2. (PhD in Animal Nutrition & Physiology)
By: A. Riasi
(PhD in Animal Nutrition & Physiology)
Advanced
Digestive Physiology
(part 4)
Isfahan University of Technology
Isfahan, Iran

3. Rumen and reticulum properties3 3

4. Rumen and reticulum properties4 4

5. The reticulum has a role in mechanical digestion of food.
Ruminoreticular wall structure
Ruminoreticular wall architecture: In the rumen the mucosal layer of the finger-like papillae containing highly vascularized connective tissue core and covered by stratified squamous epithelium with slight keratinzation. The epithelium is involved in the absorption of short-chain fatty acids. This transepithelial transport is highest in areas where the cells appear to be swollen. Muscularis mucosae are absent in this part of the forestomach.
In the reticulum the mucosa contains rows of folds (laminae) that form square honeycomb-like compartments called cells. On the surface of the laminae are short, conical projections called papillae. The stratified squamous epithelium also absorbs short-chain fatty acids. Near the margins of the folds are strands of smooth muscle fibers forming the muscularis mucosae. Contractions of the honeycomb cells, with the purse-string action of the smooth muscle strands, help the mechanical digestion of the ingesta. Food and sometimes non-food “objects” drop in here first (hardware disease, traumatic reticuloperitonitis).
The muscularis external in the ruminoreticular wall contain two layers of smooth muscle fibers (internal oblique fiber along longitudinal fiber or circular fiber (Fig. 6). Myenteric plexus can be found between these layers.
The reticulum has a role in mechanical digestion of food. 5 5

6. The venous blood drains into the hepatic portal vein.
Blood circulation of forestomachs
Celiac artery supply the blood flow to forestomach and the most part of abomasum.
The venous blood drains into the hepatic portal vein.
Blood circulation of forestomachs: The ruminant stomach is highly vascularized, and the blood flow to the luminal epithelium is greatly increased when fermentation end products are being absorbed. The arterial supply to the forestomach and most of the abomasum is via the left gastric branch of the celiac artery. The venous blood drains into the hepatic portal vein and passes to the liver before being returned the caudal vena cava by the hepatic veins. 6 6

7. The innervations of fore stomachs:
Innervations and the receptors
The innervations of fore stomachs:
Vagal nerves (10:1 afferent/efferent ratio)
Splanchnic nerves (3:1 afferent/efferent ratio)
Innervations and the receptors: The ruminant stomach is innervated by vagal and splanchnic nerves, both of which provide sensory (afferent) and motor (efferent) pathways. The vagal motor (parasympathetic) nerve fibers to the forestomach originate in the right and left gastric centers within the dorsal vagal nuclei of the medulla oblongata in the hindbrain. The right and left vagi in the thoracic region divide into dorsal and ventral branches. The dorsal branches of each side unite to form the dorsal vagal trunk, which innervates all region of the ruminant stomach. Likewise, the two ventral branches unite to the form the ventral vagal trunk, which supplies all regions of the ruminant stomach except the dorsal ruminal sac. Discharges in the vagal motor nerves are essential for the major contraction cycles of the forestomach (the primary and secondary cycles). The spanchnic motor (sympathetic) nerve fiber supply all region of ruminant stomach. When stimulated they inhibit motility, but when they are removed or blocked with drugs no effect is observed and thus they appear not be tonically active.
The ratio of afferent/efferent fibers is 10:1 in the vagi and 3:1 in the spalnchnic nerves. Therefore, although most emphasis is placed on the motor roles of the nerves, on numerical grounds they are predominantly sensory nerves! The vagal nerves transmit sensory information from two known kinds of sensory receptor: (1) tension receptors and (2) epithelial/mucosal receptors. In all stomach compartments the tension receptors are slowly adapting mechanoreceptors located in the muscle layer "in series" with the contractile elements (smooth muscle cells). They are excited by passive distention of the viscus and specially by active contraction of the smooth muscle. The epithelial receptors of the forestomach lie close to the basement membrane of the luminal epithelium and behave both as rapidly adapting mechanoreceptors and as chemoreceptors. Their grates excitation is produced by repetitive light moving, tactile stimuli (rapid light brushing) and by a range a chemicals: acids, alkali, hypotonic salt solutions. Both tension receptors and epithelial/mucosal receptors sensory nerve fibers project to the left and right to the dorsal vagal motor nuclei and to the reticular formation lying lateral and dorsal to these nuclei. The splanchnic nerves transmit sensory information from serosal receptors and possibly from tension receptors. The serosal receptors are particularly dense at the attachment of the mesenteries. They are slowly adapting mechanoreceptors, and their responses to many forms of stimuli resemble, and could be confused with, those of tension receptors. 7 7

