Many owners feel they are depriving their horses if they aren't feeding them a bunch of supplements. “If a little is good, a lot is better” is often the philosophy, with the reasoning being a horse will simply release extra nutrients in waste with no harm done. Experts say this is a common misconception and one that can be detrimental to the horse's health and the owner's pocketbook. What owners fail to realize is that even if a nutrient isn't toxic in large quantities, a horse's body still has to work harder to process nutrients that are fed in excess of its needs. Besides dietary inefficiency, piling on too many supplements could increase the risk of accidentally overloading a horse on one nutrient which could cause health problems or toxicity.
Dr. Carey Williams, nutritionist and associate professor of animal science at Rutgers University in New Jersey, said horsemen run into trouble when they feed several supplements without considering the cumulative amount of nutrients in the horse's overall diet. Vitamin A is one example.
“Vitamin A is regularly put in a lot of supplements, regardless of what they're for,” Williams said. “People don't realize that if they're feeding four or five supplements, they're at toxic levels, especially if the horse is on a really good-quality forage and a high-grain diet.”
Too much Vitamin A in a young horse's diet can hamper growth, cause skin conditions, and interfere with bone development.
Williams said although fat-soluble vitamins A, D, E, and K are stored in the body instead of being excreted, making them potentially toxic, very high levels usually are required to cause a problem.
Williams said protein is the nutrient she most often finds being fed in excess. Horseman associate protein and its building blocks, amino acids, with support for muscles. But coping with excess protein taxes the kidneys and places a general strain on the horse's body.
Too much protein actually can slow down a racehorse because it causes them to produce too much ammonia. According to a 2010 study of racetrack feeding practices published in the Equine Veterinary Journal by the University of Maryland's Dr. Michael Glade, every kilogram of crude protein ingested over the recommended level caused horses to finish one to three seconds slower. Researchers theorized energy that should have been devoted to performance was instead utilized to detoxify the body of excess ammonia.
Excess ammonia from urine in a horse's stall can cause eye irritation, hoof problems, and respiratory disease.
Selenium can reach toxic levels quickly. Williams said the normal equine diet should contain one to three parts per million (ppm) of the mineral, but just 18 ppm is toxic. Feeding two or three supplements containing selenium could add up. This becomes more dangerous if the horse is in an area of the country where selenium is naturally abundant in soil.
“If you're in a selenium-deficient area and you're feeding a couple of supplements, that will be okay,” Williams said. “[Toxicity] usually happens when you're in an area where the soil is high in selenium or you have selenium-retaining plants, along with the high selenium in the diet.”
The first sign of selenium toxicity is loss of mane and tail hair; in extreme cases, the hooves will slough off and the horse will have to be euthanized.
Magnesium is another commonly overfed mineral. The mineral is touted as a calming agent, and many horse owners with nervous or difficult horses automatically turn to these supplements as a quick fix.
“Magnesium seems to be in a lot of substances, and some forages can get really high in magnesium,” she said.
Although little is known about the toxic effects of magnesium in horses, data from other species suggests it may cause clinical signs similar to calcium deficiency — developmental orthopedic disease, loose teeth, and weight loss.
Copper and zinc are also minerals of concern because they must be given in balance with each other. Feeding too much of one will cause a deficiency of the other. Other nutrients in the diet must also be fed in the proper ratio, such as calcium to phosphorus.
If testing shows your horse is suffering from heavy metal toxicity, Williams said to look to pollution in its environment, not its feed tub. Industrial pollutants may creep into the pasture or contaminate water.
Supplementing a deficiency
Williams warned that treating a deficiency in a horse is more complicated than simply supplementing the diet with that nutrient. If the amount of the nutrient in the horse's diet is adequate, some underlying cause may be present.
“That's a really big thing with iron, especially in horses that have anemia,” she said. “People will think they just need more iron because they're anemic. Horses aren't that way; they don't metabolize that way either. So if a horse is anemic, there's a reason for it, and it's typically not the iron.”
Williams advised horsemen to look at the ingredients of all the supplements they feed, the guaranteed analysis of their horse feed, and their forage analysis, if available, to calculate the cumulative amount of specific nutrients their horses consume. Then they can compare their findings with the National Research Council's recommendations, Nutrient Requirements for Horses, which is available on the Internet and in hard copy.
She added that a horse compromised by a condition or a disease might require a special diet, so the owner should consult a nutritionist or a veterinarian when formulating or changing that horse's diet.
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