AAEP 2002: Recent Developments in Equine Nutrition
A lot has happened in the field of equine research in the last five years. Ginger Rich, PhD, of Rich Equine Nutritional Consulting in Eads, Tenn.; and Leslie Breuer, PhD, of LH Breuer and Associates, updated veterinarians and others who attended the Current Concepts in Equine Nutrition in-depth session at the 2002 American Association of Equine Practitioners’ (AAEP) Convention. Not all of the research done from 1998-2002 could be included in their presentation titled “Recent Developments in Equine Nutrition with Farm and Clinic Applications,” so Rich and Breuer chose what they felt were some of the most important research findings.
To organize their presentation, they divided it into these topics--water; energy; minerals; protein; vitamins; growth; broodmare nutrition; performance horse nutrition; nutritionally related diseases; new products, ingredients, and processing methods; and miscellaneous topics.
Research at New Bolton Center determined that during the winter when warm and cold water are both available, a horse will prefer to drink the cold water, but in lesser amounts than if only warm water were available. When only provided with either warm or cold water, horses will consume more warm water than cold water. Therefore, in order to get a horse to drink more water during the winter, lessening the risk for dehydration, only provide warm water. In addition, researchers also observed that confined horses drank the most shortly after a grain meal or within an hour of eating hay. It is suggested that lack of availability during this time might contribute to poor performance or colic.
In a Kentucky study, it was found that resuming exercise delayed further absorption of water. Geldings performed in a simulated endurance training exercise to determine if high-intensity exercise influenced fluid uptake from the gastrointestinal tract. During a one-hour rest period, some water was absorbed and rehydration began. However, when the horses resumed exercise, exercise then delayed absorption. Rich said there is a risk of colic and founder when hot horses drink cold water and then stand around. Rich recommended that hard-working horses in prolonged exercise be allowed periodic rests of at least one hour with free access to water after a cool-down period.
In other research, feeding fat to performance horses has been shown to have beneficial effects on water retention and water available for sweating, which allows for better heat dissipation during hot, humid weather.
Forages are the most important source of energy, and many horse owners do not work on improving pastures, said Breuer. Attention to soil fertility, weed control, and grazing management is of utmost importance. Due to an increase risk of laminitis and colic caused by increased hydrolyzable and rapidly fermenting carbohydrate content of early spring and fall pastures, he recommended doing forage analyses year-round. Researchers in Virginia found peak levels of these carbohydrates in forage samples taken in October and March. He suggested that owners restrict access to pasture for at-risk horses, and that pasture management can help control lush pastures. Forage analysis will also allow owners to supplement for any deficiencies.
Glycemic response is the relative response in blood glucose and insulin after eating a starchy food. Research at Kentucky Equine Research determined that the highest glycemic response is from sweet feed, then oats, corn, and a high-fiber feed. Adding fat can reduce the glycemic response. Feeds with a low glycemic response might be beneficial to horses, according to Breuer.
The theory is that feeding a high-glycemic feed to any horse that has problems utilizing glucose or has insulin resistance, or is laminitic, might be detrimental. Studies are also underway to determine if feeds with a high glycemic index are detrimental to young horses.
The effect of feed processing on glycemic response was also measured. Steam-flaked corn was found to have the highest glycemic response, ground corn was next, and cracked corn has the lowest glycemic response.
In another study, giving glucose and fructose to horses did not affect the glycemic response. This is because fructose is well-absorbed in horses and is rapidly converted to glucose. Glucose and fructose sources such as molasses are well used by the horse.
Further research into the feeding of fat discovered that there was almost 100% absorption of various fat sources--corn oil, peanut oil, soy oil, soy lecithin, tallow, and fat blends. These fat sources did not depress digestion of fiber or nutrients. Breuer recommended choosing a fat source based on availability and cost, although many researchers recommend vegetable sources of fat over animal sources due to increased palatability.
In another study, it was confirmed that corn oil had little or no effect on feed intake or digestibility of other nutrients; however, a study in The Netherlands showed that diets very high in fat might cause enough fat to enter the large intestine to depress fiber fermentation by bacteria, according to Breuer.
