The rumen, also known as a paunch, is the largest stomach compartment in ruminants and the larger part of the reticulorumen, which is the first chamber in the alimentary canal of ruminant animals.[1] The rumen's microbial favoring environment allows it to serve as the primary site for microbial fermentation of ingested feed.[1] The smaller part of the reticulorumen is the reticulum, which is fully continuous with the rumen, but differs from it with regard to the texture of its lining.
The rumen is composed of several muscular sacs, the cranial sac, ventral sac, ventral blind sac, and reticulum.
The lining of the rumen wall is covered in small fingerlike projections called papillae, which are flattened, approximately 5mm in length and 3mm wide in cattle. The reticulum is lined with ridges that form a hexagonalhoneycomb pattern. The ridges are approximately 0.1–0.2mm wide and are raised 5mm above the reticulum wall. The hexagons in the reticulum are approximately 2–5cm wide in cattle. These features increase the surface area of the reticulorumen wall, facilitating the absorption of volatile fatty acids. Despite the differences in the texture of the lining of the two parts of the reticulorumen, it represents one functional space.
Digestion
Digestion in the reticulorumen is a complex process. Digestion occurs through fermentation by microbes in the reticulorumen rather than the animal per se. The reticulorumen is one of the few organs present in animals in which digestion of cellulose and other recalcitrant carbohydrates can proceed to any appreciable degree.
The main substrates of digestion in the reticulorumen are non-structural carbohydrates (starch, sugar, and pectin), structural carbohydrates (hemicellulose and cellulose), and nitrogen-containing compounds (proteins, peptides, and amino acids). Both non-structural and structural carbohydrates are hydrolysed to monosaccharides or disaccharides by microbial enzymes. The resulting mono- and disaccharides are transported into the microbes. Once within microbial cell walls, the mono- and disaccharides may be assimilated into microbial biomass or fermented to volatile fatty acids (VFAs) acetate, propionate, butyrate, lactate, valerate and other branched-chain VFAs via glycolysis and other biochemical pathways to yield energy for the microbial cell. Most VFAs are absorbed across the reticulorumen wall, directly into the blood stream, and are used by the ruminant as substrates for energy production and biosynthesis.[2] Some branched chained VFAs are incorporated into the lipid membrane of rumen microbes. Protein is hydrolysed to peptides and amino acids by microbial enzymes, which are subsequently transported across the microbial cell wall for assimilation into cell biomass, primarily. Peptides, amino acids, ammonia, and other sources of nitrogen originally present in the feed can also be used directly by microbes with little to no hydrolysis. Non-amino acid nitrogen is used for synthesis of microbial amino acids. In situations in which nitrogen for microbial growth is in excess, protein and its derivatives can also be fermented to produce energy, yielding ammonia.
Lipids, lignin, minerals, and vitamins play a less prominent role in digestion than carbohydrates and protein, but they are still critical in many ways. Lipids are partly hydrolysed and hydrogenated, and glycerol, if present in the lipid, is fermented. Lipids are otherwise inert in the rumen. Some carbon from carbohydrate or protein may be used for de novo synthesis of microbial lipid. High levels of lipid, particularly unsaturated lipid, in the rumen are thought to poison microbes and suppress fermentation activity. Lignin, a phenolic compound, is recalcitrant to digestion, through it can be solubilized by fungi. Lignin is thought to shield associated nutrients from digestion and hence limits degradation. Minerals are absorbed by microbes and are necessary to their growth. Microbes in turn synthesize many vitamins, such as cyanocobalamin, in great quantities—often great enough to sustain the ruminant even when vitamins are highly deficient in the diet.
Stratification and mixing of digesta
Digested food (digesta) in the rumen is not uniform, but rather stratified into gas, liquid, and particles of different sizes, densities, and other physical characteristics. Additionally, the digesta is subject to extensive mixing and complicated flow paths upon entry into the rumen. Though they may seem trivial at first, these complicated stratification, mixing, and flow patterns of digesta are a key aspect of digestive activity in the ruminant and thus warrant detailed discussion.
