Nutrition and feeding of the geriatric horse

Vet Clin Equine 18 (2002) 491–508

Nutrition and feeding of the geriatric horse Paul D. Siciliano, PhD Department of Animal Sciences, Colorado State University, Fort Collins, CO 80523-1171, USA

One estimate suggests that geriatric horses make up 15% of the horse population in the United States [1], which is a significant number of horses given that the total horse population in the United States has been estimated at approximately 7 million. Little information is available regarding nutrition and feeding of geriatric horses. The purpose of this article is to review the literature regarding nutrient requirements of horses, with an emphasis on application to geriatric horses in an effort to provide feeding guidelines for geriatric horses. The nutrient requirements for horses are published in the National Research Council’s (NRC) Nutrient Requirements for Horses, which was last published in 1989 as a fifth edition [2]. The publication provides minimum rather than optimum nutrient requirements for maintenance, growth, work, gestation, lactation, and reproduction. Maintenance requirements typically make up the largest proportion of a horse’s total requirement and may be the only requirements that idle mature horses have. Because geriatric horses often but not always fall into the category of an idle horse, this article is limited to a discussion of maintenance nutrient requirements. The sections are categorized by nutrient class and formatted as follows: a brief discussion of general nutrient function, current requirement for the nutrient, and possible modifications necessary for the geriatric horse. Digestible energy Consumption of energy for processes like ion transport, synthesis and degradation of biomolecules, thermoregulation, and other energy-consuming processes is continual and results in the need for a continuous supply E-mail address: [email protected] (P.D. Siciliano). 0749-0739/02/$ - see front matter Ó 2002, Elsevier Science (USA). All rights reserved. PII: S 0 7 4 9 - 0 7 3 9 ( 0 2 ) 0 0 0 2 8 - 7

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of substrates used in the production of energy. Feedstuffs contributing to digestible energy (DE) requirements account for 60% to 70% of typical equine rations. Maintenance energy requirements for horses have been well described [2,3]. The current recommendations are expressed as megacalories (Mcal) of DE. Maintenance energy requirements are defined as the amount of DE required for zero body weight (BW) change and to provide for a minimal amount of physical activity. Maintenance DE requirements are described by the following equations [2]: Mcal of DE=d ¼ 1:4 þ 0:03 BW for horses ranging in body weight from 200 to 600 kg of BW or Mcal of DE=d ¼ 1:82 þ 0:0383 BW
0:000015 BW2 for horses with BW greater than 600 kg These equations are considered as guidelines only [2] in that they are dependent on the single independent variable BW. In reality, other factors like body composition, environment, and perhaps breed and sex also influence maintenance energy requirements [4]. Based on these NRC [2] equations, a mature light horse weighing approximately 500 kg (1100 lb) has a maintenance DE requirement of 16.4 Mcal of DE per day. Forages common to the United States range from 1.6 Mcal of DE per kilogram for mature grass hay to 2.24 Mcal of DE per kilogram for alfalfa hay harvested at an early-bloom stage. Therefore, the maintenance requirement of a mature horse can be met by consuming 7.32 kg (16.4 Mcal of DErequired/2.24 Mcal of DE/kgforage) to 10.25 kg (16.4 Mcal of DErequired/1.6 Mcal of DE/kgforage) of forage. The aforementioned range of feed intake is well within the expected daily feed intake of a mature horse (ie, 1.5% to 2% of BW). The primary energy substrate present in forage is fibrous carbohydrate (eg, cellulose, hemicellulose, pectin). Fibrous carbohydrate is fermented in the hindgut (cecum and large colon), yielding volatile fatty acids like acetate, propionate, and butyrate [5]. Volatile fatty acids are then absorbed and subsequently metabolized for energy. Some estimates suggest that volatile fatty acids contribute as much as 80% of the maintenance energy needs of horses [6]. The DE content of feeds varies considerably, and only estimates are generally available. As a result, initial calculations aimed at determining an amount of feed necessary to meet DE requirements may be high or low. Body condition scoring is a system used to assess the adequacy of DE intake. Henneke et al [7] developed a body condition scoring system for use in horses. This system uses a visual assessment of body fat covering, which reflects the energy status of the horse (Fig. 1). Periodic evaluation of body condition provides a horse owner or equine practitioner with a tool to assess a ration’s adequacy in terms of DE and enables modification of the diet accordingly.

