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Nutritional Management of Equine Diseases and Special Cases offers a concise, easy-to-comprehend text for equine veterinarians with questions about commonly encountered nutritional problems. * Assists veterinarians in supporting equine patients with special nutritional needs * Focuses on nutritional problems and impact on different body systems * Covers ponies, miniature horses, draft horses, donkeys, and mules * Offers complete coverage of common diseases and problems helped by nutrition * Includes useful chapters on poisonous plants and mycotoxins

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Table of Contents


Title Page



1 Miniature horses and ponies

1.1 Miniature Horses

1.2 General Feeding of Miniature Horses

1.3 Pony Feeding


2 Draft horses, mules, and donkeys

2.1 Draft Horses

2.2 Donkeys

2.3 Mules


3 Gastrointestinal system

3.1 The Association between Nutrition and Colic

3.2 Nutritional Plans for Horses with Colic

3.3 Routes for Feeding Horses Recovering from Colic

3.4 Diets for Specific Diseases


4 Muscular system

4.1 Myopathies Associated with Nutritional Deficiencies

4.2 Nutrigenomics


5 Endocrine system

5.1 Equine Metabolic Syndrome

5.2 Feeding Horses with Pituitary Pars Intermedia Dysfunction

5.3 Pearls and Considerations


6 Respiratory system

6.1 Effects of Inhaled Dust and Potential Aeroallergens on Equine Respiratory Disease

6.2 Respirable Dust Deposition in the Airways

6.3 Effects of Soaking Hay

6.4 Effects of Steam Treating Hay

6.5 Feeding Forage Alternatives

6.6 Exercise‐Induced Pulmonary Hemorrhage

6.7 Acute Interstitial Pneumonia


7 Neurologic system

7.1 Cervical Vertebral Malformation

7.2 Botulism

7.3 Ryegrass Staggers

7.4 Equine Degenerative Myelopathy and Neuroaxonal Dystrophy

7.5 Equine Motor Neuron Disease

7.6 Effect of Form and Dose of Vitamin E on Serum and Cerebrospinal Fluid Concentrations


8 Mycotoxins

8.1 Aflatoxins

8.2 Fumonisins

8.3 Slaframine

8.4 Trichothecenes

8.5 Mechanism of Action

8.6 Zearalenone

8.7 Treatment

8.8 Concluding Remarks



9 Poisonous plants

9.1 Excessive Salivation Induced by Plants

9.2 Colic and Diarrhea‐Inducing Plants

9.3 Photodermatitis‐Inducing Plants

9.4 Neurologic Disease‐Inducing Plants

9.5 Lameness and Muscle Weakness‐Inducing Plants

9.6 Plant‐Induced Calcinosis

9.7 Selenium Toxicosis

9.8 Anemia‐Inducing Plants

9.9 Teratogenic Plants

9.10 Sudden Death‐Inducing Plants

9.11 Larkspur

9.12 Monkshood

9.13 Poison Hemlock

9.14 Water Hemlock

9.15 Yew

9.16 Death Camas

9.17 Avocado

Supplemental Reading



End User License Agreement

List of Tables

Chapter 03

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

Table 3.2 Equine Body Condition Scoring System

Table 3.3 Factors that can influence the resting energy expenditure of the critically ill horse

Table 3.4 Nutritional composition of commonly fed forage alternatives that can provide adequate fiber length to horses with dental problems or those who cannot chew and swallow hay.

Table 3.5 A partial list of commercial complete rations available for horses, including pelleted, powdered, and forage based diets. The feeds listed in this table are provided as representative examples only and are not the only complete feeds available for horses. Nutrient analysis is provided to allow for dietary formulation, as well as weight calculations for initial reintroduction after colic. Initial ration weight is based on a digestible energy requirement (DER) of 16,400 kcal/day for a 500‐kg horse and should increase gradually to the full DER over 2–4 days. After recovery from colic, the feed type and amount should be reevaluated based on each individual horse’s needs and long stem forages should be added to the diet if possible.