After being swallowed, food travels down the oesophagus and is deposited in the dorsal part of the reticulum. Contractions of the reticulorumen propel and mix the recently ingested feed into the ruminal mat. The mat is a thick mass of digesta, consisting of partially degraded, long, fibrous material. Most material in the mat has been recently ingested, and as such, has considerable fermentable substrate remaining. Microbial fermentation proceeds rapidly in the mat, releasing many gases. Some of these gases are trapped in the mat, causing the mat to be buoyant. As fermentation proceeds, fermentable substrate is exhausted, gas production decreases, and particles lose buoyancy due to loss of entrapped gas. Digesta in the mat hence goes through a phase of increasing buoyancy followed by decreasing buoyancy. Simultaneously, the size of digesta particles–relatively large when ingested–is reduced by microbial fermentation and, later, rumination. Incomplete digestion of plant material here will result in the formation of a type of bezoar called Phytobezoars. At a certain point, particles are dense and small enough that they may “fall” through the rumen mat into the ventral sac below, or they may be swept out of the rumen mat into the reticulum by liquid gushing through the mat during ruminal contractions. Once in the ventral sac, digesta continues to ferment at decreased rates, further losing buoyancy and decreasing in particle size. It is soon swept into the ventral reticulum by ruminal contractions.
In the ventral reticulum, less dense, larger digesta particles may be propelled up into the oesophagus and mouth during contractions of the reticulum. Digesta is chewed in the mouth in a process known as rumination, then expelled back down the oesophagus and deposited in the dorsal sac of the reticulum, to be lodged and mixed into the ruminal mat again. Denser, small particles stay in the ventral reticulum during reticular contraction, and then during the next contraction may be swept out of the reticulorumen with liquid through the reticulo-omasal orifice, which leads to the next chamber in the ruminant animal's alimentary canal, the omasum.
Water and saliva enter through the rumen to form a liquid pool. Liquid will ultimately escape from the reticulorumen from absorption through the wall, or through passing through the reticulo-omasal orifice, as digesta does. However, since liquid cannot be trapped in the mat as digesta can, liquid passes through the rumen much more quickly than digesta does. Liquid often acts as a carrier for very small digesta particles, such that the dynamics of small particles is similar to that of liquid.
The uppermost area of the rumen, the headspace, is filled with gases (such as methane, carbon dioxide, and, to a much lower degree, hydrogen) released from fermentation and anaerobic respiration of food. These gases are regularly expelled from the reticulorumen through the mouth, in a process called eructation.
Reticulorumenal microbes
Bacteria dominate rumen microbiome; composition can change substantially with diet.
Microbes in the reticulorumen include bacteria, protozoa, fungi, archaea, and viruses. Bacteria, along with protozoa, are the predominant microbes and by mass account for 40-60% of total microbial matter in the rumen. They are categorized into several functional groups, such as fibrolytic, amylolytic, and proteolytic types, which preferentially digest structural carbohydrates, non-structural carbohydrates, and protein, respectively. Protozoa (40-60% of microbial mass) derive their nutrients through phagocytosis of other microbes, and degrade and digest feed carbohydrates, especially starch and sugars, and protein. Although protozoa are not essential for rumen functioning, their presence has pronounced effects.
Ruminal fungi make up only 5-10% of microbes and are absent on diets poor in fibre. Despite their low numbers, the fungi still occupy an important niche in the rumen because they hydrolyse some ester linkages between lignin and hemicellulose or cellulose, and help break down digesta particles. Rumen Archaea, approximately 3% of total microbes, are mostly autotrophic methanogens and produce methane through anaerobic respiration. Most of the hydrogen produced by bacteria, protozoa and fungi is used by these methanogens to reduce carbon dioxide to methane. The maintenance of low partial pressure of hydrogen by methanogens is essential for proper functioning of the rumen. Viruses are present in unknown numbers and do not contribute to any fermentation or respiration activity. However, they do lyse microbes, releasing their contents for other microbes to assimilate and ferment in a process called microbial recycling, although recycling through the predatory activities of protozoa is quantitatively more important.
