(see diagram below for reference)
The classical four chambered stomach of the cow can be divided into the rumen, the reticulum, the omasum and the abomasum. The rumen is by far the largest compartment and is often defined to encompass everything between the oesophagus and the reticulo-omasal orifice as it is continuous with the reticulum, hence it is commonly known as the reticulo-rumen. The rumen and reticulum are only separated by the ruminoreticular fold, a rumen pillar rising from the bottom of the stomach to prevent direct mixing of content without rumen contraction. For size comparison the rumen is typically about 185 litres in size, whereas the reticulum is only about 16 litres.
Rumen content is layered so the heavier, finer, often more digested particles sink to the bottom to an area known as the ventral sac, and the more recently ingested, lighter feed sits on top in a layer called the rumen mat (or floating mat). Cows that are feed a diet high in grain concentrates often have a very small or non-existent rumen mat as all feed sinks to the bottom immediately. The upper compartment of the rumen, known as the dorsal sac contains the gas cap where all the methane, carbon dioxide and other gases produced by the rumen microbes rise to. Small finger-like protrusions called rumen papillae line the rumen wall and serve to increase its surface area to increase the rate of absorption of nutrients released by rumen microbes.
The reticulum is located towards the front of the cow, directly below the cardia that connects the oesophagus to the rumen. Easily identifiable by its distinctive honeycomb surface, the reticulum serves to separate out particles small enough to pass through the reticulo-omasal orifice and larger particles that require further digestion back into the main part of the rumen. A condition called “hardware disease” can occur when the cow eats non-feed objects such as screws or nails, which can lead to reticulum puncture after contraction and be fatal for the cow. The reticulum is where the eCow farmBolus is retained. Designed with a specific gravity of 2.7 the bolus quickly finds its way into the reticulum, however it is smooth and inert so doesn’t causes the animal any harm. The distance between devices residing in the reticulum and the outside air is no more than 200 mm, this helps greatly when retrieving data from these devices.
The third stomach compartment that digesta passes to directly after the reticulum is the omasum. The two main functions of the omasum are to reabsorb the large quantities of fluid (mostly saliva) flushed through the reticulo-rumen and to filter out larger particles only allowing very fine particles into the abomasum. The omasum is the smallest stomach compartment only holding about 8 litres in an adult cow.
After digesta passes through the omasum it enters the abomasum or the “true stomach”. It has this name as it essentially serves the same purpose as a stomach in monogastric animals such as pigs and humans. It is here where the cows own digestive enzymes and acids reside and break down the partially digested feed, as well as the microbes, that have been filtered through the first three compartments. The abomasum is approximately 27 litres in size.
The rumen is unlike the human stomach in that it secretes none of its own digestive enzymes or acids; these are reserved for the cows “true stomach” the abomasum. Instead the rumen uses microbes (including bacteria, protozoa and some fungi) to breakdown the tough polymer cellulose that comprises a large portion of the ingested fibre. Inside the rumen of the cow is one of the most densely populated microbial environments that exist in nature, with as many as 10 billion bacteria per ml. Conditions inside the rumen are not static but must remain within certain parameters in order to maintain a healthy microbe population. Rumen microbes are anaerobic and as such the rumen environment is anaerobic as well; any oxygen introduced by water or feed is rapidly removed by the microbes. As the rumen is essentially a fermentation chamber, large amounts of heat are continuously released and because of this the rumen is consistently 1°C above the rest of the body (therefore about 39°C). This heat is called the “heat of fermentation” and can be used by the cow as a heat source.
The normal pH range observed in the rumen ranges from 5.7 to 7.2, where conditions below 5.7 are found in herds on a high grain diet (termed acidosis, see SARA) and conditions above 7.2 are found in herds with high forage diets supplemented with urea.
Rumen microbes convert fibre as well as some starch, sugar and protein from the cow’s diet into volatile fatty acids (VFA) (sometimes called short-chain fatty acids) and microbial proteins. Rumen papillae aid in the absorption of VFA by greatly increasing the surface area allowing for faster rates of transfer into the cow’s body where they can be used as a source of energy. The main types of VFA produced by fermentation include acetate (CH3COOH) mainly produced from fibre, propionate (C2H5COOH) mainly produced from grain and butyrate (C3H7COOH) produced in lower quantities than the other two. Lactate can also be produced, particularly under acidic conditions. These fatty acids provide a large proportion of the energy requirements of the cow but are actually an unwanted by-product of these microbes, showing the symbiotic relationship between the rumen microbes and the cow.