Table 3.6 Alternative feed supplements for feeding horses after colic to provide additional energy, protein, and fiber to correct dietary deficiencies. (Adapted from National Research Council, Committee on Nutrient Requirements of Horses, 2007)

Table 3.7 The Naylor Component Diet. This diet is thick and usually fed as‐is, rather than via nasogastric tube. Total DE supplied is 2,770 kcal/5 l. (Adapted from Naylor et al., 1992.)


Table 3.8 Commercially available dextrose solutions for partial parenteral nutrition.

Table 3.9 Refeeding schedule for post‐operative small intestinal resections. The diet can be adjusted based on each individual horse’s response to feedings, in that the interval between meals can be increased, the weight of feed decreased, or both for slower reintroductions. Any horse with positive net nasogastric reflux should be held off enteral feed and water until ileus resolves.

Table 3.10 Examples of refeeding schedules for mild ascending colon impactions, when colic resolves in 12–24 hours. The interval between meals can be increased, the weight of feed decreased, or both for slower reintroductions (over 3–4 days).

Chapter 04

Table 4.1 Feeding recommendations for a 500‐kg horse with exertional rhabdomyolysis due to RER or PSSM.

Table 4.2 Potassium Content of Common Horse Feeds. Potassium content varies in all feedstuffs and these values are guidelines only. Data from ADM Alliance Nutrition, Inc. (

Chapter 06

Table 6.1 Summary of results from studies investigating the effects of soaking hay on respirable dust and nutrient composition.

Chapter 07

Table 7.1 Restricted diet for young horses with cervical vertebral malformation.


Chapter 09

Table 9.1 Mechanically injurious plants.

Table 9.2 Colic or diarrhea‐inducing plants.

Table 9.3 Primary photosensitizing plants.

Table 9.4 Hepatotoxic plants.

Table 9.5 Toxic


species in North America.

Table 9.6 Protein, sulfur‐containing amino acid, and arginine content of horse feeds.


Table 9.7 Protein, sulfur‐containing amino acid, and arginine content of horse feeds.


Table 9.8 Lameness and muscle weakness‐inducing plants.

Table 9.9 Selenium accumulator plants.

Table 9.10 Anemia‐inducing plants.

Table 9.11 Teratogenic plants for horses.

Table 9.12 Plants causing acute death.

Table 9.13 Common toxic milkweeds.

List of Illustrations

Chapter 03

Figure 3.1 Algorithm for calculation of parenteral nutrition in a horse weighing 500 kg.

Chapter 04

Figure 4.1 Serum CK activity 4 h after 30 min of exercise for 5 days a week in 5 RER susceptible Thoroughbreds when fed four different diets. ½ HS represents 2.5 kg of high starch sweet feed (40% DE from NSC, 3% DE from fat) fed with grass hay for a total daily DE of 21 MCal/day. HS represents 5 kg of high starch sweet feed fed with grass hay for a total daily DE of 29 MCal/day. HS + BC represents 5 kg of high starch sweet feed with 4% sodium bicarbonate added. FAT represents a low starch, high fat feed (Re•Leve®, Kentucky Equine Research, Versailles, KY, 10% DE from NSC and 20% DE from fat) fed with grass hay for a total of 29 MCal/day. Note that horses fed a low caloric diet (1/2 HS) or a high caloric fat diet (FAT) have lower post exercise serum CK activity than horses fed a high calorie, high starch diet with or without sodium bicarbonate added.

Figure 4.2 Postprandial glucose and insulin response (mean ± SE) for four fit RER horses after consuming a high NSC meal of 4 kg of sweet feed (HS: 40% DE from NSC, 3% DE from fat) or 4.0 kg of a low starch, high fat commercial concentrate (FAT: Re•Leve®, Kentucky Equine Research, Versailles, KY; 10% DE from NSC and 20% DE from fat). Note the lower glycemic and insulinemic response of RER horses fed the low starch, high fat feed.

