Selenium: A Balancing Act

Today's horse owners are very supplement-oriented. When a new supplement hits the market, many horse owners are tempted to use it if they think it will somehow improve their horses' health. The problem occurs when a well-intentioned owner oversupplements a horse's diet either by not knowing the horse's current intake of minerals, or by thinking if a little is good, then more must be better. In the case of selenium, more can be toxic.

The upper safe total intake of selenium per day, based on the most recent National Research Council publication Nutrient Requirements of Horses, is 20 mg for an average 1,000-pound horse. Many horses get enough selenium (or even too much) just from their regular diet; for these horses, adding even as little as 5 mg selenium per day via supplements can cause mild signs of toxicity. 

Conversely, a selenium deficiency can be just as detrimental to the horse. It's a fine line to walk, so before adding any type of supplement to a horse's diet, it is important to understand what the horse needs, how much he needs, how much selenium he’s already getting in his diet, and how selenium is metabolized in his system.

Why Selenium?

A horse's body requires oxygen to fuel chemical reactions that produce energy for the horse. Seems simple, right? Not quite. These chemical reactions produce oxidative by-products that can be toxic if they are not kept in balance by antioxidants. These oxidative by-products--often referred to as "free radicals" or pro-oxidants--can damage cell proteins and DNA. When a horse's activity level increases and the body consumes more fuel, the production of these free radicals also increases.

Antioxidants are produced to combat the detrimental effects of these free radicals. Inability of the horse to produce enough antioxidants can result in an accumulation of free radicals that causes oxidative damage (oxidative stress) to cellular structures.

"Selenium and vitamin E are antioxidant nutrients that work very closely together," explains Kate Jacques, PhD, director of nutrition at Alltech, a nutritional research company in Lexington, Ky. "They are both needed and work best when they are in the right amounts."

Selenium is considered an antioxidant because it prohibits free radicals from damaging the cell membrane. It is commonly found in soil, groundwater, and plants.

Selenium Toxicity

A horse with a supplemental selenium intake in excess of 5 mg per day might suffer from selenium toxicity if his forage and feed already provide adequate selenium; the maximum tolerable level of selenium in a horse’s total diet is only 20 mg per day. As little as 50 mg per day total intake can result in mild signs of toxicity (loss of mane and tail hairs, horizontal hoof cracks). Animals consuming more than 1 gram per day--or those that are inadvertently overdosed with selenium even once--can develop acute selenium poisoning, which will rapidly cause death (as in the 2009 deaths of several polo ponies due to a medication error). Clinical signs of acute selenium poisoning include gait abnormalities, garlic breath odor, labored breathing, and muscle tremors. This poisoning can be life-threatening depending on the amount consumed. Overdoses can also be fatal for foals.

Chronic selenium toxicity, which is more common, is the result of long-term exposure to high levels of selenium for a period of weeks to months. Chronic toxicity can cause blindness, lameness, abdominal pain, excessive salivation, teeth grating, paralysis, respiratory failure, and death.

Toxic levels of selenium can damage the cells that form keratin (keratinocytes), which is the primary constituent that makes up hair and hooves. In affected horses, selenium replaces sulfur in the keratin molecule, which is what causes the clinical signs of chronic selenium toxicity including hair loss, horizontal hoof wall cracks, and even hoof sloughing in severe cases.

Regional Differences

Some areas of the United States have very high concentrations of selenium, while other areas are selenium-deficient. Research has shown that the concentration of selenium in an area depends on the type of rock from which the soil was derived, according to researchers at CSU's cooperative extension service. The Northwest, Southeast, and Great Lakes states are considered selenium-deficient areas because the soils were derived from volcanic rock or coastal deposits. Soils originating from Cretaceous shale, such as the soils in South Dakota, Montana, Wyoming, Nebraska, Kansas, Utah, Colorado, and New Mexico, tend to have much higher selenium levels.

Seleniferous (selenium-rich) soils are widespread throughout the Rocky Mountain and Great Plains regions of the western United States (see map on page 102). High levels of selenium are commonly present in areas with arid climates (less than 20 inches of annual rainfall) and acidic soils that are developed from shale rock. However, the uptake of selenium in plants is dependent on the chemical form of selenium, soil pH, temperature, moisture, the plant species, and stage of its growth.

Selenium in Plants

Selenium in its inorganic form is minimally absorbed by plants, whereas selenates, the organic form, are present in alkaline soils and are readily absorbed by plants and animals. Therefore, the analysis of selenium concentration in the soil alone is not a reliable predictor of selenium uptake by plants; one must look at the form and not just the amount.

Some plants absorb and store more selenium from soil and water than others,
making them toxic to livestock. These plants have adapted to grow only on seleniferous soils. Some of the commonly known accumulators are milkvetches, poisonvetches, prince's plume, and goldenweeds.

