A cow has 1 stomach which is divided into 4 compartments called the rumen, reticulum, omasum, and abomasum. The first 3 compartments, the rumen, reticulum, and omasum are termed ‘forestomachs’ by veterinary anatomists. This is what leads to a common misconception that a cow has 4 stomachs.
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The 4 Cow Stomachs
The stomach of a cow is called a ruminant stomach. The name is derived from a Latin word called ruminare which means to chew over again.
A cow’s digestive system works by breaking down the food that cows eat into nutrients that can be absorbed by the cow’s body. The digestion process is made up of four stages: chewing, rumination, regurgitation, and digestion.
A cow’s digestive system is made up of the mouth, esophagus, stomach, cecum, and small and large intestines.
Meanwhile, a cow’s stomach features 4 sections, which are:
The rumen also called the ‘paunch’, is connected to the cow’s esophagus and is the largest of the four compartments of a cow’s stomach. It makes up 84% of the volume of the entire stomach.
It can hold more than 30 gallons and it nearly fills the entire left side of the abdominal cavity in a mature cow. It acts as a storage and a fermentation compartment.
The rumen and the reticulum are often referred to as the reticulorumen.
Chewed plant material travels down the esophagus to the rumen where it is stored and later regurgitated back to the mouth as cud for further chewing and grinding.
Rumen fermentation is the process that converts ingested feed into energy sources for the host. The rumen microbes: the bacteria, protozoa, and fungi, are responsible for the fermentation of feed consumed by ruminant animals.
The microbes feed on the plant material ingested by the cow to produce end products that are used by the cow. The rumen microbes require water, energy, protein, and minerals to perform their work efficiently.
There should be a consistent diet of feed to achieve the required numbers of rumen microbes for optimum digestion
The products of microbial fermentation are:
- Volatile Fatty Acids (VFAs) like acetic, propionic, and butyric
- Ammonia used to manufacture microbial protein
- Gases like carbon dioxide and methane
Acetic acid contributes 50-60% of the VFAs and predominates on a high roughage diet. It is also used for muscle metabolism and body fat and is a precursor for mammalian milk fat.
Propionic acid contributes 12-18% of the VFAs and predominates on a high concentrate diet. It provides energy through the conversion of blood glucose in the liver. It is also used in blood sugar (lactose) synthesis.
Butyric acid contributes 18-20% of the VFAs. It provides energy to the rumen wall and is used in milk fat synthesis.
The absorption of the VFAs in the rumen is made possible by finger-like projections called papillae on the interior lining of the rumen. These papillae increase the surface area for absorption.
The survival of rumen microbes depends mainly on the correct pH in the rumen. pH levels affect the rumen in the following ways:
- pH 6.2-7.0 – neutral to slightly acidic and ideal for all rumen microbes
- pH below 6.2 – fiber-digesting bacteria slow down
- pH below 5.4 – acidosis can happen at this level because of a decrease in fiber-digesting bacteria and an increase in lactic acid bacteria
Gases produced during fermentation rise to the top of the rumen above the liquid fraction to be belched out by the cow.
The reticulum is referred to as the ‘honeycomb’ because its inner lining resembles a honeycomb. It’s located underneath, towards the front of the rumen. It lies against the diaphragm and It’s divided from the rumen by a thin fold of tissue.
Its main function is to collect small digested feed particles and move them into the omasum while the larger particles are retained in the rumen for further digestion.
The reticulum is responsible for holding heavy objects such as pieces of rocks, metals, and wires that may have been swallowed accidentally by the animal.
It is referred to as the ‘hardware stomach’ and a common health issue that affects this part of the stomach is hardware disease.
The omasum is a globe-shaped structure referred to as ‘manyplies’ because its inner lining has folds that resemble the pages of a book. These folds provide a larger surface area for the absorption of nutrients from feed and water.
The omasum makes up about 12% of the total volume of a cow’s stomach.
The omasum has a canal-like structure that passes between the reticulum and the abomasum called the omasum canal. The canal has smooth mucosa and is used in food transport.
The omasum contains laminae which are covered in omasal papillae. The omasal papillae resemble either a blunt or a claw-like formation to increase surface area for absorption and provide increased friction during the squeezing of ingesta (food material).
4. Abomasum (The Real Stomach)
The abomasum is the last compartment of a cow’s stomach and is often referred to as the ‘true stomach’ because it operates similarly to a non-ruminant stomach.
It represents about 4% of the total volume of a cow’s stomach.
It lies upon the abdominal floor but it may be displaced and differ in its position within the abdomen depending on several factors like:
- The fullness of the other stomach compartments
- Intrinsic abomasal activity
- Contractions of the rumen and the reticulum
- Age and pregnancy status of the cow
The abomasum is large in newborn calves because it is the only one that digests feed intake before the other compartments develop fully.
The abomasum digests its feed chemically unlike the other 3 compartments. It is lined with glands that release hydrochloric acid and digestive enzymes like pepsin that break down proteins.
It also receives pancreatic lipase secreted from the pancreas, which is used to break down fats. The pH in the abomasum normally ranges from 3.5 to 4.0.
The abomasum is heavily coated by mucous to protect its wall from acid damage. The abomasum also digests previously missed feed that was not digested earlier. The little remaining feed leaves the abomasum and is passed on to the small intestines for further absorption.