8. Innervations and the receptors
The vagal nerves transmit sensory information from two known kinds of sensory receptors:
Tension receptors
Epithelial/mucosal receptors 8 8

9. The splanchnic nerves transmit sensory information from:
Innervations and the receptors
The splanchnic nerves transmit sensory information from:
Serosal receptors
Possibly tension receptors 9 9

10. The movements serve to:
Mix the ingesta
Aid in eructation of gas
Propel fluid and fermented foodstuffs into the omasum.
A cycle of contractions occurs 1 to 3 times per minute.
Ruminoreticular motilities
Ruminoreticular motilities and their control: An orderly pattern of ruminal motility is initiated early in life and, except for temporary periods of disruption, persists for the lifetime of the animal. These movements serve to mix the ingesta, aid in eructation of gas, and propel fluid and fermented foodstuffs into the omasum. If motility is suppressed for a significant length of time, ruminal impaction may result.
A cycle of contractions occurs 1 to 3 times per minute. The highest frequency is seen during feeding, and the lowest when the animal is resting. Two types of contractions are identified:
Primary contractions originate in the reticulum and pass caudally around the rumen. This process involves a wave of contraction followed by a wave of relaxation, so as parts of the rumen are contracting, other sacs are dilating.
Secondary contractions occur in only parts of the rumen and are usually associated with eructation.
A typical primary cycle lasts about 20 seconds and consists, in turn, of (1) biphasic (double) contraction of the reticulum, (2) a caudally moving monophasic contraction of the dorsal ruminal sac, and (3) a contraction of the ventral ruminal sac. The reticulum relaxes completely in cattle and incompletely in sheep between the two phases of its biphasic contraction.
Usually at the end of alternate primary cycles the secondary cycle, eructation cycle, may occur and consist of sequential contractions of (1) the caudoventral ruminal blind sac, (2) a cranially moving contraction of the caudodorsal ruminal blind sac followed by the middorsal ruminal sac, and (3) a contraction of the ventral sac. 10 10

11. Two types of contractions are identified:
Ruminoreticular motilities
Two types of contractions are identified:
Primary contractions
Secondary contractions 11 11

12. Typical primary cycle consist of:
Ruminoreticular motilities
Typical primary cycle consist of:
Biphasic (double) contraction of the reticulum
Caudally moving monophasic contraction of the dorsal ruminal sac
A contraction of the ventral ruminal sac 12 12

13. Secondary cycle may occur and consist of sequential contractions of:
The caudoventral ruminal blind sac
A cranially moving contraction of the caudodorsal ruminal blind sac followed by the middorsal ruminal sac
A contraction of the ventral sac.
Ruminoreticular motilities 13 13

14. Contraction sequences in the ruminoreticulum of the resting sheep, based on drawing derived from tracing made directly from radiographs. Dashed lines represent ruminal pillar. Stippled regions represent the gas layer. The heavy dotted line represents the point of attachment of the rumen to the dorsal abdominal wall. The heavy solid line represents the region of wall actually contracting. 1. Resting stage between contraction cycles Primary cycle, commencing with the caudal wall of the reticulum and ruminoreticular fold (1) followed in turn by a double contraction of the reticulum (3-4). A contraction of the cranial sac, cranial pillar, and dorsal sac (5-7). A contraction of the caudodorsal blind sac and of the cranial and caudal pillar (8). A contraction of the longitudinal folds and cranial region of the ventral ruminal sac (9). Spreading caudally (10-14) and then cranially (15-16) , Secondary cycle, typically occurring after alternate primary cycles. Staring with a contraction of the caudal pillar and caudoventral blind sac (17) followed in turn by contractions of the caudodorsal blind sac (18). The (mid-) dorsal sac (19). The caudoventral blind sac (20), and the cranial region of the ventral sac (21). Eructation occurs occasionally during the primary cycle when the cardia is covered by a gas layer (8), but it occurs more commonly during a secondary sac (19). 14 14