In Finland, researchers fed fat to fat-adapted horses and non-fat adapted horses. They found depressed glycogen repletion (rebuilding of the glycogen stores) after an exercise test in non-fat adapted horses. But they also found glycogen repletion was not depressed after horses were fed fat for three weeks.
The benefits of omega-3 fatty acids have been touted in human and canine nutrition. Omega-3 and omega-6 are recognized contributors to normal cell function and help prevent immune disorders, skin conditions, inflammatory conditions, and certain disease conditions. There is considerable interest in the dietary ratio of omega-6:omega-3 fatty acids; in particular, diets that promote an increase in omega-3 relative to omega-6 fatty acids are advocated because of purported health benefits. One equine study showed that feeding fish oil (a good source of omega-3 fatty acids) increased omega-3 fatty acids in the horses’ blood serum profiles. As a result, the omega-6 fatty acid concentration was lower, as was the omega-6:omega-3 fatty acid ratio. “Changes in blood levels of insulin, free fatty acids, and glucose during an exercise test may indicate that insulin sensitivity and glucose metabolism was affected by the fish oil (for the better),” according to the authors of the study.
“In the past five years, there has been abundant literature on bone mineral content, availability of organic versus inorganic sources, emerging significance of previously considered minor minerals, and effects of growth, exercise, or sedentary status on mineral metabolism,” Rich said.
She mentioned the use of a biopsy of the 12th rib as a diagnostic tool in evaluating mineral status, especially calcium. In addition, computed tomography can also estimate bone mineral content and differences in bone density.
Chromium--This mineral is not well understood, but a new study has shown that chromium tripicolinate supplementation can increase the rate at which glucose is metabolized. However, further research failed to show an effect on growth rate and development in yearlings, and there was little effect on metabolic, hormonal, and immune response in mares feed a Bermuda grass hay diet. In addition, supplementation of growing, sedentary, and geriatric horses with chromium tripicolinate or chromium-L methionine had no consistent effect on growth, immunity, or glucose/insulin responses.
Molybdenum--Despite reports in other species of a detrimental effect on copper absorption and retention with molybdenum supplementation, horses seem to be immune to this.
Aluminum--Short-term addition of aluminum to the diet did not affect the digestibility and mineral metabolism of calcium, phosphorus, magnesium, zinc, copper, and boron.
Silicon--In a study involving young Quarter Horses in race training, zeolite A, a silicon-containing supplement, was fed. Researchers found less bone-related injuries in the supplemented group. In addition, supplemented yearlings had increased plasma silicon concentrations and decreased bone resorption. However, the study did not determine if bone strength was improved. Rich said that there seems to be promise in supplementing injured young horses with silicon.
Iron--Liver biopsies of horses fed excessive amounts of iron as ferrous sulfate showed no abnormalities. Therefore, it was concluded that horses or ponies are unlikely to develop iron toxicosis by over-consumption from this source of iron.
Manganese--One study confirmed that horses require the level of manganese recommended by the National Research Council (NRC). This level is 40 ppm, and Rich said that horses in regions known to be deficient should be supplemented. In the study, horses receiving 40 ppm gained more weight, had better feed efficiencies, higher serum copper, higher serum hydroxyproline (an indicator of bone resorption), and a higher total bone mineral content than those fed a diet deficient in manganese.
Copper, Zinc, and Manganese--A study comparing exercising horses to sedentary horses supplemented with copper, zinc, and manganese found that exercising horses have an increased need for zinc. Exercise had no effect on the digestibility or maintenance requirements of copper and manganese. Rich said that NRC values for zinc in working horses might be underestimated.
According to Rich, replacing half of the inorganic form of copper, zinc, and manganese with an organic form (also known as chelated or proteinated) in the diet did not affect liver concentrations of these minerals, did not improve immune response, and did not improve hoof wall growth rates, hardness, tensile strength, or trace mineral content in the hoof. Replacement in broodmare diets was not beneficial. In general, it was found that inorganic forms of these minerals were just as effective as organic forms.
Another study found that organic copper and zinc might increase copper and zinc retention and have slight benefits for copper digestion.
