Last Updated on February 21, 2022 by Allison Price
Over the years, the importance of selenium for equine nutrition have changed dramatically. In the 1930s, scientists first recognized selenium as a toxin. Research showed that selenium overdose in horses caused “alkali diseases”. In the 1950s, further research showed that selenium could prevent white muscular disease in horses, cattle, and horses. This disproved the notion of selenium being a toxin, and established it as an essential nutrient for horses when given in limited amounts.
Selenium is an environmental hazard because of its potential to cause toxicity. Horse feed must contain a minimum amount of selenium. The selenium requirements for horses could be seen as a compromise between political considerations and an accurate assessment of the mineral’s physiological requirements. It is not clear whether manufacturers can add selenium to horse feed in the quantities required for optimal performance, particularly in areas that are selenium-deficient forage.
Why is Selenium important?
There are many roles that selenium plays in the body. But, the most important role is that of an antioxidant. The metabolic process of oxidation, where fats, carbohydrates, and proteins are converted into carbon dioxide, water, and energy (burned for energy). The oxidative process doesn’t always know which substrates are intended for energy production and which ones are structural.
It is dangerous to oxidize the body’s structural or functional components (cell membranes and enzymes, as well as other intracellular substances). Superoxides (unstable substances) and free radicals are created during the oxidation process. These free radicals can be powerful oxidizing agents that, if not destroyed, can damage cell structures such as proteins and lipids. Particularly vulnerable are unsaturated fatty acid, which is the main component of all cell membranes. Their oxidation can be very damaging to the cell’s function. Lipid peroxides are formed when free radicals attack lipids. Antioxidants can either prevent the formation of peroxides or treat them once they have formed. This is the role of selenium.
Selenium is an essential component of the cellular antioxidant defense system. It is also a component in the enzyme glutathione oxidase. This enzyme is widely distributed throughout the body. This enzyme converts glutathione reduced to oxidized glutathione, and destroys peroxides through the conversion to harmless alcohols. This converts peroxides to harmless alcohols, which prevents them reacting with lipid membranes, causing damage and loss of membrane integrity.
Selenium can also be found in select proteins of the body. One of the selenoproteins, iodothyronine-deiodinase, plays an important part in activating thyroid hormone. Horses have not been tested for selenium deficiency, but rats may show a link to thyroid function. There is speculation that some clinical manifestations of hypothyroidism could be secondary to selenium deficiencies.
Another selenoprotein, a muscle protein, is another that can cause muscular degeneration. This is the link between selenium, muscle integrity and selenium’s antioxidant qualities. Some horses have had muscle disorders (tying up) prevented by vitamin E and selenium supplementation. Although selenium deficiency was a cause of some cases years ago of tying up, commercial grain mixes have seen a significant decrease in the incidence of selenium-induced tie-up. It is still a good idea to evaluate the selenium status of horses prone to tying up.
The development of the acquired immunity system is dependent on selenium. Selenium deficiency can affect different classes of antibodies in different ways. Animal age, sex, and other antigens all have an impact on the extent to which the antibody production responds to supplementation. Although the exact amount of selenium required for an optimal immune response in horses is unknown, it seems to be close to the daily recommended intake.
It has been suggested that selenium may play a role as a preventative for certain types of cancer. Research is underway to increase our understanding of the exact mechanism that selenium has for anticarcinogenic effects. Researchers are currently investigating two scenarios to determine the mechanism. One is to provide adequate selenium to people with low selenium diets in order to reduce the risk of developing cancer. The other is to increase intakes of modified Selenium to protect against tumorigenesis.
Vitamin E and Selenium: The Relationship
Vitamin E is an essential component of selenium. It can be replaced if there is sufficient supply. Vitamin E in the cell membrane will reduce the formation of cholesterol peroxides. The intracellular fluid is rich in selenium, which will eliminate any lipid peroxides.
Low levels of vitamin E and selenium can lead to increased oxidation-induced damages, which in turn can cause similar effects as deficiency. These effects can be treated or mitigated by administering either vitamin E or selenium. The animal will need to consume the appropriate amount of each depending on how much of the other is available. Insufficient vitamin E can lead to more peroxide formation, so more selenium is required. In contrast, insufficient selenium can cause more peroxides to be formed and therefore, it is necessary to have more vitamin E. To minimize tissue damage from oxidation, the optimal amounts of each are required.
Toxicity and deficiency
Selenium-deficient soils are common in many areas with high concentrations of horses in the United States. The soil should be deficient in selenium so that forage can be grown there. Acidic soils are not good for plants. Therefore, areas with low levels of soil selenium will make forage more difficult to absorb. The U.S. horse population is not receiving sufficient amounts of selenium through forage.
Fee manufacturers and horse owners are responsible for providing adequate selenium to horses’ diets. Before starting selenium supplementation, it is important to ensure that the horse’s forage is sufficient in the region of origin. In horses that are pastured in selenium-deficient areas, there are no obvious signs of selenium deficiency.
It is easy to overlook subclinical signs that selenium deficiency. Deficiency in selenium would first affect cellular integrity, as it plays a major role in the oxidative defence system. Although it is hard to quantify, this damage may manifest as work intolerance and poor hair care, along with other symptoms that are usually associated with aging. Hypothyroidism could also be a sign of selenium deficiencies, though this has not been proved in horses. Deficiency can lead to myositis, cardiomyopathy (muscle inflammation) and white muscle disease (weak, pale muscle). Gross signs of a gross deficiency include growth retardation and cataract formation.
