Selenium - The Goldilocks Mineral
- May 4
- 4 min read
Selenium is one of those tiny‑but‑mighty minerals that horses can’t live without, yet it barely gets a mention until something goes wrong. It’s a trace element, which means horses only need it in very small amounts, (we’re talking milligrams) but those small amounts do some very big jobs.
The easiest way to think about selenium is as part of the body’s internal clean‑up crew. Every time a horse moves, eats, digests, or even just exists, their cells produce oxidative by‑products - little chemical sparks that can damage muscle tissue, cell membranes, and even DNA if they build up. Selenium helps neutralise those sparks. It works hand‑in‑hand with vitamin E to protect muscles, support the immune system, and keep cells functioning smoothly.
Because it’s so tied to muscle integrity, one of the earliest things to slip when a horse is low in selenium is their ability to recover from work. They might feel a bit flat, a bit “not quite right,” or just lack their usual stamina. In more pronounced deficiency, the signs become harder to ignore: weak or sore muscles, a poor topline that doesn’t improve with feeding, or a coat that loses its shine. Foals born to selenium‑deficient mares can develop white muscle disease, where the muscles are literally too weak to function properly. In adults, deficiency can also show up as reduced fertility, lowered immunity, or a general sense that the horse is ageing faster than they should.
Selenium Deficiency or Excess
Selenium problems are sneaky because both deficiency and toxicity produce vague, overlapping, “not quite right” symptoms. Selenium has very small margins between too much and not enough - which is why it's a 'Goldilocks mineral'. That means owners - and sometimes even professionals - often chase the wrong diagnosis while the real culprit is a mineral imbalance hiding in plain sight.
Here are some issues commonly blamed when selenium could be the underlying problem:
1. Muscle disorders (tying‑up, PSSM flare‑ups, general stiffness)
Low selenium compromises the enzymes that protect muscle cells from oxidative damage. Horses can look tight, sore, or reluctant to move, so people assume it’s a training issue, magnesium deficiency, a PSSM episode, or a saddle problem. Excess selenium can also cause muscle pain and weakness, which muddies the waters even further.
2. Neurological issues
Abnormal gait, stumbling, weakness, and difficulty coordinating movement are all signs of selenium deficiency. These signs often get blamed on EPM, wobblers, or “idiopathic lameness,” when the real issue is oxidative stress affecting nerves and muscles.
3. Poor performance or early fatigue
Horses low in selenium struggle to recover from work and fatigue quickly. Owners often blame fitness, saddle fit, ulcers, or laziness. Toxicity can also cause lethargy and reduced performance, so both ends of the spectrum look similar.
4. Respiratory effort or “heaving” appearance
Difficulty breathing is a documented sign of deficiency. It’s easy to mistake this for asthma, allergies, or a viral infection.
5. Weak foals or poor suckling
Foals with selenium deficiency may be too weak to stand or nurse properly. This is often blamed on prematurity, dystocia, or infection, when the underlying issue is white muscle disease.
6. Coat and body condition issues
A dull coat, poor topline, or a horse that just looks older than it should often gets blamed on diet quality, parasites, or ageing. Selenium deficiency can be the quiet driver behind all of these.
7. Behaviour changes
Both deficiency and toxicity can cause irritability, dullness, or a horse that suddenly feels “off.” Owners often blame pain, ulcers, magnesium deficiency or training problems.
Australia adds an extra twist to the story. Much of our soil is naturally low in selenium, which means the pasture and hay grown on it are often low as well. A horse can be standing in a paddock full of lush green feed which is supplying plenty of vitamin E, and still not be getting enough of this one mineral. That’s why most horses here rely on a balanced supplement to meet their needs - not because their diet is “bad,” but because the landscape simply doesn’t provide it.
Enter Sound Advice Trace Mixes
Where things get messy is when owners start layering products that were never designed to be used together.
A complete feed may already contain selenium. A ration balancer may also contain selenium. Both are formulated on the assumption that they’re the only source of vitamins and minerals in the diet. So the moment you feed them together, or feed either one at anything other than the exact recommended rate, the maths falls apart. You can end up doubling the selenium without realising it, or underfeeding a complete feed and leaving the horse short. Because selenium has such a narrow safety margin, those swings matter.
Make life easier.
We always recommend single‑ingredient feeds paired with a predictable mineral mix like our Sound Advice Trace Mix, which supplies all the minerals that are needed in milligram doses at optimum ratios. You’re not guessing what’s hidden in a pellet or trying to reverse‑engineer a manufacturer’s recipe - you’re adding fibre, calories, or protein as needed, and the trace minerals stay steady, accurate, and safe every single day.
You can read some of the research about selenium here:
In short, selenium is small, essential, and easy to overlook - but when it’s missing, the whole system feels it.
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