EQUINE APPLIED and CLINICAL NUTRITION Content Strategist: Robert Edwards Content Development Specialist: Nicola Lally/Veronika Watkins Project Manager: Sruthi Viswam Designer/Design Direction: Miles Hitchen Illustration Manager: Jennifer Rose Illustrator: Richard Tibbitts Equine Applied Clinical and Nutrition health, welfare and performance Edited by RAYMOND J. GEOR BVSc, MVSc, PhD, Diplomate ACVIM (Large Animal), Diplomate ACVSMR, Diplomate ACVN (Honorary) PATRICIA A. HARRIS MA, PhD, Diplomate ECVCN, VetMB, MRCVS MANFRED COENEN Prof., Dr. med. vet., Diplomate ECVCN Foreword by HRH Princess Anne Edinburgh London New York Oxford Philadelphia St Louis Sydney Toronto 2013 © 2013 Elsevier Ltd. All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Details on how to seek permission, further information about the Publisher’s permissions policies and our arrangements with organizations such as the Copyright Clearance Center and the Copyright Licensing Agency, can be found at our website: www.elsevier.com/permissions. This book and the individual contributions contained in it are protected under copyright by the Publisher (other than as may be noted herein). ISBN 978-0-7020-3422-0 British Library Cataloguing in Publication Data A catalogue record for this book is available from the British Library Library of Congress Cataloging in Publication Data A catalog record for this book is available from the Library of Congress Notices Knowledge and best practice in this field are constantly changing. As new research and experience broaden our understanding, changes in research methods, professional practices, or medical treatment may become necessary. Practitioners and researchers must always rely on their own experience and knowledge in evaluating and using any information, methods, compounds, or experiments described herein. In using such information or methods they should be mindful of their own safety and the safety of others, including parties for whom they have a professional responsibility. With respect to any drug or pharmaceutical products identified, readers are advised to check the most current information provided (i) on procedures featured or (ii) by the manufacturer of each product to be administered, to verify the recommended dose or formula, the method and duration of administration, and contraindications. It is the responsibility of practitioners, relying on their own experience and knowledge of their patients, to make diagnoses, to determine dosages and the best treatment for each individual patient, and to take all appropriate safety precautions. To the fullest extent of the law, neither the Publisher nor the authors, contributors, or editors, assume any liability for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions, or ideas contained in the material herein. Working together to grow libraries in developing countries www.elsevier.com | www.bookaid.org | www.sabre.org The publisher’s policy is to use paper manufactured from sustainable forests Printed in China Foreword BUCKINGHAM PALACE These days people are constantly being reminded of the link optimal performance but in a way that avoids health risks, between diet and conditions such as food-related allergy, (particularly disturbances to digestive tract function), while obesity, and heart disease. We are becoming increasingly still promoting positive behaviours. aware that there is also a strong association between the With the increasing knowledge of the importance of diet feeding and the health status of our companion animals. As in the health, welfare, performance and behaviour of the those responsible for their diet, it is our responsibility to horse comes the responsibility of sharing this information ensure they are appropriately fed. so that all horses can benefit. I am therefore very pleased In the case of the horse, we need to be sure that any to provide the foreword for this book Equine Clinical and feeding practice is tailored towards a species which evolved Applied Nutrition. It has been written by experienced clini- for prolonged foraging on predominantly poor-quality cians, researchers and nutritionists from all over the world. forage in a seasonally variable environment. Whilst for Their united aim is to provide all those interested in the horses and ponies used for pleasure activities, the mimick- scientific basis and practical application of equine nutrition ing of this circumstance is relatively easy to achieve, we with the most current information available on