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Lipid Metabolism in Ruminant Animals PDF

451 Pages·1981·37.927 MB·English
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Other publications of interest Books CHRISTIE Lipid Analysis PATTON & JENSEN Biomedicai Aspects of Lactation Review Journal Progress in Lipid Research LIPID METABOLISM IN RUMINANT ANIMALS Editor WILLIAM W. CHRISTIE Department of Biochemistry The Hannah Research Institute Ayr, Scotland PERGAMON PRESS OXFORD NEW YORK TORONTO · SYDNEY PARIS FRANKFURT U.K. Pergamon Press Ltd., Headington Hill Hall, Oxford OX3 OBW, England U.S.A. Pergamon Press Inc., Maxwell House, Fairview Park, Elmsford, New York 10523, U.S.A. CANADA Pergamon Press Canada Ltd., Suite 104, 150 Consumers Road, Willowdale, Ontario M2 J1P9, Canada AUSTRALIA Pergamon Press (Aust.) Pty. Ltd., P.O. Box 544, Potts Point, N.S.W. 2011, Australia FRANCE Pergamon Press SARL, 24 rue des Ecoles, 75240 Paris, Cedex 05, France FEDERAL REPUBLIC Pergamon Press GmbH, 6242 Kronberg/Taunus, OF GERMANY Hammerweg 6, Federal Republic of Germany Copyright © 1981 Pergamon Press Ltd All Rights Reserved. No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means: electronic, electrostatic, magnetic tape, mechanical, photocopying, recording or otherwise, without permission in writing from the publishers First edition 1981 British Library Cataloguing in Publication Data Lipid metabolism in ruminant animals. 1. Ruminants—Physiology 2. Lipids—Metabolism I. Christie, William Walker 636.2Ό892397 QL737.IJ5 80-41823 ISBN 0-08-023789-4 Published as supplement No. 1 1981 to the review journal Progress in Lipid Research Printed in Great Britain by A. Wheat on & Co. Ltd., Exeter PREFACE RUMINANT animals, specifically cattle, sheep and goats, constitute one of the more impor­ tant elements of agriculture in most parts of the world and meat and milk products from these animals are of vital importance in human nutrition. Microbial fermentation in the rumen or forestomach anterior to the true stomach permits ruminants to efficiently digest and thrive on low-quality forages, often on land that is unsuitable for any agricul­ tural purpose other than grazing. These fermentation processes in the rumen also modify the nature of the metabolites, in particular the lipid and lipogenic components, made available to the host animal so that a number of aspects of lipid metabolism in ruminants are very different from those in monogastric animals. For example, short-chain fatty acids are synthesized in the rumen in large amounts and they, rather than glucose, are the principal substrates for lipogenesis in ruminant tissues. Also, dietary unsaturated fatty acids are hydrogenated in the rumen and branched- and odd-chain fatty acids are synthesized there with important consequences for the lipid composition and metabolism of the tissues of the host animal. The authors, members of the Departments of Biochemistry and Physiology of the Hannah Research Institute, believe that these unusual aspects of lipid metabolism in ruminant animals should be brought to the attention of a wider audience. In preparing this book, we have taken joint decisions on the material to be included in each chapter and have read and criticized each other's work in an attempt to ensure uniformity of treatment and as comprehensive a coverage of the topic as possible with the minimum of duplication. Although two of our number departed for Australasia after the work was commenced, we have continued to adhere to this policy. The chapters appeared separately as they were ready in Progress in Lipid Research but the earlier ones have been brought up to date (end of 1979) by the inclusion of additional material prior to publication here. W. W. CHRISTIE A. W. BELL C G. HARFOOT J. H. MOORE R. C. NOBLE R. G. VERNON V ANATOMY, PHYSIOLOGY AND MICROBIOLOGY OF THE RUMINANT DIGESTIVE TRACT C. G. HARFOOT Department of Biological Sciences,University of Waikato, Hamilton, New Zealand CONTENTS I. INTRODUCTION 1 11. FEATUROEFS R UMINANTASN D THEIR ALLIES 2 111. ANATOMYOF THE RUMINANFTO RESTOMACH 3 A. Rumen 3 B. Reticulum 4 C. Omasum 5 D. Abomasum 5 IV. PASSAGOEF FOODT HROUGHT HE RUMINANSTT OMACH 6 A. Ingestion and swallowing 6 B. Salivation 6 1. The salivary glands 6 2. Secretion 6 3. Composition of saliva 7 V. MIXINGO F DIGESTAIN THE RETICULC-RUMEN 7 VI. RUMINATION 8 VII. ENTRYO F DIGESTIANT O THE OMASUM AND ABOMASUM 8 VIII. MICROBIAFLE RMENTATINI OTHNE