Relevance of the NRC to today’s horse industry

Current status of the horse industry

The horse industry entered the new millennium during a period of strength and popularity not seen since possibly the beginning of the previous century. This upward trend followed several years of a declining horse population beginning in the mid-1980s. The general decline of the horse industry following a serious economic downturn was reflected in high horse slaughter during this time, with over 300,000 horses per year being processed by abattoirs. Since then, horse slaughter has declined by 75-80% and has so far shown no sign of increasing despite the current economic slowdown and the general consensus that horse populations are increasing in most parts of the United States. The improvement in the status of the horse industry is indicated by strong feed sales by several major horse feed manufacturers and the continued strength of sales of equine related goods and services such as trucks, trailers, tack, pharmaceuticals and veterinary and farrier services.

Although the public auction sale prices of the highest quality select horses have declined significantly since 2001, a reflection of world events and the depressed stock market, the entry level, companion horse, youth and amateur horse markets remain quite strong. Fewer low-quality horses exist today compared to two decades ago, and today’s horse owner, both new and experienced, values quality, convenience and the aesthetic properties of feeding and caring for horses like never before. The Farm Bureau’s recent emphasis on the equine industry through appointment of county, state and national equine committees suggests a fundamental change in the perception of the equine industry by traditional agricultural organizations. As the swine, dairy, poultry and beef industries consolidate and fewer livestock producers maintain membership in professional agricultural organizations, the horse industry membership will continue to be cultivated by these organizations.


Status of the NRC Nutrient Requirements of Horses

The fifth revision of the National Research Council (NRC) Nutrient Requirements of Horses is now 15 years old. Released in May of 1989, it is the most current edition of recommended nutrient allowances for horses. It contains several tables that present nutrient requirements for 19 different classes of horses, based upon age, rate of growth, level of physical activity, age/training interaction, stage of pregnancy and lactation. These tables are replicated for ponies and horses of different expected mature bodyweights of 200, 400, 500, 600, 700, 800 and 900 kg.

The 1989 NRC publication continues to provide a basic framework for ensuring that nutrient requirements of most categories of horses are generally met and not exceeded to the point of toxicity. However, research findings of the past 15 years and fundamental changes in the horse industry must be addressed by a new revision of Nutrient Requirements of Horses. To their credit, the NRC has convened an expert committee to review and revise the publication, and the first meeting of this committee is scheduled for April 2004.


Major considerations for the new NRC Equine Committee

Although not an exhaustive list, the following topics will be important for the new NRC committee to consider during the development of the updated Nutrient Requirements of Horses:

 

ENERGY METABOLISM/REQUIREMENTS

The energy requirements for most livestock species are given as metabolizable energy (ME), while requirements for horses are currently given as digestible energy (DE). Energy requirements for lactating dairy cattle are given as net energy for lactation (NEL). One of the first discussions of the NRC Equine Committee regarding energy requirements might be about the merits of expressing energy requirements for horses of various stages of growth, gestation, lactation or exercise as DE versus ME or NE. An evaluation of various equations proposed in research to estimate energy requirements of horses in various life stages and exercise intensities will be an important consideration of the NRC committee.


CARBOHYDRATE METABOLISM

The horse is fundamentally a grazing herbivore that spends as much as 12-16 hrs per day eating forage when on pasture or rangeland. It is a hindgut fermenter with substantial cellulolytic microbial populations in the cecum and large colon. Structural carbohydrates from forage are the most ‘natural’ carbohydrates for the horse to digest with assistance from hindgut microorganisms. However, as the horse is expected to grow rapidly, lactate or perform intense exercise for competition, the ability of structural carbohydrates to meet the horse’s energy requirement is exceeded and starches and sugars from concentrate mixes become necessary. Research has been conducted to evaluate the glycemic responses of horses to nonstructural carbohydrates from various types of grains as well as comparative responses to diets that provide supplemental energy high in starch and sugar versus diets high in fat and fiber. Diets high in starches and sugars have been implicated in numerous metabolic bone problems in growing horses, digestive problems related to acidification of the hindgut, and to laminitis due to toxin production in the hindgut. However, recent studies have confirmed that horses and ponies are quite capable of metabolic adaptation to either high starch and sugar diets or high fat diets (Schmidt et al., 2001; Hoffman et al., 2003).