the subject. face a particular challenge with respect to the feeding of I applaud their efforts to bring scientifically confirmed high-performance competition horses. Here the challenge knowledge into daily use, for the benefit of the health as well appears to be slightly different, in that we need to ensure as the welfare of horses and related species. I am happy to sufficient energy and nutrients are provided to support be associated with their efforts. Preface Knowledge pertaining to the nutrition and feeding manage- were truly comprehensive. We hope that we have succeeded ment of horses and other members of the genus Equus in meeting the requirement for a comprehensive reference (ponies, donkeys, and mules) continues to expand in concert text that integrates the basic and applied aspects of equine with the growth in the use of these animals for a wide range nutrition. That said, we recognize that this book is far from of purposes, particularly competition and other recreational perfect and will not meet the needs of all potential readers. riding activities. Appropriate nutrition and feeding manage- The book is divided into five major sections. In the first ment is both a science and an art. The science encompasses section on Nutritional Foundations, detailed reviews on diges- understanding of (1) feeding behavior, (2) the physiology of tive physiology, metabolic and endocrine physiology as well nutrient digestion and utilization (especially digestive phys- as factors affecting feed intake are followed by chapters that iology), (3) the nutrient requirements of various physiologi- describe the function and requirements of each of the major cal classes, and (4) the composition, digestibility and safety nutrients (e.g., amino acids, minerals, etc.). The second of feedstuffs available for use in horse rations. Other impor- section, Nutrition for Life-stage, Type or Function, has chapters tant areas are the role that nutrition in its broadest sense (i.e., on feeding broodmares and stallions, growing horses, ath- not just overt deficiency or excess of one or more nutrients) letic horses, and the older horse. In addition, this section plays in maintenance of health, welfare and behavior, as contains a chapter devoted to the nutritional management well as the effects of disease on nutrient requirements and of donkeys and mules. The third and fourth sections on the impact of nutrition on recovery from illness – although Applied Nutrition cover a number of topics relevant to the our understanding in these areas is lacking in comparison development and assessment of feeding programs, includ- to the body of knowledge that underpins the feeding of ing chapters on feedstuffs, pastures and pasture manage- healthy horses. The art of equine nutrition is especially rel- ment, the assessment of feed quality and hygiene, ration evant to feeding management, recognizing that in many evaluation, and the assessment of nutritional status, among situations a “one size fits all” approach does not work and others. The last main section, Clinical Nutrition, has 15 chap- tailoring to the individual animal is required to ensure ters that describe current knowledge of nutritionally-related attainment of nutritional goals. The art component also conditions as well as recommendations for dietary manage- takes in the human side of the equation because ultimately ment of horses with these conditions. Although it is recog- it is the owner, trainer, barn manager, etc. who has respon- nized that there are many gaps in our knowledge of equine sibility for feeding management. We firmly believe that criti- clinical nutrition, we are confident that this section provides cal application of evidence-based information is essential for a great deal of useful information on dietary management sound feeding management of equids. As such, a major for the prevention or treatment of disease. Finally, there is impetus of this book was to accurately summarize and syn- an appendix that summarizes nutritional requirements and thesize current scientifically-based information in the recommendations for different physiological states accord- context of feeding management. ing to North American (NRC 2007) and German (GEH 2013) Although contemporary reference materials on equine authorities, as well as more adjusted recommendations of nutritional requirements are available (e.g., German Equine the editors. Also included in the appendix are examples Feeding Standards, Institut National de la Recherches