Selenium – an important part of a horse's diet

Selenium is an essential mineral that horses need in small amounts. It is part of the body's normal antioxidant function and helps maintain important processes in muscles, metabolism and the immune system.

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Scroll down for the academic review with key data, tables and references.

What does the harness do in the horse's body?

✔️ Part of the body's natural defense against oxidative stress
✔️ Contributes to normal metabolism of metabolic hormones
✔️ Helps maintain normal body functions

Where do horses get their harness from?

Selenium comes from the soil and is found in the grass and roughage that horses eat. In many parts of the world – including Denmark and Iceland – the soil is very poor in selenium. This means that the feed rarely contains enough selenium to cover the horse's needs without a supplement.

👉 In our experience, we have never seen a roughage from Denmark or Iceland that alone has been able to cover the horse's selenium needs.

How do you assess whether the horse is getting enough harness?

The vet can measure selenium status via a blood test. Often, two analyses are combined:

• Selenium in blood plasma – shows the current supply
• GPx (a selenium-dependent enzyme) – says something about the body's long-term level

If the values ​​are outside the normal range, it may be a sign that the horse is not getting what it needs – or that the level needs to be adjusted.

What are normal levels?

Blood sample (plasma)	What it indicates
Below 80 µg/L	Low level – should be assessed further
100–200 µg/L	Normal level
Above 300 µg/L	High level – should be followed by a veterinarian

Signs of too high selenium intake over time may include:

• Garlic-like odor from breath
• Hair loss on the mane and tail
• Changes in hoof quality
• Low energy or fatigue
• Colic-like behavior

⚠️ At very high doses over a long period of time, selenium can put a strain on the body - this is especially true at daily amounts above 20–40 mg, which typically only occurs when the dosage is incorrect.

Suspected selenium poisoning should always be evaluated by a veterinarian.

How much selenium does a horse need?

An adult horse (approx. 500 kg) typically needs:
➡️ 1–3 mg of selenium per day , depending on activity and other feeding.

Selenium is found in two forms:

• Inorganic selenium (e.g. sodium selenite) – moderate absorption
• Organic selenium (e.g. selenium-enriched yeast) – higher absorption and storage in the body

Is there anything special about Icelandic horses?

Icelandic horses have lived on selenium-poor soils for centuries, and some studies show that they can function with lower levels in their blood. However:

- There is no documentation that Icelandic horses have lower needs
- They tolerate the same levels in the blood as other horses.
- Low levels do not necessarily mean that everything is in balance – therefore measurement is recommended

❌ Myth: “Icelandic horses need less harness than others”

Studies show that Icelandic horses can function with lower values ​​– but that does not mean they should be fed lower.

👉 There is no evidence that you achieve health benefits by deliberately keeping Icelandic horses low in the harness
👉 There are also no documented side effects from selenium levels of e.g. 150 µg/L in the blood of Icelandic horses.

Our recommendation

🔹 If in doubt, have both blood selenium and GPx measured
🔹 Use a supplement with documented selenium content, adapted to the horse's other feed
🔹 Only adjust the amount in consultation with a professional - not based on assumptions or myths

Do you need help with a feeding plan?
Contact us here

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Selenium status in Icelandic horses:
A professional clarification

Introduction
Selenium is an essential micronutrient involved in important biological functions, including antioxidant defense (glutathione peroxidase, GPx) and activation of metabolic hormones (Deiodinase 1, DiO1). Low selenium status is well documented in grass and roughage from Denmark, Iceland and large parts of Northern Europe. This places all horses in the region at potential risk of suboptimal selenium supply without supplementation.

The enduring myth: "Icelandic horses should not wear the same harness as others"
A common misconception is that Icelandic horses have a lower physiological requirement for selenium than other breeds, or that they cannot tolerate normal levels in the blood. This is primarily based on cross-sectional studies that have shown lower plasma selenium and GPx activity in Icelandic horses raised on volcanic soil in Iceland. However, these observations only document adaptation to low intake, not that it is optimal.

Evidence-based key figures

Parameters	Very low	Low / marginal			Normal / adequate		High	Toxic
Soil (mg/kg DM)	<0.1	0.1–0.3			0.3–0.6		0.6–2.0	>2.0 (–)>
Assessment, land	Can cause selenium deficiency in plants and feed	Often insufficient for grazing animals			Grass/feed can cover needs in moderate quantities		Risk of oversupply in certain soil types	Risk of poisonous plants (e.g. Astragalus spp.)
Feed (mg Se/kg DM)	<0.05	0.05–0.10			0.10–0.30		0.30–2.0	>2.0
Assessment, feed	Deficiency, supplement recommended	Probable deficiency with long-term intake			Suitable for most horses		Should be carefully assessed in relation to total ration	Risk of poisoning, depends on absorbability
Blood (plasma/serum) µg/L	<60–70–80	70–100			100–200		200–300	>300
Assessment, blood	Deficiency, risk of muscle damage	Insufficient, especially during stress or breeding			Normal range for horses		Possible oversupply, monitoring recommended	Risk of clinical symptoms
GPx in whole blood U/g Hb *	<100	100–150			150–300		>300	—
Assessment, GPx	Deficiency, poor antioxidant protection	Possible subclinical deficiency			Good antioxidant status		Possible oversupply, but not toxic on its own	—
Parameters			Iceland	Germany		Remark
Selenium in roughage (mg/kg DM)			0.01–0.06	0.01–0.04		Comparatively low level
Plasma selenium (mean ± SD, µg/L)			66 ± 47	117 ± 82		After 8 weeks of stay in Germany including subsidy
GPx activity (U/g Hb)			Often low	Often normal		Depends on duration and type of subsidy

There is no documentation that plasma selenium of 150 µg/L causes clinical side effects in Icelandic horses.

Interpretation and practice
There is no evidence that Icelandic horses are harmed by having blood selenium levels in the normal reference range for other horses (100–200 µg/L). There is also no data to support that horses with, for example, 150 µg/L plasma selenium show signs of oversupply or clinical side effects, either in the form of hoof problems or fatigue. On the contrary, the existing data base indicates that levels up to 150 µg/L are within safe limits – and in some cases possibly more appropriate.

FVST-compliant assessment (in accordance with the Danish Veterinary and Food Administration guidelines)
Based on the available data, Icelandic horses should be assessed individually based on:
• Blood tests (plasma selenium and GPx)
• The feed's documented selenium content
• Fitness level and metabolic status

There is no scientific basis for routinely keeping Icelandic horses low in the harness. Suspicion of oversupply should be based on objective measurements, not solely on breed or country of origin.

Conclusion
The myth that Icelandic horses do not tolerate the same selenium levels as other horses lacks scientific support. Until longitudinal studies are available documenting a lower physiological need, Icelandic horses should be treated like all others : with individual assessments based on analyses and evidence-based feeding planning.

Selected sources
• Rosenberger et al. (2007): Feeding management of Icelandic horses in Iceland and Germany. ResearchGate
• Brendemuhl et al. (2008): Selenium status of horses in the Czech Republic. ResearchGate
• Kentucky Equine Research (KER): Assessment of selenium status. ker.com
• Laboklin (2022): Referenztabellen für Pferde. laboklin.com
• Mad Barn (2023): Selenium and Vitamin E in Horse Diets. madbarn.com