Microbes in the reticulorumen eventually flow out into the omasum and the remainder of the alimentary canal. Under normal fermentation conditions the environment in the reticulorumen is weakly acidic and is populated by microbes that are adapted to a pH between roughly 5.5 and 6.5; since the abomasum is strongly acidic (pH 2 to 4), it acts as a barrier that largely kills reticulorumen flora and fauna as they flow into it. Subsequently, microbial biomass is digested in the small intestine and smaller molecules (mainly amino acids) are absorbed and transported in the portal vein to the liver. The digestion of these microbes in the small intestine is a major source of nutrition, as microbes usually supply some 60 to 90% of the total amount of amino acids absorbed. On starch-poor diets, they also provide the predominant source of glucose absorbed from the small intestinal contents.
Ruminal Acidosis: In cattle the ruminal acidosis is characterized by the decreased blood pH and bicarbonate due to the over production of ruminal D-lactate. It can appear as acute rumen acidosis due to lactic acid accumulation to sub-acute acidosis due to accumulation of VFAs in the rumen, although acute acidosis is more fatal than sub-acute rumen acidosis. The decrease in ciliated protozoal population of the ruminal fluid is a common feature of both acidosis and can be a good indicator of the ruminal acidosis. Acidosis in rumen occurs when the cattles consume excessive amount of rapidly fermentable non-structural carbohydrates and high grain-based diets with low fiber content. This leads to rapid fermentation of the organic substrates contributing to massive formation of VFAs and lactic acids leading to lower pH and subsequent reactions by the microbes of rumen.
Major differences between the two clinical forms of acidosis:
Ruminal Acidosis
Remarks
Acute acidosis
Sub-acute acidosis
Presence of clinical signs
Yes
Maybe
Mortality
Yes
No
Ruminal Changes
a.Rumen pH
Below 5
5.0-5.4
b.Lactic acid
Increase
Normal
c.Volatile Fatty Acids
Decrease
Increase
d.Gram negative bacteria
Decrease
Normal
e.Gram positive bacteria
Increase
Normal
f.Streptococcus bovis
Increase
Normal
g.Lactobacillus spp
Increase
Normal
h.Lactic acid producers
Increase
Increase
i.Lactic acid consumers
Decrease
Increase
Blood parameters
a.Blood pH
Low
Borderline
b.Bicarbonate
Low
Borderline
c.Lactate
Increase
Normal
Human uses
The feed contained within the reticulorumen, known as "paunch waste", has been studied as a fertiliser for use in sustainable agriculture.[4]
Rumen development
At birth, the rumen organ, rumen epithelium, and rumen microbiota are not fully developed. Developing the rumen after birth is a critical aspect of the overall development of ruminants. This early, sensitive period of rumen development, will have life-lasting effects on the overall health and productivity of the animal. Generally, the most receptive time for rumen development is between the postnatal and weaning periods. Over this period, rumen organ and epithelium growth, along with the establishment of rumen microbiota, will prove to be essential to rumen development.[5]
Papillae, or small, round projections protruding out of the tissue lining, growth in rumen epithelium is essential for rumen functionality. Papillae increase the surface area inside of the rumen. Therefore, papillae allow for a considerable increase in nutrient absorption inside of the rumen. Distinguishing a developed from an undeveloped rumen is simplified by observing the carpeting of tissue surrounding the interior of the rumen. An undeveloped rumen maintains a smooth, papillae-lacking outer surface. A developed rumen possesses thick, papillae-full walls.[6]
Due to ruminants being born with a sterile gastrointestinal tract, the developing rumen must be exposed to an array of microflora at an early stage. This is why a specific diet in which microflora that promote an anaerobic environment in the rumen are favored. Furthermore, feeds must be tailored to the needs of the specific ruminants. Developing ruminants who have been on a strict liquid feed diet will possess different microflora when compared to that of a developing ruminant fed with a combination of a dry and liquid feed. This is due to the nutrients ingested by the animal not entering into the rumen stomach compartment, as it is instead bypassed by the reflexive closure of the esophageal groove.[5]
