Selenium Levels for Horses

Horse named Henry Muscles running representing Selenium Levels for Horses

As a horse owner, you may have heard about selenium levels and that you need to be concerned with them.  But why?

What is Selenium?

selenium is a key trace mineral in equine diets because it is a major component of glutathione peroxidase, which is an anti-oxidant enzyme, as well as several other enzymes.

  • Selenium also has an important interaction with Vitamin E.
  • Selenium and Vitamin E together are essential for both anti-oxidant benefits and for reducing the risk of certain muscle problems such as white muscle disease and exertional rhabdomyolysis syndrome.

Why Are Selenium Levels Important for Horses?

Selenium can be toxic at higher intakes. It is also the only trace mineral regulated by the FDA.  Most of the U.S. is selenium deficient in soils (and thus toxicity is not an issue in those areas), but there are some areas with high selenium in the soil where some plants accumulate unacceptable levels of selenium in forage and may cause chronic toxicity. If you aren’t sure about the levels where you live, ask area horse owners, farmers, or your local extension office.

How much is in the feed?

In the case of selenium, the label guarantee, which is “added” selenium, is listed in ppm = parts per million = milligrams per kilogram. The actual content will be slightly higher as there is naturally occurring selenium in the ingredients. The labeling requirement is based on added selenium per FDA guidelines.

  • Premium horse feeds are commonly tagged at 0.6 ppm selenium, or 0.6 mg/kg of feed.
  • This equals 0.273 mg per pound of feed. (0.6 mg/kg divided by 2.2 kg/lb. = 0.273 mg per pound of feed)
  • Thus, if you feed 5 lbs per day of a 0.6 ppm selenium feed, you are providing 1.365 mg of selenium per day.

FDA requirement limits added selenium in feed to a maximum of 0.3 ppm concentration in total diet. This is why the maximum on complete feeds – feeds that include enough fiber to replace the hay/pasture portion of the diet – is 0.3 ppm.

On feeds which can be fed at a maximum of 50% of the diet (not that horse owners generally feed it that high), the limit is 0.6 ppm added selenium. Chronic selenium toxicity occurs at about 10X the FDA limit, so there is a pretty good safety margin.

How Much Selenium Does a Horse Actually Need?

A horse’s basic daily requirement for selenium is 1-3 mg per day.  Some chronic selenium symptoms might appear above 10-15 mg/head per day.

To figure out what your horse is consuming, have your hay/pasture supply tested, and then add the amount it is consuming from the hay, to the amount it is consuming from any feed & supplements, and you will know if you need to make any adjustments to your horse’s overall diet.  Depending on your area, it may also be useful to consider intake from water sources.

At Nutrena, we believe proper nutrition plays the biggest role for a lifetime of health and happiness for every horse. That’s why Nutrena horse feeds are specifically formulated for every life stage and activity level. 
Ready to ensure your horse is getting the optimum nutrition at feeding time, every time? Find the perfect feed formulated specifically for horse’s needs with our Feed Selector Tool.
Feed Selector tool

18 Replies to “Selenium Levels for Horses”

  1. So, question: Is there a selenium supplement out there that isn’t a multi-vitamin type supplement? Or is the multi supplement the way to go?

    1. Hello Jeff, Very often you will find a Selenium & Vitamin E supplement combination – you want these two nutrients to stay in the proper ratio together. Otherwise, there are many multi-supplements on the market as well.
      Thank you ~ Gina T.

    2. Platinum Performance has a straight organic selenium supplement that is inexpensive if your horses otherwise get Vit. E through pasture or other supplementation. I give my endurance horses the human natural E from Costco. They consume it in their mash with no problem and it is more economical this way.

  2. My horses feed has .6ppm of selenium per 2 lbs.How much extra should I supplement for a day?

    1. Hi Meggan, Thanks for the question. If you feed 2 lbs per day, then your horse is receiving 0.546 mg of selenium just from the feed. The missing information is, what is your hay providing? You’ll need to have it tested to be sure, but as an example, if it contained 0.1 ppm of selenium, 15 lbs of that hay would provide 0.68 mg of selenium, putting the total diet at 1.22 mg of selenium per day – right within the 1-3 mg per day recommended range, and thus not requiring any additional supplementation.
      Thank you ~ Gina T.