PROTEIN/AMINO ACID REQUIREMENTS

Limited data are available regarding the protein digestibility of specific feed ingredients in horses. Amino acid absorption occurs mainly in the small intestine of the horse, but some ammonia absorption also occurs in the hindgut. Therefore, protein requirements in the horse are expressed as crude protein (CP). Non-protein nitrogen feed ingredients are not efficiently utilized by the horse due to the location of fermentative digestion sites near the end of the gastrointestinal tract. The CP requirement for mature, sedentary horses under maintenance conditions is quite low, at less than 8%. Horses at maintenance can meet CP requirements on moderatequality forages and grains, but growing horses and especially lactating mares require protein supplementation unless a high-CP forage containing legumes is a major component of the diet.

Amino acid requirements for horses are not well defined, but some work conducted since the publication of the last NRC Nutrient Requirements of Horses has suggested that threonine is the secondlimiting amino acid (lysine is first-limiting) in a yearling horse diet of corn, oats, soybean meal and coastal bermuda grass/hay (Graham et al., 1994).

There is little question that protein quality is of concern for growing horses and lactating mares, and for the horse in general, as compared to ruminant species. Adequate lysine concentrations should be present in diets high in soybean meal, but diets supplemented with cottonseed, peanut or flax meal could require additional lysine when fed to growing horses or lactating mares. Greater commercial availability and affordability of essential amino acids and advances in plant breeding that alter the amino acid profiles of some feeds are factors to consider when making recommendations about protein/amino acid recommendations for horse diets. Dietary protein and amino acid recommendations for working horses may some day consider the acidogenic effects of excess dietary protein (Graham-Thiers et al., 2001), although data are still limited in this area.


MINERAL REQUIREMENTS

Mineral concentrations of feedstuffs may vary significantly with soil mineral concentrations, soil fertilization, plant species, harvesting conditions and stage of plant maturity. Special attention should be paid to providing trace mineral supplements formulated specifically for horses, not other species.

For instance, trace-mineralized salt for sheep is typically devoid of a bioavailable copper source for horses, since copper oxide is usually the only source of copper. A more bioavailable source should be provided instead. Information on trace mineral bioavailability in the horse is not as extensive as data collected in other species (Ammerman et al., 1995).

However, a substantial amount of research has been done in recent years regarding equine mineral nutrition that the new NRC Equine Committee will need to evaluate. Calcium and phosphorus status of equine bone has been found to be related to physical activity as much as dietary status. Extreme variations in the Ca:P ratio of different forages have a great impact on the composition of supplements recommended to complement the basic forage ration.

Trace mineral nutrition has received substantial attention from researchers in recent years, and mixed results have been obtained regarding the relative bioavailability of different trace minerals fed as inorganic versus ‘organic’ forms of trace minerals that are bound to an organic molecule.


VITAMIN REQUIREMENTS

Fresh green forages and exposure to sunlight are all that is necessary to meet the vitamin needs of most horses. Horses under confinement conditions may lack one or both of these factors, however. Forages preserved with propionic acid or stored for extended periods of time may lose much of their vitamin A activity. It is not unusual for show horses or for horses being prepared for sale to be housed indoors for extended periods of time during summer daylight hours in order to avoid dulling of the hair coat from the sun. These horses may lack vitamin D if goodquality, sun-cured forage is not available. Vitamin E could also potentially be deficient if poor-quality forages are fed for extended periods.