Selenium--Inorganic and organic forms of selenium were compared in exercising horses. The inorganic form of selenium-enriched yeast was more digestible with greater retention than sodium selenite.
Calcium, Phosphorus, and Magnesium Involved in Bone Density--Horses which have been confined to a stall or deconditioned for at least 12 weeks with minimal exercise experienced a decrease in bone mineral content. Supplemention at twice the currently recommended calcium level made no difference. “After 12 weeks of stall confinement, loss of mineral content might have weakened bones,” said Rich. “Great care should be used in the reconditioning process to avoid skeletal injuries.”
Further research found that higher levels of calcium and phosphorus in the diet allowed greater bone turnover in all ages, but young animals have the highest turnover. According to Rich, inactivity results in lower bone density, and when exercise begins, there is an increased risk of injury in older horses.
In one study, fillies were either fed a diet of 40% alfalfa hay or soybean meal supplemented diets. Growth measurements were the same for both diets, but the group fed the soybean meal retained more nitrogen, suggesting more protein muscle gain. However, in a study where 60% alfalfa hay cubes were the supplemental source of protein, growth rates were satisfactory in stock-horse type weanling fillies. Breuer pointed out that it is important to provide supplemental phosphorus and trace minerals in diets with large amounts of alfalfa.
A study was done to determine the pre-cecal digestibility of protein and lysine in alfalfa cubes and in mixed concentrates with graded levels of protein and lysine. It was found that pre-cecal digestibility of protein and lysine in high-quality alfalfa was only 60% of that of the concentrate mixes.
According to Breuer, most mature horses are fed more protein than is needed. Therefore, protein levels for mature performance horses can be reduced if adequate levels of amino acids are maintained. Researchers fed horses two different diets--one with 14.5% crude protein and one with 7.5% crude protein with supplemental amino acids. There was no deficiency observed with the lower protein diet. “Horses require amino acids, not proteins,” he said.
A study done at Rutgers showed that supplementing weanlings with 10 grams per day of vitamin C and 800 international units (IU) per day of vitamin E after a long trip improved vaccine response, reduced upper respiratory infections, and reduced days that they were off feed compared to non-supplemented weanlings. Another study found serum ascorbate and tocopherol concentrations were higher in horses that had been supplemented with vitamins C and E. Rich recommended that performance horses receive supplementation.
Another study found that oral and natural form d-alpha-tocopherol was the most effective form of vitamin E.
Another time when supplementation might be needed is for broodmares and foals. Broodmares lacking vitamin A--such as those on a dry lot or with poor-quality hay--are likely to have foals with lower birth weights, slower growth, an increased chance of foals with congenital contracted tendons, and an increased risk of retained placenta. One study showed that supplementation with retinyl palmitate at twice the NRC values resulted in higher pregnancy and foaling rates. Mares did not respond well to water-dispersible beta-carotene.
Rich recommended that lactating mares and foals be supplemented with folate (folic acid) until the foal is three months of age, since it has been demonstrated that mares’ milk folate levels decline during the first three months of lactation, leaving foals deficient. However, when foals begin to eat forage, their folate levels go back up.
Rich discussed how vitamin K is important for blood clotting and bone growth. Rich said that horses might have enough vitamin K in their diets for blood clotting, but lack enough for proper bone metabolism.
Growth and Development
The current trend in the horse industry is to accentuate growth in young horses; however, some fear developmental orthopedic disease (DOD) if growth is too rapid or foals become too large. Additional support for this theory was obtained in recent research done on a Kentucky Thoroughbred farm in which 271 foals were evaluated over four years. It was found that foals that developed DOD were larger at birth and grew more rapidly than foals without DOD.
If foals are fed for rapid weight gain, they will have an increase in bone mineral content, according to one study. However, the study showed that there were no differences in bone mineral quality in foals fed for rapid weight gain versus those fed for slow weight gain. Foals that do gain weight rapidly might have an increase in serum osteocalcin, indicating that more bone growth is taking place. However, they observed an increased incidence of physitis in foals fed for rapid gain.
