Anatomy, physiology and microbiology of the ruminant digestive tract

Print. Lipid Res. Vol. 17. pp. I 19. Pergamon Press. 1978. Printed in Great Britain

ANATOMY, PHYSIOLOGY A N D MICROBIOLOGY OF THE R U M I N A N T DIGESTIVE TRACT C. G. HARFOOT Department of Bioloyical Sciences, University of Waikato, Hamilton, New Zealand CONTENTS I. INTRODUCTION

1

II. FEATURES OF RUMINANTS AND THEIR ALLIES

2

III. ANATOMY OF THE RUMINANT FORESTOMACH

3

IV.

A. Rumen B. Reticulum C. Omasum D. Abomasum PASSAGE OF FOOD THROUGH TIlE A. Ingestion and swallowing B. Salivation 1. The salivary glands 2. Secretion 3. Composition of saliva

3 4 5 5 RUMINANT STOMACH

V. MIXING OF DIGESTA IN THE RETICULO-RUMEN

VI. RUMINATION VII. I~NTRY OF DIGESTA INTO THE OMASUM AND ABOMASUM VlIl. MICROBIAL FERMENTATION IN THE RETICULO-RUMEN A. The rumen environment B. The microbial population of the reticulo-rumen C. The holotrich protozoa D. The entodiniomorph protozoa E. The bacteria F. Metabolic activities of the rumen bacteria G. Utilization of end-products of microbial metabolism by the ruminant H. Unusable metabolic end-products

6

6 6 6 6 7 7 8 8 8

8 10 10 10 11 12 14 14

IX. PROCESSES IN THE OMASUM AND ABOMASUM

15

A. Omasum B. Abomasum X. DIGESTIVE PROCESSESIN THE RUMINANT HIND-GUT A. Small intestine 1. Secretion into the small intestine 2. Digestive processes in the small intestine 3. Absorption from the small intestine B. Large intestine !. Digestive processes in the large intestine and caecum 2. Absorption from the large intestine and caecum XI. REFERENCES

15 15 15 15 15 16 16 17 17 18 18

I. I N T R O D U C T I O N R u m i n a n t s are d i s t i n g u i s h e d from s i m p l e - s t o m a c h e d o r m o n o g a s t r i c a n i m a l s by the d e v e l o p m e n t of a series of p o u c h e s a n t e r i o r to their true gastric s t o m a c h . O f these p o u c h e s , the r u m e n is the largest a n d m e t a b o l i c a l l y the m o s t i m p o r t a n t . In the r u m e n , the c h e m i c a l c o n s t i t u e n t s of p l a n t origin which a r e ingested by the r u m i n a n t u n d e r g o m i c r o b i a l f e r m e n t a t i o n to p r o d u c e b o t h m i c r o b i a l cells, which are s u b s e q u e n t l y utilized as sources of p r o t e i n a n d o t h e r n u t r i e n t s by the h o s t a n i m a l , a n d the waste p r o d u c t s of m i c r o b i a l m e t a b o l i s m , m a n y o f which can also be utilized by the a n i m a l for either energy o r biosynthesis. As a result o f this c o n v e r s i o n of p l a n t cellular c o n s t i t u e n t s i n t o m i c r o b i a l cells, the m e t a b o l i s m of the r u m i n a n t a n i m a l is different f r o m t h a t of the s i m p l e - s t o m a c h e d a n i m a l , a n d t h e tissue c o m p o s i t i o n , p a r t i c u larly the lipid c o m p o s i t i o n , of r u m i n a n t s is distinctive. F o r e x a m p l e , it is in the r u m e n

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,18.21.35,49.63 to Phillipson's review article in Dukes' Physiology of Farm Animals 54 and to Hungate's book The Rumen and its Microbes. 34 II. FEATURES OF RUMINANTS AND THEIR ALLIES F r o m 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 c o m p a r t m e n t s anterior to the region corresponding to the simple stomach of non-ruminants. These c o m p a r t m e n t s are referred to as the rumen, reticulum, o m a s u m and abomasum. The a b o m a s u m corresponds in both physiology and anatomy to the simple stomach of non-ruminants. (b) Within the rumen and reticulum, there occurs extensive microbial fermentation of ingested food. (c) During the period following ingestion of food, there occurs regurgitation 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. F r o m 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 relationships 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 ArtiodactylaJ Including Some Representative Living Species and Showing the Degree of Stomach Complexityb Representative Living Species

Suborder

Family

Suiformes

Suidae

Tylopoda

Tayassuidae Hippopotamidae Camelidae

Pigs, wart hogs, forest hogs Peccaries Hippopotamus Camels, llama, alpaca vicuna

Ruminantia

Tragulidae

Chevrotains, mouse-deer

Ruminantia

Cervidae

True deer, moose (elk)

Girattidae

caribou (reindeer) Giraffe, okapi Pronghorn

Antilocapridae Bovidae

a After Simpson. 6°

bAfter Walker.66

Cattle, sheep, goats, buffalo, eland, antelopes, gazelles etc.

Stomach Anatomy, Rumination True stomach may have l or 2 diverticula. No true rumen reticulum, omasum or abomasum Suiformes do not ruminate. 3-chamberedstomach present; corresponding to rumen reticulum and abomasum. Equivalent to omasum lacking. Camelids and tragulids both ruminate. All cervids, giraffids antilocaprids and bovids have true 4-chambered ruminant stomach, comprised of rumen, reticulum, omasum and abomasum. All species in these four families ruminate.

