1.   DIGESTION IN THE RUMINANT

We will now look at digestion in the ruminant animals, those that have compound stomachs.

Look at the diagram of the ruminant stomach on the next page. You will see that it is made up of four compartments, the reticulum, the rumen, the omasum and the abomsurn. The rumen is by far the largest compartment, and the abomasum is the true stomach, the equivalent of the stomach in the non-ruminant.

The natural food for ruminants is grass, and grass is a food high in fibre. It is a fibrous food, and the fibre consists of cellulose which is the cell wall of the plant cell. This cell wall must be broken down before the contents of the cell can be made available to the animal. The other big difference between ruminants and non-ruminants is in their actual eating habits. Ruminants have no incisor teeth in the top jaw, only a toughened area of gum called the hard pad. They do not bite off the grass, but simply tear it off and swallow it without chewing. Non-ruminants bite off their food, chew it well, and then swallow it, so that the food reaching the stomach is broken down into small particles. The food reaching the rumen is not broken down at all, but is raw grass pulled off and swallowed. The task of the compound stomach is to break down this raw grass into small particles, destroy the cellulose of the plant cell walls and release the nutrients in the cells which then pass into the true stomach or abomasum and are digested in the same way as those of the non-ruminant animal. This breakdown of grass and cellulose is done in two ways, by mechanical action and by the action of micro-organisms, chiefly bacteria.

MECHANICAL ACTION

The reticulum, rumen and omasum have muscular walls and various projections sticking out of their inside linings; the honeycomb of the reticulum, the papillae of the rumen and the long muscular projections of the omasum. The food passes down the oesophagus into the reticulum, from there into the rumen. Once in the rumen, it is churned and mixed by the muscles of the walls, passed back into the reticulum, back to the rumen, into the omasum, back to the rumen and so on, all the time being churned and shredded by the rough inside of these organs. These actions are all mechanical actions designed to break up the tough fibrous grass.

The other mechanical action which the ruminant can perform is the action of cudding, or rumination. By using the thick, muscular wall of the rumen and the action of the diaphragm the animal can cough up a lump of food from the rumen back into the mouth where it can be thoroughly chewed. You often see cattle lying down and chewing their cud, and, in fact, this can occupy one third of the animal’s day. The purpose of chewing the cud is to grind the food into small particles with the back teeth, the molars and premolars, to mix it with saliva so that when it is swallowed, it is fairly well broken down and softened.

The normal pattern of the ruminant is to graze actively for a while, simply collecting grass and swallowing it, and then have a period of rest, either standing up or lying down, when the animal regurgitates the food it has eaten, and breaks it down, then swallows it again. The rumen of an adult cow is a very large organ, holding a wheelbarrow-load of food, and this has to be thoroughly chewed up. While this is going on, gallons of saliva are being added to soften the mixture in the rumen, together with any water the animal has consumed.

Figure 1: Digestive System of a Cow

ACTION OF THE MICRO-ORGANISMS

The rumen contains vast numbers of micro-organisms, chiefly bacteria, which live there, feeding, growing and reproducing all the time. They are known as the rumen flora. These micro-organisms attack the cellulose in the rumen and break it down into fatty acids, and simple sugars which they use for food. The fatty acids are absorbed through the walls of the rumen, converted into the simple sugar glucose, and used by the animal. At the same time, large quantities of the gases, carbon dioxide and methane are produced, and these are expelled through the animal’s mouth by eructation, or belching. Sometimes the gases get trapped in the rumen, resulting in the condition called bloat, in which the animal literally blows up like a balloon. This is a dangerous condition and, unless treated, can cause death.

Although only the rumen has been mentioned, because of the mixing of the food between the rumen, reticulum and omasum, the process of bacterial breakdown, and the absorption of the fatty acids takes place in all three. The micro-organisms of the ruminant can digest proteins and fats. Their importance in ruminant digestion is their ability to break down cellulose. This breakdown provides food, in the form of the fatty acids, directly to the animal, and also releases the contents of the cells for normal digestion in the abomasum and small intestine.

The system is a good example of symbiosis; two organisms living together with each being of some benefit to the other. The ruminant animal provides the carbohydrate (cellulose), protein and other nutrients for the bacteria to live on, and the bacteria break down the cellulose for the ruminant. There is also a continuous flow of these micro-organisms into the abomasum and small intestine, where they in turn are broken down and digested by the animal.

Although there are a great many fatty acids, the three that are the result of bacterial digestion in the rumen are acetic acid, butyric acid and proprionic acid. These are very complicated organic acids, and are discussed in more detail in the Feeds and Feeding Course.

In the ruminant, the breakdown of cellulose takes place in the rumen, reticulum and omasum. In the horse, this breakdown takes place in the caecum with the end products, the fatty acids being absorbed through the wall of the caecum. A very small amount of the microbial breakdown of fibre takes place in the caecum of the pig, but it must be stressed that this is hardly significant; the pig can digest only a small amount of fibre.

