Nutritional Management of Equine Diseases and Special Cases

Preface

Water, hay, and oats are all that a horse needs.

Anonymous

Even in modern times, this is often said from the racetrack to the farm. However, from a nutritional perspective it does not hold true for all horses. Depending on hay quality and type: energy, protein, and calcium may be deficient, especially for working horses and mares in late gestation. Working horses may require such a large amount of oats to meet energy requirements that they would be at risk for hindgut acidosis and subsequent laminitis. A horse may not be physically able to eat enough hay to meet increased energy requirements, depending on the hay’s energy content and quality.

We are fortunate to have a plethora of horse feeds that are formulated by knowledgeable equine nutritionists and tested to ensure that they meet horses’ requirements. Most commercial feeds are fortified with vitamins and minerals to meet requirements if the horse is fed as directed on the bag. Modern feeding and overfeeding practices, as well as the horse’s transition from a working to a companion animal, have created problems such as Equine Metabolic Syndrome. Horses are both living longer and becoming fatter with our help.

Like most equine practices, feeding horses can become as infinitely complicated as we make it to be. Many horses receive multiple supplements and many contain the same ingredients. At least, this is expensive and unnecessary and at worst, it can result in toxicity. Unfortunately, supplementation and feeding decisions are often based on the advice of a self‐proclaimed authority or because someone else does it with apparent success.

The purpose of this book is to guide decision making and clarify the sometimes‐confusing subject of equine nutrition. Thank you to my coauthors for their contributions, including my former major professor Dr. David Pugh, whose guidance and education have been a tremendous gift.

Bryan M. Waldridge

1

Miniature horses and ponies

DG Pugh, DVM, MS, MAg, DACT, DACVN, DACVM; Nicole Passler, DVM, MS; and Sara Ziska, DVM, PhD

This chapter will discuss feeding of miniature horses and ponies, two of the smallest members of the genus, species, and subspecies Equus ferus caballus. Both miniature horses and ponies should be fed in a similar fashion as light breeds, with the obvious exception that they are smaller and therefore require less total nutrients on a body weight basis. Many of the common feeding and husbandry practices applicable for other breeds may be applicable to both miniature and pony breeds.¹

1.1 Miniature Horses

Although the American Miniature horse was declared a single breed in the late 1970s by the American Horse Association, many miniature horse breeders consider several distinct breeds to exist (e.g., Australian Miniature Pony, Dartmoor Pony, Falabella, Micro Mini, Miniature Toy Horse, etc.). These breeds can be traced back to royal families in Europe of the seventeenth century. Presently, these equids are used as pets, show, and service animals. Miniature horses usually live 25–35 years and are described as being less than 97 cm (38 in) in height at the withers (probably all carry some genes for dwarfism). Many non‐guide miniature horses may weigh up to 90 kg, while the minis used as service animals should be less than 66 cm (26 in) in height and weigh between 24–45 kg (55–100 lb).

1.2 General Feeding of Miniature Horses

Unlike ponies, there are few controlled scientific studies on nutritional requirements of miniature horses.¹–³ Minis are considered an easy keeper breed and should not be overfed to prevent obesity. As with other horse breeds, energy requirements for maintenance usually can be met by feeding 1.0–1.8% of their body weight daily in dry matter derived from good quality forages or 1–2 kg of good quality dry forage daily. Miniature horses can be fed small amounts of grass or hay and concentrates or used to graze or mow lawns. Supplemental grain should be fed only as needed and added to the diet based upon body condition score (BCS). The principles of body condition scoring are the same for miniatures as for other breeds of horses. Miniature horses should be maintained at an ideal BCS of 5–6/9 (ribs can easily be palpated, but not seen, and there are no obvious fat deposits on the neck, shoulders, withers, or tail base). Body condition scoring should be used to determine energy intake adequacy or lack thereof.¹ When BCS falls below 5/9, caretakers should consider increasing either the quality and/or quantity of forages or slowly introducing a small amount (0.25–0.5 lb/day) of concentrate. Common mistakes made with concentrate feeding include overestimation of body weight and underestimation of concentrate offered. Both mistakes can result in obesity and possibly one of many metabolic disorders (e.g., equine metabolic syndrome, laminitis, hyperlipemia, etc.) seen in overweight miniature horses. Owners of miniature horses should be encouraged to purchase scales to actually weigh feed to avoid overfeeding. Scales used by fishermen to weigh fish are inexpensive and can be readily purchased. As for other horses, access to fresh clean water is critical to ensure adequate feed intake, minimize colic risk, and maintain overall health. The general guidelines for water, energy, protein, mineral, and vitamin requirements as a percent of diet are based upon age, growth, production status (e.g. early, mid, late gestation, or lactation), and use of miniature horses, and are similar to other breeds.¹