Anatomy. physiologyand microbiologyof the ruminant digestivetract

3

digits with the mid-line passing between digits 3 and 4 (digit l 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 l 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, consisting 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 Ruminantia 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 musculature 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 |II. 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 fermentation 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 otherwise 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 indicate 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 LP ATP

~ D

-

VRF

RO0 -

~

O

A

O

FIG. I. 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 transverse 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 (papillae) which greatly increase the surface area available for absorption. The papillae differ in size both with the specie~ 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 l 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 TASTE 2. Weights and Proportions of Regions of the Gastro-intestinal Tract in Domestic Ruminant Species Milk cow: body wt 520 kff'

7-month old bull: body wt 204 kff'

Wt (kg)

°o live wt

°o total stomach wt

Reticulo-rumen Omasum Abomasum Total

65.4 7.5 2.3 75.2

12.6 1.4 0.4 !4.4

87.5 9.7 2.8 I~

Small intestine Caecum Large intestine Total gastrointestinal tract

6.3 a 5.4

1.2 _ 1.0

2.9

1.4

3

7.5 1.3 1.8

86.9

16.6

50.0

24.4

30

37.6

Organ

=Data compiled from Hungate. 34 Data compiled from Maynard and Loosli. 4s Not applicable. u No data.

Wt (kg)

°o live

°o total stomach wt

40.0 2.5 1.1 43.6

19.6 1.2 0.5 21.3

92.0 5.6 2.4 1~

3.5

1.7

Sheep; body wt 80 kg b

Wt (kg)

°~ live wt

0g total stomach wt

17

21.3 1.3 2.5 25.1

84.9 5.1 10.0 100

1

2 20 6 1

Anatomy.physiologyand microbiologyof the ruminantdigestivetract

5

J 7 O

Fen

....----- V R F Re,,culum

1/

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; O13, 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 frequently 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. Ornasum

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, respectively. 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, is 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. Inyestion 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. 7a 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 100g, 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. 1).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 I. 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 epithelium. 3-' 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/I with percentage composition in parenthesesp Salivary gland Parotid Submaxillary Sublingual

HCO~

HPO~-

Cl-

Na ÷

K÷

Ca -'+

110.8 (38.9) 15.7 (19.2) 104.3 (34.4)

22.4 (7.9) 0.5 (0.6) 17.0 (5.6)

10.6 (3.7) 30.9 (37.8) 17.6 (5.8)

123.0 (43.2) 13.6 (16.6) 142.0 (46.8)

14.7 (5.21 13.9 (17.0) 19.4 (6.4)

3.4 (1.2) 7.1 (8.7) 3.2 (l.1)

~'Data from Phillipson and Mangan. 5~

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 f o o d . 2° 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. 19,2a To a lesser extent, salivation was stimulated by pressures within the rumen. 56

Anatomy, physiology and microbiology of the ruminant digestive tract

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The more or less continuous secretion of alkaline saliva by the parotid glands is responsible for the large volume of saliva secreted by ruminants compared with that by non-ruminants. Because the volume of saliva secreted is dependent on both the ration and the frequency of feeding, estimates of the total amount of saliva secreted vary but, in general terms, the volume of saliva secreted per day appears to be roughly equivalent to the volume of the rumen. 34

3. Composition of Saliva The ionic compositions of the saliva secreted by the alkaligenic glands of cattle and sheep are shown in Table 3. It can be seen that the concentration of Na ÷ ions in bovine parotid saliva is approximately 10 times that of K ÷ ions. In sheep, the difference is even greater. In animals deprived of Na÷, the K ÷ concentration increases, maintaining the combined Na ÷ and K ÷ concentration at about 150 m-equiv/l. 2° The saliva is roughly isotonic with blood although there are differences in the concentrations of individual ions. 36 The osmotic pressure of the secretion of the mucogenic glands is much lower than that of the blood, however, and contains less HCO3 and H P O ~ - than the alkaline saliva. 55 Although the output of any one type of salivary gland varied according to whether or not the animal was resting, feeding or ruminating, the composition and osmotic pressure of the salivary secretion remained fairly constant, la'a6

V. MIXING o F DIGESTA IN THE RETICULO-RUMEN After being swallowed, the bolus of ingested food that has entered the rumen is mixed with the digesta already present. Mixing is achieved by means of a cycle of events involving both the reticulum and the rumen. The cycle can be considered to start with two contractions of the reticulum; the first contraction lasts approximately 2-3 sec and at its maximum the reticular volume is reduced by about half. This contraction is succeeded by a brief period of relaxation in cattle, but in sheep the second contraction of the reticulum begins before the reticular muscles have been relaxed. 54 The second contraction is complete and has the effect of forcing most of the reticular contents over the reticulo-ruminal fold into the rumen after which the reticulum relaxes. The pair of contractions is often referred to as the biphasic contraction of the reticulum and takes some 10 sec from start to finish. It is repeated at intervals of roughly one minute in the resting or ruminating animal, and rather more frequently while feedingfl 2 During the second part of the biphasic contraction, the anterior transverse pillar of the rumen contracts, closely followed by the longitudinal pillars, dorsal sac, dorsal coronary pillar and dorsal blind sac in that order (Fig. 1). The effect of this posteriorlymoving wave of contractions is to reduce the volume of digesta in the dorsal sac of the rumen and to transfer it to the now relaxed ventral sac of the rumen, s8 The mixing cycle in the rumen ends with the contractions of the ventral coronary pillars and the ventral blind sac, thus causing the level of the liquid in the ventral sac of the rumen to rise and, by immersing the more solid particles of digesta (including the recentlyswallowed bolus of food), brings this material into close contact with the rumen microorganisms. The rise in level of the liquid also results in the latter spilling over into the reticulum, taking with it the finer particles. As the rumino-reticular fold tends to restrain the flow of the larger particles of digesta, the reticulum contains more liquid and less solid material than does the rumen. It is not easy to assess the efficiency with which these movements mix the digesta; certainly the mixing is insufficient to prevent the marked stratification of digesta in the rumen which results from coarse particles being borne to the surface by the small bubbles of gas arising from microbial fermentation. 7 Furthermore, it is frequently observed during sampling from the rumen that there are regions which are predominantly of fibrous material interspersed within a mainly liquid phase.