15. Ruminoreticular motilities
When ingesta enter the foestomach, heavy objects fall into the reticulum and lighter material enters the rumen.
Added to this mixture are voluminous quantities of gas produced during fermentation.
During and shortly after feeding the rate of primary and secondary cycles almost double. During rumination, the rate of primary cycles has a value lying between the resting (not feeding, not ruminating) rate and the feeding rate, and the primary/secondary cycle rations are as at rest.
Ingesta enter the foestomach through the cardia lying in the dorsomedial wall of the reticulum. Heavy objects (grain, rocks, nails) fall into the reticulum, while lighter material (grass, hay) enters the rumen proper. Added to this mixture are voluminous quantities of gas produced during fermentation. All these materials within the rumen partition into three primary zones based on their specific gravity. Gas rises to fill the upper regions, grain and fluid-saturated roughage ("yesterday's hay") sink to the bottom, and newly arrived roughage floats in a middle layer. As fermentation proceeds, feedstuffs are reduced to smaller and smaller sizes and microbes constantly proliferate.
Most ingesta have a low density because of their fiber content and trapped air. Ingesta float high up in the reticulum and cranial sac of the rumen until the next biphasic reticular and cranial sac contractions carry them caudally to joint the fibrous raft. There they become enmeshed in the raft as it is slowly rotated anti clockwise (as seen from the left side) during the powerful dorsal ruminal sac contractions.
Saliva, ingested water, and swallowed cud join the soupy material in the reticulum. This flows in turn into the cranial sac and then either back into the reticulum or on into the dorsal and ventral sacs. The contraction of the ventral sac forces its soupy fluid contents along the ventral and cranial surfaces of the fibrous raft to mix with the fluid in the cranial ruminal sac and reticulum. The material that leaves the reticulum via the reticuloomasal orifice is a random portion of the soupy material that is common to the reticulum, cranial sac, and ventral sac and happens to be in the reticulum at the time the orifice is open and when there is a suitable pressure gradient from the reticulum to the omasum. The particles in the omasum are the same size as those in the reticulum, so the orifice does not have a sieving role. 15 15

16. Ruminoreticular motilities16 16

17. The forestomachs possess a rich enteric nervous system.
Ruminoreticular motilities
The forestomachs possess a rich enteric nervous system.
Contractions coordination need the central input.
Motility centers in the brainstem control both the rate and strength of contraction via vagal efferents.
The forestomachs possess a rich enteric nervous system, but coordinated contractions require central input. Motility centers in the brainstem control both the rate and strength of contraction via vagal efferents. There are also vagal afferents from the rumen to the motility centers which allow stretch receptors and chemoreceptors in the rumen to modulate contractility. Because of their dependence on activity in extrinsic (vagal) nerves, the primary and secondary cycles are often referred to as extrinsic contractions and, after the destruction of more than 50 percent of the vagal nerve supply to the ruminoreticulum, extrinsic contractions are proportionately abolished. Total vagotomy or cholinergic blockade results in total loss of contractions (ruminoreticular stasis). Feeble intrinsic contractions responsible for the smooth muscle tone in the forestomach wall arise from nervous activity in its intrinsic nerve networks. When extrinsic contractions are absent the intrinsic contractions become more forceful, but they do not produce a recognizable cyclical sequence and can not compensate for the loss of extrinsic contractions. In such a case the animal dies, unless, experimentally, it is given special feeding directly into the ruminoreticulum and abomasum.
The gastric centers do not have spontaneous activity and need to be driven by excitatory inputs in excess of inhibitory inputs from other parts of the nervous system. Inputs from unidentified higher brain-stem centers are excitatory, and those from lower brain-stem centers are inhibitory. Splanchnic nerve inputs do not appear to be tonically active but are potentially inhibitory. The principal inputs to the gastric centers are from the forestomachs, abomasum, and duodenum by way of vagus nerves, the excitatory inputs normally dominating the inhibitory inputs. When the animal is feeding and ruminating, a particularly potent excitatory input to the gastric centers arises from buccal mechanoreceptors during the act of chewing, which accounts for the more frequent and more forceful cyclical movements that occur at these times. 17 17