It is not easy to distinguish between the toxic and selenium requirements. According to the National Research Council, the minimum requirement for selenium is 0.1 mg/kg diet (one mg per day for an average horse), depending on age and use. Horses can have toxic levels as high as 2 mg/kg diet. Some soils in the United States are very rich in selenium. Forages and crops grown in these soils may be toxic. There are also selenium accumulator plant, such as milk vetch and woody aster that can superconcentrate selenium in the soil. The potential for toxicity of selenium accumulator plants is evident at 5-50 mg/kg. These plants have an unpleasant garlic-sulfur smell that horses will avoid unless forage availability is restricted. Alkaline soils seem to have more selenium available for plants. Horses that graze in areas with high levels of soil selenium, or selenium accumulator, plants can develop “alkali diseases”. This is characterised by hair loss, sloughing hooves, joint erosion, and lameness. Excessive levels of selenium can cause damage to the liver, heart, and skeletal muscles, as well as degeneration of bones, joints, and ultimately death.
Certain minerals, such as copper, silver and mercury, can reduce the toxicity of selenium. They can either increase or interfere with selenium’s absorption. Horses can forage in soil with high levels of selenium if they also consume these minerals. Although this is not ideal, it is a reason why some horses can survive on soil with high levels of selenium.
Selenium availability and absorption
The horse can have either organic selenium (found in grains and forages) or inorganic selenium (salts added to supplement). Selenium is not all equal for horses. The absorption of selenium appears to be quite high (>50%). According to limited research, organic selenium absorbs slightly more than inorganic. Selenium-enriched yeast (organic), was found to be more digestible and retain more selenium salts than inorganic. The soil content will affect the levels of selenium in plants and grains. Grains are a good source of selenium, as they are often grown in areas with adequate selenium. Garlic may also contain high levels of selenium. Selenium is a good source in both wheat and rice brans. Horse feeds are mainly made up of sodium selenite which is easily absorbed.
What is enough?
1989 National Research Council defines horse selenium requirements as 0.1 mg/kg diet. (Mg/kg is the equivalent to ppm). This amount is enough to prevent any signs of selenium deficiencies in an idle mature horse. This amount is also the legal limit for horse feeds. A 1000-pound horse would need to consume 1 milligram per day of selenium if he ate 10 kilograms of grain and forage per day. This would suffice for horse health. What impact does age, health, performance have on this requirement? The selenium requirement of the athletic horse should be increased because exercise increases oxygen delivery to tissues, oxidation of energy substrate, and results in the production of peroxides. The recommended daily intake of selenium for the performance horse could be between 2.5 and 3.5 mg depending on weight, work intensity, and other environmental factors. If the feed rate is sufficient, this amount can be supplemented with a commercial grain mixture.
How to determine selenium status
To determine selenium status in horses, concentrations can either be measured in serum or plasma, whole blood, liver, or whole blood. While serum, plasma and whole blood can be used to measure the horse’s selenium status, liver is more difficult and may not be practical for a live horse. The activity of the selenium dependent enzyme glutathione oxidase, which is found in whole blood, can also be used to measure selenium levels. The red blood cells are home to the majority of selenium as glutathioneperoxidase, which is the main source of whole blood selenium.
There is a strong correlation between whole-blood selenium and glutathione oxidase activity in almost all animals. Serum and plasma levels of selenium may be different from whole blood or glutathione oxidase activity. The concentrations in serum and plasma are more sensitive to changes in selenium status than whole blood and glutathione oxidase activity, which are more persistent indicators.
Changes in selenium intake will cause serum and liver concentrations to increase or decrease quickly. However, whole blood glutathione oxidase activity may slow down and remain elevated for up to nine months after the cessation of selenium supplementation. Selenium concentration will be affected by age. The normal levels of selenium for young horses are lower than that of adults.
Whole blood levels are higher than serum and plasma, and have different normal values. If supplementation history is not known, serum and whole blood should both be tested for selenium.
A few easy steps can help horse owners ensure that their horses have enough selenium .
1) A blood test for selenium is the best way to determine if your horse is receiving enough. The long-term effectiveness of supplementation with selenium can be assessed using whole blood. However, serum and plasma are sufficient if this is not possible.
2) Determine how much horse is receiving from the grain. The horse will not get enough selenium from forage if the hay is from an area that is deficient in selenium. There are many variables in the amount of selenium found in straight grains, such as oats, corn and barley. Although some laboratories can analyze feed and forage for selenium, the cost of this assay is high.
3. Find out the amount of selenium in horse feed. Many manufacturers include the amount of selenium in commercial mixes. Manufacturers should include this information if selenium isn’t listed.
A horse that is only receiving small amounts of a commercial feed may not be getting enough selenium. Commercial feeds are intended to be fed at a specific feeding rate. A horse that receives 1 pound of a commercial mix designed to be fed at 5 lbs per day will only get 1/5 of the required amount of selenium. Selenium supplementation might be necessary in such cases.
4) Make sure to check the amount of selenium in any supplements you are taking. Supplementing selenium or vitamin E to horses with a history in tying-up may be an option.
5) The daily intake of selenium should range from 1 to 10 mg depending on the horse’s size and how much work they do. Avoid consuming more than 20 mg daily, as toxicities can develop.
Selenium is essential for normal body function. Even though it is not essential, selenium plays an important role in antioxidant defense, immune functioning, thyroid hormones production and may have anticarcinogenic properties. Vitamin E can be used to compensate for a lack of selenium. However, adequate amounts should be given for optimal health and well-being horses.