Ruminant stomach structure
The most abundant bacteria present in the rumen microbiome include Prevotella, Butyrivibrio, and Ruminococcus.[7] This is due to ruminant organisms ingesting high-forage, commonly grass-based diets. Their typical high-forage diets cause this significant demand for cellulose digesting bacteria to be ever-present. Other bacteria, such as Lachnospira multiparus, Prevotella ruminicola, and Butyrivibrio fibrisolvens, play essential roles in the creation of volatile fatty acids (VFAs).[7] Specific feeds can stimulate this extensive bacterial growth in the rumen and therefore aid in the production of these volatile fatty acids, which play a major role in rumen epithelium growth, capillary development, and papillae formation.[8] Previous research identified the significant impact of volatile fatty acids on rumen development through the effects of the inter-ruminal insertion of acetate, propionate, and butyrate.[9] The most visually notable and impactful of these volatile fatty acids was butyrate, which is synthesized naturally in ruminants through multiple anaerobic fermentation pathways of dietary substrates.[10] Butyrate, mainly expressed in epithelial tissue lining, is involved in regulating a plethora of ruminant epithelial cell genes. Generally, butyrate regulates gene expression by acting on cell cycle control pathways.[11] In the epithelial wall of the rumen, butyrate regulates epithelial cell gene expression to increase blood flow and papilla proliferation.[9]
Rumen microbiome genetics
Developing feeds to support the microbiome growth of both production and pet ruminant animals is vital; both for the overall health of the maturing animal and for reducing the costs associated with raising that animal. In the production animal realm, feeding can account for up to 75% of the overall cost associated with that animal, making it crucial to identify and satisfy the nutritional demands of the rumen.[12] Sampling microbial DNA from rumen epithelial cells has led to the identification of microbial genes and functional pathways associated with animal growth factors.[13] Microbial clusters in the rumen possess genes associated with many animal growth-related factors. Protein encoding genes that encode for bacterial cell functions, such as aguA, ptb, K01188, and murD, also are associated with the animal’s average daily weight gain.[12] Furthermore, vitamin B12 related genes, including cobD, tolC, and fliN, are also related to the daily feed intake of the animal.[12]
Related Research Articles
Digestion is the breakdown of large insoluble food compounds into small water-soluble components so that they can be absorbed into the blood plasma. In certain organisms, these smaller substances are absorbed through the small intestine into the blood stream. Digestion is a form of catabolism that is often divided into two processes based on how food is broken down: mechanical and chemical digestion. The term mechanical digestion refers to the physical breakdown of large pieces of food into smaller pieces which can subsequently be accessed by digestive enzymes. Mechanical digestion takes place in the mouth through mastication and in the small intestine through segmentation contractions. In chemical digestion, enzymes break down food into the small compounds that the body can use.
Ruminants are herbivorous grazing or browsing artiodactyls belonging to the suborder Ruminantia that are able to acquire nutrients from plant-based food by fermenting it in a specialized stomach prior to digestion, principally through microbial actions. The process, which takes place in the front part of the digestive system and therefore is called foregut fermentation, typically requires the fermented ingesta to be regurgitated and chewed again. The process of rechewing the cud to further break down plant matter and stimulate digestion is called rumination. The word "ruminant" comes from the Latin ruminare, which means "to chew over again".
Lactic acidosis is a medical condition characterized by a build-up of lactate in the body, with formation of an excessively low pH in the bloodstream. It is a form of metabolic acidosis, in which excessive acid accumulates due to a problem with the body's oxidative metabolism.