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Fig. 1. Body condition scoring of horses. (Adapted from Henneke DR, Potter GD, Kreider JL, et al. Relationship between condition score, physical measurements and body fat percentage in mares. Equine Vet J 1983;15:371–2; with permission.)

Chronically low body condition is a common problem seen in some geriatric horses [8–11] indicating that meeting maintenance energy requirements is at times difficult in the geriatric horse. Several factors may contribute to chronically low body condition in geriatric horses. Among them are

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decreased intake, reduced digestibility, and environmental factors that increase energy requirements. A reduction in feed intake, and therefore energy intake, may occur for a number of reasons in geriatric horses. Ambulation may be compromised, which limits the horse’s ability to compete for feed in a group-feeding setting. In addition, compromised ambulation also reduces the horse’s ability to forage for feed in a pasture setting. This problem is easily remedied by feeding the geriatric horse individually, thereby removing the need for competition with other horses and the necessity to travel relatively long distances as in foraging. As indicated previously, consumption of 1.5% to 2% of BW of typical forage is generally adequate to meet a horse’s maintenance DE requirement. There is one report that suggests fiber digestion in geriatric horses may be less than that of younger horses, however [11]. A reduction in crude fiber apparent digestibility of approximately 5% was reported for geriatric horses (ranging in age from 20–35 years of age) compared with younger horses (2–3 years of age). These authors also stated that the percentage of fiber digestion of the geriatric horses included in this study was similar to that of the younger horses after resection of the left colon, suggesting a reduction in digestive capacity in the hindgut of geriatric horses. Additionally, in the study involving large colon resection, it was found that horses fed timothy hay (lower quality forage) were in a negative energy balance (based on a decline in BW) during a 21-day period after surgery, whereas those fed alfalfa hay maintained a neutral to slightly positive energy balance [12–14]. Application of this finding to geriatric horses with a potentially reduced ability to digest fiber suggests that feeding higher quality forages to geriatric horses is prudent when maintaining adequate body condition is a problem. Ralston and Breuer [9] concluded that a complete feed (ie, a feed containing adequate roughage so as to allow it to be the sole feed in the ration) consisting of a mix of pelleted and extruded feeds and formulated to meet theoretic specifications for geriatric horses [15] was more beneficial in maintaining energy balance in geriatric mares with low body condition scores (mean body condition score of approximately 2) than was a textured sweet feed mix of corn, oats, barley, alfalfa, and molasses. The benefit of improved energy status resulting from this complete feed was not evident in a group of geriatric mares having a mean body condition score of approximately 4.5. Cold weather can increase maintenance energy requirements of horses as the result of an increase in energy consumed for maintaining body temperature [16]. Failure to account for this increase can result in a negative energy balance and a subsequent loss of body condition. Furthermore, the lack of body fat covering present in horses with a body condition of 4 or less may exacerbate the problem because of lowered insulation [16]. Cymbaluk and Christison [16] provide an equation for calculating DE requirements during cold weather as follows:

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DE ðMcal=dÞ ¼ 1:1  ½ð1:4 þ 0:030 BWÞ þ ððLCT
ATÞ  0:00082 BWÞ where BW is measured in kilograms, LCT is lower critical temperature in degrees centigrade, and AT is ambient temperature in degrees centigrade. Although little is know regarding optimal energy status/body condition for geriatric horses, it would still seem prudent to maintain them in a moderate body condition as is the case for younger horses. Excess body condition may result in excess force on the skeletal system that could exacerbate lameness caused by old injuries. In addition, reduction of excess energy has been shown to decrease the incidence and the growth of tumors in rodents [17,18] as well as slowing the aging process and extending the lifespan in short-lived mammals [19]. Maintaining moderate body condition (ie, body condition score 5; see Fig. 1) may thus be prudent in geriatric horses.