18. Ruminoreticular motilities
There are also vagal afferents from the rumen to the motility centers which allow stretch receptors and chemoreceptors in the rumen to modulate contractility. 18 18

19. Feeble intrinsic contractions responsible for the smooth muscle tone in the forestomach wall arise from nervous activity in its intrinsic nerve networks.
Ruminoreticular motilities 19 19

20. Ruminoreticular motilities
The gastric centers do not have spontaneous activity and need to be driven by
Excitatory inputs
Inhibitory inputs 20 20

21. The principal inputs to the gastric centers are from:
Ruminoreticular motilities
The principal inputs to the gastric centers are from:
Forestomachs
Abomasum
Duodenum by way of vagus nerves 21 21

22. Stimulus
Receptors
Projections
Effect

23. Stimulus
Receptors
Projections
Effect

24. Stimulus
Receptors
Projections
Effect

25. The known sensory receptor mechanisms are responsible for the vagal inputs.
The tension receptors are located in the muscle layer of different parts.
The epithelial receptors are located closed to the basement membrane of the luminal epithelium of the forestomachs.
Ruminoreticular motilities
The known sensory receptor mechanisms (tension receptors and epithelial/mucosal receptors) are responsible for the vagal inputs. The tension receptors are located in the muscle layer of all parts of the forestomachs, abomasums, and intestine and monitor the tension present in the muscular wall. The epithelial receptors are located closed to the basement membrane of the luminal epithelium of the forestomachs, and the mucosal receptors are located close to the luminal mucosal of the abomasum and small intestine. They are unusual in that they are excited both by mechanical stimuli and by certain chemical stimuli. They have a low threshold to mechanical stimulation and are particularly sensitive to any moving light tactile stimulus. The acid sensitivity of these receptors is not due to the hydrogen ion concentration (pH) of an individual acid solution but is more closely related to its acidity. The epithelial receptors in the forestomachs and the mucosal receptors in the abomasum lie about 150 µm below the luminal surface, and this long diffusion distance favors low-molecular-weight weal acids, particularly those with high membrane permeabilities, such as butyric acid. Some high-molecular-weight acids (e.g. citric acid) and acids with low permeabilities (e.g. lactic acid) are relatively ineffective in the direct excitation of epithelial/mucosal receptors. When mixed with ruminal contents they may be more effective in exciting epithelial receptors than when they are used in isolation. This is due to an indirect action whereby the VFAs in the ruminal fluid are rendered more undissociated by a stronger acid (such as lactic acid), so that the titratable acidity of the VFAs increase above the threshold for receptor activation. 25 25

26. Conditions inside the rumen can significantly affect motility.
Acidic ruminal contents
High roughage diet
Ruminoreticular motilities
Sustained epithelial receptor discharges are also encountered in experimentally induced "ruminal acidosis" and reflexly lead to progressive reductions in ruminoreticular motility to the point of total stasis. It seems unlikely that the chemosensitive properties of these receptors have any role in regulating normal ruminal pH, but their activation may become important during the early stages of clinical ruminal acidosis. The reduction in ruminoreticular motility may, in turn, reduce fermentation and thereby reverse the developing acidosis.
Low and moderate levels of passive reticular distension and ruminoreticular fold stretching have been shown to excite tension receptors and gastric center neurons, leading to increase in the rate and amplitude of primary and secondary cycle contractions. These excitatory inputs also go to salivary centers to produce the increased salivation that occur under these circumstances. Tension receptor inputs are evoked actively during relatively isometric contractions, particularly when the ruminoreticular contents are insufficiently fluid. These give beneficial effects through reflexly compensatory increase in salivation and in the force and amplitude of the contraction themselves. High levels of reticular distension, in contrast, lead reflexly to a reduction in ruminoreticular motility and salivation, even though tension receptor activity is at peak. This paradox may be due to the excitation of separate high threshold tension receptors with an inhibitory action, although no such receptors have been observed.
Conditions inside the rumen can significantly affect motility. If, for example, ruminal contents become very acidic (as occurs in grain engorgement), motility will essentially cease. Also, the type of diet influences motility: animals on a high roughage diet have a higher frequency of contractions than those on a diet rich in concentrates. 26 26