8

c.G. Harfoot VI. RUMINATION

Rumination is a complex process and little more than a general outline of the mechanism can be given here; for further information, the reader is referred to reviews by Phillipson s4 and by Stephens and Sellers. 63 During rumination, there occurs an additional contraction of the reticulum which precedes the normal biphasic contraction. This contraction raises the level of the contents of the reticulum above the level of the cardia. The cardia relaxes, thereby enlarging the opening to the oesophagus. Although the changes in pressure within the rumen and reticulum during the mixing cycle described above are sufficient to propel digesta into the oesophagus. 9 there is evidence that the animal also contracts the diaphragm and closes the glottis, reducing the pressure in the thorax to less than atmospheric, 17 thus producing a pressure gradient in the oesophagus. Once in the oesophagus, the bolus is carried to the mouth by reverse peristalsis. The liquid present in the bolus is swallowed and enters the reticulum along with the fine particles suspended therein. The fibrous material of the bolus is then chewed with the lateral grinding motion of the jaw that is characteristic of ruminants (see Section II). This chewing is very important as it results in (a) extensive maceration of the plant tissues with saliva; the resulting physical damage to the plant cells increases the rate of subsequent microbial digestion in the rumen, (b) comminution of the plant material, thereby increasing the surface area exposed to microbial attack and (c) thorough interpenetration of the bolus with any fluids transported with it from the rumen; this ensures extensive and intimate inoculation of the plant material with rumen microorganisms. The ruminated bolus is swallowed, enters the rumen and is there mixed with the rumen contents in the same manner as ingested food. Immediately after the entry of a ruminated bolus into the rumen, a second bolus is regurgitated and, since regurgitation precedes the mixing cycle of the reticulo-rumen, rumination is synchronous with mixing, one bolus being ruminated per mixing cycle. Although regurgitation does not completely discriminate between ruminated and non-ruminated digesta, the ruminated bolus contains little food that has been recently ingested as the latter collects in the dorsal sac of the rumen and takes some time to mix. VII. ENTRY OF DIGESTA INTO THE OMASUM AND ABOMASUM Because the mixing cycle in the reticulo-rumen involves emptying of the contents of the reticulum into the rumen and the subsequent spilling over of the more liquid contents of the rumen into the reticulum, the latter tends to contain fewer large particles per unit volume than does the rumen. The liquid nature of the contents of the reticulum is further increased by the swallowing of saliva and the liquids from the regurgitated bolus. The separation of fine particles from the coarse fibres is far from complete but an even more effective partition takes place at the reticulo-omasal orifice. During the relaxation of the reticulo-rumen after the second part of the biphasic contraction, the reticulo-omasal orifice opens and the fine particles contained in the reticulum enter the omasum as a slurry containing some 90-95~o water. 8 The reticulo-omasal orifice then closes and remains so during the first part of the biphasic contraction. During this closure, coarse particles present in the reticulum accumulate at the entrance to the omasum where they aid in elicitation of the rumination reflex. VIII. MICROBIAL FERMENTATION IN THE RETICULO-RUMEN A. The Rumen Environment

Before considering the processes taking place in the reticulo-rumen and the microorganisms responsible for those processes, it is pertinent to describe the environment within the reticulo-rumen. However, as both the environment and the magnitude of

Anatomy, physiology and microbiology of the ruminant digestive tract

9

the processes taking place are affected by the diet of the animal, only a generalized consideration of the reticulo-rumen ecosystem will be given here. For information concerning the effect of diet on ruminal fermentation, the reader is referred to Chapter 10 of Hungate's book. 34 The liquid phase of the rumen contents contains about 10-20~o by weight of organic matter and has a pH generally within the range of pH 5.8-6.8. With large amounts of readily fermentable carbohydrate entering the rumen, however, the pH may fall to around pH 4.0. Conversely, on very poor forages, the pH may rise to pH 7.5 or more, but these are the extremes of the pH range. 33 The pH of the reticulo-rumen is maintained at a fairly constant level by the alkalinity (pH = 8.0) of the large volumes of saliva entering the rumen, by the buffering capacity of the HCO3 content of the saliva and by removal of the acidic end-products of microbial fermentation through the rumen wall at a rate approximately equal to that at which they are produced. 53 The temperature of the contents of the reticulo-rumen is stable at around 39°C. The gas phase above the digesta in the reticulo-rumen consists of approximately 65~o CO2, 25~o CH4, 7~ N2 and trace amounts of H2 and 02 .43 The CO2 and CH4 are derived from microbial fermentation as is the small amount of H2. The N2 and 02 enter the gas phase of the reticulo-rumen along with ingested forage. As the liquid phase of the reticulo-rumen has an oxidation-reduction potential of about - 3 5 0 mV, 6t it is clear that the reticulo-rumen environment is extremely reduced and almost devoid of oxygen. However, its stability is such that it will tolerate addition of considerable amounts of oxygen without marked or prolonged changes in oxidationreduction potential or in microbial fermentation, t° The inorganic solutes present in the reticulo-rumen are derived from the saliva (see Section IV.B.3). Of the cations present in the rumen, only Na ÷ is transported across the reticulo-ruminal wall in appreciable amounts against the concentration gradient which occurs between the rumen and the bloodstream. As a consequence of this, the ionic content of the rumen closely reflects that of the saliva. 2t'24 The major organic solutes present in the reticulo-rumen are the short-chain (C2-C5) monocarboxylic acids produced as a result of microbial fermentation. The rumen liquor of domestic ruminants contains from about 50 mM to 150 mM concentrations of these acids, with the concentration being roughly proportional to the extent of feeding. The proportions of different fatty acids vary greatly with diet as can be seen from Table 4, but as a very rough approximation the concentration of short-chain fatty acids in the rumen is about I00 mu and the molar proportions of individual acids as a percentage of the total are acetic, 65; propionic, 20; n-butyric, 12; iso-butyric, 1; n-valeric, 1; isovaleric and 2-methyl-butyric, 1 (compiled from a range of values given by Hungate34). Due to the high rates of turn-over resulting from rapid microbial metabolism, soluble sugars, amino acids and NH~ are present in solution in the reticulo-rumen at low TABLE 4. The Effect of Ration on the Volatile Fatty Acid Concentrations in the Reticulo-rumen i

Animal Sheep

Cattle

Ration

Total VFA concentration (mM)

Acetic

81-178 65 49-109 52-193 101-187 n.d. 78 56-60 n.d. 120

65.5 68 61-69 68-69 50-62 56 68 58-69 52 63

Grass pasture Dried grass Wheat hay Alfalfa hay Grass + clover Pasture Pasture Hay + concentrates Silage Alfalfa hay

a Data from Hungate 3'* Table IV--4. n.d. = no data.