Crude protein and amino acids Proteins consist of amino acids; are important structural components of all organ systems; and play many regulatory roles as enzymes, hormones, and other biomessengers. Protein makes up approximately 22% and 80% of the composition of a mature horse on a fat-free and fat-free moisture-free basis, respectively [20]. A daily supply of proteins is necessary because of continual turnover of body proteins resulting from a balance between synthesis and degradation and a limited ability to store considerable protein [21]. The current dietary protein requirement for maintenance in mature horses has been estimated to be 40 g of crude protein per megacalorie of DE [2]. Therefore a 500-kg mature horse with a maintenance requirement for DE of 16.4 Mcal requires 656 g of crude protein per day. This is equivalent to approximately 6.6% of the total ration, assuming an intake of approximately 2% of BW or 10 kg. One other estimation of the protein requirements of adult horses suggests an amount approximately 40% less than the aforementioned [21]. Although the amino acid requirements of mature horses have not been studied, lysine and threonine are considered the first and second limiting amino acids, respectively, in equine diets [2, 21,22]. Quality of the protein (ie, amino acid composition) can influence the total amount of protein required in that higher quality sources of dietary protein are required in smaller amounts presumably because of their superior amino acid profiles [21]. Ralston et al [11] reported that the apparent digestibility of crude protein in geriatric horses ([20 years of age) was less than that of younger horses (2–3 years of age). Subsequently, these authors suggested that geriatric horses may perform better, in terms of maintaining BW and condition, when fed diets containing 14% to 16% crude protein in their diets. This is approximately double that required by mature horses.

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A progressive loss of muscle mass with ageing has been described in human beings [23–26] and rodents [23,27]. Loss of muscle mass is also a common characteristic of geriatric horses [8]. Although the loss of muscle mass that occurs with ageing may be somewhat attributed to decreased level of activity, nutrition has also been implicated. Dardevet et al [23] demonstrated that geriatric rats have a defect in postprandial stimulation of protein synthesis and that this defect was overcome with supplementation of the amino acid leucine. The mechanism by which leucine stimulates protein synthesis is thought to involve enhancing the efficiency of translation but is not fully understood [23]. Alfalfa hay is relatively high in leucine as well as in crude protein and therefore may be a useful feed in preventing loss of muscle mass in geriatric horses. This is only speculative, however, because there are no reports to confirm that muscle wasting horses is influenced by dietary leucine.

Minerals Dietary minerals can be classified as either macro- or microminerals (ie, trace minerals). Macrominerals are present in the diet in larger amounts (ie, grams) relative to microminerals (ie, milligrams). The mineral concentration of common equine feeds varies considerably and is deficient relative to requirements in many cases. Therefore, most minerals are supplemented to some degree in equine rations. Minerals play a variety of roles in horses, including structural (eg, calcium and phosphorus as components of hydroxyapatite in bone), regulatory (eg, the role of calcium in muscle contraction), and the generation of energy (eg, the role of phosphorus in the formation of high-energy phosphate bonds present in adenosine triphosphate [ATP]). Storage of minerals by the body is limited; therefore, daily consumption is necessary to maintain proper mineral status and subsequently prevent deficiency symptoms from occurring over time [28]. The requirements for macrominerals (calcium, phosphorus, magnesium, sodium, and potassium) in horses have been described [2]. These requirements are generally based on determination of endogenous daily loss of the mineral, which is used to establish the daily metabolic need; this amount is then divided by the apparent digestibility for each mineral, which establishes the daily amount of the mineral required in the diet. Forages generally contain levels of potassium and magnesium well in excess of requirements; in addition, legumes contain calcium in concentrations well above that required. Although grasses tend to have much lower levels of calcium, they still often have concentrations that meet requirements for mature horses at maintenance. Phosphorus is typically supplemented to equine diets consisting primarily of forages but may be adequate in diets containing cereal grains. Phosphorus present in cereals is in the form of phytate, which is relatively low in availability in nonruminant animals [29]; however, microbial phytase activity in the hindgut of horses may improve the availability of

