A: Selenium is a trace mineral, meaning that it is required in the horse’s diet in trace amounts measured in milligrams. Examples of other trace minerals include copper, zinc, iron, manganese, and iodine. In comparison macrominerals—such as calcium, phosphorous, magnesium, sodium and potassium—are needed in gram quantities.
Even though selenium is needed in tiny quantities it plays a number of very important roles. It is an essential component of the enzyme glutathione peroxidase, regulating its activity. This enzyme keeps cells safe from oxidative damage caused by hydrogen and lipoperoxides (a type of free radical) that can occur during metabolic processes. Selenium is also important to the function of type I iodothyronine 5-deiodinase, which removes iodine from the thyroid hormone T4 and gives rise to most of the circulating T3 hormone—the form which acts on practically every cell in the body.
Glutathione peroxidase and the deiodinase enzymes require the amino acid selenocysteine, a cysteine with a selenium molecule where there would regularly be a sulfur molecule. Therefore, if diets are deficient in selenium, horses have a reduced ability to create selenocysteine and the subsequent enzymes.
I often hear a lot of concern about excess selenium—people are often worried about giving their horses too much. This is because while some trace minerals such as manganese and iron have an upper tolerable amount of 400 to 500 mg/kg in the diet, respectively, selenium has a maximum tolerable limit of 2 mg/kg dry matter in the diet per the National Research Council’s (NRC) Nutrient Requirements of Horses. This is clearly significantly lower than the tolerable limits for other trace minerals.
What does this really mean in terms of how much selenium is safe for a horse to consume each day? For a 1,100-pound horse consuming 2% of body weight per day as dry matter (which amounts to 10 kilograms or 22 pounds of feed), a safe upper limit would be 20 mg per day, according to the NRC’s recommendations of 2 mg/kg per day. This is far greater than the 1 to 2 mg consumed each day by many horses. So while owners are right to be cautious about selenium, and while it does have a lower safe daily intake as compared to other trace minerals, there is still some room to play with in many horse’s diets before you hit the upper safe limit.
The U.S. Food and Drug Administration has set a daily recommended intake of selenium for an average horse (weighing 1,100 pounds) per day as 3 mg or 0.3 ppm in the diet (ppm stands for parts per million and is equivalent to milligrams per kilogram). The NRC states that there is no justification for going over 0.5 ppm in the diet and sets the requirement for a 1,100-pound horse at 1 to 1.25 mg per day. However there is some research suggesting an improved immune response when horses are fed 3 mg per day. I generally work to keep horses’ levels between 1 and 3 mg per day.
That said, there are times when clients come to me feeding closer to 5 or 6 mg each day with their horses showing no negative signs. In these situations I typically request that they have their veterinarian test their horses’ blood selenium levels to see whether the animals would benefit from a reduction in how much they consume. Interestingly, in the endurance horses I have worked with whose dietary selenium levels are this high, their blood work has come back in the high end of normal; this could suggest that, perhaps due to their very heavy work levels, they have higher selenium demands. For this reason I test when I’m in doubt.
Typically if you are feeding a good ration balancer or other commercial feed correctly (at the manufacturer’s recommended feeding rates), you will be providing 1-3 mg of selenium per day and should not need further supplementation. This is where care is needed if you’re adding additional supplements that could contain selenium which is especially the case with some vitamin E products. It can be easy to accidentally surpass the NRC requirement and, while it’s unlikely to be close to the maximum tolerable level, as the NRC stated there is limited justification for going higher. If you have concerns about your horse’s selenium status I would encourage you have your veterinarian perform blood tests before adding additional supplements.
It is also worth remembering that forages grown in some parts of the country may contain high selenium levels. Here in California the hays I test often have levels too low to register, yet every now and again I come across a hay with high selenium levels; this is why it’s risky to make assumptions. In fact, this past year I came across several hays with selenium levels over 0.5 ppm. I do not know why these hays’ selenium levels are higher, but speculate that it might have something to do with the drought changing soil alkalinity and causing plants to put down deeper roots. On the opposite end of the scale are forages grown in other areas such as the Pacific Northwest, which are generally accepted to be low in selenium. For these reasons it is important to know where your forages come from. I once heard of a barn in San Diego that was having trouble with selenium deficiency, which they were finding baffling. It turned out they were purchasing and feeding timothy hay grown in the Pacific Northwest.
Three forms of selenium are used in feeds and supplements: inorganic sodium selenite, inorganic sodium selenate, and organic selenium yeast. The yeast form has been found to be more bioavailable than the inorganic forms, more readily raising horses’ serum selenium levels. The organic forms found in plants such as selenocysteine and selenomethionine are quite bioavailable.
In summary selenium is an essential trace mineral that plays a number of vital roles in your horse’s body. It is inadvisable to add selenium from multiple sources to your horse’s ration due to the potential for toxicity. If you have concerns about your horse’s selenium status first consider whether you are feeding fortified feeds correctly. If not consider changing your feeding rate or selecting a different feed that you can feed correctly. If you still have concerns ask your veterinarian to test your horse’s serum selenium levels before adding additional sources of selenium to the ration. Should your horse be confirmed as being selenium deficient adding a supplemental source of selenium yeast to the ration will likely rectify the issue the most quickly.