Concentration of individual acids (moles ~o) Propionic Butyric 19.5 19 17-24 15--20 21-30 18 21 16-24 24 23

15 13 13-15 10-16 8-16 26 11 12-14 24 14

Higher n.d. n.d. n.d. n.d. n.d. n.d. n.d. 2.8-4.1 n.d. n.d.

10

C.G.

Harfoot

concentrations only but these concentrations vary considerably with time in relation to feeding. From the data given by Lewis,4~ rough estimates of the concentrations of free dissolved amino acids and dissolved ammonia in the rumen can be made, i.e. of the order of 25 mM each.

B. The Microbial Population of the Reticulo-rumen The microorganisms most adapted to grow and metabolize in the environment described above are anaerobes and facultative anaerobes capable of hydrolyzing the structural carbohydrate polymers of forage and of utilizing the released soluble sugars as sources of energy and of carbon for biosynthesis. The population density within the reticulo-rumen is extremely high: there are between l01° and l0 ~ bacteria/ml and between l05 and l06 ciliate protozoa/ml of rumen contents. Population densities as high as this are only possible if the population is of considerable metabolic diversity and if the respective metabolic activities of physiologically different organisms are closely integrated. Both of these conditions occur in the reticulo-rumen.

C. The Holotrich Protozoa The ciliate protozoa of the reticulo-rumen are of two types, generally referred to as holotrichs and entodiniomorphs. The holotrichs, which are represented by the genera lsotricha and Dasytricha, are members of the subclass Holotrichia and are characterized by the possession of cilia over the whole body surface. The holotrichs metabolize soluble sugars as sources of carbon and energy, generally polymerizing hexoses to amylopectin and thus sequestering potential carbon and energy sources against periods of relative scarcity. The major fermentation products of the holotrichs are acetic, butyric and lactic acid together with gases CO2 and H: which are produced in more or less equal amounts (Table 5).

D. The Entodiniomorph Protozoa The entodiniomorphs are represented by many genera, chief among them being Entodinium, Epidinium, Eudiplodinium, Diplodinium, Polyplastron and Ophryoscolex. The distribution and proportions of these genera in the reticulo-rumen are greatly influenced by the diet of the animal and the reader should refer to Chapter 3 of Hungate's book 34 for a detailed account of their identification and of factors affecting their distribution. The entodiniomorphs are members of the subclass Spirotrichia, order Entodiniomorphida, and all genera are characterized by their cilia being confined to specific regions of the body surface, usually at the anterior end. They are predominantly particle-feeders, TABLE 5. P r o p o r t i o n s ( m o l e s ° o ) o f the E n d - p r o d u c t s of F e r m e n t a t i o n P r o d u c e d by S o m e of the P r o t o z o a of the R u m e n Protozoan (substrate) Mixed holotrichs (glucose) M i x e d isotrichs (glucose)

CO2

H2

Formic acid

Acetic acid

Propionic acid

Butyric acid

Lactic acid

Ref.

26.0

34.8

--

9.3

--

10.6

19.3

27

33.7

30.9

--

4. 5

--

6.9

24.0

26

30.8

36.0

--

8.0

--

3.6

19.6

26

46.4

--

--

14. l

--

6.5

33.0

29

29.7

25.7

1.8

15.2

4.4

22.2

1.0

30

49.5

24.1

--

14.2

1.4

10.9

--

44

Dasytricha (glucose)

Dasytricha (galactose)

Entodinium (undefined)

Opho,oscolex) (ground wheat)

Anatomy, physiology and microbiology of the ruminant digestive tract

ll

ingesting both plant and bacterial cells and starch grains; soluble carbohydrates are only used as sources of carbon and energy when particulate food is unavailable. Whether the entodiniomorphs utilize cellulose or not has been in doubt for some time. Cellulolytic activity was demonstrated in Diplodinium spp. by Hungate 31'32 and in Polyplastron by Abou Akkada et al. ~ More recently Coleman 16 using 14C-labeled cellulose has demonstrated cellulolytic activity by a number of species of entodiniomorphs but it should be noted that the activity was possible due to the presence of bacteria within the endoplasm of the protozoa. Certainly starch is rapidly hydrolyzed by the protozoa and probably serves as the major carbon and energy source for the rumen entod!niomorph protozoa. 34 The products of carbohydrate fermentation by the entodiniomorphs are CO2 and H 2, various volatile fatty acids and lactic acid. The nature and proportions of the end-products differ with different genera (see Table 5). Unlike rumen bacteria, most of which appear to use NH,~ as a source of nitrogen for protein biosynthesis, the entodiniomorphs utilize amino acids of plant and bacterial origin without prior d e a m i n a t i o n ; N H ~ does not appear to s e r v e as a nitrogensource.14'~ 5 F r o m the point of view of lipid metabolism in the rumen, the entodiniomorphs are the more important of the groups of protozoa in that they ingest particulate material of plant origin, including chloroplasts, and could potentially protect the unsaturated fatty acids of these from hydrogenation by rumen bacteria. Both groups of protozoa are, of course, eukaryotic cells and therefore have a lipid composition that more closely resembles that of animal cells than does the lipid composition of the cells of prokaryotes. E. The Bacteria

The bacterial flora of the reticulo-rumen is very complex. Table 6 shows a number of species that are important both in terms of their occurrence in the rumen and in their metabolic activities. N o indication is given in the table of the distribution of the bacteria within the reticulo-rumen but it is important to remember that only approximately half of the bacteria in the reticulo-rumen are present in free suspension in the liquid. ~2 The rest, a m o n g them important cellulolytic species, are found adhering to the surfaces of the plant particles. 2's2 TAaLE 6. The Physiological Niches, Substrates Utilized and Major Products of Fermentation of the More Important Bacterial Species of the Rumen Substrates Utilized