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phytate phosphorus [30]. Sodium concentrations in natural feedstuffs used for horses are typically deficient. Therefore, sodium requirements (and presumably chlorine) [2] are met via supplementation with sodium chloride. The only apparent modification in macromineral requirements necessary in geriatric horses may be phosphorus. Apparent digestibility of phosphorus was less in geriatric horses (actually slightly negative at
4  19 [mean  SD]) when compared with that in younger horses aged 2 to 3 years [11]. The apparent digestibility reported for the younger horses in this study was considerably lower (11  6 [mean  SD]) than that previously reported (35%), however [2]. Fecal samples in this study were collected from the ground, and although care was taken to prevent contamination from urine, it is possible that some contamination occurred, leading to the low apparent digestibility of phosphorus. Nonetheless, because phosphorus plays a multitude of key roles in cellular biochemistry ranging from structural components of cell membranes to derivation of energy used by cells [31], its levels in the diet should be closely monitored so as to provide adequate amounts. The need for additional dietary electrolytes (sodium, potassium, and chlorine) may also be a consideration for geriatric horses that are still used in athletic competition or are undergoing some type of work that results in sweating. Sweating results in considerable loss of electrolytes, particularly sodium, potassium, and chloride [32]. McKeever and Malinowski [33] demonstrated that the aerobic capacity of older exercising horses is less than that of younger horses. Older horses work harder than younger horses at a given speed and, in turn, may also sweat more at lesser intensities than younger horses, which may increase their need for electrolytes even when working at relatively low intensities. This is only speculative, however, and has not been demonstrated to the author’s knowledge. Nonetheless, following dietary electrolyte recommendations for that of performance horses may be prudent for geriatric horses performing even relatively moderate workloads. A commonly recommended electrolyte supplement for exercising horses is 3 oz/h of exercise of a mixture containing equal parts of common salt (sodium chloride) and ‘‘lite’’ salt (mixture of sodium chloride and potassium chloride) [34]. The nutritional requirements for microminerals or trace minerals are not as well defined as those for macrominerals [2]. Nutritional requirements of the horse for many of the microminerals are estimated based on observations of concentrations found in typical horse feeds and the lack of deficiency symptoms in horses consuming these feeds. Few are based on the dietary need of the trace mineral relative to a physiologic/biochemical function (eg, selenium) [35,36]. As a result, speculation on requirements for geriatric horses is even more difficult. Geriatric horses were found to have reduced immune function relative to younger horses [37]. Chromium has been demonstrated to influence immune function in cattle [38–40]; however, in geriatric horses, indices of immune function were either unaffected or, in some cases, decreased [41]. Interleukin-2, an important mediator of immune response, was increased in geriatric mares

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fed supplemental chromium (chromium-L-methionine at a rate of 0.02 mg/kg of BW). The status of trace minerals, such as copper and zinc, which also influence immune function [42,43], may have an impact on the health of geriatric horses. Chromium has been shown to potentiate the action of insulin on glucose uptake [44]. Ralston et al [45] reported that supplemental chromium (chromium-L-methionine at a rate of 0.02 mg/kg of BW) might improve some indices of glucose/insulin response in aged horses (ie, mares[20 years old). Several trace minerals are components of biomolecules involved in antioxidant defense mechanisms [46] that are considered important in attenuating the aging process [19]. Additionally, selenium has been demonstrated to have anticarcinogenic properties [47]. Vitamins Vitamins are categorized as either fat-soluble or water-soluble vitamins. Fat-soluble vitamins include vitamins A, D, E, and K. Water-soluble vitamins consist of the B-vitamins and vitamin C. Vitamins play a wide variety of regulatory roles ranging from coenzymes to regulation of gene expression [48]. Fat-soluble vitamins are stored to some degree within the body, which enables vitamin status to remain within normal limits even if vitamins are lacking in the diet. These stores are limited, however, and require replenishment. Water-soluble vitamins are not typically stored within the body for any significant period and thus must be replenished daily. Vitamin requirements for horses are generally based on dietary amounts of a vitamin necessary to maintain adequate body stores and prevent associated deficiency symptoms [2]. Vitamin A is involved in three primary physiologic functions: visual perception, cell differentiation, and immune function [49]. b-Carotene, or provitamin A, is the predominant source of vitamin A for horses consuming forage-based diets; however, preserved forages stored for longer than 1 year or those damaged by rain may have considerably lower vitamin A activity [2]. The vitamin A status of horses varies with the season and tends to be lowest in the winter months [50–54]. Horses maintained on pasture have a greater vitamin A status than those maintained on a dry lot and fed hay, even when the horses maintained on a dry lot receive supplemental vitamin A [52]. Therefore, supplementation of vitamin A should be considered important in horses not maintained on pasture. Retinyl-palmitate was a more efficient means of improving vitamin A status in horses than a synthetic water-soluble form of b-carotene [52]. Vitamin E is the major lipid-soluble antioxidant within cell membranes and is important in protecting them against lipid oxidation and subsequent damage [55]. Deficiencies of vitamin E tend to manifest in tissues that are highly metabolic, such as the skeletal muscle (eg, nutritional muscular dystrophy), central nervous system (eg, equine motor neuron disease), and