Miniature horses are susceptible to many of the nutritionally related conditions seen in other breeds, but may be more prone to enteroliths⁴ and hyperlipemia.⁵,⁶ Caregivers should be cognizant of normal horse feeding practices and adopt well‐conceived, basic feeding programs as discussed in other chapters of this text.

1.3 Pony Feeding

Ponies are horses less than 147 cm (14.2 hands or 58 in) in height at the withers. Pony breeds typically have shorter heads with broader foreheads, thicker necks, wider barrels, and shorter legs compared to other horses. Pony breeds are used as pets, show, riding, and working animals. There are many distinct breeds of ponies with varying uses that help determine proper feeding programs (e.g., carriage ponies vs hunter/jumper ponies).

Because of their size and availability, ponies have been utilized in many equine nutrition research projects. Therefore, much information is available specifically discussing pony nutrition.⁷–¹¹ Feeding practices for other light breeds are usually applicable to ponies.¹ Many pony breeds will reach 75% of their mature weight by 12 months of age, while Thoroughbred horses only reach approximately 69% of mature weight at the same age.¹ Therefore, feeding practices should be adjusted for ponies compared to other breeds because of their faster growth rate. Because most pony breeds were selected and evolved under conditions of sparse or poor quality pasture and rugged terrain, they tend to be easier to maintain than other horse breeds. With the possible exception of working, lactating, and growing ponies, most ponies will rarely require concentrates and easily become obese. Ponies are predisposed to many metabolic conditions, such as hyperlipemia and equine metabolic syndrome.¹,¹²,¹³ Increased fat supplementation with soybean oil at 10% of dry matter intake was associated with glucose intolerance in Shetland ponies.¹⁴ Ponies appear to have a higher voluntary intake than other horse breeds.¹,¹¹,¹⁵ In one study, ponies consumed 3.9 kg of alfalfa hay per 100 kg of body weight (3.9% of body weight in dry matter intake).¹¹ Caretakers should be cautious and utilize high energy feedstuffs only when necessary. When providing a concentrate or concentrates, the BCS should be continuously monitored to minimize obesity.

As for miniature horses, ponies should be fed good quality forages at 1.0–1.8% of their body weight in dry matter daily. Body condition scores should be estimated for ponies as for other breeds, with diet modifications implemented to maintain ideal body condition near 5–6/9. Ponies at maintenance (neither gaining nor losing weight) usually can survive on hay only or grass pastures, while those used for light work may require 20% of their dietary intake in the form of a concentrate. Feed should be withheld from ponies only when medically indicated and with strict observation. Prolonged periods of inadequate energy intake result in hyperlipemia, which is exacerbated by preexisting conditions such as illness, pregnancy, and/or obesity.¹

References

1. National Research Council. Nutrient Requirements of Horses, 6th ed. National Research Council, The National Academies Press, 2007.

2. Hoyt JK, Potter GD, Greene LW, et al. Mineral balance in resting and exercised miniature horses. J Equine Vet Sci 1995;15(7):310–314.

3. Hoyt JK, Potter GD, Greene LW, et al. Copper balance in miniature horses fed varying amounts of zinc. J Equine Vet Sci 1995;15(8):357–359.

4. Cohen ND, Vontur CA, Rakestraw PC. Risk factors for enterolithiasis among horses in Texas. J Am Vet Med Assoc 2000;216:1787–1794.

5. Moore BR, Abood SK, Hinchcliff KW. Hyperlipemia in 9 miniature horses and miniature donkeys. J Vet Intern Med 1994;8(5):376–381.