Major End-products of Fermentation

Bacteroides succinogenes Ruminococcus spp

Cellulose, cellobiose, glucose. C02 Cellulose, cellobiose, xylose. C02

Succinate, acetate, formate Succinate, lactate, acetate,

Butyrivibrio fibrisolrens

Cellulose, wide range of sugarsa

Butyrate, lactate, formate, CO:

Lachnospira multiparus

Pectin, cellobiose, glucose

Succinivibrio dextrinosolvens

Pectin, dextrin, maltose, xylose

Formate, lactate, acetate, CO2, H2 Acetate, succinate, lactate

Starch, maltose, CO2 Starch, wide range of sugarsa

Succinate, acetate, lactate Lactate

CO2, H2, formate acetate, other VFA

CH4 production biosynthesis

Wide range of sugarsa, COs Lactate, glucose, fructose, maltose, sugar alcohols Wide range of sugarsa. glycerol

Succinate. acetate, formate Range of VFA, H2, COs

Niche/Organism Cellulose degradation

formate. H 2

Pectin degradation

Starch degradation Bacteroides amylophilus Streptococcus boris

Methane production M ethanobacterium ruminantium

Miscellaneous fermentations Bacteroides ruminicola Megasphaera (Peptostreptococcus) elsdenii Selenomonas ruminantium

aActual sugars utilized depends on the particular strain of the species.

Acetate, propionate, CO2 butyrate

12

c.G. Harfoot

F. Metabolic Activities of the Rumen Bacteria The metabolic activities of the tureen bacteria can be considered to be of two types, namely (a) degradation of plant polymers (cellulose, hemicelluloses and proteins) to their respective monomeric subunits and (b) fermentation and subsequent utilization of these subunits to yield energy, carbon skeletons and NH~ for bacterial cell biosynthesis. Obviously, the anaerobic nature of the reticulo-rumen precludes oxidative phosphorylation and the consequent complete oxidation of carbon sources to CO2 with the result that both with respect to energy-yielding processes and in order to regenerate the oxidized forms of co-factors such as NAD ÷ and NADP ÷, bacterial metabolism results in the production of a diverse range of end-products of fermentation (Table 6, Figs 3 and 4). Through the hydrolysis of cellulose and hemicellulose to their respective glucose and mixed hexose and pentose monomers, the carbon and energy present in plant carbohydrates is made available to the ruminant by way of microbial fermentation. The extent to which cellulose and hemiceUulose are degraded in the rumen depends to a large measure on the age of the forage. Estimates have been made of the extent of degradation of cellulose and hemicellulose present in mature forages of different ages. Over a 24-hr period, the extent of degradation of cellulose ranged from 40-60%; the corresponding values for hemicellulose were 45-70%. 34 Recent studies using very young forages of exceptionally high digestibility showed that 90% of the structural carbohydrate and 93% of the soluble carbohydrate of these forages were degraded over a 24-hr period in the sheep rumen. 64 The metabolism of hexoses to pyruvate by the rumen microflora appears to take place largely by means of the enzymes of the glycolytic pathway, at least in the limited range of organisms so far studied a9 while that of the pentoses present in hemicellulose occurs via hexose synthesis through the agency of transaldolase and transketolase enzymes in a manner similar to that of the pentose phosphate pathway. 65

Carbohydrate

2NADH~ '~2ATP

ATP? - Formate

Pyruvote

J-IMethane

Lactate

"~

cetyl CoA

Motote

H20 I

J ~'- hydroxybutyryl CoA

~,~H20

2A~iate

H20 Cb

Fumarote

I

2H"~ATP?

/oo,e

Crotonyl CoA

Butyryl CoA

I °',rate

]

Methylmalonyl CoA

)

CoA

1 I

I

FIG. 3. Diagram of the pathways leading to the major metabofic end-products of fermentation in the reticulo-rumen.

13

Anatomy, physiology and microbiology of the ruminant digestive tract

Corbohy0rate polymers

(CH20)n

I I

Monosecchorides

riCH 0

I f

•I ~-

VFA =

Waste

Lactate

/

Formate Waste

I I Protein

, NH+ F "~ \

Bacterial cel IS

\

'

•

=

L

Amino

'

I

Host

I = Host

~ . _ ~

Fatty acids ~

Unsotur]ted

= HOSt

Soturated

= Host

FIG. 4. Integration of major metabolic activities occurring in the reticulo-rumen El, of plant origin; II, major end-product of metabolism; --, degradation; - - , synthesis; --, Host, used by host animal; ....~ Waste, Waste product.

Proteins of plant origin are rapidly hydrolyzed in the rumen to amino acids which in turn are rapidly deaminated to NH2; the latter serves as the major nitrogen source for bacterial protein biosynthesis. This extensive degradation of plant protein followed by re-synthesis by the bacteria may result in some 70% of the protein present at any one time being microbial, a4 This microbial protein subsequently passes on to be digested in the abomasum and hindgut of the ruminant. In detailed studies with sheep, Pilgrim TABLE 7. Nutritional Requirements of the More Important Bacterial Species of the Rumen

Organism

CO z

Volatile Fatty Acids

Bacteroides succinogenes Ruminococcus spp.

E E

Essential Essential

Butyritihrio fibrisolvens

S

Essentialc

Lachnospira multiparus Succinivibrio dextrinosolvens Bacteroides amylophilus Streptococcus boris

S S E S

Acetate S None required None required Stimulatory"

Methanobacterium ruminantium

E

Essential

Bacteroides ruminicola Megasphaera elsdenii Selenomonas ruminantium

E S S

Essentialc Acetate S Essentialc

' Degree of stimulation depends on particular strains of the species. b p-Aminobenzoic acid. c Essential for some strains only. d N O data. e Stimulatory for some strains only. E = Essential. S = Stimulatory.