Figure 4.3 Postprandial glucose and insulin concentrations (mean ± SE) for seven type 1 PSSM‐affected horses fed hay with either a high (17% NSC), medium (11% NSC), or low (4.4% NSC) NSC content. Glycemic and insulinemic responses for PSSM horses were significantly higher when fed high NSC hay than medium or low NSC hay.

Figure 4.4 Three isocaloric diets were formulated to provide a total digestible energy of 24 MCal/day, either in the form of hay supplemented with grain (HS), corn oil with hay cubes (FAT‐CO: 1.5 ml/kg/day), or 2.5 kg of a low starch, high fat concentrate (FAT‐RE: Re•Leve®, Kentucky Equine Research, Versailles, KY). The oil provided 30% of digestible energy (DE) from fat whereas Re•Leve® provided 15% of DE from fat. Type 1 PSSM horses (n = 8) were fed each diet for 3 weeks and exercised for up to 20 min at a walk and trot for 5 days per week. Note that 15% DE from fat significantly reduced serum CK activity in PSSM horses that were exercised and that it was not necessary to provide over 25% of DE from fat to have a significantly lower serum CK activity compared to the grain diet.

Chapter 05

Figure 5.1 A pony with generalized obesity, body condition score of 9, and the Equine Metabolic Syndrome phenotype.

Figure 5.2 A pony exhibiting enlargement of adipose tissue within the neck region (“cresty neck”). Cresty neck score 4 (crest grossly enlarged and thickened and can no longer be cupped in one hand or easily bent from side to side. The crest may have wrinkles perpendicular to topline).

Figure 5.3 Factors suggested in pathogenesis of laminitis in equids with equine metabolic syndrome.

Figure 5.4 The amount of hay should be measured at each feeding using a scale for the most precise allotment and for successful weight loss.

Figure 5.5 Prevention of obesity or overweight body condition normalizes insulin sensitivity and represents the most effective protection from EMS associated laminitis.

Chapter 06

Figure 6.1 A horse eating from a haynet with its nostrils buried into the hay. This illustrates the close contact that a horse has with its forage and how respirable dust from forage enters the breathing zone.

Figure 6.2 A commercially available hay steamer.

Chapter 08

Figure 8.1 Chemical structures of aflatoxins.

Figure 8.2 Chemical structure of fumonisin B



Figure 8.3 Fumonisin induced ELEM in horse brain.

Figure 8.4 Activation of slaframine to ketoimine in liver.

Figure 8.5 Slaframine poisoned horse.

Figure 8.6 Chemical structure of Deoxynivalenol (DON).

Figure 8.7 Chemical structure of T‐2 toxin.

Figure 8.8 Chemical structure of zearalenone.

Chapter 09

Figure 9.1 Foxtail barley (

Hordeum jubatum

) seed heads.

Figure 9.2 Fruits of horse chestnut or buckeye (



Figure 9.3 Field bindweed (

Convolvulus arvensis


Figure 9.4 Gambel’s oak (

Quercus gambelii

) showing typical leaves and acorn.

Figure 9.5 Mountain laurel (

Kalmia latifolia


Figure 9.6 Azaleas (

Rhododendron catawbiense


Figure 9.7 Fetterbush (



Figure 9.8 Mountain pieris (



Figure 9.9 Maleberry (



Figure 9.10 Pokeweed berries (

Phytolacca americana


Figure 9.11 Buttercup (



Figure 9.12 Castor oil plant (

Ricinus communis

) leaves and spiny fruit capsules.

Figure 9.13 Rosary peas (

Abrus precatorius

) with characteristic black patch on the scarlet seeds.

Figure 9.14 Black locust (

Robinia pseudoacacia

) flowers, compound leaves, and thorns on branches.

Figure 9.15 Jimson weed or thorn apple (

Datura stramonium

) showing characteristic trumpet‐shaped flowers and spiny fruit (thorn apple).