Fortunately, most of these plants are not readily consumed by grazing animals because they are unpalatable and emit a garlic-sulfur odor. This deters horses from eating them unless forced by overgrazing or starvation. These plants are important indicators of seleniferous soils on rangeland and can be used to identify areas where selenium poisoning might occur.

Different plants are able to grow in areas void of selenium, but when in selenium-rich areas, they also can store selenium in high concentrations. These plants--including curlycup, saltbushes, broom snakeweed, asters, and gumweed--are more likely to cause selenium toxicity in horses because they are palatable. However, they become less palatable as levels of selenium uptake increase.

Since grasses contain lower levels of selenium during the fall and winter months, seleniferous pastures should be used during those periods, and their use should be limited or avoided in spring and summer. Controlling plants that can take up high levels of selenium is also helpful in reducing the risk of selenium toxicity to horses. Most of these plants are broad-leaf weeds that can be killed by using pasture-safe herbicides such as Roundup.

Sarah Ralston, VMD, PhD, Dipl. ACVN, associate professor of animal science at Rutgers University in New Jersey, recommends that horses in seleniferous areas or which are supplemented with selenium should be observed for changes to their manes, tails, and hoof condition.

"Mane and tail hair loss are the most visible signs associated with selenium toxicity," Ralston says. "Horizontal cracks in the hoof that start at the heel are also a common sign of selenium toxicity."

If an owner suspects a horse is suffering from selenium toxicity caused by grazing in seleniferous pastures, Ralston recommends the horse be removed from the area and placed in a dry lot with grass hay until a veterinarian is able to examine him and have the pasture tested for selenuim.

Selenium in Hay and Water

Jacques warned that horse owners should also be aware of the selenium content in their hay sources. "Hay shipped from other areas that have high concentrations of selenium in soils could contain toxic levels of selenium," Jacques said. Alfalfa hay grown in seleniferous areas can be especially high in selenium. Grass or alfalfa hay with high levels (higher than 5 ppm) should not be fed to horses. Supplementing with feeds grown on non-seleniferous soils might be necessary to dilute the total dietary intake of selenium. Feeding hay with sulfur amino acids such as cysteine or methionine might reduce the effects of a high-selenium diet. Selenium replaces amino acids, such as sulfur. By increasing your horse's amino acid intake, you can dilute the selenium levels.

Forage or feed suspected to contain a high selenium concentration can be analyzed to determine total selenium content. Horses continually fed forages containing marginal levels of selenium might show signs of chronic selenium toxicity.

Water can also be a source of selenium concentrations high enough to produce toxic effects in horses. Horses using water supplies such as stock dams or streams in seleniferous areas are at risk of chronic or acute selenium toxicity. Horses shouldn't be permitted to access water supplies that have a selenium concentration greater than 0.05 ppm.

Selenium Deficiency

Despite its risk of toxicity, selenium is still an essential nutrient that is needed to protect the integrity of cell membranes. In areas that are selenium-deficient, horses might need to be supplemented with selenium. Selenium deficiency may be a factor in causing a degenerative muscle condition known as rhabdomyolysis, or tying-up. In young horses, it can manifest as a condition known as white muscle disease. A horse with rhabdomyolysis will experience severe muscle cramps that result in stiffness, sweating, and an increased pulse rate. Affected foals will develop gait abnormalities and will not be able to swallow normally. Examining selenium levels in horses can be tricky, Ralston says. Cornell and Michigan State Universities have the best laboratories in the United States to determine whole blood selenium levels in horses. "Selenium is very difficult to analyze in the horse's blood because it is present in such small levels," explained Ralston.

"To determine your horse's intake of selenium from grazing, you should contact your local county extension agent and have your pastures analyzed for selenium content," Jacques suggests. Most extension agencies will test soils at no charge. "You should also contact your feed companies, because yeast-based selenium is often already added to feed mixtures and adding extra supplements on top of that could be dangerous."

Supplementing Selenium

"I think selenium is way oversupplemented in the U.S.," Ralston says, adding that owners should discuss their horses' dietary intake of selenium with a veterinarian or equine nutritionist to determine if additional supplementation is required. For horses with a selenium-deficient diet, Ralston recommends using a yeast-based supplement. "At least the yeast-based supplement is a natural form of selenium," she comments.

The yeast-based organic supplement can be easily poured over a horse's feed or premixed into the grain by the feed supplier. Organic selenium is much safer to handle because it isn't absorbed through your skin like inorganic selenium, according to Jacques.

Take-Home Message

If your horse is eating a balanced diet of grains and forages, he probably consumes enough selenium to offset soil deficiencies. Before adding any type of supplements to your horse's diet, discuss his dietary needs with your veterinarian or equine nutritionist and consider having his forages tested.

About the Author

Chad Mendell

Chad Mendell is the former Managing Editor for .

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