6. Golenz MR, Knight DA, Yvorchuk‐St Jean KE. Use of a human enteral feeding preparation for treatment of hyperlipemia and nutritional support during healing of an esophageal laceration in a miniature horse. J Am Vet Med Assoc 1992;200(7):951–953.

7. Vermorel, M; J Vernet; W Martin‐Rosset. Digestive and energy utilisation of two diets by ponies and horses. Livest Prod Sci 1997;51:13–19.

8. Kane E, Baker JP, Bull LS. Utilization of corn oil supplemented diet by the pony. J Anim Sci 1979;48:1379–1384.

9. Goodson J, Tyznik WJ, Cline JH, et al. Effects of an abrupt diet change from hay to concentrate on microbial numbers and physical environment in the cecum of the pony. Appl Environ Microb 1988;54(8):1946–1950.

10. Cuddeford D, Pearson RA, Archibald RF, et al. Digestibility and gastro‐intestinal transit time of diets containing different proportions of alfalfa and oat straw given to thoroughbreds, Shetland ponies, highland ponies, and donkeys. 1995; Anim Sci 61:407–417.

11. Pearson RA, Archibald RF, Muirhead RH. The effect of forage quality and level of feeding on digestibility and gastrointestinal transit time of oat straw and alfalfa given to ponies and donkeys. Brit J Nutr 2001;85:599–606.

12. Treiber K, Carter R, Gay L, et al. Inflammatory and redox status of ponies with a history of pasture‐associated laminitis. Vet Immunol Immunopathol 2009;129(3–4):216–20.

13. Hughes KJ, Hodgson DR, Dart AJ. Hyperlipaemia in a 7‐week‐old miniature pony foal. Aust Vet J 2002;80(6):350–1.

14. Schmidt O, Deegen E, Fuhrmann H, et al. Effects of fat feeding and energy level on plasma metabolites and hormones in Shetland ponies. J Vet Med 2001;48A:39–49.

15. Argo C McG, Cox JE, Lockyear C, et al. Adaptive changes in the appetite, growth, and feeding behaviour of pony mares offered ad libitum access to a complete diet in either a pelleted or chaffbased form. Anim Scis 2002;74:517–528.

2

Draft horses, mules, and donkeys

DG Pugh, DVM, MS, MAg, DACT, DACVN, DACVM; Sara Ziska, DVM, PhD; and Nicole Passler, DVM, MS

Draft horses, donkeys, and their hybrid crosses are discussed together in this chapter, as all three are traditionally thought of as working animals or beasts of burden. Draft horses, mules, and donkeys still are used as working animals, but also as pets, for trail riding, cart pulling, showing, and other recreational uses. As all three are of the genus Equus, this chapter will review some of the differences between them and other horse breeds with respect to feeding.

2.1 Draft Horses

There are approximately 30 breeds of draft or draught horses found in the world today. These large horses (550–1180 kg or 1400–2600 lb) are utilized in farming and logging industries, blood or plasma donation, biological and pharmaceutical production, advertising campaigns, as carriage horses, show horses, and pets. The most popular breeds of draft horses in the United States, Belgians, Clydesdales, Percherons, and Shires, all originated in Western Europe. These breeds were selected for their tall stature, heavy bone and frame structure, muscular hindquarters, and patience to haul large loads.

Traditionally, these large working animals were thought to have a similar nutrient metabolism as pony breeds. Historically, draft breeds have been fed slightly less feed per kg of body weight than light breeds. The most recent National Research Council feeding guidelines for horses¹ suggested that idle, mature, healthy draft horses could subsist on 30.3 kcal of digestible energy (DE)/kg of body weight daily. This energy requirement is slightly lower than 33.3 kcal/kg of body weight daily recommended for light horse breeds. Obviously, during work, growth, lactation, or other periods of increased energy expenditure, the energy requirements are greater. The total energy requirement is higher for draft horses (700 kg or greater) than light horse breeds (Thoroughbred, Quarter Horse, etc.), as these breeds may weigh substantially less (425–480 kg). Mature draft horses should be fed a minimum of 1.5% of their body weight in roughage daily, with a total dry matter intake between 1.5–3.0% of their body weight daily. Still, these breeds can be fed using many of the general guidelines applicable to light breeds.