Cud is a portion of food that returns from a ruminant's stomach to the mouth to be chewed for the second time. More precisely, it is a bolus of semi-degraded food regurgitated from the reticulorumen of a ruminant. Cud is produced during the physical digestive process of rumination.
A monogastric organism has a simple single-chambered stomach. Examples of monogastric herbivores are horses and rabbits. Examples of monogastric omnivores include humans, pigs, hamsters and rats. Furthermore, there are monogastric carnivores such as cats. A monogastric organism is comparable to ruminant organisms, such as cattle, goats, or sheep. Herbivores with monogastric digestion can digest cellulose in their diets by way of symbiotic gut bacteria. However, their ability to extract energy from cellulose digestion is less efficient than in ruminants.
Streptococcus bovis is a species of Gram-positive bacteria that in humans is associated with urinary tract infections, endocarditis, sepsis, and colorectal cancer. S. gallolyticus is commonly found in the alimentary tract of cattle, sheep, and other ruminants, and may cause ruminal acidosis or feedlot bloat. It is also associated with spontaneous bacterial peritonitis, a frequent complication occurring in patients affected by cirrhosis. Equivalence with Streptococcus equinus has been contested.
The omasum, also known as the bible, the fardel, the manyplies and the psalterium, is the third compartment of the stomach in ruminants. The omasum comes after the rumen and reticulum and before the abomasum. Different ruminants have different omasum structures and function based on the food that they eat and how they developed through evolution.
Enteric fermentation is a digestive process by which carbohydrates are broken down by microorganisms into simple molecules for absorption into the bloodstream of an animal. Because of human agricultural reliance in many parts of the world on animals which digest by enteric fermentation, it is the second largest anthropogenic factor for the increase in methane emissions directly after fossil fuel use.
Cecotropes are a nutrient filled package created in the gastointestinal (GI) tract, expelled and eaten by rabbits to get more nutrition out of their food. The first time through the GI tract, small particles of fiber are moved into the cecum, where microbes ferment them. This creates useable nutrients which are stored and expelled in cecotropes. The cecotropes are eaten & the nutrients are absorbed in the small intestine.
The reticulorumen represents the first chamber in the alimentary canal of ruminant animals. It is composed of the rumen and reticulum. The reticulum differs from the rumen with regard to the texture of its lining. The rumen wall is covered in small, finger-like projections called papillae, whereas the reticulum is lined with ridges that form a hexagonal honeycomb pattern. The ridges are approximately 0.1–0.2 mm wide and are raised 0.5 cm above the reticulum wall. The hexagons in the reticulum are approximately 2–5 cm wide in cattle. Despite the differences in the texture of the lining of the two parts of the reticulorumen, it represents one functional space.
In biology, syntrophy, syntrophism, or cross-feeding is the cooperative interaction between at least two microbial species to degrade a single substrate. This type of biological interaction typically involves the transfer of one or more metabolic intermediates between two or more metabolically diverse microbial species living in close proximity to each other. Thus, syntrophy can be considered an obligatory interdependency and a mutualistic metabolism between different microbial species, wherein the growth of one partner depends on the nutrients, growth factors, or substrates provided by the other(s).
The reticulum is the second chamber in the four-chamber alimentary canal of a ruminant animal. Anatomically it is the smaller portion of the reticulorumen along with the rumen. Together these two compartments make up 84% of the volume of the total stomach.
Fibrobacter succinogenes is a cellulolytic bacterium species in the genus Fibrobacter. It is present in the rumen of cattle. F. succinogenes is a gram negative, rod-shaped, obligate anaerobe that is a major contributor to cellulose digestion. Since its discovery in the 1950s, it has been studied for its role in herbivore digestion and cellulose fermentation, which can be utilized in biofuel production.