of typical “real-world” rations, with graphical depiction of Agronomique INRA, National Research Council [NRC] how the nutrients provided match with the requirement Nutrient Requirements of Horses, 6th revised edition), we rec- data as well as a brief narrative on selected aspects of each ognized the need for a comprehensive source of information ration. on equine nutrition and feeding management for nutrition- Our sincere gratitude is extended to the authors of chap- ists, veterinarians, undergraduate and graduate students in ters in this book for their willingness to contribute knowl- these fields, as well as well-informed lay horsemen and edge in their area of expertise and in recognition of the women. Our primary goal was to develop a book that spans considerable time and effort required to prepare a compre- from the basic foundations of equine nutrition (digestive hensive review. We also thank Robert Edwards, Nicola and metabolic physiology; nutrient functions and require- Lally, and Veronika Watkins at Elsevier and, most impor- ments), to nutritional management by life stage or function, tantly, our families for their assistance, support and toler- plus the assessment of feedstuffs and feeding programs, and ance during the development of this book. finally to clinical nutrition. A second goal was for the book to be international in scope, drawing upon the knowledge Raymond J. Geor and expertise of nutritionists and veterinarians from around Patricia A. Harris the world and ensuring that reviews of scientific literature Manfred Coenen Contributors Caroline McGregor Argo BSc, BVSc, PhD, Diplomate ECAR, Patricia A. Harris MA, PhD, Diplomate ECVCN, VetMB, MRCVS MRCVS Senior Lecturer, Veterinary School and Dept of Obesity and Director of Science for MARS Horsecare; Head of Equine Endocrinology, University of Liverpool, Neston, UK Studies Group WALTHAM Centre for Pet Nutrition, Waltham-on-the-Wolds, UK; Adjunct Professor of Equine Ruth Bishop BSc(Hons), Dip Eq S Studies at Virginia Polytechnic Institute and State Ruth Bishop Consulting Limited, Shrewsbury, UK University, USA; Visiting Professor, University of Liver- pool, UK; Honorary Professor in Equine Nutrition, Uni- Faith A. Burden BSc, PhD versity of Nottingham, School of Veterinary Medicine Head of Research, The Donkey Sanctuary, Sidmouth, UK and Science UK; Adjunct Professor at University of Queensland, Australia Elizabeth A. Carr DVM, PhD, Diplomate ACVIM, Diplo- mate ACVECC Rhonda M. Hoffman PhD, PAS, Diplomate ACAN Associate Professor, Department of Large Animal Clinical Professor, Horse Science, Middle Tennessee State Univer- Sciences, College of Veterinary Medicine, Michigan State sity, School of Agribusiness & Agriscience, Middle Ten- University, East Lansing, MI, USA nessee State University, Murfreesboro, TN, USA Rebecca A. Carter PhD Becky Hothersall MSc, PhD Health & Nutrition Scientist, The Nutro Company, Franklin, Research Assistant, School of Veterinary Science, University TN, USA of Bristol, Langford, UK Manfred Coenen Prof. Dr. med vet., Diplomate ECVCN Véronique Julliand DVM, MS, PhD Professor of Animal Nutrition and Dietetics, Institute of Professor of Animal Science, AgroSup Dijon; Head of the Animal Nutrition, Nutrition Diseases and Dietetics, Research Unit “URANIE” equine nutrition, AgroSup Faculty of Veterinary Medicine, University Leipzig, Dijon; Head of the Master of Equine Science and Busi- Leipzig, Germany ness, AgroSup Dijon, France Derek Cuddeford BSc, MSc, PhD Josef Kamphues Prof. Dr. med. vet., MAgrSc, M.V. Sc, Honorary Fellow, Royal (Dick) School of Veterinary Studies, Diplomate ECVCN/ECPHM University of Edinburgh, Roslin, UK INSTITUTE of Animal Nutrition, University of Veterinary Medicine, Hannover, Foundation; Hanover, Germany Nadia F. Cymbaluk BSc.Ag, MSc, DVM Director, Veterinary Research/Field Compliance, Pfizer John A. Keen BSc, Bvet.Med, MSc, CertEM(Int Med), Diplo- Canada Inc, Manitoba, Canada mate ECEIM, MRCVS Senior Lecturer in Equine Medicine, Dick Vet Equine Hos- Alexandra H.A. Dugdale MA, VetMB, DVA, Diplomate pital, Royal (Dick) School of Veterinary Studies and ECVAA, PhD, PGCert(LTHE), FHEA, MRCVS Roslin Institute, University of Edinburgh, Roslin, UK Senior Lecturer in Veterinary Anaesthesia, School of Veteri- nary Science, University of Liverpool, Neston, UK Ellen Kienzle Dr. med. vet., Diplomate ECVCN Professor of Animal Nutrition and Dietetics, Ludwig-Maxi- Catherine E. Dunnett BSc, PhD, R.Nutr milians-University Munich, Veterinary