Concerns over a connection between glucose intolerance and insulin resistance to development of osteochondritis dissecans (OCD) lesions prompted the development of an oral dextrose challenge test for glucose intolerance. Research at Rutgers found that there might indeed be an association of glucose intolerance and OCD.
Studies at Kentucky Equine Research on the effects of feed on the incidence of OCD indicated that providing feed with a low glycemic response could reduce the incidence of OCD.
Owners of young horses strive to make weaning time as stress-free as possible, and researchers wondered what the effects of weaning time are on body weight, wither height, and bone density. It was found that weaning had no effect; however, a decline in weight gain at one and three weeks after weaning is normal.
If young horses enter race training, there will be a period of bone demineralization, then a period of re-mineralization. A study of yearling Quarter Horses showed a significant decrease in the density of metacarpal bone believed to be caused by the exercise-induced bone remodeling. “The research indicated the onset of speed work often coincides with the time of greatest demineralization (50-60 days into training), which may account for the high incidence of skeletal injuries in 2-year-old horses in training,” according to the authors of the study. “Training methods and protocols should be designed to reflect the findings that significant demineralization of metacarpal bone occurs in the first 60 days of training.”
Further research shows that feeding higher levels of phosphorus, calcium, and magnesium than those recommended by the NRC might increase bone mineralization. And even for long yearlings not in training, calcium levels might need to be increased to 115% of NRC recommendations, according to Oklahoma researchers.
For many young horses, once training is begun, they are then confined to stalls. However, research suggests this might be detrimental. Michigan researchers found that confined long yearlings and 2-year-olds had a loss of bone mineral content in comparison with those on pasture. Rich recommended allowing young horses free access to exercise on pasture, or if this is not an option, then training should be modified to account for the bone loss. One product that might help prevent bone mineral loss is exogenous somatopropin (eST).
Rich said research in the past five years has provided the following tips for broodmare owners:
• Feed a mare a fat and fiber diet prior to weaning to help her handle weaning stress better than if she was on a carbohydrate-based diet.
• Mares kept in fat condition during fall and winter months are more hormonally prepared for breeding than thin mares. Fatter mares will continue to come into estrus, while thin mares will go into a deep anestrous state.
• Keep mares in a moderate-to-fleshy body condition before foaling to reduce the risk of poor pregnancy rates.
• Mares fed a fat and fiber concentrate will have healthier foals than those on a starch and sugar concentrate. They will also have increased linoleic acid in their milk, which might reduce the incidence of foal gastric ulcers and enhance passive immunity.
• In addition, certain digestive disorders might be reduced in mares fed a fat and fiber feed in place of grain.
• It is not necessary to feed more copper, zinc, and iron to pregnant and lactating mares than is recommended by the NRC. A study showed that overfeeding has no beneficial effect.
• If selenium supplementation is necessary, supplementing with 3 mg/day of selenium yeast is preferred over sodium selenite.
• To enhance colostrum and thus passive transfer, and to improve blood levels of vitamin E in mares and foals, supplement 160 IU of an oral dietary vitamin E/kg of body weight for four weeks before foaling and four weeks after foaling.
Performance Horse Nutrition
More tips were offered by Breuer on performance horse nutrition.
• Feed a high-fiber diet to endurance horses and other competition horses in prolonged exercise to help maintain fluid and electrolyte levels, resulting in fewer cardiovascular and thermoregulation problems.
• To achieve a short-term hyperhydration, providing a hypertonic solution will help stimulate the horse to drink more water and will temporarily reduce the amount of urine output.
• Administering a hypertonic electrolyte supplement just before and during strenuous exercise will help the horse drink more, lose less weight, and maintain higher blood electrolyte levels.
• An isotonic electrolyte solution similar to equine sweat is preferred over an isotonic glucose-glycine solution for fluid loss and plasma electrolyte restoration in exercise-dehydrated horses.
• Glucose solutions appear to have no benefit when trying to maintain electrolyte and fluid balance.
• Feeding an 11.8% fat diet appears to adapt horses for greater fat utilization, which might enhance performance and allow less lactate accumulation in the body.