27. Ruminants are well known for "cud chewing“
It provides effective mechanical breakdown of roughage and increases substrate surface area.
Rumination is a unique characteristic of:
True ruminants (deer, giraffes, and bovidae)
Pseudoruminants (camels and llamas).
Rumination and its components
Rumination and its components
Ruminants are well known for "cud chewing". It provides for effective mechanical breakdown of roughage and thereby increases substrate surface area to fermentative microbes. Rumination is a unique characteristic of the true ruminants (deer, giraffes, and bovidae) and pseudoruminants (camels and llamas). It is most pronounced in the relatively nonselective, coarse grazing members of the bovidae (cattle) fed high-roughage diets, when it may occupying up to 10 hours a day. With low- roughage diets or diets in which the roughage is finely ground, total rumination time may be as short as 3 hours a day or less.
Rumination occurs predominantly when the animal is resting and not eating, but that is a considerable fraction of the animal's lifespan. The highest incidences of rumination occur during the afternoon and in the middle of the night. However many lactating ruminants ruminate while they are suckling their young or are being milked. 27 27

28. Rumination occurs in resting.
Rumination and its components
Rumination occurs in resting.
The highest incidences of rumination occur during afternoon and middle of the night.
Many lactating ruminants ruminate while they are suckling their young or are being milked. 28 28

29. The time spent ruminating by a given animal depends on:
Rumination and its components
The time spent ruminating by a given animal depends on:
The texture of the food
The amount of food ingested
Cattle may ruminate from 35 to 80 minutes per kilogram of roughage consumed.
The urge to ruminate is strong and leads to single or successive cycles of rumination lasting about 60 seconds, which, all together represent as much as 8 hours per day. The time spent ruminating by a given animal depends on the texture of the food and the amount of food ingested. Ruminants fed concentrates or pelleted hay ruminate less than those that receive hay. When the texture of food is unchanged, the rumination time is related to the quantity eaten. Cattle may ruminate from 35 to 80 minutes per kilogram of roughage consumed.
Rumination is centrally mediated by the "gastric centers" of the medulla oblongata and the ventral hypothalamic area. Not all the factors that stimulate or inhibit rumination have been clearly defined. Tactile stimulation of the reticular and ruminal epithelia is a powerful stimulus for rumination, especially when digesta move over the ruminoreticular fold during contraction. The carniocaudal pillar complex harbors receptors to monitor the volume and texture of ruminoreticular digesta.
The time allowed for the physical break down of food through rumination is controlled by sensory information about the digesta that is perceived at or in pillars. The types of sensory information are digesta texture, from tactile stimulation, digesta consistency, from the resistance opposed to the contracting pillars; and rumen fill, from the stretch of the pillars. In an assay, intravenous injection of a very low dose of epinephrine (3 µg/kg) induced rumination in sheep by enhancing the afferent input from the reticular wall and rumination was inhibited in cattle and sheep by a large dose of epinephrine. 29 29

30. Rumination and its components
Pharmacological agents have been used to examine the physiological mechanisms involved in evoking rumination.
Volatile fatty acids
Catecholamines
Gastric hormones
Opioids
Autacoids
Pharmacological agents such as volatile fatty acids, catecholamines, gastric hormones, opioids and autacoids have been used to examine the physiological mechanisms involved in evoking rumination. Alpha-2 adrenoreceptor agonists, pentagastrin and serotonin have been found to evoke rumination. Naloxone facilitated the evocation of rumination by adrenaline. The exact means whereby these pharmacological agents exert their effects.
The intravenous injection of a very low dose of epinephrine (3 µg/kg) induced rumination in sheep
Epinephrine enhances the afferent input from the reticular wall.
Rumination was inhibited in cattle and sheep by a large dose of epinephrine. 30 30

31. Rumination is centrally mediated by the "gastric centers“ located at:
Rumination and its components
Rumination is centrally mediated by the "gastric centers“ located at:
Medulla oblongata
Ventral hypothalamic area
Tactile stimulation of the reticular and ruminal epithelia is a powerful stimulus for rumination. 31 31