  3. I’m feeding platinum performance, one scoop twice a day, and then feeding about 1-2 lbs of a high performance concentrate that says .8 mg twice a day as well, I also feed alfalfa twice a day probably about 10-15 lbs a day, I’m curious when your calculating MG is it in every .999 almost equals 1.0mg? Also is the daily amount the same for every horse or does it go by weight of the horse as to how much they can tolerate at a safe level every day? I’m just wondering cause I have a 6 month old weanling as well

  4. Hello,
    I am considering getting a new supplement for my horse that says it contains 2.1 ppm of selenium per serving. The supplement she is currently on says it has 2 mg of selenium per serving. I know this is a silly question, but are mg and ppm the same thing?

    1. Hi Yara,
      So, if the ‘serving’ of the supplement is 1 oz (typical of a small supplement scoop in a container), then you have to do a little math to convert the amounts.
      • 2.1 ppm = 2.1 mg/kg
      • 2.1 mg/kg divided by 2.2 kg/lb gets you to 0.95 mg/lb
      • There are 16 ounces in a lb, so 0.95 mg/lb divided by 16 ounces gives you 0.059 mg in a 1 oz serving.

      At the end of the day, what your horse needs is a total of 1 to 3 mg per day. You will need to look at the total selenium in the diet to see where your horse is at, to ensure you don’t get yourself in to a toxicity situation. If you were to feed these two supplements in combination at one ounce each, for example, you’d be at 2.059 mg per day already, and that is not including any hay, pasture, or grain. You’ll need to get those tested to find out what they are providing, and combine that with the supplements.

      Good luck!

  5. Hi I had my horse tested for his selenium levels and they returned low. The range is 14 to 24 and he was 11. I feed elevate liquid and would just like to some selenium to help raise his values into the normal range as his breed is prone to muscle problems. What is the best way to go about knowing how much to give and to keep him at a safe level as to not risk toxicity.


    1. Hi Marcy,
      Thank you for your interesting question about selenium. There are a number of Selenium/Vitamin E supplements on the market. As long as you do not combine more 2 supplements following the directions for each, you should not get into toxicity. The FDA limit for added selenium is 0.3 ppm (mg/kg) on a total diet basis. For a 500 kg horse (1100 pounds) consuming 2% of bodyweight, this would be about 3 milligrams of added selenium per day. There is a very good safety margin in this calculation and you would expect to see any toxicity issues at 2-3 times this intake. The upper safe limit for selenium per the NRC Nutrient Requirements of Horses is 2 mg/kg of dry matter in the diet, which is more than 5X the FDA limit.
      Best of luck,
      Roy J.

  6. We purchased a retired racing horse from KY. We live in SD. He was tested for Selenium toxicity recently and his level was high, but not yet toxic. We have taken him off his grain and no more treats. He needs to gain about 50 – 100 lbs. He has needed the weight gain since we bought him a couple of months ago. What kind of grain can we get for him that doesn’t have selenium added?

    1. Hi Melissa,
      Se toxicity is a legitimate concern, and should be monitored closely, especially since serum levels are on the high end. Some areas have high Se in soil, and thus forages from that region. I would recommend testing soil and forage to see where they are at first, considering forage typically comprises the highest intake of the total diet. If Se is high in forages, then I recommend seeking good quality forage from another region that isn’t so high in Se, and/or providing a commercial forage replacer.
      Good luck!
      Emily L.

      1. You didn’t answer the question of what grain to feed. Is there any that don’t have Selenium in them?

        1. Hi Cindy,

          Thank you for your question about grains/feeds that do not have selenium in them. The Food and Drug Administration has established the regulations that if a feed contains added selenium, it must be stated on the tag and this regulation is specified by the American Association of Feed Control Officials (AAFCO) and enforced by the state Feed Control officials. The FDA has also established that complete diets cannot contain more than 0.3 ppm added selenium on a total diet basis. If you are looking for a feed that does not contain added selenium, you would be able to see from both the tag and the ingredient list if the feed does contain selenium.