RETICULO-RUMEN 8 A. The rumen environment 8 B. The microbial population of the reticulo-rumen 10 C. The holotrich protozoa 10 D. The entodiniomorph protozoa 10 E. The bacteria 11 F. Metabolic activities of the rumen bacteria 12 G. Utilization of end-products of microbial metabolism by the ruminant 14 H. Unusable metabolic end-products 14 Ix. PROCESSES IN THE OMASUM AND ABOMASUM 15 A. Omasum 15 B. Abomasum 15 x. DIGESTIVPER OCESSES IN THE RUMINANTH IND-GUT 15 A. Small intestine 15 1. Secretion into the small intestine 15 2. Digestive processes in the small intestine 16 3. Absorption from the small intestine 16 B. Large intestine 17 1. Digestive processes in the large intestine and caecum 17 2. Absorption from the large intestine and caecum 18 XI. REFERENCES 18 I. INTRODUCTION Ruminants are distinguished from simple-stomached or monogastric animals by the development of a series of pouches anterior to their true gastric stomach. Of these pouches, the rumen is the largest and metabolically the most important. In the rumen, the chemical constituents of plant origin which are ingested by the ruminant undergo microbial fermentation to produce both microbial cells, which are subsequently utilized as sources of protein and other nutrients by the host animal, and the waste products of microbial metabolism, many of which can also be utilized by the animal for either energy or biosynthesis. As a result of this conversion of plant cellular constituents into microbial cells, the metabolism of the ruminant animal is different from that of the simple-stomached animal, and the tissue composition, particu- larly the lipid composition, of ruminants is distinctive. For example, it is in the rumen 1 2 C. G. Harfoot that the volatile fatty acids are synthesized that are subsequently utilized for lipogenesis in the tissues of the host animal. In the rumen, dietary unsaturated fatty acids are hydrogenated with important consequences for the lipid composition of the tissues of ruminants. In the sections of this article which follow, a generalized account is given of the anatomy and physiology of the ruminant digestive tract, together with a summary of microbial processes in the rumen. Because of the economic importance of the ruminant animal, the literature available is very extensive; references have been kept to a minimum and for those requiring more comprehensive reviews, attention is drawn to the review articles in the Handbook of Physiology,18,21,35A9>63 to Phillipson's review article in Dukes' Physiology of Farm Animals54 and to Hungate's book The Rumen and its Microbes.34 II. FEATURES OF RUMINANTS AND THEIR ALLIES From the viewpoint of digestive physiology, the distinguishing features of the ruminant animal are as follows: (a) There is extensive enlargement of the cardiac region of the stomach to form a series of compartments anterior to the region corresponding to the simple stomach of non-ruminants. These compartments are referred to as the rumen, reticulum, omasum and abomasum. The abomasum corresponds in both physiology and anatomy to the simple stomach of non-ruminants, (b) Within the rumen and reticu­ lum, there occurs extensive microbial fermentation of ingested food, (c) During the period following ingestion of food, there occurs régurgitation of some of the contents of the rumen. The regurgitated bolus of rumen contents is mixed with saliva in the mouth, chewed and re-swallowed. This process is referred to as rumination, or more popularly "chewing the cud", the "cud" being the regurgitated bolus of food. From the viewpoint of the zoologist, the ruminants (Ruminantia) form a Suborder of the even-toed Ungulates or "cloven-hoofed" animals (Order Artiodactyla) (Table 1). As zoological classification is essentially phylogenetic, that is, it is designed to demonstrate evolutionary relation­ ships between organisms as well as arranging the living species into morphologically similar groups, more emphasis is placed on skeletal structure than on the anatomy of soft parts which leave no fossil record. The principal distinguishing feature of the Artiodactyla is the skeleton of the foot, which consists of a number of well-developed TABLE 1. A Simplified Classification of the Artiodactyla3 Including Some Representative Living Species and Showing the Degree of Stomach Complexity0 Representative Living Suborder Family Species Stomach Anatomy, Rumination Suiformes Suidae Pigs, wart hogs, forest True stomach may have 1 