32. Sensory information from digesta that is perceived in pillars:
Rumination and its components
Sensory information from digesta that is perceived in pillars:
Digesta texture
Digesta consistency
Rumen fill 32 32

33. Rumination and its components Four phases of a rumination cycle:
Regurgitation
Remastication
Reinsalivation
Reswallowing
Regurgitation starts with an inspiration effort.
A rumination cycle includes four phases of activity: regurgitation of ingesta from the ruminoreticulum, followed by remastication, reinsalivation, and reswallowing (or redeglutition) of the finely rechewed bolus.
Rumination starts with regurgitation phase, which is the swift movement of semiliquid digesta from the ruminoreticulum, through the cardia, up the esophagus, and into the mouth. Regurgitation starts with an inspiration effort, with the tongue and soft palate raised to block the buccal cavity and nasopharyngeal orifice, respectively, so that an increased negative pressure (-25 to -40 mm Hg) develops in the thorax. The bolus is carried into the mouth by reverse peristalsis. Rapid antiperistalsis is provided by the esophageal striated muscle of ruminants. The antiperistaltic wave of regurgitation passes over the esophagus at a velocity of 0.2 m per second (i.e., two times {in cattle} to five to six times {in sheep} faster than the velocity of the normal peristalsis of deglutition). In about two seconds, the "retained" bolus has the fluid squeezed out of it. This fluid is immediately swallowed, and the remainder is chewed, reinsalivated, and subsequently swallowed. The "tail" bolus (i.e., the excess fraction in the pharynx and esophagus of the slurry) is reswallowed within 1 second of regurgitation. Regurgitation is heralded by a long-lasting (2 to 4 seconds in sheep, 1 to 1.5 seconds in cattle) extra contraction of the reticulum, which is immediately followed by the normal biphasic reticular contraction. The reticular extra contraction in conjunction with relaxation of the distal esophageal sphincter, allows a bolus of ingesta to enter the esophagus. Electromyographic and fluoroscopic evidence shows that the influx of ruminoreticular digesta into the esophagus occurs when the lower esophageal sphincter (LES) is actively opened. Also, at this moment, the upper esophageal sphincter (UES) is relaxed.
The remastication phase of rumination is characterized by chewing movements occurring at a slower and more regular rate than those associated with ingestion of food (primary mastication). The pattern of mandible movements are different during eating and during rumination (fig 18). During rumination cycle that last about 60 seconds, most of the time (50 seconds) is taken by the chewing and salivation phases, and the remaining 10 seconds are for regurgitation and redeglutition. The rate and duration of rumination chewing are controlled by the texture (coarseness) and quantity of the food. Rumination involves a great number of remastication movements (9000 to daily). 33 33

34. Rumination and its components
The bolus is carried into the mouth by reverse peristalsis.
Antiperistaltic waves of regurgitation passes over the esophagus at a velocity of 0.2 m per second. 34 34

35. Rumination and its components
In about two seconds, the "retained" bolus has the fluid squeezed out of it.
This fluid is immediately swallowed, and the remainder is chewed, reinsalivated, and subsequently swallowed. 35 35

36. Rumination and its components36 36

37. Rumination and its components
The rate and duration of rumination chewing are controlled:
The texture (coarseness)
Quantity of the food
The reinsalivation phase of rumination coincides with the remastication phase. The serous rumination saliva comes mostly from the parotid gland on the side of chewing during rumination and does not contain any secretion from the mandibular glands. By contrast, ingestion (primary) saliva contains more mucous (buccal glands) and seromucous (sublingual and mandibular glands) secretions, to coat and lubricate the boluses of forage for easier swallowing. Rumination saliva and ingestion saliva are secreted at about the same rate and about 2.5 times the resting rate. The latter is maintained by activation of receptors of the ruminoreticular epithelium by tactile stimuli. Raising the percentage of dry matter (DM) or fiber in the food increases saliva secretion during ingestion, resting, and rumination. When widely disparate feedstuffs are fed, particle size seems more important in determining the rate of salivation.
A rumination cycle is concluded with the redeglutition phase of the now reinsalivated and rechewed portion of ruminoreticular content that was regurgitated. In sheep, the esophageal peristalsis of redeglutition propels reswallowed fluids at the rate of 35 cm per second and the remasticated bolus at 21 cm per second. 37 37