Science Depart- Independent Equine Nutrition, Mildenhall, UK ment, Oberschleißheim, Germany Andy E. Durham BSc, BVSc,CertEP, DEIM, Diplomate Laurie M. Lawrence PhD ECEIM, MRCVS, RCVS and European Specialist in Provost’s Distinguished Service Professor, Department of Equine Internal Medicine Liphook Equine Hospital, Animal and Food Sciences, University of Kentucky, Lex- Liphook, UK ington KY, USA Andrea D. Ellis Diplomate EuHum, BSc (Hons), PGCert Ed, Jan Erik Lindberg PhD PhD Professor, Swedish University of Agricultural Sciences, Director, UNEQUI Ltd. Research Education Innovation, Department of Animal Nutrition and Management, Southwell, UK Uppsala, Sweden Raymond J. Geor BVSc, MVSc, PhD, Diplomate ACVIM, Annette C. Longland BSc, PhD, DIC Diplomate ACVSMR, Diplomate ACVN (hon) Head of Research, Equine and Livestock Nutrition Services, Professor and Chairperson, Department of Large Animal Tregaron, Ceredigion, UK Clinical Sciences, College of Veterinary Medicine, Michi- gan State University, East Lansing, MI, USA Nanna Luthersson DVM Equine Practitioner, Hestedoktoren, Denmark Contributors Tim Mair BVSc, PhD, DEIM, DESTS, Diplomate ECEIM, Harold C. Schott II. DVM, PhD, Diplomate ACVIM Assoc ECVDI, MRCVS Professor and Equine Division Head, College of Veterinary Partner, Bell Equine Veterinary Clinic, Mereworth, Maid- Medicine, Michigan State University, Large Animal Clin- stone, UK ical Sciences, East Lansing, MI, USA Bruce McGorum BSc, BVM&S, PhD, CEIM, Diplomate David G. Smith PhD, MSc, BSc (Hons), Cert. Ed (FE), FHEA ACVIM, MRCVS Lecturer, School of Nursing, Midwifery and Social Care, Head of Equine Section, Dick Vet Equine Hospital, Royal Faculty of Health, Life and Social Science, Edinburgh (Dick) School of Veterinary Studies & Roslin Institute, Napier University, Edinburgh, UK Midlothian, UK William B. Staniar, MS, PhD C. Wayne McIlwraith BVSc, PhD, DSc, FRCVS, Diplomate Assistant Professor, The Pennsylvania State University, ACVS, Diplomate ECVS, Diplomate ACVSMR Department of Animal Science, The Pennsylvania State University Distinguished Professor, Barbara Cox Anthony University, University Park, PA, USA University Chair in Orthopaedics, Director Orthopaedic Research Center, Colorado State University, Ft. Collins, Sarah J. Stoneham BVSc, CertESM, MRCVS CO, USA Veterinary Surgeon, Lone Oak Stud, Hilborough, UK Alfred M. Merritt AB, DVM, MS, Diplomate ACVIM (hon) Kristine L. Urschel PhD Professor Emeritus, Department of Large Animal Clinical Assistant Professor, Department of Animal and Food Sciences, College of Veterinary Medicine, University of Sciences, University of Kentucky, Lexington KY, USA Florida, Gainesville FL, USA Ingrid Vervuert Dr. med. vet. Jenifer A. Nadeau MS, PhD Docent of Animal Nutrition and Dietetics, Faculty of Veteri- Associate Professor of Animal Science, Department of nary Medicine, University of Leipzig, Institute of Animal Animal Science, University of Connecticut, Connecticut, Nutrition, Nutrition Diseases and Dietetics, Leipzig, USA Germany Christine J. Nicol, MA, DPhil Kelly R. Vineyard MS, PhD, PAS Professor of Animal Welfare, School of Veterinary Science, Research Equine Nutritionist, Land O’Lakes Purina Feed, University of Bristol, Langford, UK Gray Summit, MO, USA Brian D. Nielsen PhD, PAS, Diplomate ACAN Lori K. Warren MS, PhD, PAS Professor, Equine Exercise Physiology, Department of Associate Professor, Department of Animal Sciences, Insti- Animal Science, Michigan State University, East Lansing, tute of Food and Agricultural Sciences, University of MI, USA Florida. Gainesville, FL, USA Robert S. Pirie BVM&S, PhD, Cert EP, Cert EM (Int Med), Carey A. Williams PhD Diplomate ECEIM, MRCVS Associate Professor, Department of Animal Science, Rutgers, Senior Lecturer in Equine Medicine, Dick Vet Equine Hos- the State University of New Jersey, New Brunswick, NJ, pital, Royal (Dick) School of Veterinary Studies & Roslin USA Institute, University of Edinburgh, Roslin, UK Annette Zeyner DAI, DNM, Dr. Sarah L. Ralston VMD, PhD, Diplomate ACVN Professor of Animal Nutrition, Professor of Animal Nutri- Associate Professor, Department of Animal Sciences, School tion, Department of Animal Nutrition, Halle, Germany of Environmental and Biological Sciences, Rutgers, the State University of New Jersey, New Brunswick, NJ, USA José Luis L. Rivero DVM, PhD Full Professor of Veterinary Anatomy, Departamento de Anatomía y Anatomía Patológica Comparadas, Facultad de Veterinaria, University of Cordoba, Cordoba, Spain 1  SECTION A