Vitamins Essential Biotin Biotin, PABA, folic acid Biotin, PABA, pyridoxal 0 .d None required Biotin Unidentified vitamin Some B vitamins .d None required

Stimulatory = PABA b Thiamin, riboflavin Nonstimulatory / d Nonstimulatory Thiamine, pantothenic acid Nonstimulatory d Nonstimulatory

14

C.G. Harfoot

et al. 57 estimated that of 22.9 g of dietary nitrogen entering the rumen along with 5.0 g

of nitrogen derived from the nitrogen being recycled in blood and saliva, 2.7 g (9.7%) appeared as protozoal nitrogen and 10.8 g (38.7~o) as bacterial nitrogen. The undigested plant nitrogen which passed on to the abomasum constituted about 18~o of the dietary nitrogen entering the rumen, or about 15~0 of the total nitrogen (dietary plus recycled). It has been pointed out that the end-products of fermentation in the reticulo-rumen are a consequence of the anaerobic conditions present. These end-products may serve any one of a number of functions. NH~ is used almost universally as a nitrogen source for the biosynthesis of bacterial protein and CO2 is similarly required by a large range of bacteria as a "primer" for biosynthetic processes (Table 6). Other end-products of metabolism are C4 and C5 branched-chain fatty acids which are required by species and strains of Bacteroides, Ruminococcus and Methanobacterium that are unable to synthesize the branched-carbon skeletons for themselves. Although products of biosynthesis rather than end-products of fermentation, a similar situation exists with respect to the supply and demand of B-vitamins in the reticulo-rumen (Table 7). 35 G. Utilization of End-products of Microbial Metabolism by the Ruminant

From the point of view of the host animal, among the most important of the endproducts of microbial metabolism in the reticulo-rumen are the C2-C4 fatty acids, which are absorbed into the bloodstream of the ruminant through the reticulo-ruminal wall at a rate approximately equal to their rates of production, s3 The contribution of these fatty acids to the metabolism of the host animal is considerable. For example, some 50-60~o of the animals' energy requirements are met by the acetate produced by microbial fermentation.34 Approximately half of the glucose used by the ruminant is synthesized by way of methylmalonyl CoA, succinate and oxaloacetate and subsequent gluconeogenesis from propionate originating in the rumen. It had been suggested earlier that prior to its conversion to glucose, some 20~o of the propionate leaving the rumen was converted to lactate by the rumen epithelium.4° Recent studies have shown this not to be the case. Weigand et al. 7t showed that only about 5~ of the propionate leaving the rumen of the sheep was converted to lactate and studies by Weekes 6a'69 and by Weigand et al. 7° both in vitro using tissue incubation and in vivo have indicated that the metabolism of propionate by the rumen epithelium of sheep accounted for only about 15°4 of the total lactate synthesized by the animal. It would thus appear that propionate is transported to the liver for conversion to glucose without extensive prior metabolism to lactate. Butyrate is metabolized by the rumen epithelium to acetoacetate, acetone and fl-hydroxybutyrate, apparently without prior cleavage to C2 units. 3 Weigand et al. 71 estimated that 50~o of the butyrate leaving the reticulo-rumen of the sheep was metabolized to acetoacetate, acetone and fl-hydroxybutyrate, of which the last comprised 75~o. Metabolism of butyrate also occurs in the liver, yielding largely acetyl CoA with some fl-hydroxybutyrate. The acetyl CoA derived from butyrate may be used ultimately as an energy source by the ruminant, thus sparing the oxidation of propionate to acetyl CoA and reserving propionate for gluconeogenesis. H. Unusable Metabolic End-products

From a microbiological point of view, it is the integration of the different metabolic pathways within the reticulo-rumen which makes the rumen ecosystem so efficient. Similarly, it is the utilization by the host animal of the volatile fatty acids produced by microbial fermentation which makes the symbiotic relationship between the rumen microorganisms and the host so effective. However, it is clear that C02 is produced at a rate greater than that required by microorganisms for biosynthesis and that required by the host for maintaining HCO~ levels in saliva; consequently, it constitutes some two-thirds of the gas phase of the reticulo-rumen.

Anatomy, physiologyand microbiologyof the ruminant digestivetract

15

A further major end-product of microbial metabolism is methane. In the anaerobic conditions present in the reticulo-rumen, methane cannot be utilized as a source of either carbon or energy by any of the microorganisms present nor can it be utilized subsequently by the host animal. Like CO2, it is expelled from the rumen through eructation. Methane production represents a not inconsiderable loss of the animal's total carbon intake, 7-970 of which is thus lost. It should also be borne in mind that ammonia, produced by deamination of amino acids at a rate greater than that of its utilization for amino acid biosynthesis by the rumen microflora, is transported across the rumen wall to the bloodstream and is subsequently detoxified in the liver. However, under feeding regimes in which nitrogen supply to the microflora is limiting, there is a flow of urea produced by the animal into the rumen by way both of the saliva and of diffusion from the bloodstream across the rumen wall.

IX. PROCESSES IN THE OMASUM AND ABOMASUM A. Omasum

T h e large size of the reticulo-rumen has already been mentioned (Table 2) and, as the turnover rate of the reticulo-rumen is of the order of 1-1.5 volumes per day, it is clear that a very large volume of liquid enters the omasum from the reticulo-rumen. The material entering the omasum contains 90-95~/o water 8 and the primary function of this organ is the remove water, thus reducing the liquid content of the digesta passing into the abomasum. Estimates of the quantity of water removed by absorption in the omasum vary, but a not unreasonable estimate based on the figures cited by Badawy et al. 6 and Gray et al. 2s is about 50Y/o, the extremes of the range reported being 33 and 6570. Whether the water is removed by being physically expressed from the digesta entering the omasum or solely by absorption is not clear; possibly both processes take place. 34 In addition to removing water, the omasum also absorbs volatile fatty acids. 25 Masson and Phillipson 47 estimated that the concentration of volatile fatty acids present in the digesta leaving the abomasum was 5-15~o of the concentration present in the reticulorumen. B. Abomasum

The abomasum corresponds in structure and function to the fundic region of the stomach of non-ruminants. The abomasal epithelium possesses cells which secrete electrolytes, especially hydrochloric acid, pepsin and mucus. The pH of this secretion is in the range of pH 1.0-1.354 and the overall pH of abomasal contents is about pH 2.0.34 The low pH of abomasal contents is responsible for the death of the bacteria and for the death and disintegration of the protozoa entering the abomasum; it also provides optimum conditions for activity of the peptic enzymes responsible for the digestion of microbial protein in the abomasum.