B-complex vitamins are synthesized in the horse’s hindgut and usually do not require supplementation. Although no dietary biotin requirement has been established beyond what is synthesized in the gut, research continues into the possible use of supplemental biotin for improving poor-quality hooves. Two studies have found improved hoof quality in horses with initially poor hoof quality when fed 5-10 mg of supplemental biotin per 100 kg bodyweight per day for periods ranging from one to six years (Josseck et al., 1995; Geyer and Schulze, 1994). Another study found a 15% increase in the rate of hoof growth in ponies with 0.12 mg/kg bodyweight of biotin as compared to ponies on a control diet without supplemental biotin (Reilly et al., 1998). Feeding recommendations for vitamin E could be viewed both in terms of requirement levels needed to avoid deficiency symptoms and also as optimal levels for minimizing exercise-induced oxidative damage to cells through free-radical formation and for maximizing immune function.


WATER REQUIREMENTS

A series of recent studies at Michigan State University (Butudom et al., 2002; 2004) found that horses dehydrated by either exercise or frusemide administration drank more fluid when presented as either 0.45% NaCl or 0.9% NaCl solutions as compared to regular water. Their work also supported previous observations that horses quench their thirst faster and delay rehydration when consuming cold fluids after dehydration as compared to warm fluids, and that the primary stimulus of thirst is an increase in plasma tonicity rather than hypovolemia (Butudom et al., 2003). Horses consumed more 20°C water than 10°C water after being dehydrated. Geor et al. (1998) found that pre-exercise hyperhydration provided no thermoregulatory advantage to horses and Sosa et al. (2002) found that hyperhydration using an isotonic solution at 6% of body weight resulted in arterial hypoxemia, possibly from pulmonary edema. These recent studies and others will likely help to guide the NRC Equine Committee’s recommendations regarding provision of electrolyte solutions versus water for rehydration of already dehydrated horses.


DIETARY FATS AND OILS

The use of dietary fats and oils has been one of the most active areas of research since the publication of the fifth revision of the National Research Council (NRC) Nutrient Requirements of Horses. Horses can utilize fats and oils effectively, and inclusion rates of up to 15% of the concentrate ration may be well accepted by the horse. One study found that the jejunal microflora of ponies was not affected by the addition of up to 20% coconut oil in the diet (Kollarczik et al., 1995). Research has suggested that fats and oils in equine diets may provide benefits to horses in addition to increased caloric density of the diet for weight gain or maintenance in ‘hard keepers’. Dietary fats and oils may also help to improve hair coat quality, spare muscle glycogen in exercising horses, reduce post-exercise lactic acid accumulation, and reduce the chance of enterotoxemia, colic and founder. Not all studies have found beneficial effects, however. Excess fats and oils reaching the hindgut may inhibit fiber digestion (Jansen et al., 2000) and diets high in fat and fiber when fed to growing foals may reduce the absorption of calcium and other minerals necessary for bone development (Hoffman et al., 1999). The preponderance of research data, however, appears to support the continued expanded use of dietary fats and oils for many classes of equines. Research suggesting that added dietary fat may reduce the heat production of performance horses working under hot and humid conditions (Kronfeld, 1996), and enhance the oxidative capacity of muscle (Orme et al., 1997) continues to stimulate interest in this dietary supplement for horses.


GERIATRIC HORSES

With more and more elderly horses represented in the equine population every year, and more commercial diets being formulated for geriatric horses, this area of research seems to be a promising one for equine nutritionists, yet little work has been done in determining nutrient requirements and feeding management strategies of geriatric horses. This is most likely due to the fact that so many different age-related infirmities may be related to poor nutritional performance in geriatric horses. Poor teeth, malabsorption, endocrine problems such as Cushing’s disease, chronic pain, etc. may all affect digestion, absorption, and metabolism of nutrients. Such variability among geriatric horses makes controlled study of this group of animals difficult.

The typical feed formulation strategies being employed by feed manufacturers include processing of both forages and concentrates to improve foregut digestion of starches and improving hindgut digestion with highly fermentable fiber like beet pulp and decreasing particle size of fiber to enhance hindgut fermentation. Addition of microbial products to further enhance hindgut digestion and adding fats and oils to improve caloric density have also been employed. There is opportunity for further research in this field.


