Digestion in Grass Eating Animals: Ruminant Digestive system, Physiology

Digestion is one of the important physiological processes that convert the food nutrients in the form so that they can be easily absorbed into the blood and can reach each body cell to provide energy for several other living activities. Have you ever noticed that cows, buffaloes, sheep, and goats keep chewing something continuously even when they are not eating? These animals are called ruminants and mainly feed on plants and grasses. Like humans, ruminants also have a well-developed digestive system for the digestion of food. How does digestion in grass-eating animals (ruminants) take place? How does it differ from non-ruminants? To know all about this let’s read the article till the end.

What is Digestion?

Definition: Digestion is the process of breaking down complex insoluble food molecules (carbohydrates, proteins, and fats) into soluble simple food molecules. This process is catalysed by digestive enzymes.

Ruminant Digestive System

Ruminants have a well-developed digestive system like humans. The digestive tract of grass-eating animals consists of the following parts:

1. Mouth
2. Oesophagus
3. Four-chambered stomach
4. Small Intestine
5. Caecum
6. Large Intestine

Mouth: The mouth of ruminants is adapted for grazing. Adult ruminants have 32 teeth. These include 8 incisors in the lower jaw, 3 premolars on both the sides in the upper and lower jaw (12) and 3 molars on both the sides in the upper and lower jaw (12). The tongue of ruminants is highly muscular and facilitates to rip of the grasses from the ground.

Oesophagus: It is a muscular tube that is commonly divided into cervical, thoracic, and abdominal sections. Oesophagus is made up of striated muscles that facilitate the bidirectional movement of food in ruminants. (From mouth to the rumen and from the rumen to mouth).

Stomach: Ruminant’s stomach is divided into four main parts, namely, Rumen, Reticulum, Omasum, and Abomasum.

1. The rumen is the largest part of the stomach that consists of several sacs. It also serves as a food reservoir for ruminants.
2. The reticulum is a bag-like structure located closer to the heart.
3. Omasum is a globe-shaped structure that is concerned with the absorption of food.
4. The abomasum is the terminal part of the stomach that is lined with the digestive glands. Abomasum can be compared to the stomach of humans.

Small Intestine: It is a long, highly coiled tubular structure that is divided into three parts, namely, duodenum, jejunum, and ileum. It is the site of digestion and absorption of food. The inner wall of the small intestine is similar to that of humans. It comprises several inward folding called villi that facilitate the absorption of food.

Secretions from the pancreas and gallbladder help in the digestion of food within the small intestine by pouring their secretion into the small intestine.

The caecum is the junction between the small intestine and large intestine.

Large intestine: It is the end part of the digestive tract through which the undigested food passes for elimination. This part is mainly concerned with the absorption of water from undigested food.

Fig: Digestive System of Ruminants

Practice Ruminant Digestive System for

Physiology of Digestion in Ruminants

Digestion of food in ruminants is a stepwise process that involves the functioning of organs of the digestive tract in a sequential manner. The entire process of digestion in grass-eating animals (ruminants) can be described as follows:

1. The digestion in ruminants starts with swallowing and chewing the foods (grasses, hay, or straw) into their mouth. However, ruminants do not completely chew the food. Instead, they quickly engulf the food and mix it with the saliva. The food mixed with saliva reaches the rumen (first chamber of the stomach). The food is transferred to the rumen through the peristaltic movement of the oesophagus.
2. Digestion in the rumen: The saliva mix food is received from the mouth through the oesophagus. The saliva maintains a pH between 6.2-6.8 that is ideal for the digestion of forage and facilitates the growth of microbes that assist in the process of digestion. A large amount of swollen food remains stored in the rumen. Hence it is also known as the storage compartment of ruminants.
   a. Longer storage of food in the rumen facilitates the fermentation of food through bacteria, protozoans, and fungi. It is called microbial fermentation.
   b. Microbes also produce proteolytic enzymes (proteases and peptidases) that cause the breakdown of proteins into ammonia, amino acids, and other products.
   c. Ammonia is absorbed in the rumen wall and converted to urea and further mixed into the blood as blood urea nitrogen (BUN).
   d. About 40-75 % of protein feed is broken down in the rumen. The remaining enters the small intestine for further digestion and absorption.
   e. The food ingested by ruminants has high cellulose content. Later the microbes present in the rumen also produce another enzyme, cellulase, that partially digests cellulose.
   f. The by-products of cellulose digestion mainly include volatile fatty acids that can be readily absorbed across the ruminal mucosa into the blood and further provide energy for other life processes.

Summary

Grass eating animals like cows, goats and buffaloes are also known as ruminants. These animals swallow a portion of the grass quickly and store it in a sac-like structure in the abdomen called the rumen. Rumen forms the first stomach of the ruminant and is four-chambered. Here, food is partially digested and is converted into something called the cud. Plants contain large quantities of cellulose. It is a complex structure and it is broken down into simpler particles in the rumen. Rumination is the process in which cud returns to the mouth in small lumps and the ruminants chew it and afterwards completely digest it in their reticulum.

Bacteria present in the Rumen aids in Rumination which breaks down cellulose in plants. Then, the digested food is passed into the reticulum. Animals including humans, which are not ruminants, cannot digest cellulose for its complex structure.

However, some animals including Rabbits and Horses have a large sac-like structure known as caecum present between the food pipe (oesophagus) and the small intestine. Like in the rumen, Cellulose present in the food is digested by the action of bacteria in the caecum.