or 2 hogs diverticula. No true rumen Tayassuidae Peccaries reticulum, omasum or abomasum Hippopotamidae Hippopotamus Suiformes do not ruminate. Tylopoda Camelidae Camels, llama, alpaca 3-chambered stomach present; vicuna corresponding to rumen reticulum and abomasum. Equivalent to omasum lacking. Ruminantia Tragulidae Chevrotains, mouse-deer Camelids and tragulids both ruminate. Ruminantia Cervidae True deer, moose (elk) All cervids, giraffids caribou (reindeer) antilocaprids and bovids have Giraffidae Giraffe, okapi true 4-chambered ruminant Antilocapridae Pronghorn stomach, comprised of rumen, reticulum, omasum and abomasum. Bovidae Cattle, sheep, goats, All species in these four buffalo, eland, antelopes, families ruminate. gazelles etc. a After Simpson.' "After Walker.6< Anatomy, physiology and microbiology of the ruminant digestive tract 3 digits with the mid-line passing between digits 3 and 4 (digit 1 is present only in fossil forms and digits 2 and 5 are either vestigial or absent). This produces the "cloven hoof" typical of the group. Table 1 shows the major systematic divisions of the Artiodactyla, and includes the common names of some representative members of each family. In each of the three families shown, there has been a tendency for species to evolve multicompartmented stomachs,46 ranging in complexity from the situation found in the Suiformes where there are simply diverticula present adjacent to the opening of the oesophagus into the simple stomach, to the situation in the Tylopoda and the primitive ruminant family Tragulidae where there is a three-chambered stomach with the equivalent of the omasum lacking. The greatest complexity is found in the remaining families of the Ruminantia, all of which possess the characteristic four-chambered stomach. On this basis, the Ruminantia are sometimes divided into two groups or infra-orders; the Tragulina, con­ sisting of the family Tragulidae, and the Pecora which contains the remaining four families.62 The three orders show differences in dentition, particularly of the upper jaw. The Suiformes have well-developed incisors and canines, the latter often tusk-like. In the Tylopoda, the young animal has a full set of incisors in the upper jaw, but the adult possesses only the third incisor on each side.46 In the Ruminantia, there is total loss of the upper incisors. The upper canines are also normally absent, but where they persist they become much enlarged. The upper incisors and canines are replaced by a thickened callous pad against which the lower incisors and canines bite. The Ruminan­ tia have retained all the lower incisors and, in general, the lower canines have become incisor-like; this feature is presumably of evolutionary advantage in that it extends the length of the row of cropping teeth. Ruminating is common to both Tylopoda and Ruminantia orders (Table 1), and although much of the adaptation to the ruminating habit is associated with the muscu­ lature and the neuromuscular reflexes of the rumen and associated organs, one skeletal adaptation that has taken place is an extreme shallowness of the depression in the skull into which the mandibular condyle fits. This permits the lower jaw to move from side to side, resulting in the rotary motion of the lower jaw that can be seen in animals that are ruminating and which greatly increases the efficiency of grinding of food. From the above account, it can be seen that the term "ruminant" does not describe a single zoological group; rumination is not confined to the Ruminantia, neither do all members of the Ruminantia possess the characteristic four-chambered stomach. The situation is further complicated by the fact that many herbivorous mammals of diverse zoological affinities have developed ruminant-like features through convergent evolution; presumably because the possession of these features confers ecological and evolutionary advantages.49 III. ANATOMY OF THE RUMINANT FORESTOMACH In this and subsequent sections of this article, a generalized account is given of the anatomy and physiology of the ruminant digestive tract and of the microbial fermen­ tation that takes place within it. Most of the findings described have been made with cattle and sheep because of the economic importance of these animals but, unless other­ wise specified, the following account applies to all true ruminants. Figure 1 shows in diagrammatic form a section taken through the median vertical plane of a