Figure 9.16 Jimson weed seeds (

Datura stramonium


Figure 9.17 Kentucky coffee tree (

Gymnocladus dioica


Figure 9.18 Photosensitization in a horse showing dermatitis affecting the nonpigmented skin only.

Figure 9.19 St. John’s wort or Klamath weed (

Hypericum perforatum


Figure 9.20 Buckwheat (

Fagopyrum esculentum

) showing the typical heart‐shaped leaves and white flowers.

Figure 9.21


flower showing the characteristic single layer of bracts surrounding the petals.

Figure 9.22 Tansy ragwort or stinking willie (

Senecio jacobaea


Figure 9.23 First‐year growth or rosette stage of hound’s tongue (

Cynoglossum officinale

). Inset shows flowers and fruits.

Figure 9.24 Fiddleneck or tarweed (

Amsinckia intermedia

) showing its characteristic fiddleneck shape of the inflorescence.

Figure 9.25 Rattlebox or rattlepod (

Crotalaria spectabilis

) showing the pea‐like flowers and seed pods.

Figure 9.26 Creeping indigo (

Indigofera spicata

) young flowering plant showing prostrate form.

Figure 9.27 Lantana (

Lantana camara

) showing the flowers and ripe fruits.

Figure 9.28 Sand sage (

Artemisia filifolia


Figure 9.29 Fringed sage (

Artemisia frigida


Figure 9.30 Locoweed (

Astragalus molissimus

) showing the typical flowers, leaves, and pea‐like seed pods.

Figure 9.31 White locoweed (

Oxytropis sericea


Figure 9.32 Purple locoweed (

Oxytropis lambertii


Figure 9.33 Yellow star thistle (

Centaurea solstitialis

) showing the long spiny bracts surrounding the flowers.

Figure 9.34 Russian knapweed (

Acroptilon repens

) with spineless papery bracts surrounding the purple thistle‐like flowers.

Figure 9.35 Horsetail, marestail, horserush, or snake grass (

Equisetum hymale

) with spore‐producing heads.

Figure 9.36 White snakeroot or richweed (

Eupatorium rugosum


Figure 9.37 Bracken fern (

Pteridium aquilinum


Figure 9.38 Johnson grass (

Sorghum halepense


Figure 9.39 Sudan grass hybrid (

Sorghum sudanense


Figure 9.40 Black walnut (

Juglans nigra

) leaves and nuts.

Figure 9.41 Walnut wood shavings.

Figure 9.42 Hoary Alyssum (

Berteroa incana


Figure 9.43 Sicklepod cassia (

Cassia obtusifolia


Figure 9.44 Day‐blooming jessamine (

Cestrum diurnum


Figure 9.45 Night‐blooming jessamine (

Cestrum nocturnum


Figure 9.46 Flatweed or false dandelion (

Hypochoeris radicata


Figure 9.47 Approximate geographic distribution of selenium‐rich soils in the continental United States.

Figure 9.48 Two‐grooved milkvetch (

Astragalus bisulcatus

) showing the two characteristic grooves on the seed pods.

Figure 9.49 Woody aster (

Xyorrhiza glabriuscula


Figure 9.50 Prince’s plume (

Stanleya pinnata


Figure 9.51 White prairie aster (

Aster falcatus


Figure 9.52 Snakeweed, broomweed, matchweed, or turpentine weed (

Gutierrezia sarothrae


Figure 9.53 Gumweed or resinweed (


spp.) showing the whitish gummy resin in the tops of the flower buds.

Figure 9.54 Saltbush (


spp.) showing four‐winged fruits.

Figure 9.55 Indian paintbrush (



Figure 9.56 Beard tongue (



Figure 9.57 Horse with “bob‐tail” due to hair loss from chronic excess selenium consumption.

Figure 9.58 Circular horizontal cracks in the hoof wall of all feet characteristic of chronic excess selenium consumption.

Figure 9.59 Red maple leaves (

Acer rubrum


Figure 9.60 Yellow sweet clover (

Melilotus officinalis


Figure 9.61 Skunk cabbage or western false hellebore (

Veratrum californicum


Figure 9.62 Serviceberry or Saskatoon berry (

Amelanchier alnifolia

) flowers.

