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NRC panel updates Nutrient Requirements for Horses
The computer program helps determine nutrient requirements and formulate rations for various classes needed.
It's been 18 years since the National Research Council (NRC) revised its Nutrient Requirements for Horses – a manual widely used by equine practitioners. . Now the Sixth Revised Edition (2007) is out, published by the NRC's Committee on Nutrient Requirements for Horses, chaired by Laurie Lawrence, PhD, professor at the University of Kentucky's Department of Animal Sciences.
The volume, which this time comes with a Web-based computer program, updates what was published in 1989 on the various nutrient classes.
It offers expanded chapters on energy, carbohydrates, fats and fatty acids, protein and amino acids, minerals and vitamins.
Besides covering basic nutrients, there is a chapter on water and water-quality issues; feeding and feed processing, including pasture and forage; feed additives; feed analysis; and on formulating and evaluating rations.
The most detailed new information is in the chapters on "Feeding Behavior and General Considerations for Feeding Management," "Unique Aspects of Equine Nutrition" and one on nutrient needs of donkeys and other equids.
The innovative computer program helps users determine nutrient requirements and formulate rations for horses and ponies of various physiological classes (maintenance, exercise, reproduction, lactation and growth).
Most information is aimed at light horse breeds, so the authors suggest that "the recommendations for ponies and draft horses be applied with discretion."
As in past editions, most information is said to reflect the latest scientific knowledge with a proviso that it is subject to further review. Here are some highlights:
Energy is discussed mainly as apparent digestible energy (DE) – the amount of energy in a food as determined by comparing gross energy or energy intake minus that contained in the feces, which includes not only that not absorbed, but also sloughed-off intestinal material and digestive secretions.
The result is therefore apparent DE, not truly non-absorbed energy, since it includes endogenous losses.
Although for some animal species energy is discussed as metabolizable energy (ME) or net or recovered energy (NE = GE-[DE+ UE+HE]), the revised volume shows DE is sufficient and a good basis of the energy needs of horses.
The committee decided that metabolizable energy or net energy was difficult to discern, given the variability and complication of calculating and evaluating energy lost via urine and gaseous losses, and that HE (heat energy or heat production) was even more convoluted for the horse.
Heat energy – energy lost to the environment – includes that for basal metabolism, digestion and absorption, the heat of fermentation, product formation (anabolism), voluntary activity (chewing, movement to retrieve nutrients, i.e., grazing), thermal regulation and waste formation and excretion.
The net energy, by definition, is a recovered entity or product, as in the energy of work, or in milk, or in fetal tissues.
With this unit as the basis of energy requirements for horses, the apparent DE for maintenance, growth, reproduction and exercise is discussed.
Maintenance energy is that exclusive of any of the other physiological states. There is an extensive section on the energy needs for exercise, including that for light, moderate, heavy and very heavy work. Heavy work is defined as four to five hours per week – 20 percent walk, 50 percent trot, 15 percent canter and 15 percent gallop – in ranch work, polo, show horses (frequent, strenuous), low to medium eventing and race training (middle stages).
Very heavy work is defined as that ranging from one hour per week of speed work to six to 12 hours per week of slow work, including racing (Thoroughbred, Standardbred, Quarter Horse, endurance) and elite three-day event work.
The main energy providers in horse diets are discussed in chapters on carbohydrate and fat.
Carbohydrates, from pasture, stored forage and grain, are the prime suppliers of energy for the horse, absorbed in the small intestine, and those converted to volatile fatty acids (VFAs) (proprionic, butyric, acetic) and absorbed in the hindgut (cecum and colon). The microbial population in the cecum can meet up to 30 percent of the horse's maintenance energy needs with additional VFAs produced in the colon.
The chapter on fats covers needs and research findings, along with the value of fats added to the horse's diet as oils, especially for exercise, and the benefit of the longer-chain fatty acids (omega-3, omega-6).
Omega-3's linolenic from linseed and flax, and DHA and EPA, the 20- and 22-carbon chain EFA's from fish oil, are now finding their way into horse diets via EFA supplements. According to the new NRC, "the data from studies in which horses were fed diets enriched with omega-3 (n-3) fatty acids (linseed, flaxseed or fish oils) have demonstrated modulation of inflammatory mediator synthesis by cells harvested from blood, peritoneal fluid or respiratory secretions.