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immune system (eg, impaired cellular and humoral immune response) [56–58]. Pro-oxidants like reactive oxygen species (oxygen-derived free radicals and their derivatives) are capable of damaging all classes of biomolecules, with lipids being extremely susceptible [59]. Reactive oxygen species are considered to contribute to the aging process [19]. Therefore, maintaining an adequate vitamin E status in geriatric horses seems prudent. Additionally, vitamin E has been shown to attenuate the age-related increase in the proinflammatory compound prostaglandin E2 and therefore has implications in the prevention of inflammatory diseases in aged horses [60]. Vitamin E, like vitamin A, is found in relatively high concentrations in fresh forages as well as in forage preserved under adequate conditions (eg, hay that has not been rained on or stored outdoor and exposed to the elements and sunlight). The activity of vitamin E deteriorates over time in stored forage; therefore, hay stored for longer than 1 year may have minimal vitamin E activity [61]. Vitamin E is relatively nontoxic; therefore, supplementation is generally recommended as a safeguard. Vitamins D and K are involved calcium homeostasis [62] and processes involving vitamin K-dependent carboxylases [63] (eg, blood clotting, bone metabolism), respectively. Requirements for vitamins D and K have not been described in the horse [2]. Vitamin D activity is produced photochemically through the action of sunlight on the 7-dehydrocholesterol that is present in skin [62]. In addition, some vitamin D activity is present in some sun-cured forages [2]. As a result, it is assumed that horses receiving adequate sunlight maintain adequate vitamin D status. Compounds containing vitamin K activity are present in forages and are also produced in the hindgut of the horses via microbial fermentation. Deficiencies in vitamins K and D are not apparent in mature horses fed practical diets [2]. It is not apparent how the requirements for these two nutrients might change for geriatric horses. B-vitamins are involved in a host of biochemical processes, including metabolism of carbohydrates, lipids, proteins, amino acids, and nucleic acids. Requirements for B-vitamins are not well defined in horses [2]. In fact, requirements have been established for only two of nine B-vitamins (ie, thiamin and riboflavin) [2]. B-vitamins are produced by microbial fermentation via microbes present in the hindgut of horses [64,65]. The combination of Bvitamins synthesized by microbes and those occurring naturally in forages is generally thought to meet the horse’s requirement for B-vitamins [2]. Although the requirement for the B-vitamin biotin has not been established, there is evidence suggesting that supplementation of 15 to 30 mg/d can increase hoof wall growth as well as integrity [66–69]. It should also be noted that the positive effects of biotin supplementation occurred over a period of 5 to 12 months or more and should not be expected in the short term. This may be relevant for geriatric horses afflicted with diseases affecting hoof growth (eg, chronic laminitis). Vitamin C is involved in collagen synthesis [70,71] and is an important antioxidant defense mechanism [59]. No requirement for vitamin C exists

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for horses [2]. Vitamin C can be synthesized from glucose [72]. Plasma ascorbic acid concentration was lower in aged horses ([20 years old) compared with younger horses fed a similar diet; however, some of the aged horses were affected by viral infection and endocrine disorders that may have affected their ascorbic acid status [10]. Vitamin C status has been shown to affect immune function [73] and attenuation of cartilage degradation and to reduce the risk of osteoarthritis progression [74,75]. Therefore, supplementation of vitamin C may be prudent in geriatric horses. Snow and Frigg [76] reported that ascorbyl palmitate was absorbed from the intestine more effectively than ascorbic acid.