Figure 9.63 Wild blue flax (



Figure 9.64 Western chokecherry (

Prunus virginiana

) flowers.

Figure 9.65 Elderberry (


spp.) flowers and fruits.

Figure 9.66 Arrow or goose grass (



Figure 9.67 Milkweed flowers (

Asclepias speciosa

) showing the characteristic horn‐like modified petal.

Figure 9.68 Milkweed pods and seeds (

Asclepias subverticillata


Figure 9.69 Narrow leafed or whorled milkweed (

Asclepias subverticillata


Figure 9.70 Foxglove (

Digitalis purpurea


Figure 9.71 Oleander (

Nerium oleander

) flower with seed pod.

Figure 9.72 Yellow oleander (

Thevetia peruviana

) flower and fruit.

Figure 9.73 Pheasant’s eye (

Adonis aestivalis


Figure 9.74 Larkspur or poison weed (


spp.) showing the characteristic spur of the flower.

Figure 9.75 Monkshood or aconite (

Aconitum columbianum


Figure 9.76 Poison, European, or spotted hemlock (

Conium maculatum

) showing the carrot‐like leaves and spots on the stems.

Figure 9.77 Water hemlock (

Cicuta douglasii

) tuberous roots and hollow partitioned stems.

Figure 9.78 Water hemlock (

Cicuta maculatum

) umbel inflorescence and leaves with serrated edges.

Figure 9.79 Yew (


spp.) showing the fruits that resemble a pitted olive.

Figure 9.80 Death camas (

Zigadenus paniculatus




Table of Contents

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Nutritional Management of Equine Diseases and Special Cases


Bryan M. Waldridge

Park Equine Hospital at Woodford,Versailles, KY, USA










This edition first published 2017 © 2017 by John Wiley & Sons, Inc.

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This book is dedicated to my wife, Sonja; my parents; and family who never complain and always understand when I have to go see a sick horse.


Iveta Becvarova, DVM, MS, DACVNHill’s Pet Nutrition,Prague, Czech RepublicRamesh C. Gupta, DVM, MVSc, PhD, DABT, FACT, FACN, FATSToxicology Department,Breathitt Veterinary Center,Murray State University,Hopkinsville, KY, USAPeter Huntington, BVSc (Hons), MANZCVSKentucky Equine Research,Mulgrave, Victoria, AustraliaAnthony P. Knight, BVSc, MS, DACVIMProfessor Emeritus,Colorado State University,Fort Collins, CO, USAAmelia Munsterman, DVM, MS, DACVS, DACVECCDepartment of Surgical Sciences,University of Wisconsin,Madison WI, USANicole Passler, DVM, MSEquine Source Plasma Project,College of Veterinary Medicine,Auburn University,Auburn, AL, USADG Pugh, DVM, MS, MAg, DACT, DACVN, DACVMAlabama Veterinary Diagnostic Laboratory System,Auburn, AL, USAStephanie J. Valberg, DVM, PhD, Diplomate ACVIM, ACVSMRMary Anne McPhail Equine Performance Center,Michigan State University,East Lansing, MI, USABryan M. Waldridge, DVM, MS, DABVP, DACVIMPark Equine Hospital at Woodford,Versailles, KY, USASara Ziska, DVM, PhDCollege of Veterinary Medicine,Auburn University,Auburn, AL, USA


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


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

1Miniature 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.5,6Caregivers 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


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.

2Draft 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.1Caregivers 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.


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


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


4. Stromberg B. A review of the salient features of osteochondrosis in the horse.

Equine Vet J


5. Riley CB, Scott WM, Caron JP, et al. Osteochondritis dessicans and subchondral cystic lesions in draft horses: a retrospective study.

Can Vet J


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


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


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


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


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


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


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


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.