2.  ABSORPTION AND UTILISATION

As we already know, the end products of microbial digestion in the ruminant, the fatty acids are absorbed through the wall of the rumen, reticulum and omasum, converted into glucose and carried into the blood-stream. The remaining products pass into the abomasum and from then on, they undergo the same treatment as in the case of the non-ruminant. No actual food is absorbed in the stomach, and most absorption takes place in the small intestine. The food, or chyme, at this stage is a fluid, broken down by mechanical chewing and churning, and acted upon by gastric juice, succor entericus, pancreatic juice and bile. Lining the inside of the small intestine are the villi, small projections filled with blood capillaries, which lead to the arteries and veins, and lacteals, which lead into the lymphatic system. These are illustrated in the diagrams below:

Figure 2: A Villus

Figure 3: A cross section of the small intestine

The Villi are bathed in a solution of chyme, and the nutrients which are dissolved in the chyme, either as ions or as molecules, pass through the walls of the Villi, these walls being made up of semi-permeable membranes. This passage is achieved in a number of ways. Please refer to this lecture to refresh your memory.

The simple sugars, and in particular glucose in the chyme, pass through the walls of the villi and into the blood capillaries which carry them to the liver via the hepatic portal vein.

The fats in the chyme, pass in this form through the walls of villi and enter the lacteals which lead into the blood system and are carried to the lymphatic system. It is important to remember that the sugars, mainly in the form of glucose pass directly to the liver. Here they are converted, by the action of an enzyme, into a substance called glycogen, which is then stored in the liver. When required in the bloodstream, the glycogen is converted back into glucose and transported around the body. The liver acts as a storage and regulatory organ, the amounts of sugar in the blood being controlled by the hormones, insulin and glucagon, which are produced by the pancreas. This is situated close to the liver.

To sum up, the end product of the digestion of carbohydrate is the simple sugar, glucose, which is either circulated around the body by the bloodstream, or stored in the liver in the form of glycogen. The end product of the digestion of fats is the breakdown into glycerol and fatty acids, the reforming into fats which are either circulated around the body in the bloodstream, or stored in the body, to act as a reserve.

The protein part of the food is broken down into amino acids by the digestive process, and these pass into the blood capillaries and are carried to the liver.

After passing through the small intestine, where most of the absorption takes place, the remainder of the chyme enters the large intestine, made up of the caecum and the colon.  In the ruminant and most non-ruminants, there is very little in the way of nutrients to absorb at this stage, and the primary function of the colon is to absorb water from the chyme back into the body. Once this has been done, the remainder, which is all waste products, passes into the rectum and out through the anus.

UTILISATION OF THE END PRODUCTS OF DIGESTION

Involuntary: done without will or conscious control

If you set fire to a bale of hay, the result is the production of carbon dioxide, water vapour, and energy in the form of heat. After the bale has completely burned, some solid matter in the form of ash is left, and this ash consists mainly of minerals. The animal similarly disposes of the food that it eats. It burns up, producing carbon dioxide, which escapes from the body through breathing, water vapour, and energy. This is used to heat the body, repair tissues, and maintain the body functions like breathing and beating of the heart and the involuntary muscular movements. It is also required for movements like walking and running. In order to achieve this, the animal needs fuel, in the form of glucose, and oxygen which it gets from the air in breathing. The glucose is carried to the cells where energy is needed, particularly by the muscle cells, by the bloodstream. The oxygen required is also carried to the cells by the bloodstream, and the reaction takes place inside the cells. The waste products, the carbon dioxide and any residues are carried away from the cells by the bloodstream, and the carbon dioxide is passed out of the body via the lungs. Every time the animal breathes out, it is expelling carbon dioxide from its body. The energy is used up on the spot. The process sounds simple, but getting the right amount of glucose and oxygen to the right place is very complicated. Glucose is the basic fuel for the body, and the amount in the bloodstream is regulated by the liver. Any excess sugar is stored in the liver in the form of glycogen. However, the liver has only a limited capacity for storing glycogen, and any further excess of glucose is converted into fat, and stored under the skin and around some of the body’s organs such as the kidneys and intestines. This fat acts as a further reservoir, and can be converted back into glucose and used when required. As animals get older, they take less exercise, but eat about the same amount of food. They thus require less energy but still provide themselves with plenty of glucose. The result is that they get fat.

The fats in the body that have been produced by digestion are also converted into glucose, and in fact, produce more energy than carbohydrates. Any excess fats are stored in the body.

Chyme: the pulpy acidic fluid which passes from the stomach to the small intestine, consisting of gastric juices and partly digested food.   Glycogen: a substance deposited in bodily tissues as a store of carbohydrates. It is a polysaccharide which forms glucose on hydrolysis.

The proteins have a different role to play. They are broken down by digestion into amino acids, pass into the bloodstream and are carried around the body. Some of these amino acids are then built back into protein, and these go to form muscle tissue, or meat. The proteins form the lean meat, and the fat on meat is stored carbohydrates. Proteins also form hair, wool, feathers, hooves, horns and claws. They also replace enzymes and hormones that have been used up. Any proteins that are not required for these tasks are taken to the liver where the nitrogen is removed and the remainder is circulated back into the bloodstream and used to produce energy exactly like carbohydrates.

The nitrogen is converted into ammonia gas which is then turned into a chemical called urea. This is then passed out of the body via the kidneys as urine. Excess protein cannot be stored in the body. The body takes what it needs and the rest is used to produce energy or is passed out as urine.

The utilisation of the end products of digestion can be summarised as follows:

You must always remember that the breakdown, absorption and utilisation of food is a very complicated process. You have been given a simplified version of what takes place.