Good quality grass‐legume mixed pastures or hay will usually suffice for draft horses at maintenance (neither gaining nor losing weight). The caregiver should always be cognizant of carbohydrate concentrations in the forage and pastures, as with any breed of horse, to minimize the risk of colic and laminitis.

Feeding to maintain a body condition score (BCS) of 5–6/9 is optimal in most circumstances. The energy density of the diet and/or use of supplemental high energy feedstuffs (e.g., concentrates) should be adjusted to support growth, production, lactation, late gestation, work, and needs for increased energy use with the goal of maintaining a BCS between 5–6/9. Total dietary energy required will depend on the type of work, duration of work, weight of loads, or the amount of force exerted to perform work. Again, body condition should be used to adjust energy intake to meet demands and maintain BCS in the 5–6/9 range. Unfortunately, draft horses may be prone to Polysaccharide Storage Myopathy, Exertional Rhabdomyolysis, Equine Metabolic Syndrome, and other diet‐related conditions.¹–³ Thus, feeding diets high in carbohydrates should be done with extreme caution, and then only when necessary. Nutritional myopathies are discussed in Chapter 4.

Overall, these breeds seem less prone to developmental orthopedic disease.¹,⁴,⁵ Of the draft horses, Clydesdales and Percherons appear to be the breeds most affected with metabolic bone diseases.⁵ The caregiver is cautioned to follow feeding practices that minimize developmental orthopedic disease in growing animals.

Unfortunately, the large size of draft horses presents other management issues that directly affect feeding. The authors have observed more catastrophic outcomes when draft breeds develop laminitis and increased heat stress with obesity, as compared to lighter breeds. The caregiver should strive to maintain a BCS of 5–6/9 and carefully monitor obese animals, particularly in times of warm weather or when laminitis is a concern.

Due to their impressive body weight, it is not uncommon for draft horses to require 24 gallons (91 L) of fresh, clean water daily. Dehydration may result if caregivers are unable to meet these extreme demands, which increases the risk of developing intestinal impactions and other potentially life‐threatening conditions.

2.2 Donkeys

Donkeys or asses (Equus africanus asinus or Equus asinus) are traditionally thought of as working animals, and in many parts of the world are depended upon in this manner. In modern‐day North America, donkeys or burros are used for work, show, cart and/or carriage pulling, competitive riding, drug smuggling, guard animals, training animals, and pets. There are 15–20 breeds of donkeys, including miniature, standard, large standard, and mammoth stock, which vary greatly in height (81–157 cm or 32–62 in). The female is commonly referred to as a jenny or jennet. Intact males are commonly called a jack or jackass. These animals characteristically have longer ears and make loud vocal noises (bray), as compared to horses. Their reproductive cycle has similarities to that of the horse and has been described.⁶

Donkeys are believed to have evolved in arid to semi‐arid climates and show extraordinary tolerance for heat and dehydration. They seem able to continue eating for several days in the absence of drinkable water. This is in contrast to horses, which decrease forage and feed consumption in the face of dehydration. In modern agricultural husbandry practices, as with other equids, clean, fresh water should be offered to donkeys free choice, despite their relative hardiness.

Donkeys appear to readily adapt to new environments and feedstuffs, which is not common in other equids.¹ It is an accepted husbandry practice to feed donkeys less than horses on a body weight basis, as they are not simply small horses.¹ Donkeys have a narrow muzzle and mobile lips, which allow for greater feed selection in comparison to most horses. Therefore, they can selectively consume higher quality portions of available forages. Donkeys will subsist on more mature forages than are willingly consumed by most horses (e.g., bark on trees and shrubs), but can and will consume traditional feedstuffs.¹,⁷,⁸ On poor quality forage diets, donkeys appear to have a lower dry matter intake requirement than ponies.⁹,¹⁰ Reported voluntary dry matter intake ranges for donkeys have been between 0.83–2.6% of body weight, depending on the type and quality of the feed stuff, along with physiologic requirements.¹¹,¹² However, dry matter intake between 1.75–2.25% of body weight of moderate to good quality forage will routinely meet maintenance requirements in mature donkeys.¹ When offered moderate to good quality forages, donkeys will readily adapt to consume complete diets and employ their selective grazing habits only when offered mixed forage diets of differing quality.¹³