Feed additives Osteoarthritis is a common problem in horses used for athletic performance [77]. Glucosamine and chondroitin sulfate are two feed additives used in horses aimed at slowing the progress of osteoarthritis; however, their efficacy remains uncertain. The use of oral glucosamine has been reported to slow the deterioration of articular cartilage occurring with osteoarthritis in human beings [78]. Glucosamine was recently found to inhibit the production of several metabolites involved in cartilage degradation occurring with osteoarthritis (ie, nitric oxide, prostaglandins, gelatinase/collagenase activity) using equine articular cartilage explants [79]. In vivo studies in horses on the effect of glucosamine and chondroitin sulfate (another component of articular cartilage for which glucosamine is a precursor) have been conducted; however, the results are not definitive [80]. Probiotics are live organisms (eg, bacteria, fungi) and their products that contribute to intestinal microbial health [21]. Factors having a positive influence on microbes within the intestine may enhance the process of microbial fermentation and the subsequent production of nutrients useful to the horse (eg, volatile fatty acids, B-vitamins). Addition of a yeast culture (Saccharomyces cerevisiae) has been shown to improve fiber digestibility in yearlings [81] and pregnant and lactating mares [82] as well as improving nitrogen retention in growing horses [83]. Hall et al [84] reported no effect on fiber digestibility and nitrogen retention in mature horses fed three different levels of yeast culture (S cerevisiae), however. Addition of yeast culture (Aspergillus oryzae) to an in vitro system modeling cecal fermentation did not improve fiber digestion, and actually decreased fiber digestion when included in the system at 10 times the recommended amount [85]. In summary, the benefit of adding probiotics to equine diets remains uncertain. Rations for the geriatric horse Geriatric horses free from disease, having adequate dentition, and having no difficulty in maintaining body condition may thrive on rations similar to

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those fed to younger mature horses [86]. Geriatric horses that are afflicted by various diseases (eg, Cushing’s disease, chronic laminitis, arthritis) and have poor dentition may require modified rations to accommodate their unique needs, however. Mastication is the first step in the digestion of nutrients present in feedstuffs. Mastication increases the surface area of ingested feeds and disrupts protective coatings present in some feedstuffs, thereby allowing greater access by enzymes and other digestive secretions. Age-related changes in dentition can lead to a reduction in digestibility of nutrients. One common approach to this problem is the use of processed complete feeds. Complete feeds are those that can be fed as the sole ration. They are generally a combination of roughage, energy concentrates, protein supplements, and mineral-vitamin supplements. Roughage in complete feeds often consists of dehydrated alfalfa meal and/or dehydrated beet pulp, both of which are relatively high in DE compared with other sources of roughage. Energy concentrates present in complete feeds include cereal grains (eg, corn, oats, barley) and fat (ie, various vegetable oils, animal fat). Cereal grains in complete feeds often undergo processes involving heat treatment (eg, extrusion, pelleting), which results in gelatinization of starch molecules and a subsequent increase in their availability for digestion and absorption in the small intestine [87,88]. Prececal digestion of starch is beneficial in reducing the risk of starch overload in the hindgut and subsequent problems that arise from it, such as colic and laminitis [89–91]. Fat contains 2.25 times more calories than starch and is an effective means of increasing the energy density of rations. Dietary fat is well utilized by horses [92]. In addition, there is no (or at least extremely minimal) risk of digestive upset with fat as opposed to energy-dense rations containing primarily cereal grains (ie, starch). Ralston and Breuer [9] found that a complete feed containing extruded ingredients with a nutrient composition similar to that shown in Table 1 was more effective at maintaining the weight of geriatric horses with low body condition scores (ie, \3) when compared with a more traditional ration (containing similar nutrient concentrations) consisting of a grain-mix concentrate and timothy-alfalfa hay. Several complete feeds formulated for geriatric horses are commercially available and are an effective means of providing nutrients and improving body condition to geriatric horses with poor dentition and low body condition scores in the author’s experience. Long-stem forage can be fed with these rations to prevent boredom. Other options for feeding geriatric horses with poor dentition and low body condition are chopped hay or ensiled forage. In either case, the forage should be of high quality to account for the possibility of reduced digestibility; therefore, even if digestibility is slightly reduced, the total amount of nutrients absorbed is still adequate. Alfalfa or a mixture of a high-quality grass hay and alfalfa generally meets these criteria. The quality of the forage is determined by its stage of maturity at harvest. The more mature the forage is at harvesting, the lower is its nutrient concentration and digestibility.