X. DIGESTIVE PROCESSES IN THE RUMINANT HIND-GUT A. Small Intestine 1. Secretion into the Small Intestine

That portion of the small intestine immediately distal to the abomasum is referred to as the duodenum. The duodenum receives both bile from the gall-bladder and pancreatic secretions from the pancreas via a duct which at the point of entry into the d u o d e n u m is c o m m o n to both organs since the bile duct and pancreatic duct fuse s o m e 2-3 cm from this point. J.P.C.F. 17/I--a

16

C.G. Harfoot

Bile consists largely of bile acids and bile pigments, with small amounts of cholesterol, lecithin, electrolytes and protein. Bile acids do not enter the duodenum in the free form, but as conjugates of either glycine, giving glycocholic acid, or taurine, giving taurocholic acid. Esterification takes place at the terminal carboxyl group of the parent acid. The significance of the bile conjugates (bile acids) in the digestion and metabolism of lipids is discussed in the review article by Noble. 5°a The secretions of the pancreas include the proteolytic enzymes trypsinogen (converted to the active form, trypsin, by enterokinase secreted by the duodenum), chymotrypsinogens and pro-carboxypeptidases (both activated by trypsin) and carboxypeptidase. Proteolytic enzymes constitute some 70% of the total protein secreted by bovine pancreas.37 Also present in the pancreatic secretions are DNAase, RNAase, pancreatic lipase and an s-amylase similar to that present in saliva. These enzymes, together with the pepsin secreted by the abomasum, are responsible for the degradation of the microbial cells entering the region of the gastro-intestinal tract posterior to the reticulo-rumen. There appears to be little information available on the range and activity of the enzymes secreted by the ruminant duodenum but it has been suggested, on the basis of the observed continuous proliferation and exfoliation of the intestinal mucosal cells, that the enzymes present in the small intestine are derived from these cells rather than being true secretions, s4 The secretions present in the small intestine appear to consist largely of electrolytes, particularly Na + and C1-. The pH of the small intestine differs along its length, ranging from pH 7.1 in the region distal to the duodenum and known as the jejunum to approximately pH 8.0 at the ileum, which region constitutes the greater portion of the small intestine. These pH values are maintained through the presence of HCO3 ions in the intestinal secretion; the concentration of HCO~ at the jejunal end of the small intestine was 11.8 mM and that at the ileal end 41.5 r a M . 59 2. Digestive Processes in the Small Intestine

The extent of the contribution of the microbial flora of the small intestine to the degradation of digesta in this organ is not known. It is assumed that this contribution is small compared with that of the rumen because of the very much smaller bacterial population density of some 105-106 cells/ml compared with the density of 101°-10 ~I cells/ml in the rumen. Extensive degradation of microbial cells takes place in the small intestine as is evidenced by the presence of fatty acids with odd-numbers of carbon atoms and of double bond positional isomers characteristic of bacterial fatty acids. However, degradation of carbohydrate polymers (other than starch) which have escaped microbial degradation in the reticulo-rumen probably does not take place to a large extent because of the relatively low microbial population density and of the relative short residence time of digesta in the small intestine compared with that in the rumen. This relatively short residence time is a consequence of the relative sizes of the reticulorumen and the small intestine (Table 2). 3. Absorption from the Small Intestine

Measurement of the rate or extent of absorption from the small intestine is rendered difficult in some cases by the low concentration of organic solutes present. The concentration of volatile fatty acids entering the small intestine from the abomasum, for example, is only 5-20 mM, and that leaving the jejunum only 2--4 mu compared with ruminal concentrations of the order of 100-150 mM. Clark et al. ~2 found that, in sheep fed poor hay with and without a protein supplement, the concentration of amino-nitrogen entering the small intestine varied between 70 and 150mg/100ml, with the higher concentration obtained in the sheep fed:':the protein supplement. As a consequence of the conversion of plant protein into microbial protein in the reticulo-rumen, this amino-nitrogen is markedly different in composition

Anatomy, physiologyand microbiologyof the ruminant digestive tract

17

from that of the diet and does not reflect variations in the diet in its composition. It was found in the experiments of Clark et al. t2 that the amino-nitrogen leaving the small intestine ranged in concentration between 80 and 100 mg/100ml. The differences in concentrations of amino-nitrogen entering and leaving the small intestine led Clark et al. to calculate that, over a period of 24 hr, 2-10g of amino-nitrogen was absorbed by the small intestine. These values did not take any secretion of amino-nitrogen into the lumen of the small intestine from the mucosa into account. As mentioned in Section X.A.I, electrolytes make up the major proportion of the secretions of the small intestine. 59 Studies by Bruce et al., t t who compared the concentration of ions at the duodenal and ileal ends of the small intestine of sheep, indicated that about 50% of the Na ÷ entering the small intestine was absorbed over its length. The corresponding values for Cl- and PO 3- were 70-80~o in each instance. Similarly, by monitoring the rate of flow through the small intestine, Clark et al. 12 calculated that about 40~o of the water entering the small intestine was absorbed over its length. As in the amino-nitrogen experiments, these values did not allow for any net secretion of ions or water that may have taken place from the mucosa into the lumen of the small intestine.