38. Rumination and its components
The reinsalivation phase of rumination coincides with the remastication phase.
Rumination saliva and ingestion saliva are secreted at about the same rate and about 2.5 times the resting rate. 38 38

39. Eructation
Eructation is the physiological process of expelling ruminoreticular gases:
Carbon dioxide, 65%;
Methan, 25%;
Nitrogen, 7%;
Oxygen, 0.5%;
Hydrogen, 0.2%;
Hydrogen sulfide, 0.01%.
Eructation
Fermentation in the rumen generates enormous, even frightening, quantities of gas. We're talking about liters per hour in adult cattle and about 5 liters per hour in a sheep or goat. Eructation (belching) is how ruminants continually get rid of fermentation gases.
Eructation is the physiological process of expelling ruminoreticular gases (carbon dioxide, 65%; methan, 25%; nitrogen, 7%; oxygen, 0.5%; hydrogen, 0.2%; hydrogen sulfide, 0.01%) through the cerdia and the esophagus. Rumen gases, particularly methane, are increasingly in the news because of their contribution to greenhouse gas and climate change.
The eructation is associated with almost every secondary ruminal contraction. Eructated gas travels up the esophagus at 160 to 225 cm per second (Stevens and Sellers, Am J Physiol 199:598, 1960) and, interestingly, a majority is actually first inspired into the lungs, then expired. Eructation occurs at a rate of one to three times per minute but is not audible or evident at the nostrils or the mouth. Because of closure of the nasopharyngeal sphincter, a large portion of the eructated gases (carbon dioxide, methane) is inspired and recycled into the organism by absorption into the lungs. This explains how strong volatile flavors from ruminoreticular degradation of plants (garlic, onion, leek) reach the mammary glands and are incorporated into milk.
Most fermentation occurs in the raft, and it is therefore the site of origin of most of the gases of fermentation. During the kneading of the raft by the dorsal ruminal sac contractions, its dorsal surface splits, and this facilitate the release of free gas into the gas layer above the raft. Not much of the gas is absorbed, and most must be eliminated by eructation. During the secondary cycles, to a greater extent than during the primary cycles, the dorsal sac contraction causes the gas layer to be moved cranially into the reticulum while the raft and fluid material are forced ventrally and held back primarily by the cranial pillar in cattle or by the ruminoreticular fold in sheep. If the gas layer reaches the cardia and clear it of fluid, the eructation mechanism is evoked. 39 39

40. Eructated gas travels up the esophagus at 160 to 225 cm per second
Eructation
The eructation is associated with almost every secondary ruminal contraction.
Eructated gas travels up the esophagus at 160 to 225 cm per second
Interestingly, a majority is actually first inspired into the lungs, then expired. 40 40

41. Eructation
A large portion of the eructated gases is inspired and recycled into the organism by absorption into the lungs. 41 41

42. Eructation
The rumen raft is the site of origin of most of the gases of fermentation.
When dorsal surface of raft splits, releasing of free gas into the gas layer occur.
Not much of the gas is absorbed, and most must be eliminated by eructation. 42 42

43. Eructation
If the gas layer reaches the cardia and clear it of fluid, the eructation mechanism is evoked.
Eructation is a vagovagal reflex, with centers in the medulla ablongata.
Mechanical receptors to detect distention are present in
Rumen dorsal sac
Reticular groove
Around the cardia and esophagus
Eructation is a vagovagal reflex, with centers in the medulla ablongata. Mechanical receptors to detect distention are present in the:
Rumen dorsal sac
Reticular groove
Around the cardia
Esophagus.
The effector response involves clearing the submerged cardia of digesta by contraction of the cranial and caudal pillars of the rumen, simultaneously relaxing the cardia and contracting the dorsal sac, and a biphasic esophageal phase. The esophageal first experience a filling phase, with an increase in esophageal pressure between the closed diaphragmatic and lower esophageal sphincters. The second esophageal phase bring clearing contractions (i.e., retroperistalsis bring up gases) from the diaphragmatic sphincter of the esophagus, and an immediate abroad peristalsis to clear the esophagus of remaining fluids.
Experimental insufflation of the rominoreticulum stimulate the eructation reflex, which is accompanied by motility of the ventral sac of the rumen to displace the gases dorsally into the rumen and cranially toward the relaxed cardia. This ruminal motility is an example of the secondary type of contraction of the rumen, which starts caudally (dorsal blind sac) and spread oral. The primary type of rumen contraction always follows the biphasic reticular contraction and spread caudally. In cattle, 66 percent of eructations coincide with the secondary type of rumen contraction, and 20 percent with the primary type. In sheep, eructation may occur independently of the rumen (4 percent), concurrently with secondary-type rumen contractions (37 percent), and most of the time with the primary type (60 percent).
Anything that interferes with eructation is life threatening to the ruminant because the expanding rumen rapidly interferes with breathing. Animals suffering ruminal tympany (bloat) die from asphyxiation. 43 43