Traditional donkey feeding practices infer that donkeys are more efficient in digestion than horses. Donkeys appear to have higher apparent digestibility for dietary dry matter and fiber than ponies and horses, particularly when fed poor quality forages (e.g., oat straw).¹¹,¹⁴ The higher digestibility ability of donkeys could be attributed to longer gut retention time or greater microbial cellulolytic activity in the cecum, compared to other equids.¹⁵,¹⁶

Some reports have shown that donkeys may require only 3.8–7.4% protein in their diet, due to efficient dietary protein utilization.¹⁷,¹⁸ Despite these findings, the authors recommend that dietary protein intake in donkey diets should be fed similar to recommendations for horses. Consequently, feeding as for horses should meet requirements for donkeys, with respect to protein intake.

In parts of the world where high quality feedstuffs are plentiful, obesity in donkeys is a major concern. Caregivers should be mindful to feed donkeys only to a desired body condition and avoiding over‐conditioning. Energy‐protein malnutrition, mineral deficiencies, and emaciation are of most concern in many tropical areas of the world where donkeys are used as work animals.

Donkeys fed to obesity will develop a fat roll over the neck (pones) and fat on the barrel and hips, which are quite unsightly. Donkeys, much like pony breeds, may be prone to hyperlipemia during stress and feed deprivation.¹ Caregivers should closely monitor donkeys for feed intake in times of stress (e.g., changes in weather, illness, etc.). Because of the stoic disposition of donkeys, close attention to dietary intake and body condition is imperative. Body condition scoring systems for donkeys have been described.¹⁹,²⁰ The Vall system assigns a score from 1 (emaciated) to 4 (good), with emphasis placed on the appearance of the flank and back.¹⁹

Diets appropriate and practical for horses can typically be fed to donkeys, with caregivers mindful to avoid obesity. Diets should include 6–10% protein intake for maintenance needs, free access to fresh clean water, and a good quality mineral mixture designed for horses.

2.3 Mules

Mules are the offspring of a male donkey (jack or jackass) and a female horse (mare). Their size, shape, and use are often determined by the breed characteristics of both the sire and dam. Thus, mules can be found in all statures, colors, and types of conformations. Mules were used primarily for riding, packing, and/or working animals. Today, mules are still used for these purposes, as well as guard animals for small ruminants, showing, recreation, and pets. The female mule is traditionally referred to as a female, mare mule, molly, or molly mule, whereas the male mule is traditionally referred to as a male, gelded/stud mule, or john mule. Mules are considered more sure‐footed, patient, hardier, and slower than horses, and less obstinate than donkeys. As donkeys have 62 chromosomes and horses have 64 chromosomes, mule hybrids are rarely fertile. The cross between a stallion and a jenny is called a hinney; hinnies tend to be more donkey‐like and much less common than mules.

Mules are routinely fed less than horses but more than donkeys on a body weight basis. Regrettably, there are few controlled studies on nutritional requirements for mules.²⁰ Mules are physiologically more similar to horses than donkeys and feeding practices should take this into account. Accordingly, mules from Quarter Horse mares and used for Quarter Horse‐like purposes should be fed in a similar manner for the mare. However, caregivers must remain cognizant that these mules are also part donkey and therefore may require less total dietary intake than Quarter Horses of similar size. This principle is applicable across lines of mules; that is mules should be fed in a similar fashion to that of the dam. As with donkeys, obesity can be a major problem in mules, so caution should be exercised when feeding high energy diets. Overall, energy, protein, and mineral requirements in mules appear to be very similar to horses, with the exception that mules may digest feedstuffs more efficiently than horses.

References

1. National Research Council, Nutrient requirements of horses, 6th ed. National Research Council: The National Academies Press, 2007.