B. Large Intestine 1. Digestive Processes in the Large Intestine and Caecum

Our extensive knowledge of the processes occurring in the reticulo-rumen reflects the importance of that organ in the digestive physiology of the ruminant. In contrast, comparatively little is known of the range and magnitude of the processes taking place in the large intestine and caecum of ruminants, compared with those in the large intestine and caecum of animals such as the pig aa and horse 4 which lack a rumen. Nonetheless, studies have shown that, in general, the environmental conditions in the large intestine and caecum are not dissimilar to those in the rumen, both having redox potentials of the order of - 3 5 0 m V and a typical pH range of the order of 6.5-7.0. 4s Mann and Orskov 4s compared the bacterial flora present in the rumen and caecum of sheep fed similar diets. In both, the bacterial population density was of the order of 2 x 10l° bacteria/ml, Of the bacteria in the caecum, the population was dominated by an organism apparently identical to Bacteroides ruminicola of the rumen. Other species of bacteria isolated from the caecum were also similar to their tureen counterparts, including a Butyrivibrio species which was the dominant cellulose-decomposing organism in the caecum, a physiological niche similar to that which it occupied in the rumen 34 (Table 6). The volatile fatty acids which occur as the major end-products of microbial fermentation in the tureen were also found in the large intestine and caecum but at a lower total concentration. The concentration of volatile fatty acids in the rumen was about 10ff-150mM. Ward et al. 67 found that the corresponding concentrations in the caecum and in other regions of the large intestine were 60 mM and 7 mM, respectively. As well as being present at different concentrations, the volatile fatty acids are also present in slightly different proportions to those found in the rumen. Mann and Orskov 4s recorded the following average molar percentages of C2-Cs fatty acids in the caecum of the sheep: acetic, 73.8; propionic, 15.4; iso-butyric, 2.4; n-butyric, 4.9; iso-valeric, 2.6 and n-valeric, 0.9. These proportions were more easily changed by alterations in the dietary regime or by different feeding practices than were the proportions of the corresponding compounds in the rumen. On feeding the adult ruminant food either in solution or in suspension from a bottle, the animal brings the lips of the oesophageal groove together to form a channel which by-passes the rumen by leading the liquid directly from the oesophagus into the vicinity of the reticulo-omasal orifice (Fig. 2). When the rumen was by-passed in this way, Mann and IDrskov4° found that

18

C.G. Harfoot

the molar percentage of propionate in the caecum dropped from 15.4 to 11.4 and that of butyric acid increased from 4.9 to 12.5. Orskov et al. 5t had previously found that adding a slurry of starch to the a b o m a s u m of sheep, by-passing the microbial fermentation in the rumen, caused a marked increase in the numbers of Butyrivibrio species in the caecum to the point where it became the dominant organism. A major feature of the metabolism of Butyrivibrio is the production of butyric acid as a major end-product of fermentation by the organism (Table 6). That certain organisms present in the rumen appear to be excluded from the caecum has been shown by Lysons et al. 42 who established a population made up of eight species of rumen bacteria in the rumens of germ-free sheep. O f these eight species only one, a species of Ruminococcus, was not subsequently isolated from the caecum of these animals. It appeared that the two cellulose-hydrolyzing general Butyrivibrio and R u m i n o coccus were able to co-exist in the rumen, whereas in the caecum Butyrivibrio grew to the exclusion of Ruminococcus. N o explanation is available for these variations in caecal populations and in the concentrations of metabolic end-products, but it is logical to assume that nutrient limitations are more likely in the caecum than in the tureen. An important aspect of microbial fermentation in the large intestine and caecum from the host animal's point of view is that the microbial cells synthesized in these regions of the intestinal tract are not subjected to subsequent digestion, and therefore are not available as potential sources of protein to the host animal. 2. Absorption f r o m the L a r g e Intestine and C a e c u m

F r o m the studies of Myers et al., 5° it appears that volatile fatty acids produced in the caecum of sheep are rapidly transported across the caecal wall into the bloodstream. This does not occur against a concentration gradient, and Myers et al. 5° suggest that transport is by simple diffusion. The extent of the contribution to the metabolism of the host animal of these fatty acids is not known, but from the data available 34"4°'67 it would not be unreasonable to suggest that the volatile fatty acids of hind gut and caecal origin contribute about 30% of the total acids entering the ruminant bloodstream, the remaining 700/0 being largely of ruminal origin. Water is also absorbed from the large intestine and caecum, values of 1.0-2.5 l/day in the sheep being recorded by Goodall and Kay, 23 and relatively large amounts of amino-nitrogen are also absorbed by the large intestine, values ranging from 0.5-1.6 g amino-nitrogen/day being obtained by Goodall and K a y 23 and by Bruce et al. II with sheep. As with the studies on absorption from the small intestine, these values did not take into account any additions made to the contents of the large intestine and caecum by secretion or sloughing of the epithelial tissue lining the gut. (Received in revised f o r m 7 April 1978)

REFERENCES [. ABOUAKKADA.A. R., EADIE,J. M. and HOWARD,B. H. Biochem. J. 89, 268-272 (1963). 2. AKIN.D. E.. BURDICK.D. and MICHAELS.G. E. Appl. Microbiol. 27, 1149-1156 (1974). 3. ANNISON,E. F. and ARMSTRONG,D. G. In Physiology of Digestion and Metabolism in the Ruminant, pp. 422-437 (PHILLIPSON. A. T., ed.) Oriel Press, Newcastle-upon-Tyne, 1970. 4. ARGENZIO,R. A.. SOUTHWORTH.M. and STEPHENS,C. E. Ant. J. Physiol. 226. 1043-1050 (1974). 5. ASH,R. W. and KAY.R. N. B. J. Physiol., Lond. 149, 43-57 (1959). 6. BADAWY, A. m., CAMPBELL R. M.. CUTHBERTSON,D. P. and MACKIE,W. W. Br. J. Nutr. 12, 391-403 (1958). 7. BALCH,C. C. and KELLY,A. Br. J. Nutr. 4, 395-398 (1950). 8. BALCn.C. C.. KELLY,A. and HELM,G. Br. J. Nutr. 5, 207-216 (1951). 9. BELL, F. R. J. Physiol., Lond. 142, 503-515 (1958).

10. BROnERG,G. Nord. VetMed. 9, 918-930 (1957). 11. BRUCE, J., GOODALL, E. D., KAY, R. N. B., PHILLIPSON, A. T. and VOWLES, L. E. Proc. R. Soc. B 166,

46-62 0966). 12. COATS,D. A. and WRIGar, R. D. J. Physiol.. Lond. 135, 611-622 (1957).

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