Nutritional Foundations Gastrointestinal physiology Alfred M. Merritt, Véronique Julliand Enteric nervous system 3 Small intestine 13 Mouth 4 Cecum and colons 21 Esophagus 6 References 27 Stomach 6 This chapter is directed primarily at the normal function of Concerning GI motility, virtually all of the parasympa- the gastrointestinal tract (GIT) of the prototype 500 kg adult thetic (vagal) input into the gastro intestinal tract horse. Readers interested in the GIT of foals are advised to (GIT) is processed through the ENS, with acetylcholine consult the various available texts on equine neonatology. (ACh) playing a predominant role in intra-neuronal com- The authors have focused on information that is specifically munication (Berthoud 2006, Furness 2006, Goyal & Hirano available for the horse and data applicable to other species 1996). Parasympathetic input is stimulatory whereas sym- is only presented to make a point of comparison or contrast. pathetic (adrenergic) input, with respect to controlling This chapter therefore differs in some aspects from previous motility, is inhibitory. The majority of the sympathetic input reviews on this topic which inferred that certain mecha- is directly from the sympathetic ganglia, and mediated at nisms have been shown to be present in horses when, in fact, the neuromuscular junction by norepinephrine (NE). There they are not. This chapter considers the different anatomical is a smaller component of the sympathetic input to the ENS regions of the GIT in sequence (i.e. mouth, esophagus, that down-regulates ENS-mediated ACh stimulation via stomach, small intestine, and large intestine). It must be intrinsic α′ -adrenergic pathways (Scheibner et al 2002). 2 remembered from the outset that the horse is a classic “hind Synaptic transmission, paracrine signaling and hormonal gut fermenter” with a particularly large cecum and complex signaling are forms of chemical information transfer within large colon structure in which digestion of plant fiber occurs the ENS. Motor neurons that stimulate muscle contraction (Fig. 1.1). may express, in addition to ACh, either substance P (SP) or Each section starts with a short description of the respec- the peptide motilin at the neuromuscular junction. Those tive anatomy and then covers secretion, digestion, motility neurons from the ENS that inhibit contraction can express a and absorption, in that order, as appropriate. The initial variety of neurotransmitters including nitric oxide (NO), introductory section presents a brief overview of the enteric vasoactive intestinal peptide (VIP) or adenosine triphos- nervous system (ENS) which is essential to the functioning phate (ATP). These three neurotransmitters, along with SP, of the entire GIT and, hopefully, emphasizes that, despite an comprise the non-adrenergic, non-cholinergic (NANC) com- arbitrary subdivision, to facilitate discussion, it is one inte- ponent of the ENS (Burnstock 2009, Goyal & Hirano 1996, grated system. Specific effects of diet have been included, Kunz & Furness 1999). Calcitonin gene-regulated peptide where appropriate, in the discussion of a particular process (CGRP) and numerous subtypes (e.g. 5-HT, 5-HT, 5-HT, 1 2 3 (e.g. gastric receptive relaxation; intragastric fermentation of etc.) of serotonin (5-HT) act within the ENS to either a diet high in soluble CHO vs one high in fiber) rather than up-regulate or down-regulate the activity (Berthoud 2006, being considered as a separate section. Goyal & Hirano 1996, Kunz & Furness 1999). Various recep- tors within the GIT mucosa that constantly monitor the wall tension and the physicochemical characteristics of the Enteric Nervous System digesta evoke either stimulatory or inhibitory responses via the ENS intrinsic afferent neurons; again, CGRP and 5-HT In mammals, all GIT function is constantly monitored and are important mediators in this process (Braun et al 2007, modified by the highly complex enteric nervous system (ENS), Cooke 1986, Holzer et al 2001, Lundgren 2004 Schemann & commonly referred to as the “brain of the gut”. It functions Mazzuoli 2010) (Fig. 1.2). both on an independent local level and in conjunction with In horses, all of the mentioned neurotransmitters have modulatory input from the central nervous system (CNS) been found, with varying degrees of expression depending via the vagus nerve (parasympathetic) and the sympathetic upon the segment of the GIT (Ceccarelli et al 1995, Chio- ganglia. Essentially, the ENS itself is made up of