However, the physiological importance of these findings is unclear and further research is needed to determine the effects of n-3 fatty acid supplementation in the treatment and prevention of inflammatory diseases in horses (e.g., recurrent airway obstruction)."
Chains of amino acids – proteins – are important constituents of tissues, enzymes, hormones and antibodies. The protein chapter discusses protein digestibility, bioavailability and the need for protein to maintain growth, pregnancy, lactation and exercise.
In summary, "total tract and prececal digestibility vary with protein source and protein concentration in the diet. It is important to consider amino acid profile and prececal digestibility of feedstuffs in addition to total crude protein, especially in rations fed to growing horses and those in high states of production."
Minerals and vitamins
The new volume states that "the horse obtains most of the necessary minerals from forages and concentrates. The mineral content of feeds and the availability of minerals vary with soil concentrations, plant species, stage of maturity and conditions of harvesting.
"The resulting variation in feed mineral content should be considered in assessing an animal's mineral status and formulating appropriate diets, as minerals are elements that cannot be created or destroyed under normal circumstances and must be provided in the ration."
Current information is provided on each mineral's (macro and micro) function, source and factors influencing absorption, signs of deficiency or excess and recommendations.
Besides minerals addressed in the past, new details are provided on special minerals – fluorine, chromium and silicon.
The vitamin requirements of horses "have been estimated using several response variables (e.g., prevention of specific deficiency symptoms, maximizing tissue stores, and optimization of various biological functions)."
The requirements for certain vitamins may change, depending on the response variable used.
For example, the vitamin E requirement to maintain erythrocyte stability in growing horses is different from the vitamin E requirement for an immunostimulatory effect in older horses.
Some vitamin needs – i.e., vitamins A, D, E, thiamin and riboflavin – have been estimated from substantial data, while the data on which the estimates for vitamin K, niacin, biotin, folate, vitamin B12, vitamin B6, pantothenic acid and vitamin C are insufficient for absolute certainty.
Upper limits for all the vitamins are given, defined as the "estimated (based on literature) upper range of vitamin intake that can be presumed to be safe, but not necessarily the maximum tolerance level of vitamin intake (NRC 1987)." For each vitamin discussed, its function, dietary sources, deficiency, toxicity and requirements are noted for maintenance, growth, breeding, gestation, lactation and work/exercise.
Water, the horse's most essential nutrient – critical at a deficiency of 8 percent to 10 percent – is discussed, including its need and the horse's drinking habits.
"Water consumption by horses is episodic and circadian," states the NRC. "Drinking patterns are modified by water source, water availability and age of the horse. Each drinking episode is biphasic – a long draught is followed by sips of shorter duration."
The total daily drinking time for the horse is short, and the duration of individual drinking bouts is brief. From several studies cited by the NRC, horses drank two to eight, 16 to 21, or 18 to 39 times per day for about 10 to 60 seconds, 10 to 52 seconds or 13 to 26 seconds per bout, for stalled horses, or pregnant mares, respectively.
The balance of the chapter explains the best methods of providing fresh, clean water to horses and the drinking behavior of foals, pregnant/lactating mares, exercising horses, dependent on weather conditions, ambient temperature, sun exposure, etc. The average water consumption is estimated at 5 liters per 100 kg body weight (25 liters for an 1,100-pound adult horse).
The next chapters are excellent for equine practitioners: "Feeds and Feeding;" "Feeding Behavior" and "Unique Aspects of Equine Nutrition."
"Feeds and Feeding" discusses the chemical composition and nutrient content of forages and the specific nutrient benefits of pastures. "The feed value of pastures for horses is a function of pasture intake and forage nutrient composition, digestibility and bioavailability," the NRC states.
Estimates for voluntary dry matter intake for grazing horses averages 1.5 to 3.1 percent of body weight. Average daily DM intake for grazing mares is high in comparison with other physiological classes of horses. Mares consumed about 2.8 percent of body weight while most other horses consumed about 2 percent of body weight.