• Racehorses consuming predominately alfalfa diets might be at a higher risk for testing high for plasma TCO2. (an indicator of the total carbon dioxide content in the blood, which can indicate that illegal substances have been given to the horse).
• Diet composition or short-term feed restriction might affect the ability of horses to maintain blood glucose concentrations during exercise.
• Supplementing fat can help a horse’s body use fat in place of glucose stores during exercise.
• Any potential benefits of feeding a horse grain within three hours of exercise are outweighed by a risk of hyperinsulinemia that might interfere with energy utilization in the performance horse.
• Grain should be withheld from horses before exercise, but small amounts of hay should be fed to ensure proper gastrointestinal tract function.
• Hay should not be reduced to less than 1% of body weight.
Nutritionally Related Diseases
Muscle disorders--Equine rhabdomyolysis syndrome (tying-up) can affect any horse. Affected horses can be divided into two groups--those with a dysfunction of the muscle excitation-contraction process (sporadic tying-up) in a previously normal horse, and those with a defect in the carbohydrate storage-use process (chronic/recurrent form).
Recurrent tying-up can then be divided into two different forms, according to research done at the University of Minnesota. Polysaccharide storage myopathy (PSSM) is found in heavily muscled horses whose muscle biopsies show abnormal polysaccharide accumulation. Affected horses are unable to use starch correctly, have increased glucose clearance, and increased sensitivity to insulin.
PSSM horses usually do not need grain, especially if good-quality hay is provided, said Rich. She recommended small amounts of fat. Research supports feeding two pounds of rice bran per day or one cup of vegetable oil over alfalfa pellets, along with 600-1,000 IU/day of vitamin E. A PSSM horse in training should have gradual increases in exercise time and intensity. Daily turnout is critical, she said.
Lighter muscled horses such as Thoroughbreds, Standardbreds, and Arabians are more susceptible to recurrent exertional rhabdomyolysis (RER), in which there is normal polysaccharide accumulation. Researchers believe that RER might be an inherited dominant trait that can be affected by diet, rest, lameness, and training. Another study found that with RER, there is abnormal intercellular calcium, which is not related to calcium in the diet.
Rich recommended several management methods for controlling RER. She suggested replacing grain with a high-fat, high-fiber, low-starch diet, which would still meet the caloric requirement of the horse. Horses should also be exercised daily and have minimal excitement. She recommended a Hallway Feeds’ product called ReLeve.
Fescue Toxicosis--Tall fescue--a hardy, abundant, cool-season perennial--can be infected with the endophyte Acremonium coenophialum, which has been known to be harmful to late-gestation mares. A recent study found no detrimental effects on early or mid-gestational mares. However, exercising adults were found to have vasoregulatory and thermoregulation side effects when grazing on endophyte-infected fescue. These horses had a harder time dissipating heat and recovering from exercise during hot weather.
Laminitis--High amounts of fructan, which is found in lush pasture grass, is now thought to be one reason for laminitis. As fructan enters the large intestine undigested, its presence causes “good” bacteria to die, which causes the production of toxins. These toxins might then be a cause of laminitis. Rich recommended that horses at risk for laminitis should not be given free access to lush pasture. If access is given, then it should only be given for a short time, and turnout should not be in the middle of the day when fructan concentrations are the highest.
Gastric Ulcers--Several things have been discovered in the past five years about gastric ulcers. Feeding alfalfa hay every five to six hours is thought to provide a protective effect on the non-glandular squamous mucosa, which is where ulcers typically develop. Depriving feed for longer than 12 hours can contribute to the development of ulcers. Rich also mentioned a new product called Pronutrin, which is a commercial lecithin-pectin mixture. When horses in a study were given 300 grams per day for 10 days, there was a positive effect on gastric lesions. However, since lesions did not heal completely in 10 days, it is recommended to use this product for 14 days.
Sand Colic--It was previously thought that underfeeding was a cause of sand consumption, which could possibly lead to sand colic. However, a recent study demonstrated that diets deficient in energy, protein, or both did not influence sand intake in confined horses. In addition, mineral consumption was unrelated.