intrinsic chetti et al 2009, Domenghini et al 2004, Fink et al 2006, afferent neurons, ascending and descending interneurons Freytag et al 2008, Kotz & Van Aswegan 1990, Malone et al and motor neurons. At least twenty different subtypes of 1999, 2000, Merritt et al 1998, Pearson 1994; Rakestraw et al these motor neurons have been identified (Furness 2006, 1996, Rutkowski et al 1998, Sasaki & Yoshihara 1998, Sasaki 2008, Goyal & Hirano 1996). et al 2000, Sellers et al 1984b, Solinger & Sonea 2008). There 4 Section A Nutritional Foundations Cow Horse Dog Figure 1.1 GIT anatomy of: 1) the cow, representing herbivores that ferment ingested plant fiber within the 62.3% compartmentalized forestomach, 70.8% primarily within the rumen; 2) the horse, Rumen 8.5% representing herbivores that ferment plant fiber within the hindgut, primarily 23.3% within the cecum and colon, and; 3) the 18.5% dog, representing the carnivores. The percentage of total GIT volume that each of 30.2% these sections comprises in each of the species is indicated. Total GIT volume values 15.9% RVC are for a 500 kg cow or horse, and a 20 kg dog. The major divisions of the doubly folded ascending (great) colon of the horse, starting Cecum at the cecocolic orifice, are: right ventral colon 2.8% (RVC), left ventral colon (LVC), left dorsal LDC colon (LDC) and, right dorsal colon (RDC). Final desiccation of the ingesta to form the LVC RDC 1.3% fecal balls takes place in the descending (small) colon (SC).  7.9% 13.1% 45.4% SC Total volume 350 L Total volume 200 L Total volume 7 L cholecystokinin (CCK), somatostatin (SST), gastrin-releasing Central nervous system peptide (GRP), neuropeptide Y (NPY) and various opioids Central autonomic neural network (Berthoud 2006, Cooke 1986, Furness 2006, Wood & Galligan 2004). Finally, orexin A, a neurotransmitter found to be involved in appetite control in rats, has also recently been found within the ENS of the horse (Dall’Aglio et al 2009). Nodose Dorsal root ganglia ganglia Key Points Sympathetic • The ENS monitors and modifies all aspects of equine GIT ganglia function, with modulatory input from the CNS via the vagus nerve and the sympathetic ganglia. Parasympathetic Sympathetic • Numerous neurotransmitters are involved in ENS function nervous system nervous system that manifest their effects by neurocrine, paracrine and hormonal routes. Parasympathetic ganglia Mouth GI tract Enteric nervous system The horse has three main salivary glands, parotid, mandibu- myenteric plexus, submucous plexus lar (submandibular), and sublingual, named according to their anatomical location. The parotid is the largest and most Intermediate peripheral, the dorsal end being just in front of the ear and cells the ventral end being just behind the caudoventral margin of the ramus of the mandible. The mandibular gland lies Effector systems underneath the ventral portion of the parotid gland and muscle, secretory epithelium, endocrine cells, vasculature extends from the atlantal fossa to the hyoid bone. The sub- lingual gland, which is the smallest, is located just under the Figure 1.2 Schematic diagram of the general organization oral mucous membrane between the body of the tongue and of the enteric nervous system (ENS) and its connections mandible (Sisson & Grossman 1959). with central nervous system (CNS).  Adapted from Goyal & Hirano 1996. Secretion may be some equine species specificity with respect to pre- Equine saliva is >99% water (Alexander, 1966). In general it dominance of 5-HT subtype, which has important implica- contains relatively more calcium and chloride, and less tions concerning the development of drugs to control GIT bicarbonate and sodium, than that of ruminants and is thus function in the horse, but this still needs further clarification more similar in composition to that found in carnivora and (Delesalle et al 2008, Lippold et al 2004, Nieto et al 2000, omnivora (Alexander & Hickson 1970, Stick et al 1981). Prause et al 2009). Resting electrolyte concentrations in parotid saliva, which is In all species, secretory epithelia, endocrine cells and hypotonic to plasma, are listed in Table 1-1. Concentrations vasculature within the GIT are also under ENS control. Neu- of Na+, Cl− and HCO− increase in a linear relationship to the 3 rotransmitters involved, in addition to those mentioned rate of secretion. Adult horses may secrete up to 35–40 above, include numerous well-known peptides such as liters/day with a pH of 8.6–9.1, the majority originating