2. Valentine BA, Credille KM, Lavoie JP, et al. Severe polysaccharide storage myopathy in Belgian and Percheron draft horse. Equine Vet J 1997;29:220–225.

3. Valentine BA, Habecker PL, Patterson JS, et al. Incidence of polysaccharide storage myopathy in draft horse‐related breeds: a necropsy study of 37 horses and a mule. J Vet Diagn Invest 2001;13:63–68.

4. Stromberg B. A review of the salient features of osteochondrosis in the horse. Equine Vet J 1979;11:211–214.

5. Riley CB, Scott WM, Caron JP, et al. Osteochondritis dessicans and subchondral cystic lesions in draft horses: a retrospective study. Can Vet J 1998;39:627–633.

6. Wilborn RR, Pugh DG. Donkey reproduction. In: McKinnon AO, Squires EL, Vaala WE, et al., eds, Equine reproduction. 2nd ed. Ames: Wiley‐Blackwell, 2011;2835–2838.

7. Mueller PJ, Protos P, Houpt KA, et al. Chewing behavior in the domestic donkey (Equus asimus) fed fibrous forage. Appl Anim Behav Sci 1998;60:241–251.

8. Suhartanto B, Tisserand JL. 1996. A comparison of the utilization of hay and straw by ponies and donkeys. 47th EAAP meeting, Lillehammer.

9. Pearson RA, Merritt JB. Intake, digestion and gastrointestinal transit time in resting donkeys and ponies and exercised donkeys given ad libitum hay and straw diets. Equine Vet J 1991;23:339–343.

10. Tisserand JL, Pearson RA. Nutritional requirements, feed intake and digestion in working donkeys: a comparison with other work animals. In: Pearson RA, Lhoste P, Saastamoinen M, and Martin‐Rosset W, eds, Working animals in agriculture and transport. a collection of some current research and development observations. EAAP Technical Series No 6. Wageningen, Netherlands: Wageningen Academic Publishers, 2003;63–73.

11. Pearson RA, Archibald RF, Muirhead RH. The effect of forage quality and level of feeding on digestibility and gastrointestinal transit time of oat straw and alfalfa given to ponies and donkeys. Br J Nutr 2001;85:599–606.

12. Pearson RA. Effects of exercise on digestive efficiency in donkeys given ad libitum hay and straw diets. In: Pearson AA, Fielding D, eds, Donkeys, mules and horses in tropical agricultural development. Edinburgh: University of Edinburgh Press, 1991;79–85.

13. Maloiy GMO. The effect of dehydration and heat stress on intake and digestion of food in the Somali donkey. Environ Physio Biochem 1973;3:36–39.

14. Araujo LOD, Goncalves LC, Rezende ASC, et al. Digistibilidade aparente em equideos submetidos a dieta composta de concentrado e volumosos, fornecido com diferentes intervalos de tempo (Apparent digestibility in equids of diets differing in concerntration and volume when fed over different time periods). Arquivo Brasileiro de Medicina Veterinaria Zootecnia 1997;49:225–237.

15. Cuddeford D, Pearson RA, Archibald RF, et al. Digestibility and gastro‐intestinal transit time of diets containing different proportions of alfalfa and oat straw given to thoroughbreds, shetland ponies, highland ponies, and donkeys. Animal Science 1995;61:407–417.

16. Suhartanto B, Julliand V, Faurie F, et al. Comparison of digestion in donkeys and ponies. In: Proceedings of the 1st European Conference on Equine Nutrition. Pferdeheilkunde Sondergabe 1992;158–161.

17. Muller PJ, Protos P, Houpt KA, et al. Voluntary intake of roughage diets by donkeys. In: Bakkoury M, Prentis A, eds. Working equines. Rabat, Morocco: Aetes Editions, 1994 137–148.

18. Schlegal ML, Miller M, Crawshaw G, et al. Practical diets and blood mineral and vitamin concentrations of captive exotic equids housed at Disney’s Animal Kingdom and the Toronto Zoo. Second Annual Crissey Zoological Nutrition Symposium, December 10–11, Raleigh, North Carolina, 2004; 39–46.