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Table 1 Nutrient concentration of a mineral-vitamin supplement and complete feed for use in feeding geriatric horsesa,b Nutrient

Mineral-vitamin supplement

Complete feed

Digestible energy (Mcal/lb) Crude protein (%) Lysine (5%) Calcium (5%) Phosphorus (5%) Copper (ppm) Zinc (ppm) Manganese (ppm) Selenium (ppm) Vitamin A (IU/lb) Vitamin D (IU/lb) Vitamin E (IU/lb) Riboflavin (ppm) Thiamin Vitamin C (ppm)

— 10 — 3.25 2.25 275 750 650 1.8 15,000 3,700 325 20 30 —

2.86 14 .65 .6 .4 55 220 — .3 3000 100 20–40 2 3 500

Abbreviation: Mcal, megacalories. a Above values are on an as-fed basis. b This type of supplement is designed to be fed at 1 to 2 pounds per head per day. The above values are valid only when the supplement is fed at this rate (1–2 pounds per head per day).

The greater the ratio of leaves to stem and the absence of seed heads or flowers indicate higher quality. The percentage of crude protein and acid detergent fiber are two chemical measures indicative of forage quality. In general, forages containing approximately 14% to 18% crude protein and 30% to 35% acid detergent fiber are of suitable quality for feeding geriatric horses. Chopping hay minimizes the amount of mastication necessary by the horse; however, it can also increase the amount of fine particles contributing to dustiness, which may lead to respiratory problems or exacerbate existing ones. Therefore, chopped hay should be moistened enough to prevent obvious dust. Ensiled forages can be either grass or alfalfa. Ensiled alfalfa is available commercially in small bags suitable for feeding horses. Nevertheless, one should understand that this type of bagged ensiled forage contains approximately 40% to 50% water and does not provide the same value pound for pound as a bale of hay containing approximately 10% water. In addition, the moisture content affects the amount of ensiled forage required per day. For example, a 500-kg horse with maintenance requirements eats approximately 2% of its BW in dry matter, which is equivalent to 11 kg of alfalfa hay containing 10% moisture or 90% dry matter (ie, [500 kg  0.02]/0.9) and 16.7 kg of ensiled alfalfa containing 40% moisture or 60% dry matter (ie, [500  0.02]/0.6). Failure to recognize this concept results in an underfed horse and eventual loss of body condition and other

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nutrient deficiencies. The ensiled forage should be free from mold, which may be detrimental to the horse’s respiratory system [93]. Although chopped hay and ensiled forage often meet energy and protein requirements, they are typically lacking in many minerals and vitamins. Therefore, mineral vitamin supplementation is generally required. A mineral vitamin supplement suitable for geriatric horses is outlined in Table 1. This type of mineral vitamin supplement is typically referred to as a mixing pellet (ie, it is a means of fortifying grain-mix concentrates with minerals and vitamins). Use of free-choice mineral-vitamin supplements is less desirable in that assurance of adequate daily intake may be uncertain, particularly in a group-feeding setting. In the event that the combination of the forage (ie, chopped hay, ensiled forage) supplemented with minerals and vitamins as described previously does not maintain adequate body condition when fed at approximately 2% of BW in dry matter (ie, near maximal intake), the addition of energy concentrates may be necessary. Fat is the energy concentrate of choice in that it requires no mastication, is higher in energy density than others, and does not increase the risk of digestive upset as discussed previously. Vegetable oils (eg, corn oil, soybean oil, canola oil) are generally the most available and efficient means of adding fat to a ration. Up to two cups can be added on top of a small amount of grain or mixing pellets. The initial amount added may need to be less than the two-cup maximum, because higher amounts may be unpalatable to some horses initially. The addition of one cup of oil is generally a good starting place. Many of the complete feeds commercially manufactured for geriatric horses contain some cereal grains. In some instances, reduction of starch in the diet may be desired, such as in horses with Cushing’s disease or chronic laminitis. Therefore, the ration described previously (forage, mixing pellet, and oil) may also be useful in feeding this class of geriatric horses. The addition of vegetable oil to this ration may or may not be necessary depending on the body condition of the horse being fed.

Summary Little is known regarding nutrient requirements and feeding of geriatric horses, and more effort should be placed on this area of equine nutrition research. That which is known suggests that some geriatric horses may not have different requirements than other mature horses, whereas others affected by disease or poor dentition may have special nutritional needs. In general, rations for geriatric horses should be based on high-quality roughage supplemented with complementary minerals and vitamins. The need for additional energy aside from that provided by the forage can be supplied by adding energy concentrates, such as cereal grains or fat, to the ration. Processing techniques involving heat, such as pelleting and

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