hypoxia mimetic; high doses toxic to the cardiovascular system 5 |
Sources & studies
The content is based on the references listed below. It does not replace individual veterinary or feeding advice.
Show list of sources (7)
- NRC – Nutrient Requirements of Horses, 6th ed.International reference for the nutrient requirements of horses (National Research Council)
- GfE (2014) – Empfehlungen zur Energie- und Nährstoffversorgung von PferdenSpecialist reference in German-speaking countries (GfE, German Society of Nutrition Physiology), Committee for Requirement Standards
- 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.
- 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.
- Towards the elimination of excessive cobalt supplementation in racing horses: A pharmacological reviewCobalt as a hypoxia mimetic (HIF-1α stabilisation, EPO release); serious cardiovascular side effects in overdose
- Verordnung (EG) Nr. 1831/2003Feed additives – authorisation and use; “daily ration” based on 12% moisture (Art. 2(2)(f)), renewal of authorisation and automatic continuation (Art. 14(4)), general conditions of use (Annex IV: natural plus added amount; complementary feed, diluted as specified, at most at the level for complete feed). Link to the current EUR-Lex consolidated version (as of 27 March 2021).
- 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).