          If selenium is NOT listed with a minimum on the tag and there is no source of selenium in the ingredient listing, then selenium has not been added. In general, you will see that in grain mixtures such as COB (corn, oats and barley mixture) that is common in the western U.S.

          Best wishes,
          Roy J.

  7. Good morning. My question is I have 6 quarter horses ranging from 8 to 30 in age. I recently started putting a lick tub called Crystalyx Stable Lyx out in the pasture for my horses. Anyway they went through a 125 lb tub in a week. Bought another 125 lb tub gone again in a week, so bought the 250 lb tub. They are going crazy on this tub. Do I have to remove it from the pasture & put it out every other day so they don’t get selenium toxicity? They are really going through this stuff. I only give 5 of my horses a 1/2 scoop of grain a day & my 30 yr old gets 1 & 1/2 scoops of senior 3 times a day. I’m sure they were lacking a lot of vitamins but can they overdue themselves on this lick tub? Thank you Paula

    1. Hi Paula,
      Thank you for your interesting question about the Crystalyx Stable Lyx. It does sound like your horses are enjoying the access to the tub. If 6 horses consume 125 pound in one week, that would be just under 3 pounds per head per day. That would be at the high end of recommended intake, but should not represent nutritional hazard.
      There are a few reasons that the horses may be consuming higher levels:
      1. Forage may becoming more limited in the pasture? If so, they would consume higher levels of the tub.
      2. If salt is not available free choice, the horses might be somewhat behind on salt consumption. You might want to offer loose salt and see if that reduces consumption. Fresh clean water must be available.
      3. Depending on the type of forage in the pasture, they might be calcium or phosphorus deficient. This can also increase the intake of the tub. You might offer a 12:12:12 non-medicated mineral if you think this could be the case.
      4. If you are in an area that is pretty warm right now, the tubs are pretty soft and easy to consume. You might want to make certain tubs are in the shade, where they might remain a bit harder and might slow down consumption.

      Check the list and let us know how what works out.
      Best of luck!
      -Roy J.

  8. What are the symptoms of selenium toxicity?
    How many mgs would be in a measuring level 1/4 teaspoon of pure organic selenium from Canada?
    How much selenium would be in a measuring level tablespoon of Brewers Yeast?
    With the two combined in morning feed and just brewers yeast in afternoon feed. Dry feed weight – 770 -800gms using oaten chaff along with other minerals, 1/2 measuring cup whole oats, 1/2 measuring cup soaked barley, 1/2 measuring cup whole black sunflower seeds, 2 heaped tablespoon ground linseed.

    1. Hello Cheryl, Thank you for the interesting question regarding selenium. Selenium is a trace mineral that is important for a number of key body functions and interacts with Vitamin E.

      Selenium toxicity is dose and time related. Low toxic levels over a long period of time produce chronic toxicity symptoms, also called “alkali disease” that include lameness, cracking of hooves, loss of tail and mane hair, dullness and depraved appetite (eating unusual items). Acute toxicity produced by high levels for shorter periods can produce diarrhea, loss of coordination, loss of appetite fever, colic, head pressing, prostration and death. Symptoms are dose related.

      A few years ago, some polo ponies in Florida died almost immediately after an injection with a high selenium supplement that had been incorrectly produced. They died very shortly, within hours, after the injection of the supplement.

      Chronic toxicity might be expected at above 16 ppm (mg/kg) in the total diet. For a 500 kg (1100 pound) horse eating 10 kg per day, that would be 160 mg selenium per day.
      Acute toxicity would be dose dependent and higher than the chronic level.

      I do not know what pure organic selenium product you are getting from Canada or what a ¼ teaspoon would weigh. You need to get that from the manufacturer or source. Something like selenium yeast will normally contain 2,000-3000 mg/kg selenium. A 3,000 mg/kg product would contain about 85 mg per ounce.

      Brewer’s yeast contains about 0.9 mg/kg selenium. Most grain products contain about 0.1-0.6 ppm selenium, depending on the ground that they were grown on.

      Your risk of selenium toxicity is primarily determined by the concentration in the pure organic selenium product and how much you are feeding. You really need to get that from the manufacturer and follow the feeding directions for the specific product. It is fairly easy to get too high on selenium with a straight selenium product.

      We hope this information is helpful, please let us know if you have further questions.

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