Proteins absorbed by microbes are broken down into amino acids and rebuilt into microbial proteins used to aid cellular replication. As these microbial proteins are locked up within the cell bodies of the microbes, they cannot be absorbed by the rumen wall. It is not until they pass through to the abomasum that they are broken down by the cows own digestive enzymes and acids, releasing the microbial proteins and allowing them to be absorbed into the cow’s body for nutrients. These proteins are high quality proteins having similar amino acid profiles to the cows own proteins, therefore converting between them is very efficient. These microbes provide the vast majority of the protein requirements for the cow but the stock in the rumen is never diminished as they proliferate within the rumen so freely.
As well as VFA production, another by-product of microbial fermentation inside the rumen is gas, most notably carbon dioxide and methane. An adult cow undergoing healthy rumination can produce as much as 50 litres of excess gas per hour, comprising approximately 60% CO2, 40% methane and less than 0.05% hydrogen gas. Carbohydrate fermentation into VFA produces CO2 and hydrogen as by-products, and methane bacteria (methanogens) use these by-products to produce methane. These gases are expelled from the cow regularly via the mouth through eructation and exhalation. Eructation (silent burping) is the process of pushing gases in the rumen back up the oesophagus using rumen contractions. Studies on ruminants have shown that methane can also be absorbed through the rumen wall, passed to the lungs and exhaled, although this is only a small proportion of the total.
Failure to remove these gases from the rumen causes bloat and can lead to asphyxiation through pressure building up and pushing on the lungs. Gas can get trapped in the rumen because of excessive foaming, due to gas being trapped inside the foam bubbles and therefore cannot easily escape. Excessive foaming can be caused by specific food types such as alfalfa or white clover, or high grain diets. Cows suffering from foaming can be treated with oils or low-foam detergents, or by adjusting diet.
Mechanics of Digestion and Rumination
In order to ensure forage ingested by the cow is exposed to sufficient rumen microbes and broken down into small enough particles to pass through the rest of the digestive tract, a complex series of rumen contractions take place. These contractions also allow the eructation of gases from the rumen out the mouth and the regurgitation and re-mastication of feed (rumination). Four main contractions take place in order to maintain digestion: a primary contraction to mix the contents of the rumen, a secondary contraction to facilitate eructation, a second type of primary contraction with rumination and a second type of secondary eructation contraction with rumination.
The primary contraction starts in the reticulum and the cranial sac, moving digesta up to the dorsal sac, followed by a contraction of the dorsal sac directing digesta into the ventral sac (clockwise motion on diagram). The ventral sac then contracts moving rumen contents back to the front of the rumen over relaxed reticulo-ruminal and cranial pillars. During the contraction of the reticulum, rumen fluid with any suspended particles is pushed through the reticulo-omasal orifice and onto the omasum, allowing fluid to be reabsorbed and fine particles to pass onto the abomasum.
The secondary contraction occurs in a similar way to the first one, however during ventral sac contraction the cranial pillar is contracted and blocks off the reticulum and cranial sac. This blockage forces digesta back into the dorsal sac, in turn forcing gas forwards towards the cardia for eructation. Digesta moves back into the ventral sac and contractions move it back to the front of the rumen.
Rumination causes a slightly different contraction cycle to allow for parts of the floating mat to re-enter the mouth. It is stimulated by pressure receptors called mechanoreceptors on the reticulum, ruminoreticular fold and cardia mucosa. This pressure on the rumen wall is often called “scratch factor”.
During rumination the contraction cycle is again very similar to the primary contraction. It differs in that there is an initial reticulum contraction where the cardia (lower oesophageal sphincter) is relaxed and a bolus of partially digested fibre (cud) from the floating mat is pushed up the oesophagus. It then makes its way to the mouth by reverse peristaltic contractions along the oesophagus and, following a complete primary contraction cycle as described above, the cud is swallowed after it has been re-insalivated and re-masticated.
Combining all these processes and contractions together the cow can perform a second type of secondary contraction with rumination and eructation. In this contraction cycle the cow expels gas just before it swallows its cud.
The mixing of rumen content by rumen contraction aids digestion by distributing saliva and rumen microbes throughout the dietary fibre as well as distributing VFA to the rumen walls. Rumination also aids digestion by decreasing particle size and therefore increasing the surface area of the fibre, allowing more contact with microbes and speeding up digestion. Another benefit of rumination is extra saliva production helping to lubricate and buffer the rumen (decrease acidity). Interestingly, decreased rumination can cause decreased appetite as the contents of the rumen is not regurgitated, remains in the rumen longer and therefore makes the cow feel fuller. Suppressed saliva production from less rumination also slows down digestion.
Rumination is a lengthy process as grazing herds spend approximately a third of their time ruminating. In the wild ruminants can use this to their advantage by quickly ingesting large quantities of food and retreating to safety to ruminate in peace.