Selenium: Safe Limits & Testing

Selenium has been shown to be a critical trace mineral for horses. The major source of selenium in the diet of most horses is their forage, which absorbs selenium from the soil. However, it is estimated that nearly 30% of all horses are deficient in this important nutrient. This is because many areas of North America (and throughout the world) have soils that are deficient in selenium. Thus, many horses need to receive additional selenium either through their feed or supplemented by other means.

Why Is Selenium Important for Horses?

For horses, selenium has been identified as an important antioxidant. Briefly, antioxidants help to prevent or limit cellular damage caused by free radicals. Free radicals are also known as damaging “reactive oxygen species” and are unstable atoms produced by the horse during normal metabolic processes. These damaging atoms are thought to contribute to oxidative stress, leading to aging and illness.

Selenium is an essential component of the antioxidant enzyme glutathione peroxidase, which has a critical role in antioxidant defenses and helps reduce cell damage. Other important antioxidant nutrients include:

  • Vitamin C and Vitamin E
  • Other trace minerals: zinc, copper, and manganese

Selenium is also thought to be important for thyroid and normal body function. A horse’s thyroid is important for hormone production and metabolic function. Selenium deficiency is thought to contribute to hyperthyroidism, which means the thyroid is not functioning properly. The other concern with selenium deficiency in horses is associated with white muscle disease and/or muscle weakness, respiratory distress and impaired cardiac function.

Conversely, selenium, if fed in too high amounts, can result in toxicity. This can result in alkali disease with damage to tissues like the heart, liver and muscle. Selenium toxicity can also lead to death. Signs a horse may be suffering from selenium toxicity could include:

  • Hair loss seen in the tail and mane
  • Lameness or joint degeneration
  • Cracks or sloughing of the hooves
  • Blind staggers (blindness

How Much Selenium Is Appropriate for Horses?

Being a “trace” mineral, selenium is meant to be received in “trace” or small amounts. The exact daily requirement of selenium in the diet of horses is still not known. However, based on a few studies, the National Research Council (NRC) estimates horses need to receive at least 0.1 mg of selenium per kg of feed per day. Thus, feeding a 1000-pound horse 2.5% of its body weight per day, or 25 pounds (11 kg) of feed per day, would equal 1.1 mg of selenium per day. Yet, this is the absolute minimum a horse needs to avoid any deficiencies. The recommended dose for most adult horses for optimal health is 3 mg of selenium per day.

The NRC states the absolute maximum tolerable levels of selenium for horses is thought to be 5 mg per kg of feed. However, the recommended safe upper limit of selenium is 2mg per kg of feed per day. Thus, feeding a 1000-pound horse 2.5% of its body weight per day, or 25 pounds (11 kg) of feed per day, would equal to a safe upper limit of 22 mg of selenium in the diet.

Sources of Selenium in the Equine Diet

As previously stated, most of the selenium a horse receives in their diet is from their forage. However, in certain regions, selenium is deficient in soils, leading to selenium deficient forages. In the United States, regions in the western and eastern parts of the country have many areas where soils are deficient in selenium. Yet, there are regions within the central United States (i.e., Colorado, Dakotas) where selenium in the soils is very high and could potentially lead to toxicity.

Because selenium levels in forages vary widely, it is worthwhile to consider getting your forages tested for selenium. There are many services available and you can contact your local extension office to find out how to take an appropriate pasture/hay sample and where to send them. Furthermore, it is always worthwhile to discuss the selenium content in your region’s soils with your local area extension office. Notably, there will be variation within even the same region. Providing selenium supplementation above what is provided in a fortified feed is not recommended even in “selenium poor” areas, unless indicated by blood work.

Other important sources of selenium in the diet are commercial horse feeds and/or supplements. The level of selenium in a feed or supplement is required to be listed on the feed tag. It is very important to remember to carefully follow the manufacturer’s instructions when feeding supplemental selenium. Selenium toxicity, as stated above, can lead to serious health problems and/or death.

Finally, research is beginning to demonstrate that the types (sources) of selenium fed to horses have an effect on their digestibility and bioavailability. Selenium can be fed to horses in inorganic (sodium selenate/selenite) or organic (selenium yeast) forms. Data in horses suggests that the organic form of selenium fed as “selenium yeast” is more readily absorbed by horses. Furthermore, horses fed selenium yeast had higher elevated levels of selenium in their blood versus those fed inorganic forms of selenium. Thus, organic forms of selenium are viewed as superior for horses. When looking at commercial horse feed tags, you can see what form (inorganic or organic) of selenium is being added in their list of ingredients.

How Do I Know if My Horse Is Getting Too Much or Too Little Selenium?

One way to determine selenium intake is to analyze the diet. This would require testing your hay and pasture, and combining this selenium intake with that coming from your horse feed and supplements. Because this is a somewhat odious task, it is more common to measure the amount of selenium in the horse’s blood and compare those to reference ranges to determine if the horse is getting too little (deficient) or too much (toxic) selenium.

Selenium is one of the only trace minerals that can be reliably tested for in the blood. It is fairly common for horse owners to have their horse’s blood selenium levels tested when horses exhibit vague, not quite right issues or even as a routine screening tool. When testing for blood selenium, it is important to understand the different types of blood tests, as these results can sometimes be confusing and lead to undue alarm. Whole blood selenium is a good indicator of the long-term selenium status of the horse, whereas plasma and serum are much more sensitive to recent changes. It is not unusual for serum or plasma to be elevated if blood is drawn within close proximity to a meal, but whole blood selenium from this same time period would fall within the normal range. Therefore, for most situations, whole blood selenium would be the most appropriate measure.

Take Home Message

Selenium is one of the most important trace minerals when considering what you are feeding your horse. This is mainly because there is no uniformity of selenium levels in forages from around the world. Thus, it is highly suggested to first test or identify the selenium levels you are currently feeding your horse and adjust your feeding plan from there. As a general rule, horses weighing around 1000 pounds require about 3 mg of selenium per day. If you have any concerns or questions about what you are feeding your horse or need advice on how to identify the selenium levels in your horse’s diet, please contact us for a free, personalized feeding plan.

National Research Council. (2007). Nutrient requirement of horses, 6th rev. ed. National Academies Press.
Pagan, J. et al. (1999). Effect of selenium source on selenium digestibility and retention in exercised Thoroughbreds. Proc. Equine Nutr. Physiol. Soc. 2-5.

Article By: Chris Mortensen, Ph.D.
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