19. Vall E, Ebangi AL, Abakar O. A method of estimating body condition score (BCS) in donkeys. In: Pearson RA, Lhoste P, Saastamoinen M, Martin‐Rosset W, eds. Working animals in agriculture and transport. A collection of some current research and development observations. EAAP Technical Series No 6. Wageningen, Netherlands: Wageningen Academic Publishers, 2003;93–102.

20. Pearson RA, Quassat M. A guide to live weight estimation and body condition scoring of donkeys. Edinburgh: University of Edinburgh Press, 2000.

3

Gastrointestinal system

Amelia Munsterman, DVM, MS, DACVS, DACVECC

Nutritional support of the critically ill patient is no longer seen only as an adjunct therapy. Recent studies in humans support that early and adequate nutritional support can reduce complications, shorten the duration and severity of disease, and improve patient outcomes. However, it is important to note that, even in human medicine, recommendations for nutritional support are limited by the heterogeneity of patient populations, their illnesses, and statistical power. In the veterinary literature, these limitations are even more restrictive. This chapter offers basic guidelines for nutritional support of adult horses with colic, based on a review of available literature and expert opinion.

3.1 The Association between Nutrition and Colic

The horse was designed to be a continuously grazing animal, with hindgut fermentation supporting the digestion of high fiber, low carbohydrate feeds. Modern horse keeping either neglects this fact or is unable to provide a lifestyle for the needs of the horse’s digestive system, which was evolutionarily refined for fiber digestion. Intermittent feedings, large boluses of cereal based feeds, and stall confinement are the norm for most modern horses. While the horse is able to adapt to some extent, it is not unexpected that the adaptations that allow horses to live among us periodically fail. Nutrition is often implicated as the cause of gastrointestinal pain, however, the multifactorial nature of the problem, including types of feed, quality of feedstuffs, and variations in feeding practices, make it difficult to pinpoint epidemiologically the true role of diet in colic.

In veterinary medicine, the search for the cause and effect of a condition is often hampered by financial limitations, small group sizes, and limited record keeping. It is important, however, to take an unbiased and critical view of all the information provided, since the knowledge we gain will be used to directly influence the treatments provided. Evidence based medicine is the practice of integrating unbiased research and clinical expertise, and applies a grading scheme to the published literature. It acknowledges that all evidence is not created equal and requires careful consideration when applying research to the clinical patient. In this chapter, only studies with a level of evidence of grade 2 or higher were included to provide statistical evidence linking colic and nutrition (see Table 3.1). When assessing the evidence, the veterinary practitioner is cautioned to keep in mind the strengths and weaknesses of the published literature in order to make decisions based only on the most valuable information available. The first step in linking feeds to colic is to analyze the evidence related to the most common feeds provided to horses, including grasses, dried forages, and concentrates.

Table 3.1 Classification levels for evidence based medicine applied to veterinary publications

(Source: Bedenice, 2007, Reproduced with permission of Elsevier).³³⁰

3.1.1 Feeds and Colic: Pastures

The horse was evolutionarily designed to graze grasses. The large colon, specifically, developed to ferment these grasses into short chain fatty acids (acetate, propionate, and butyrate) for energy.¹While grasses have the capability of storing large quantities of carbohydrates, as starches and fructans, which could upset the delicate microbial populations, continuous grazing should allow the bacteria to respond to any changes in carbohydrate content gradually.²–⁵However, access to pastures with high levels of fermentable carbohydrates has been implicated as a cause of colic and laminitis and may relate to the apparent seasonality of colic events.⁶–⁸While grazing on pasture has been regarded as protective against colic, its effect may be tempered by other factors, including water supply, weather, rate of feed intake, stocking density, quality of pasture, and supplements provided.