generalized ruminant stomach viewed from the right-hand side. Arrows indi­ cate the route taken by food during digestion and rumination. The subdivisions of the stomach will be considered in the order in which food passes through them. A. Rumen This is the largest compartment of the adult ruminant stomach although its size relative to other compartments varies from species to species (Table 2). The rumen 4 C. G. Harfoot DRF DCP VRF VCP R00 FIG. 1. Section through the median vertical plane of a generalized ruminant stomach viewed from the right-hand side. ABO, abomasum; ATF, anterior transverse fold; ATP, anterior trans­ verse pillar; D, duodenum; DBS, dorsal blind sac; DCP, dorsal coronary pillar; DRF, dorsal reticular fold; DSR, dorsal sac of the rumen; LP, longitudinal pillar; OAO, omaso-abomasal orifice; OES, oesophagus; OM, omasum; RET, reticulum; ROO, reticulo-omasal orifice; VBS, ventral blind sac; VCP, ventral coronary pillar; VRF, ventral reticular fold; VSR, ventral sac of the rumen; —►, route taken by food during ingestion and passage through digestive tract; ►, route taken by digesta during rumination. is roughly ovoid, somewhat compressed laterally and is divided internally into dorsal and ventral sacs by a series of shelf-like pillars (Fig. 1). The whole of the internal surface is covered with heavily keratinized projections (papil­ lae) which greatly increase the surface area available for absorption. The papillae differ in size both with the species and also with respect to their location in the rumen, ranging in size fron small cones approximately 1-3 mm in height to large flattened leaf-shaped structures which may be up to 1 cm in length in the ox. The distribution of the papillae is not uniform; in most ruminants, the sides of the dorsal and ventral sacs of the rumen are the most densely papillated and also have the largest papillae. The borders of the pillars of the rumen on the other hand, along with the dorsal region of the dorsal sac bear only small, widely separated papillae. B. Reticulum The most anterior of the compartments of the ruminant stomach, the reticulum, is TABLE 2. Weights and Proportions of Regions of the Gastro-intestinal Tract in Domestic Ruminant Species Milk cow; 7 -month old bull; Sheep; body wt 520 kga body wt 204 kga body wt 80 kgb % % % total total total Wt stomach Wt % stomach Wt stomach /o /o Organ (kg) live wt wt (kg) live wt (kg) live wt wt Reticulo-rumen 65.4 12.6 87.5 40.0 19.6 92.0 17 21.3 84.9 Omasum 7.5 1.4 9.7 2.5 1.2 5.6 1 1.3 5.1 Abomasum 2.3 0.4 2.8 1.1 0.5 2.4 2 2.5 10.0 Total 75.2 14.4 100 43.6 21.3 100 20 25.1 100 Small intestine 6.3 1.2 c 3.5 1.7 6 7.5 Caecum d — — — 1 1.3 Large intestine 5.4 1.0 2.9 1.4 3 1.8 Total gastro­ intestinal tract 86.9 16.6 50.0 24.4 30 37.6 aData compiled from Hungate.34 bData compiled from Maynard and Loosli.48 c Not applicable. d No data. Anatomy, physiology and microbiology of the ruminant digestive tract 5 FIG. 2. Interior view of right side of reticulum and portion of the adjoining rumen. C, cardia; DRF, dorsal reticular fold; LOG, lips of oesophageal groove; O, oesophagus; OG, oesophageal groove; ROO, reticulo-omasal orifice; VRF, ventral reticular fold. separated from the rumen by a ridge of tissue called the rumino-reticular fold which extends from the ventral right side, transversely to the left, up the left side and partially across the dorsal region from left to right. As the rumino-reticular fold does not extend to the right lateral wall of the stomach, there is no demarcation of the reticulum from the rumen on this side. In consequence of this incomplete anatomical separation and the functional similarities of the reticulum and rumen, the two compartments are fre­ quently referred to as the reticulo-rumen, the reticulum being regarded simply as an anterior pouch of the rumen. In addition to its communication with the rumen, the reticulum also communicates in its dorsal region with the oesophagus via the cardia and with the omasum via the reticulo-omasal orifice (Fig. 2). The cardia and reticulo-omasal orifice are situated at either end of two bands of tissue between which lies the oesophageal groove (see below). The mucosal lining of the reticulum is raised into a honeycomb-like pattern and is more or less uniformly covered with small conical papilla, which, like those of the rumen, are heavily keratinized. C. Omasum This compartment lies on the right-hand side of the stomach and connects with