Equine nutritionists and veterinarians currently use the selenium requirements for horses as a guideline for mules. | Getty ImagesQ: Are mules extra sensitive to selenium? Does too much selenium cause hoof problems? What are the symptoms of excess selenium?A: Mules, which are a cross between a female horse and a male donkey, might have different nutritional requirements than horses because donkeys are extremely efficient at utilizing nutrients. However, very little research exists on the specific nutrient requirements of donkeys or mules. Because of this lack of data, equine professionals currently rely on established horse requirements when formulating diets for mules.The Role of Selenium in Equid DietsSelenium is an essential micromineral, meaning equids must obtain it through the diet. It plays several critical roles in the body, including supporting immune function and thyroid hormone regulation, and it acts as an important antioxidant.Both selenium deficiency and toxicity can be problematic. Clinical signs of deficiency can include weakness, impaired movement, respiratory disease, difficulty swallowing, and impaired cardiac function. While deficiency is relatively common in horses, depending on geographic region, cases of selenium toxicity have also been reported.Acute selenium toxicity often causes abdominal pain, colic, apparent blindness, excessive sweating, lethargy, and increased heart and respiratory rates. Chronic selenium toxicity more commonly leads to hair loss and hoof changes, including alopecia and cracking around the coronary band.Selenium Requirements for MulesThe true selenium requirement in mules remains unclear. Authors of the Nutrient Requirements of Horses (NRC, 2007) recommend 0.1 milligrams of selenium per kilogram of dry matter intake to prevent classical deficiency symptoms in horses. Selenium intake can vary widely by region because soil selenium levels shape the selenium content of the local forages. Also, akaline soils tend to produce plants that accumulate more selenium.If you are concerned about your mules selenium intake or suspect toxicity, consult a qualified equine nutritionist. A nutritionist can evaluate all components of the diet, including forage, pasture, concentrates, and supplements, to estimate total daily selenium intake. Then they can compare this measurement with NRC guidelines to ensure intake meets but does not significantly exceed recommended levels.Because health concerns are involved, also consult your veterinarian. A veterinarian might recommend blood tests to assess plasma selenium concentrations and help rule out other causes of hoof abnormalities, because hoof health issues can have multiple underlying factors.Take-Home MessageAlthough equine nutritionists have limited scientific data available on donkey and mule nutrition, they often consider these animals metabolically efficient. Because the NRC does not provide selenium requirements for donkeys or mules, professionals currently use horse recommendations as a guideline. Working with both a veterinarian and a qualified nutritionist can help prevent excessive selenium intake, which has been linked to hoof issues such as cracking at the coronary band.Do you have an equine nutrition question? Do you have an equine nutrition question? The Horses editors want to hear from you! Submit your question via the form below. Name(Required) First Last Email(Required) Submit your equine nutrition question below!(Required)CAPTCHA