Pasture access was found to reduce the likelihood of colic in a case control observational study of 364 horses, which noted a three‐fold increase in the risk of colic for horses with no pasture turnout or that had a recent reduction in paddock size or time at pasture (95% CI 1.4–6.6, P = 0.007).⁹A separate case control study in the UK noted that stall confinement tended to increase the risk of colic (OR = 9.30, 95% CI 1.68–51.40, P = 0.011). Stabling for 24 hours per day was associated with the greatest risk for colic (OR = 35.2).¹⁰Lack of grazing activities may also predispose to specific types of colic, including enterolithiasis, which was noted to occur 2.8 (95% CI 1.06–7.59, P = 0.04)¹¹to 4.0 (95% CI 1.3–12.2, P = 0.02)¹²times more frequently in horses that spent less than 50% of their time outdoors. Access to pasture did not reduce the risk of colic in a report by Reeves et al.,¹³but if water was not available in the paddock, it more than doubled the risk of colic (OR = 2.2, 95% CI 1.2–4.3). Despite this published evidence, it is difficult to separate any beneficial effect of grazing activities from the patterns and timing of ingestion, the horse’s activity levels, and the effects of exercise on intestinal motility.

3.1.2 Feeds and Colic: Dried Forages

Hay often provides a large portion of the modern horse’s diet due to the confines of space and resources. While dried forages provide some consistency to the horse’s diet, there is still variability that can occur between batches due to changes in source, the type of hay fed, and even the preservation process that produced the hay. Poor quality forages with high concentrations of hemicellulose, cellulose, and lignin increase the risk of impaction‐related colic.⁹,¹⁴–¹⁶Feeding hay from round bales may also increase the risk by 2.5 times (95% CI 1.1–5.6, P = 0.028), likely due to the methods of preservation, exposure to the elements before and during feeding, and the unchecked quantities available to the horse.⁹

Abrupt changes in the type or batch of hay have been noted to be a common cause of colic. In a case control study of horses experiencing colic in Texas, it was noted that while a feed change in the previous two weeks was significantly associated with colic (OR = 5.0, 95% CI 2.6–9.7, P < 0.001), a change of hay increased the risk even further (OR = 9.8, 95% 1.2–81.5, P = 0.035).⁶This was confirmed in an additional study in 2001 (OR = 4.9 95% 2.1–11.4, P < 0.001).⁹On the eastern seaboard of the United States, a change in diet was again linked to colic, specifically with a change in hay resulting in a 2.1 times increase in colic incidence (95% CI 1.2–3.8, P = 0.01).¹⁷

The specific type of hay also may play an important role in the risk of colic. Coastal Bermudagrass hay has been implicated in one cohort study as a cause of colic (OR = 1.65, 95% CI 1.01–2.7, P = 0.045),¹⁴and has been suggested in numerous retrospective case series to cause ileal impactions.¹⁸–²⁰Coastal Bermudagrass hay was confirmed as a risk factor for ileal impactions in a retrospective case control study by Little and Blikslager,¹⁵who found that horses fed Coastal Bermudagrass had a 4.4 times higher risk for ileal impaction (95% CI 2.1–9.1) versus non‐colic controls; a 5.7 times higher risk (95% CI 2.4–13.6) for medical colic, and a 2.7 times higher risk (95% CI 1.2–6.5) for surgical colic. However, this study also noted that feeding Coastal Bermudagrass alone did not increase the risk of colic in general and diluting it with other hays did not reduce the risk of ileal impactions.

Alfalfa hay has been associated with an alkalinizing effect on the colonic ingesta, resulting in alterations in microflora and a reduction in volatile fatty acid production, which produces an environment suited to the formation of enteroliths.²¹In one study, the odds of enterolithiasis were increased if the diet was comprised of >50% alfalfa (OR = 4.2, 95% CI 1.3–12.9, P = 0.01).¹²Two additional studies confirmed this finding, supporting the restriction of alfalfa hay from the diet of horses at risk for enterolithiasis.¹¹,²²One possible explanation is that the high magnesium content of alfalfa contributes to its alkalinizing effect on colon contents.²²,²³In addition, the high protein content may decrease magnesium absorption and increase ammonia to precipitate minerals and form enteroliths.²⁴Conversely, grass hays may be useful for prevention of enteroliths and were noted to have a protective effect if fed at greater than 50% of the diet.¹¹

3.1.3 Feeds and Colic: Concentrates

Carbohydrate rich feeds, including grains, are the most commonly implicated dietary cause of colic in the horse, likely due to the well‐documented influences of this