05/29/2026
We have had 5 foals born being selenium deficient, though Bunny, Brie, and Anthem were not as severe as the filly, Avalon (One of them Razr, was additionally later found to have liver cancer when at Rood and Riddle, and had to be put to sleep).
We know that Saffron was at least born in the very low range because she got tested, but it was not until days after she'd already been given 2mls by injection of e and Se.
What the heck is Selenium, and why should I care?
First off, let us say to Not randomly add selenium to your horse's feed without having a veterinarian with knowledge on this subject run lab work first. Too much selenium can be fatal or cause serious issues.
If the map shows that you live in a potentially selenium deficient area, you can and should put a selenium infused block out with your salt block and loose minerals.
While the common train of thought is that horses aren't guided like other livestock on knowing what they need, we often notice if they eat it quite vigorously, and we have lab work run it comes back that vet- guided, additional supplementation is needed.
Many commercial horse feeds will contain selenium. Often, though, they will not contain enough to make up for an environmental deficit. Many times it is selenium selenite added, which is not a form that horses are good at uptaking.
Because too much selenium is highly toxic, how much is added is understandably strictly regulated.
For this reason, the selenium requirements established for horses may be as much a compromise between soils that are high needs and low needs areas.
This is why providing your mares (and all your horses actually) with a well rounded nutritional program, including loose minerals and salt (as well as selenium blocks if you live in one of the areas shown on the map) is extremely important.
Signs of selenium deficiency may be easily overlooked because deficiency would first compromise cellular integrity. This damage is difficult to measure, but may eventually surface as: *work intolerance (tying up), *poor hair coat, and *early onset of problems normally associated with aging
On the other hand, anxiety, tension and spookiness can be signs of too much selenium or too little.
Hypothyroidism may also be a subtle sign of selenium deficiency, it is in humans, but the connection has not been proven in horses.
Deficiency effects on the muscle can cause: cardiomyopathy, myositis (muscle inflammation and pain), and white muscle disease (weak, pale muscles).
Gross deficiency signs are growth retardation, cataract formation, retained placentas, red bag deliveries, and eventual reproductive failure.
In foals, selenium deficiency may manifest as windswept legs, severe clumsiness, being extraordinarily spooky, very rigid muscles, getting stuck at birth, poor and ineffective suckling, stupor and/or a bright red tongue and gums.
Selenium deficient foals can be mistaken for traditional "dummy" foals.
If you have a dummy foal, and you live in the area identified on the map, you might consider asking your vet about giving 2ml e plus Se and pulling a selenium level. It follows that if your foal's test comes back low, your mare is going to be low as well.
Interestingly a podcast was just on talking about severe selenium deficiency and anhydrosis. When the selenium level was corrected the anhydrosis disappeared. Thinking back, we have had two HOP horses with disappearing anhydrosis. One was The Little and the other was Valero. Makes one wonder if their much better diet finally brought their selenium into a more normal range?
Things that make you go hmmmm