FEED COMPOSITION TABLES

As novel by-product feeds, genetically modified plants and new crops are added as potential livestock dietary components, the NRC Equine Committee will need to review those products that may have an application to equine diets. Significant modification of the NRC feed composition tables will likely be necessary.


BONE METABOLISM/GROWTH

Glade (1993) found that mid-cannon bone diameters of foals born to mares fed calcium-deficient diets were thinner and mechanically weaker at birth than in foals born to mares fed NRC recommended concentrations of dietary calcium. These differences in limb bone size and strength persisted during the first 40 weeks after birth. Hoffman et al. (1999) found that bone mineral content was lower in weanlings fed a supplement high in fat and fiber compared to weanlings fed a supplement high in starch and sugar, and they speculated that “binding of calcium by fat and fiber may alter the availability of elements necessary for bone development.”

Studies comparing diet manipulations and evaluating the resulting changes in serum skeletal metabolic markers such as hydroxyproline (bone resorption), osteocalcin (bone mineralization), C-propeptide of type-II procollagen (collagen synthesis) as well as measurements of the physical properties of bone promise to further elucidate the effects of diet on bone metabolism and growth in horses.


DIETARY/GENETIC INTERACTIONS

Although feeding programs designed for horses with genetic defects should never find a large market, there has been substantial interest in managing some of the more common equine metabolic disorders with diet. The common genetic defect manifested as hyperkalemic periodic paralysis (HYPP) in Quarter Horses has been managed with both medication and low potassium diets. Polack et al. (2000) found that horses with equine motor neuron disease had greater concentrations of copper and lower concentrations of vitamin E in the spinal cord, suggesting that an imbalance of oxidant/antioxidant nutrients may have an effect on the occurrence of this poorly understood disease. Recurrent exertional rhabdomyolysis (RER) has been associated with a diet high in soluble carbohydrate. McKenzie et al. (2003) found that horses severely affected with RER that were fed a high-fat, low-starch diet had dramatically lower postexercise serum creatine kinase (CK) activity compared to horses fed a low-fat, high-starch diet. Firshman et al. (2003) found that polysaccharide storage myopathy (PSSM) in Quarter Horses can be managed by following dietary recommendations of reduced soluble carbohydrates and supplemental fats and oils combined with gradual increases in daily exercise.


DIET/EXERCISE INTERACTIONS

The NRC Equine Committee will have numerous studies to review related to the energetics of exercising horses. Many studies, though, deal with more than meeting additional energy demands.

Researchers have shown great interest in evaluating diets that may help exercising horses to maintain proper immune function, enhance aerobic metabolism of fatty acids and minimize lactate production from anaerobic glycolysis. The timing of meal feeding with respect to exercise has received a great deal of attention in recent years. St. Lawrence et al. (2002) found that 48 min of light work performed 1 hr before a meal did not affect glucose or insulin responses to the meal. Jose-Cunilleras et al. (2002) found that feeding a soluble-carbohydrate-rich meal (corn) to horses before exercise increased muscle utilization of blood-borne glucose and carbohydrate oxidation and decreased lipid oxidation compared with a meal of insoluble carbohydrate (alfalfa) or not feeding.

Carbohydrate feedings did not produce a sparing of muscle glycogen compared with fasting. Pagan and Harris (1999) found that feeding grain before exercise reduced free fatty acid (FFA) availability and increased blood glucose disappearance during exercise in Thoroughbred horses. Feeding hay either along with grain or ad libitum the night before exercise resulted in reduced plasma volume and higher lactate production and heart rates during exercise. They concluded that feeding only forage before exercise did not adversely affect performance. They also suggested that grain should be withheld from horses before exercise, but that small quantities of hay should be fed to ensure proper gastrointestinal tract function.