44. Eructation
The primary type of rumen contraction always follows the biphasic reticular contraction and spread caudally.
In cattle
66% with the secondary type of rumen contraction,
20% with the primary type. 44 44

45. Eructation In sheep 3% no dependent rumen motility
37% with secondary-type rumen contractions
60% with the time with the primary type 45 45

46. Strategies for lower methane emission
Increasing the efficiency in which animals use nutrients to produce milk or meat.
Rumen modifiers such as ionophores improve dry matter intake efficiency and suppress acetate production, which results in reducing the amount of hydrogen released.
Dietary strategies to lower methane emissions
There has been a lot of research conducted in Canada, Australia, Europe and the US on strategies to reduce methane emissions from dairy and beef operations. The main focus has been on nutritional strategies, especially cows grazing pasture. Some dietary practices that have been shown to reduce CH4 include the addition of ionophores, fats, the use of high quality forages and the increased use of grains.
These nutritional strategies reduce CH4 through the manipulation of ruminal fermentation, direct inhibition of the methanogens and protozoa or by a redirection of hydrogen ions away from the methanogens. Relatively new mitigation options have been investigated and include the addition of such additives as probiotics, acetogens, bacteriocins, organic acids, and plant extracts (i.e. condensed tannins). For the long term approach, genetic selection of cows that have improved feed efficiency have a possibility. The following gives more detail about some of the strategies that reduce CH4:
1. Increasing the efficiency in which animals use nutrients to produce milk or meat can result in reduced CH4 emissions. This can be accomplished by feeding high quality, highly digestible forages or grains. However, the emissions produced in producing and/or transporting the grain or forage should be considered.
2. Rumen modifiers such as ionophores improve dry matter intake efficiency and suppress acetate production, which results in reducing the amount of hydrogen released. In some of the published research, CH4 has been reduced by 10%, however the affect of the ionophores have been short-lived in respect to CH4 reduction. Mores research on the continued use of ionophores for this purpose is needed.
3. The grinding and pelleting of forages can reduce emissions by 40% however the costs associated with this practice may be prohibitive.
4. Dietary fats have the potential to reduce CH4 up to 37%. This occurs through biohydration of unsaturated fatty acids, enhanced propionic acid production and protozoal inhibition. The effects are variable and lipid toxicity to the rumen microbes can be a problem. This strategy can affect milk components negatively and result in reduced income for the producer.
There are several novel approaches to reducing CH4 that are not very practical at this point. An example would be the defaunation of the rumen. Removing protozoa has been demonstrated to reduce CH4 emissions by 20 percent. There may be opportunities to develop strategies that encourage acetogenic bacteria to grow so they can perform the function of removing hydrogen instead of the methanogens. Acetogens convert carbon dioxide and hydrogen to acetate, which the animal can use as an energy source. There is also research being conducted to develop a vaccine, which stimulates antibodies in the animal that are active in the rumen against methanogens.
The problems with some of these mitigation strategies to reduce CH4 are potential toxicity to the rumen microbes and the animal, short-lived effects due to microbial adaptation, volatility, expense, and a delivery system of these additives to cows on pasture. 46 46

47. Strategies for lower methane emission
The grinding and pelleting of forages can reduce emissions by 40% .
Dietary fats have the potential to reduce CH4 up to 37%. 47 47