Results on the efficacy of psyllium as a treatment for sand colic vary. One study compared psyllium, wheat bran, mineral oil, and water. Psyllium cleared the greatest amount of sand within 24 hours, but there was no difference in treatments after five days.
One important finding regarding diagnosis of sand colic was that radiography of the abdomen was found to be a useful tool for monitoring sand accumulation.
Foal Diarrhea--New evidence shows that the product arabinogalactan can help prevent foal diarrhea. This fiber is derived from the larch tree.
Thumps--At one point, low calcium concentrations were blamed for thumps. However, recent studies show that total calcium concentrations do not affect thumps. It is now believed that blood pH increases following large losses of chloride and potassium in sweat. Prevention of thumps is now achieved with sodium, chloride, and water replacement.
New Products, Ingredients, and Processing Methods
With many new products and changes in older products, horse owners can end up spending a lot of money on their animals. Research has been trying to prove what works and what doesn’t, and how certain products affect the horse.
• Lecithin, or phospholipids, are found in soybeans and other naturally occurring plants and animals. They have been found to have a calming effect on horses. In addition, they have also been found useful in the healing and prevention of ulcers. Researchers recommend giving 50 grams of de-oiled lecithin per day for a 1,000-pound horse.
• Hulless or naked oats have been found to have the same amount of energy as corn or 25% more energy than traditional hulled oats. They are also higher in fat, phosphorus, crude protein, and antioxidants. Therefore, they make a good alternative to conventional oats for performance, geriatric, and growing horses.
• Feeding 12 grams per day of the amino acid L-carnitine reduced harmful plasma lactic acid produced during exercise.
• Researchers have determined that there is no reason to supplement creatine in horses. One study failed to show any metabolic or muscle response in trotters supplemented with creatine.
• Supplementation with beta-hydroxy-beta-methylbutyrate (HMB, a metabolite of the amino acid leucine ) is not recommended at this time.
• Despite generalized thinking that horses prefer apple flavoring, a study proved that cherry flavoring was the most popular among horses fed a meal with added fruit flavor. Apple flavoring was second, with citrus and teaberry tying for third, and plain oats as the control were the least palatable to study horses.
• When corn is a large proportion of the concentrate, researchers now recommend that steam-flaked corn be used since it is more digestible.
• Beet pulp is a valuable feedstuff for horses. It is a good source of digestible fiber with low crude protein, high calcium, low phosphorus, and low vitamin content. It can be fed dry without horses “exploding,” choking, or colicking, as was once feared. When feeding wet beet pulp, discard within 24 hours to avoid mold.
• Substituting 15% of dietary energy as fat or fiber will not adversely affect performance. Feeding beet pulp at 15% of total energy works as well as feeding horses a high-starch diet. It has been found that soaking and rinsing beet pulp can lower glycemic response.
• Corn oil, rice bran, and refined dry fat of animal origin are highly digestible sources of fat. Rice bran can be supplemented to lower lactate levels and lower heart rates during exercise.
• The supplement betaine is beneficial to unconditioned horses, resulting in lowered lactate metabolism after exercise. It was not shown to have an effect on fit horses.
Feed allergies in horses are often misdiagnosed. In order to determine what is causing the allergy, a horse must be fed a single bulk food for a minimum of four weeks, adding other dietary ingredients such as oats or grain one at a time every four weeks. This is called an elimination diet.
It was found that the best way to feed group fed horses to allow more timid horses longer eating time was to place feed tubs in an equilateral triangle 11.8 feet (3.6 meters) apart.
About the Author
Sarah Evers Conrad has a bachelor’s of arts in journalism and equine science from Western Kentucky University. As a lifelong horse lover and equestrian, Conrad started her career at The Horse: Your Guide to Equine Health Care magazine. She has also worked for the United States Equestrian Federation as the managing editor of Equestrian magazine and director of e-communications and served as content manager/travel writer for a Caribbean travel agency. When she isn’t freelancing, Conrad spends her free time enjoying her family, reading, practicing photography, traveling, crocheting, and being around animals in her Lexington, Kentucky, home.
POLL: University Equine Hospitals