NUTRACEUTICALS

The term nutraceutical was coined in the 1990s by Dr. Stephen DeFelice in reference to dietary substances affecting human health. The term has gained popularity in reference to dietary products that may impart health benefits to horses as well. Dr. DeFelice defined a nutraceutical as:

As the number of geriatric horses increases, and as horses compete in more demanding competitions, the interest among horse owners in such nutraceutical products is increasing. Documentation of benefits of nutraceuticals in horse diets has been very limited.

Orth et al. (2002) found that the combination of glucosamine and chondroitin sulfate inhibited the synthesis of several mediators of cartilage degradation in equine carpal cartilage explants in vitro. Evidence of benefits to equine joint health from supplementation with glucosamine and chondroitin sulfate in vivo has been inconclusive.


NUTRITION/ENVIRONMENTAL INTERACTIONS

Horses are frequently housed in facilities that are designed more for the comfort of the horse owner than for the horse. Horse housing is notoriously poorly ventilated to the point of being airtight in some cases, and horses are forced to breathe poor-quality air. The problem is exacerbated when horses consume diets containing excessive protein, resulting in high rates of urea excretion and high aerial ammonia concentrations.

Fear of fecal contamination of hay and/or grain with the causative organism for Equine Protozoal Myelitis has prompted some horse farm managers to feed all forages and concentrates entirely as bagged cubes, pellets or texturized feeds. Hay cubes, complete hay/ grain cubes and completely pelleted diets are alternative methods for providing forage for horses. The primary motivation for using these forms of feed is the extreme ease of storage and handling under confinement situations or when frequent hauling of animals precludes transport of bulky-type feeds.

Because nutrient levels in complete feeds can be precisely defined while still providing adequate roughage, they are gaining favor as creep rations for rapidly growing foals and even as high-performance race horse diets as well as diets for geriatric horses.

Hay supplies tend to be unpredictable at racetracks, which makes ration balancing difficult. The potential for increased wood chewing, boredom, colic or founder of horses on complete feeds has been cited as being of concern. However, trainers using complete hay-grain feeds have reported that horses eat the complete pellets slower than grain alone, and that boredom and health problems have not increased. Research on a complete cubed feed tends to support these observations (Younglove et al., 1994).

Environmental stress, intense exercise training, large amounts of concentrated high-energy feeds, small rations of forage and a low feeding frequency per day are associated with gastric ulcers in horses (Feige et al., 2002).

Another consideration related to nutrition and the horse’s environment has to do with management of excess nutrients in fecal and urinary waste. Excess nitrogen, phosphorus, trace minerals such as copper and zinc and other nutrients fed in excess may be subject to greater scrutiny and governmental regulation of waste management in the future. Further precision in defining nutrient requirements of horses may help to minimize unintended environmental impacts due to excess provision of certain nutrients.


HORSE OWNER DEMANDS

The psychology of feed buying decisions by horse owners is not the primary focus of the NRC committee, but this consideration is a significant factor in determining feeding strategies, feed forms and nutritional products that horse owners are willing to offer their horses. The adequacy of dietary energy, protein, vitamins and minerals in a horse’s diet may receive considerably less conscious thought from the horse owner than the expression of diet sufficiency as a glossy hair coat, vigorous attitude, hearty appetite and soundness of limb and wind. The extent to which today’s horse owner wants her horse to be not merely healthy, but also fully contented, can be a great motivating force that drives feed-buying decisions.

This general mind-set of many modern horse owners tends to drive the demand for horse feeds that are both high in nutritional quality and in sensory satisfaction to the horse and owner. The appearance, texture, aroma, ease of handling and palatability (from the standpoint of the horse’s apparent enjoyment) are major considerations when buying feed. The value of a commercial horse feed has as much to do with serving the horse owner’s emotional needs as providing adequate nutrient content in a cost-effective manner. Successful feed companies have developed profitable new product lines based upon identifying the motivations behind the buying decisions of horse owners. Today’s horse owner has shown a tremendous willingness to pay a premium price for high-quality products that directly acknowledge their view of the horse as both a livestock equine partner and a companion.


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