Pasture digestibility is good, depending on the grass species: in other words, the more cell-wall fiber, the lower the digestibility. Spring and summer pasture is adequate for most horses; even lactating mares can gain sufficient energy, protein and trace nutrients from pasture intake.
Vitamin E and beta-carotene (the precursor to vitamin A) are adequate in fresh grass; so are vitamin D and vitamin K (phylloquinones).
The species of pasture grasses and their nutrient benefits are discussed, as well as hay and silage. The objective of haymaking is to provide nutrient-complete forages from fresh grass. Drying begins as soon as the grass is cut and proceeds rapidly with storage.
As opposed to fresh pasture, stored forage accounts for significant fat-soluble vitamin losses – up to 80 percent or 90 percent loss in cured hay, compared to the same species of fresh grass. Beta-carotene is lost at 80 percent and vitamin E to 90 percent.
Forages "conserved as hay by desiccation should contain no more than 200g moisture/kg to prevent proliferation of micro-organisms, heating (due to microbial respiration) and subsequent reduction of nutritive quality. However, under conditions of high (90 percent to 100 percent) relative humidity, which favors microbial growth, hay stored at 200g moisture/kg can become moldy."
Other aspects of hay are discussed, including hay intake and digestibility. Further topics include ensiled forages, including intake, digestibility, problems including contamination by insects and microbes.
The NRC publication discusses grains, including common grains, their digestibility and contamination by microbes.
Oats are a main staple of the horse diet, the NRC explains. Oats are less energy-dense than corn, contain more fiber, less starch and greater protein content than corn, and oat starch is more readily digested than corn starch in the small intestine. Therefore oats may produce less starch to the large intestine.
Oats tend to be higher in protein, lysine and fat than corn, but like corn have an inverted calcium-to-phosphorus ratio. Oats are palatable to horses, but the consumption of oats often is less than sweet feed or pelleted rations.
The balance of the chapter on feeds and feeding is a discussion of supplemental fats, oils and vitamin/mineral supplements and finally a section on feed manufacturing and its effects on nutrient intake and digestibility.
The chapter on feeding behavior and general feeding considerations is pertinent to equine practitioners.
"Horses are herbivores. Therefore they require a forage supply to lessen the risks of clinical disorders such as colic, laminitis, and oral and locomotory behavior problems. By acknowledging the normal feeding behaviors of the equine, appropriate feed management decisions can be made to minimize these important clinical problems and contribute to the well-being of the horse."
Voluntary intake by horses, and feeding behavior of foals and mature horses on pasture, provide a look at why horses eat the way they do and their consumption of forages and grains when available.
Animal, environment and feed/forage effects on feeding behavior provide insight on the factors that relate to the intake behaviors of horses. One section addresses "vice" feeding behaviors, such as cribbing, wood chewing, coprophagia and geophagia.
The chapter on "Unique Aspects of Horse Nutrition" discusses "nutritional management of specific disease conditions, including laminitis, colic, hyperkalemic periodic paralysis, exertional rhabdomyolysis syndrome, polysaccharide storage myopathy, developmental orthopedic disease, nutritional secondary hyperparathyroidism, gastric ulcer syndrome, enterolithiasis and recurrent airway obstruction. In addition to these disorders, nutrition of the nursing and orphaned foal, the aged horse and horses fed in extreme cold and hot environments are discussed.
Lawrence says that "several mathematical equations have been derived to provide more dynamic estimates of requirements for some physiological states, including growth, gestation and exercise.
"The requirements shown in the tables provide recommendations for broad classifications of horses, whereas the computer program allows some flexibility in calculating the nutrient requirements for a specific animal."
The committee says it used the best estimates and best actual data available, and provides notes to the user to "to recalculate requirements when they possess specific information on nutrient availability for the rations being fed in practice."
Though the committee reviewed the latest literature on equine nutrition, "it is not possible to predict every combination of variables that could influence the nutrient requirements of a specific animal," says Lawrence. "Therefore, it is incumbent upon the user to accurately access the factors that could alter requirements and then apply appropriate adjustments."
Ed Kane is a Seattle author, researcher and consultant in animal nutrition, physiology and veterinary medicine, with a background in horses, pets and livestock.