Digestive Anatomy of Ruminants
The performance and utilization of plant materials by ruminant animals for production function is related to the efficiency of their gastrointestinal tract. The handling of food materials by ruminant animals and subsequent digestive processes differ largely from that of non-ruminants. The gastrointestinal tract starts from the mouth of the animal.
The cheeks form a wall at the two sides of the mouth with the tongue as the soft organ beneath, and the hard palate above it, and is partially separated from the pharynx.
The door of the mouth is the lips which in cattle are somehow stiff and not very flexible while the lips in sheep and goats are more flexible and help in picking feed faster than in cattle. A good understanding of the digestive system of ruminants based on their anatomy is a key factor for successful ruminant production.
Digestion in the Mouth and Oesophagus
The whole process of digestion starts from the mouth of the animal. The combination of the tongue and the lips helps ruminant animals to pick and roll the plant material into the mouth. The teeth help in the mastication or chewing of the plant material.
There are two types of dentition in ruminants. The first is called deciduous dentition which is found in young animals with formula I 0/4; C 0/0; P 3/3. The second is called the permanent dentition with formula I 0/3; C 0/3; P 3/3; M 3/3 where I = Scissors, C= Canine, P= Premolar, and M= Molar.
The mouth has a salivary gland that secretes saliva with a pH of about 8.2. It stabilizes the pH of the mouth and reduces the acidity in the subsequent chamber called the rumen.
The esophagus is a tubular column through which food is swallowed and regurgitated for re-chewing. It has no sphincter valves and the muscles contract in both directions to allow movement of food.
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Digestion in the Stomach
The stomach forms the greater proportion of the ruminants’ digestive system. It has four chambers. The first is the rumen, while the second is the reticulum, the omasum is the third, and the abomasum is the fourth.
1. Digestion in the Rumen and Reticulum
The digesta move through the esophagus into the rumen. A thin wall separates the rumen and the reticulum and the contents always mix. Hence the two chambers are called reticulorumen. It is in the rumen that fermentation of the plant materials takes place.
The rumen harbors billions of microbes for this function. In the young and unweaned animal, there exists an oesophageal groove that enables the milk ingested to pass down into the abomasum which is the true stomach for digestion and subsequent absorption and utilization in the small intestine.
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2. Digestion in the Omasum
After fermentation, the digesta flows into the omasum chamber. The omasum is a spherical organ with muscular laminae and in this chamber, water and inorganic minerals are absorbed. The digesta is filtered to ensure that no harmful object enters the omasum.
3. Digestion in the Abomasum
This is the true stomach and the first glandular portion whereby digestion by enzymes takes place. Gastric juice containing hydrochloric acid, pepsin, renin, and lipase are produced.
In young unweaned animals, the abomasum is about 80% of the stomach while in the adult it is only 10%. The digesta stays in the abomasum for about 1-2 hours
4. Digestion in Small Intestine
Further breakdown of the food digesta occurs at the upper part of the small intestine. Here, pancreatic juice and bile assist in the digestion process while absorption of the end product takes place in the lower portion of the intestine through a finger-like structure called villi.
5. Digestion in the Large Intestine
The large intestine is made up of the colon and caecum. In this part of the Gastro-Intestinal Tract (GIT), some of the food residue is deposited for further fermentation in the caecum which has a blind end containing some microbes.
The absorption of water and other nutrients continues in the colon. The digesta moves until it reaches the rectum and anus. The undigested food material forms a solid mass in the colon and it is eventually expelled through the anus.
In summary, the digestive system of ruminants is made up of the mouth, the esophagus, the four compartmental stomach (rumen, reticulum, omasum, and abomasum), the small intestine, and the large intestine and also, fermentation of the plant material occurs in the rumen which harbours billions of microorganisms that attack fiber for proper nutrient utilization.
The abomasum is the true stomach where digestion takes place and absorption of water, inorganic minerals, and nitrogen takes place in the omasum and also in the small intestine, the colons, and the large intestine.
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