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Vitamins and Minerals in Horse Feed Rations, -- Closer Look at Selenium

by: Ann Swinker, PhD, Extension Horse Specialist, Penn State University Equine Science Program Penn State抯 Equine Science Program frequently receives questions about feeding horses, vitamin and mineral supplements and hay testing. When balancing or evaluating a horse抯 ration, we always use the National Research Council抯 (NRC) Nutrient Requirements of Horses, tables as a guideline to determine available nutrients to meet your horse抯 requirements (available online at This article contains many of the NRC guidelines. The NRC is currently being revised (published by the National Academy Press, Washington, D.C., in 1989) and the new edition should be available in the next year.

We need to remember, that horses evolved as a grazing animal with a digestive system that was meant to ingest grass. Nearly, 80- 100% of a horse's feed ration is hay. The general rule of thumb for feeding horses at a maintenance level is to feed 2% of the horse's body weight in pounds of average quality mixed grass hay. Therefore, the average 1,000 lb. horse would require approximately 20 pounds of mixed grass hay/day to maintain its weight. Nutritional qualities of hay can vary allot from a grass hay such as timothy, which has a digestible energy content of about 1.8 Mcal/kg and a crude protein content of about 7%, and a high-quality legume hay like alfalfa, with a digestible energy content of 2.2 Mcal/ kg and a crude protein content of 17%. The quality of hay is affected by the stage of maturity when cut, the weather conditions at the time of harvest and hay processing. To determine the nutritional content of your hay have the hay sampled and analyzed.

Additionally, the nutritional requirements of your horse will vary by age, stage of growth, pregnancy and lactation, and other factors such as climate, and intensity and duration of exercise. So, your average 1,000 lb. horse would require 20 pounds of average timothy hay or 16 pounds of the average alfalfa hay for daily maintenance. To complicate your feeding program, the performance horse and the lactating broodmare will require a higher nutrient dense feed (such as grain) to meet the energy needs. So after meeting the energy levels --how do you know what supplements to ad to the diet.

Protein and vitamin-mineral supplements are added to the diet to increase the diet抯 concentration of these nutrients. Grains are often energy supplements to a high forage diet. We only add supplements to the diet if some nutrients are needed or missing in the hay. Vitamin and mineral supplements should only be added to the diet if the ration is deficient in these nutrients.

Vitamins are needed in much smaller amounts than other nutrients, but they are important for many body functions. Each vitamin has a different job in the body. Some vitamins are in the food a horse eats while others are produced inside the horse. Depending on its diet, a horse may need vitamin supplements. Supplements are not necessary if a horse grazes on high quality pasture or eats high quality hay.

Several factors can decrease the vitamin content of horse feed. Some vitamins are sensitive to sunlight, heat, and oxidation (especially vitamins E and A). Hay stored for a year or longer, and hay that was rained on may have a decreased concentration of vitamins. In the processing of some feeds, heat and pressure can cause the loss of vitamins, so it is important to know if the vitamins were added back in, and if so how were they added. The most common vitamins added to horse feeds are A (important for reproduction), E (a natural preservative/antioxidant that helps ensure optimum function of the reproductive, muscular, circulatory, nervous, and immune systems), and H (biotin, which helps improve hoof and hair quality). Iron, copper, phosphorous, calcium and magnesium are minerals important to the horse. Without iron, blood can not carry oxygen to the body抯 cells. Without calcium and phosphorous, bones and teeth will not form properly. Calcium and phosphorous should be fed in a ratio that ranges from 3:1 (three parts calcium for each part of phosphorous) and no less than 1:1. An imbalance of these and other minerals can cause developmental bone disease in young, growing horses.

Vitamin and mineral supplements should only be added to the diet if the ration is deficient in these nutrients. Generally, the major minerals of concern in feeding horses are calcium, phosphorus and salt. In some geographical areas, the lack of selenium and, in growing horses, copper and zinc, is a concern. Other minerals are likely to be present in adequate amounts in a normal diet with high quality feeds.

