Feeding Horses with Endocrine Disorders

Nicholas Frank, DVM, PhD, Dipl. ACVIM, associate professor of large animal clinical sciences at the University of Tennessee, spoke about equine endocrine disorders that are the most manageable by dietary control. There are two main endocrine disorders to address: equine metabolic syndrome (EMS) and equine Cushing's disease (ECD or pituitary pars intermedia dysfunction, PPID). Frank addressed these at the 2007 American Association of Equine Practitioners Convention, held Dec. 1-5 in Orlando, Fla.

An EMS horse tends to be fairly young, might have some genetic predisposition, and has pronounced fat deposits, especially of the neck, shoulders, and buttocks. These horses are considered "easy keepers." Some breeds are more likely to develop EMS than others, such as Paso Finos and Arabians. Generally, these horses have a higher-than-normal blood insulin concentration. An ECD horse is typically an older or aged horse with pituitary dysfunction that results in excess secretion of ACTH (adrenocorticotropic hormone), alpha-MSH (alpha-melanocyte stimulating hormone), and other hormones. These horses have delayed or patchy hair coat shedding, and there is noticeable muscle loss. Often there is excess drinking and urination, which might go undetected.

Frank explained that insulin resistance can develop in either EMS or ECD individuals. Insulin resistance results from impaired tissue responses to insulin due to problems with insulin receptors, insulin-signaling pathways, or glucose transport systems. To compensate, many horses will secrete additional insulin, thereby worsening the problem. An obese horse has lipid accumulation within adipose (fat tissue) and skeletal muscle tissue that interferes with insulin signaling, leading to insulin resistance. Frank described some basic concepts that are important for a horse owner and veterinarian to consider:

  • Obesity predisposes to insulin resistance;
  • Insulin resistance is a diabeteslike state;
  • Dietary sugars exacerbate insulin resistance;
  • Insulin resistance decreases the threshold for developing laminitis (horses are more prone to laminitis);
  • It is important to avoid laminitis triggers such as intestinal abnormalities, seasonal variations, and dietary changes.

A goal for managing an obese horse is to induce weight loss. In a horse that is in good body condition, yet has insulin resistance, you should aim to maintain its current weight with the objective of improving insulin sensitivity. A thin Cushing's horse with insulin resistance should gain weight and improve its insulin sensitivity.

Of three possible diets to manage insulin resistance, the first Frank described is a weight loss diet in which all grain is eliminated and access to pasture is eliminated or restricted to less than two hours per day or to a small area of strip grazing. A grazing muzzle is useful to control consumption of grass. Dynamic phases of grass growth (for example, following drought conditions) should be avoided. Soaking hay for 30 minutes before feeding lowers its sugar content. Hay should be analyzed for its nonstructural carbohydrate (NSC) content, and hay should be selected that is low (less than 10-12%) in NSC. Frank notes that it is impossible to identify NSC content (simple sugars and starches) from appearance, and content varies by grass type, rainfall, soil type, season, cutting, and time of day. (As a comparison to forage, oats have a value of 50% NSC.)

General recommendations apply to most, but not all, insulin-resistant horses since insulin resistance varies in severity and there are individual variations in responses to feed.
Initially, the fat horse should be fed 2% of its current body weight in hay along with elimination of grain and pasture grass from the diet. Then, hay is lowered to 1.5% of current body weight, then offered at 1.5% of ideal bodyweight. For example, you'd give 15 pounds of forage to a 1,000-pound horse. The horse is fed at this level until an ideal body condition score is achieved. In some cases, a hay-only diet might provide insufficient protein; a diet of 8% protein is acceptable. If more protein is needed, a small amount of alfalfa hay or soybean meal can be substituted for a portion of the diet.

Exercise is important for weight loss, provided laminitis or lameness does not preclude physical activity.

The second diet Frank considered is a weight maintenance diet that is predominantly forage-based (hay) with less than 12% NSC. Hay is fed at a rate of roughly 2% of body weight to maintain a horse's weight. Vitamins, minerals, and protein are provided as needed. High-glycemic feeds should be completely avoided, such as sweet feed with molasses, as this challenges glucose and insulin metabolism and exacerbates insulin resistance. A horse experiences a higher glycemic response (blood sugar) if carbohydrates are digested in the small intestine, so it is best to feed small amounts of food more frequently (so lesser amounts of carbohydrates ferment in the small intestine at one time), and to feed hay before concentrates. Always avoid sudden changes in feed so the bacterial flora of the bowel can adjust gradually to various feed materials.

Frank stressed that "a safer feed is not necessarily a safe feed." General recommendations apply to most, but not all, insulin-resistant horses since insulin resistance varies in severity and there are individual variations in responses to feed. Body condition should be assessed every two to four weeks, and blood insulin should be monitored.

The third diet Frank described is a weight gain diet that relies on low-NSC (less than 12%) hay fed free-choice along with molasses-free beet pulp and rice bran or corn oil. Soaked molasses-free beet pulp is mostly digested in the large intestine. Presoaked, one cup is equivalent to one-quarter pound. Start with one-quarter to one-half pound of pre-soaked beet pulp and feed this twice a day, increasing gradually to 1 pound daily. Beet pulp should be rinsed twice in warm water to remove simple sugars, then soaked for 10 to 30 minutes in warm water before feeding.

Commercial low-NSC pelleted feeds are also recommended for weight maintenance or gain. Advantages of these products include the ease of feeding, regular testing of ingredients to ensure a lower sugar content, and addition of ingredients that affect absorption and palatability.

About the Author

Nancy S. Loving, DVM

Nancy S. Loving, DVM, owns Loving Equine Clinic in Boulder, Colorado, and has a special interest in managing the care of sport horses. Her book, All Horse Systems Go, is a comprehensive veterinary care and conditioning resource in full color that covers all facets of horse care. She has also authored the books Go the Distance as a resource for endurance horse owners, Conformation and Performance, and First Aid for Horse and Rider in addition to many veterinary articles for both horse owner and professional audiences.

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