3Gastrointestinal 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

Level of Evidence

Veterinary Literature Classification

Grade of Recommendation


Systematic review of randomized controlled trials



Individual randomized controlled trial (clinical patients or disease models in the horse)



Retrospective, non‐randomized cohort study or case control study



Case series



Research model in the horse or a similar species



In vitro

testing or expert opinion based on physiologic justification


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 substrate on the flora of the equine gastrointestinal tract.4,25While carbohydrates are much more abundant in grains, there is also a significant difference in the type of carbohydrate present versus forages. In general, carbohydrates can be grouped into two groups: rapidly hydrolyzable carbohydrates (starches, hexoses, disaccharides, and some oligosaccharides), which are primarily degraded in the small intestine and fermentable carbohydrates degraded by bacterial populations to short chain fatty acids (acetate, propionate, and butyrate) in the large intestine and cecum.7Grains contain larger quantities of rapidly hydrolysable carbohydrates.

Hydrolysable carbohydrates in the small intestine are normally degraded by pancreatic α‐amylase to oligosaccharides, which are further digested by brush‐border enzymes to glucose for absorption.26–28While glucose transport by enterocytes can be improved with adaptation to high starch feeds, this adaptation relies on the presence of monosaccharides produced by amylase to stimulate this adaptive process.29However, the overall activity of amylase in the horse is low compared to other species30and highly variable between horses.27,29The lack of sufficient amylase has been proposed as the rate limiting step of carbohydrate digestion in the horse.31

Research supports this conclusion by documenting starch intake exceeding 0.4% of body weight in one meal can allow hydrolyzable carbohydrates to pass into the hindgut of the horse.32In the cecum and colon, carbohydrates are fermented to lactic acid by saccharolytic bacteria, reducing colonic pH and therefore the survival of bacteria required for fiber fermentation (Clostridiaceae, Spirochaetaceae, and Fibrobacter spp.).33–35Changes in the microbial environment can alter fermentable carbohydrate digestion,36reduce the production of volatile fatty acids,34,35,37,38dehydrate the ingesta,39,40increase transit times,40and result in gas distention of the large intestine,28predisposing the horse to colic.

In clinical patients, the association of concentrates with colic was variable, having a significant effect in one study (OR = 2.6, 95% CI 0.9–7.2, P = 0.064),9but no effect in a second study.12While feeding more than 2.7 kg of oats per day increased the risk of colic in the study by Hudson et al. (OR = 5.9, 95% CI 1.6–22, P = 0.009),9previous work noted that any whole grain (oats, barley, etc.) was associated with an increased risk of colic if fed in amounts greater than 2.5 kg per meal (OR = 4.8, 95% CI 1.4–16.6, P = 0.01).17Another study found that only whole corn was a risk factor.13Pelleting grain may increase the risk of colic, but studies were not as clear.15,17,41However, changes in the concentrate fed was shown statistically to increase the risk of colic (OR = 3.6, 95% CI 1.6–5.4, P < 0.001).17Current recommendations regarding concentrates advise feeding less than 0.2% of body weight per meal to prevent adverse effects on digestion.42.

3.1.4 General Practices to Prevent Colic

Prevention of colic should be centered on providing a consistent diet, since dietary change is the factor most commonly associated with colic in the horse.6,9,14,17,43,44. Variations in the type of feed (forages, concentrates, as well as pasture turnout), quantity provided, and frequency that feed is offered all may play a role in altering the pH of ingesta and the bacterial populations that are relied upon for effective digestion.6,9,17Therefore, the horse should be provided at least 60% of its diet from forage sources, at a minimum of 1–1.5% of body weight.45Concentrates should be kept to a minimum and provided in three or more feedings per day to reduce acid production in the stomach and colon.42,45If additional energy sources are required, vegetable oil, strained sugar beet pulp, or soy hulls are good alternatives to starch‐based feeds. Finally, any change in the horse’s diet should occur over 7–10 days, to allow adjustment of the microbial population, small intestinal enzymes, and glucose transporters required for carbohydrate digestion.46