It is well known that many areas of the United States produce selenium-deficient forage. Pennsylvania抯 soils have been known to be deficient in selenium. The concentration of a particular mineral within a plant is proportional to concentrations of that mineral in the soil. So rations for livestock in Pennsylvania and surrounding areas need to pay attention to Se levels in the feed. One thing to remember, minerals are relatively stable (don't break down) in feed processing. The concentration of selenium in feedstuffs will ranges from 0.05 to 0.3 ppm and is influenced by variations in soil selenium and pH. Selenium is absorbed quite efficiently (77 percent) in nonruminants, in contrast to about 29 percent for ruminants. Selenium in forages and grains is normally present as organic selenium. Sodium selenite and sodium selenate are common inorganic sources of supplemental selenium. Although the Food and Drug Administration (FDA) has approved maximal selenium supplementation at 0.3 mg/kg of dry matter in complete feeds for cattle, sheep, and swine (FDA, 1987), selenium supplementation of equine feeds is restricted by industry practices. The selenium requirement of the horse was estimated at 0.1 mg/kg of diet; and that 140 ng of selenium/ml of plasma (or serum) was adequate to prevent problems associated with selenium deficiency (NRC 1989). The Food and Durg Admimistration recently approved organic selenium or selenium yeast for horse feeds. Researchers (KY) suggested feeding 3mg of organic selenium yeast per day for horses with a marginal Se status.

Signs of Deficiency: Selenium deficiency can cause muscle disease in both adult horses and foals. Check with your veterinarian regarding the supplementation of selenium, because the difference between the required amount and a toxic amount is very small. Be careful to avoid using three or four feed supplements that contain selenium; this can unintentionally cause toxicity. This holds true for all of the supplements. And be aware of exactly what is in the feed and how much of it, then measure what your horse needs. Selenium and vitamin E are both anti-oxidants, and selenium deficiency can be modulated by the amount of vitamin E present in the diet. Although selenium is essential, it can also be toxic, so supplementation must be controlled. Selenium deficiency can also cause weak foals. When mares are consuming selenium-deficient diets, foals can be born with white muscle disease, which affects their hearts and skeletal muscles, often resulting in death.

Signs of Toxicity: Chronic (over a long period of time) selenium toxicity梐lkali disease梚s characterized by hair loss (alopecia), especially about the mane and tail, as well as cracking of the hooves around the coronary band and signs of laminitis. Acute (major dose) selenium toxicity is characterized by blindness, head pressing, perspiration, abdominal pain, colic, diarrhea, increased heart and respiration rates, and lethargy. There are anecdotal accounts of immediate death after administration of injectable vitamin E/selenium preparations. These deaths appear due to a sensitivity of the horse to a carrier ingredient in the injectable preparations and not to the toxicity of selenium or vitamin E.

It is well known that muscle tying-up (myopathies) responds to vitamin E and selenium suggesting an interrelationship between these two nutrients. Studies have shown that vitamin E and selenium function as an antioxidant defense system.

Selenium status: The selenium status of horses can be evaluated by measuring serum, plasma, or whole blood selenium. The serum selenium of foals from selenium-adequate mares is typically much lower than their dams and ranges from 70 to 80 ng of selenium per ml of serum (Stowe, 1967). If, according to Blackmore et al. (1982), serum selenium values below 65 ng/ml are indicative of deficiency, young foals may be prone to nutritional muscular dystrophy, especially if their vitamin E status is low.

The maximum tolerable level of selenium in horses is estimated at 2 mg/kg of diet (NRC, 1989), and the LD50 for orally administered selenium is considered to be approximately 3.3 mg of selenium (as sodium selenite)/kg of body weight (Miller and Williams, 1940). For a list of adequate to toxic amounts of minerals for horses (NRC 1989) see table 1.

However you balance a ration for your horse, make sure it is one that suits the horse's age, activity level, and nutritional requirements. In addition to your veterinarian, your Penn State county extension agent can help with ration balancing and forage/feed analysis. One of the best references is (available online at

TABLE 1. Minerals for Horses and Ponies (on a dry matter basis) Adequate Concentrations in Total Rations
Minerals Maintenance Pregnant and Lactating Mares Growing Horses Working Horses Maximum Tolerance Levels
Sodium (%) 0.10 0.10 0.10 0.30 3a
Sulfur (%) 0.15 0.15 0.15 0.15 1.25
Iron (mg/kg) 40 50 50 40 1,000
Manganese (mg/kg) 40 40 40 40 1,000
Copper (mg/kg) 10 10 10 10 800
Zinc (mg/kg) 40 40 40 40 500
Selenium (mg/kg) 0.1 0.1 0.1 0.1 2.0
Iodine (mg/kg) 0.1�6 0.1�6 0.1�6 0.1�6 5.0
Cobalt (mg/kg) 0.1 0.1 0.1 0.1 10