the reticulum and abomasum via the reticulo-omasal and omaso-abomasal orifices, respect­ ively. Projecting into the omasum are a large number of plate-like folds or laminae attached to the greater curvature of the omasum and to its ends, in a manner akin to the pages of a book being attached to a binding that extends from the spine across the top and bottom of the pages. The omasal laminae bear heavily keratinized papillae that point in the general direction of the abomasum; this arrangement ensures that food material present in the omasum is propelled towards the abomasum irrespective of the muscular activity taking place in the omasum.18 D. Abomasum The abomasum is a tubular organ connecting the omasum with the small intestine. The mucosa of the abomasal wall is folded into longitudinal ridges not dissimilar from the laminae present in the omasum. The arrangement of these ridges is such that they possibly serve to prevent the contents of the abomasum from flowing back into the omasum.34 The abomasum corresponds in function to the fundic and pyloric regions 6 C. G. Harfoot of the stomach in non-ruminant animals in that the epithelium is supplied with secretory cells which produce hydrochloric acid and pepsin in the fundic region, and in the pyloric region, mucus. The peptic activity of the secretion from the pyloric region is low.54 It is in the abomasum that food is first subjected to digestive processes which are of ruminant rather than microbial origin. IV. PASSAGE OF FOOD THROUGH THE RUMINANT STOMACH A. Ingestion and Swallowing Ruminants, in common with other large herbivores, require a large bulk of food in order to satisfy their demands for substrates for biosynthesis and energy. Field studies have shown that cows spend approximately equal amounts of time grazing, ruminating and resting.73 However, the proportion of time spent in the ingestion of food is markedly less under conditions in which the animals are fed concentrates and ground and pelleted foodstuffs. During feeding, the food is briefly chewed, mixed with saliva to form a bolus, which in cattle weighs approximately 100 g, and swallowed. The bolus is propelled down the oesophagus by peristaltic contractions of the latter with such force that it falls into the rumen rather than into the reticulum (Fig. I).5 This rapid propulsion of the bolus is achieved through the action of striated muscle in the oesophageal wall of the ruminant. In contrast, smooth muscle is present in the oesophageal wall of non-ruminants. In the rumen, muscular contractions serve to mix the bolus with previously ingested material. B. Salivation 1. The Salivary Glands There are two types of salivary gland in ruminants, alkaligenic glands which comprise the paired parotid, inferior molar and buccal salivary glands and which secrete a fluid containing a high concentration of HCO^ ions with little mucoprotein and mucogenic glands, which comprise the paired submaxillary, sublingual and labial salivary glands as well as the unpaired pharyngeal gland and the numerous glands in the buccal epithe­ lium.32 The secretions of the mucogenic glands are predominantly mucoprotein. The composition of the secretion of some of these salivary glands is shown in Table 3. Of the glands secreting alkaline saliva, the most important are the parotid glands which secrete about half the salivary output in cattle.36 TABLE 3. Ionic Composition of the Saliva Secreted by Bovine Salivary Glands (Concentration in m-equiv/1 with percentage composition in parentheses)3 Salivary gland HCO3 ΗΡΟΓ c\- Na + K + Ca2 + Parotid 110.8 (38.9) 22.4 (7.9) 10.6 (3.7) 123.0(43.2) 14.7 (5.2) 3.4 (1.2) Submaxillary 15.7 (19.2) 0.5 (0.6) 30.9 (37.8) 13.6 (16.6) 13.9 (17.0) 7.1 (8.7) Sublingual 104.3 (34.4) 17.0 (5.6) 17.6 (5.8) 142.0 (46.8) 19.4 (6.4) 3.2(1.1) aData from Phillipson and Mangan.55 2. Secretion During the period when the animal is not feeding, there is a basal level of secretion of alkaline saliva but little secretion of mucoproteins. Feeding stimulates secretion of alkaline saliva and greatly stimulates that of mucoproteins. This stimulation was greatest with coarse fibrous food,20 and appeared to result from reflexes initiated by stimulation of the walls of the rumen by coarse food particles, especially those in the vicinity of the rumino-reticular fold.19'28 To a lesser extent, salivation was stimulated by pressures within the rumen.56

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