Butyrivibrio is a genus of bacteria in Class Clostridia. Bacteria of this genus are common in the gastrointestinal systems of many animals. Genus Butyrivibrio was first described by Bryant and Small (1956) as anaerobic, butyric acid-producing, curved rods. Butyrivibrio cells are small, typically 0.4 – 0.6 µm by 2 – 5 µm. They are motile, using a single polar or subpolar monotrichous flagellum. They are commonly found singly or in short chains but it is not unusual for them to form long chains. Despite historically being described as Gram-negative, their cell walls contain derivatives of teichoic acid, and electron microscopy indicates that bacteria of this genus have a Gram-positive cell wall type. It is thought that they appear Gram-negative when Gram stained because their cell walls thin to 12 to 18 nm as they reach stationary phase.
Fibrolytic bacteria constitute a group of microorganisms that are able to process complex plant polysaccharides thanks to their capacity to synthesize cellulolytic and hemicellulolytic enzymes. Polysaccharides are present in plant cellular cell walls in a compact fiber form where they are mainly composed of cellulose and hemicellulose.
Hindgut fermentation is a digestive process seen in monogastric herbivores. Cellulose is digested with the aid of symbiotic bacteria. The microbial fermentation occurs in the digestive organs that follow the small intestine: the large intestine and cecum. Examples of hindgut fermenters include proboscideans and large odd-toed ungulates such as horses and rhinos, as well as small animals such as rodents, rabbits and koalas. In contrast, foregut fermentation is the form of cellulose digestion seen in ruminants such as cattle which have a four-chambered stomach, as well as in sloths, macropodids, some monkeys, and one bird, the hoatzin.
Syntrophococcus sucromutans is a Gram-negative strictly anaerobic chemoorganotrophic Bacillota. These bacteria can be found forming small chains in the habitat where it was first isolated, the rumen of cows. It is the type strain of genus Syntrophococcus and it has an uncommon one-carbon metabolic pathway, forming acetate from formate as a product of sugar oxidation.
Ruminal tympany, also known as ruminal bloat, is a disease of ruminant animals, characterized by an excessive volume of gas in the rumen. Ruminal tympany may be primary, known as frothy bloat, or secondary, known as free-gas bloat.
Methanogens are a group of microorganisms that produce methane as a byproduct of their metabolism. They play an important role in the digestive system of ruminants. The digestive tract of ruminants contains four major parts: rumen, reticulum, omasum and abomasum. The food with saliva first passes to the rumen for breaking into smaller particles and then moves to the reticulum, where the food is broken into further smaller particles. Any indigestible particles are sent back to the rumen for rechewing. The majority of anaerobic microbes assisting the cellulose breakdown occupy the rumen and initiate the fermentation process. The animal absorbs the fatty acids, vitamins and nutrient content on passing the partially digested food from the rumen to the omasum. This decreases the pH level and initiates the release of enzymes for further breakdown of the food which later passes to the abomasum to absorb remaining nutrients before excretion. This process takes about 9–12 hours.
Anaeromyces robustus is a fungal microorganism that lives in the gut rumen of many ruminant herbivores such as cows and sheep. Previously thought to be protozoa from their flagellated zoospores, they are biomass degraders and help the animal by breaking down carbohydrates and plant materials from the food the animal ingests. This fungus, therefore, is anaerobic and lives without oxygen. Gut fungi are dramatically outnumbered by other organisms in the microbiome; however, they are important members of the gut microbiome in ruminants and hind-gut fermenters and play a key role in digestion.
↑ McCabe, Bernadette K.; Antille, Diogenes L.; Birt, Henry W. G.; Spence, Jennifer E.; BFernana, Jamal M.; der Spek, Wilmer Bvan; Baillie, Craig P. (July 17–20, 2016). An Investigation into the Fertilizer Potential of Slaughterhouse Cattle Paunch. 2016 American Society of Agricultural and Biological Engineers Annual International Meeting. Orlando, FL. doi:10.13031/aim.202460831 (inactive 31 January 2024). Paper No. 16-2460831.{{cite conference}}: CS1 maint: DOI inactive as of January 2024 (link)
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