In the lectures already completed, we have been looking at the digestive organs of the animal; the anatomy of digestion. We now have to consider the process of digestion, the absorption of the digested material by the animal and the ways in which this material is utilised by the body.

Digestion is the process by which food is broken down in particle size and rendered soluble so that it can be absorbed and utilised by the animal. It is, in fact, not one process but a vast series of processes. Furthermore, digestion is very complicated, involving a complex series of chemical and bio-chemical reactions, some of which are barely understood even today. You will be getting a simplified version of what is a difficult process, but the idea is to give you enough knowledge of the digestion of an animal to enable you to provide the correct feeds and nutrients for your farm livestock. Feeding is the most important single factor in successful livestock farming, so it is well worth taking a bit of time to learn about.

There are two important basic concepts about digestion that should be grasped at this stage, and these are:

  • Animals, like plants, can only use nutrients that are in solution; i.e. that are dissolved in water. Solids are of no use to either animal or plant. Nutrients that are in solution, that have been dissolved in water, are usually either in the form of ions, very small particles carrying an electrical charge, or are in the form of molecules, very small particles of atoms joined together and;
  • The digestive tract of any animal is a long tube stretching from the mouth to the anus. Various pipes and tubes along its length allow substances to be added to the tract but nowhere is there a pipe leading from the tract into the body cavity. This means that all the nutrients used by the animal have to pass through the wall of the digestive tract; there is no other way they can reach the rest of the body.

All feeds can be broadly classified as being compounds of one or all of the following:

CarbohydratesWhich are compounds that contain carbon, hydrogen and oxygen. Examples of carbohydrates are starch, sugars and cellulose.
ProteinsWhich contain carbon, hydrogen and oxygen, but also contain nitrogen. Remember that all proteins contain nitrogen.
  FatsOr lipids as they are called. They contain carbon, hydrogen and oxygen, but no nitrogen. However, they do have a high energy value, about 2 1/4 times that of carbohydrates.

The breaking-down process of digestion takes place in three ways, by physical or mechanical means, by the action of enzymes and of micro-organisms. We will consider each of these actions in turn.


Physical breakdown begins in the mouth with the chewing and grinding actions of the teeth which reduces the size of the food particles in order to make swallowing easier.

It is continued by the muscular movements of the stomach and intestines which help to churn and

break up the mass of food and mix it with digestive juices.


Enzymes are chemical substances which can change other substances by a process called fermentation. Fermentation is the process which occurs in the brewing of beer and in the decomposition of dead matter. Enzymes are chemicals very much like proteins in composition, with two major properties; they can bring about chemical changes in other substances without themselves being changed in the process. Also, they can achieve these changes, (which in a laboratory would require conditions of great heat and the use of strong acids or alkalis,) inside the body in conditions of mild heat and comparatively mild acidity or alkalinity. Furthermore, it only takes a very small amount of an enzyme to achieve big changes. Enzymes, therefore, are very powerful chemicals and are specific to certain tasks. One particular enzyme will carry out one and no other. Some enzymes can only break down carbohydrates while others act only on proteins.

Chemical or enzyme action starts in the mouth with the action of the enzyme ptyalin which is found in saliva and produced by the salivary glands. This enzyme acts on the starch in the food, converting it into simple sugar called maltose.

The food in the mouth, having been ground up, and mixed with saliva, and the chemical breakdown is swallowed in the form of a bolus, travels down the oesophagus into the stomach where it joins the food already there. In the stomach, there are two enzymes, pepsin and rennin, and one chemical which is hydrochloric acid with the chemical formula HCl. HCl is a strong mineral acid, produced by glands in the stomach wall, and makes the contents of the stomach very acidic. The enzymes and hydrochloric acid mixed together form the gastric juice; and the pH of this is between 1 and 2 (see Soil Science Lecture 4 for an explanation of pH). The function of the acid in the stomach is to further break down the food to prevent rotting or putrefaction of undigested and partly digested food.

The function of the enzyme rennin is to curdle milk in the stomach to prevent it from passing through too quickly. It is an important enzyme in young animals that are feeding on milk only. Pepsin acts on proteins in the food, breaking them down into simpler compounds. By the time all these actions have taken place, the food is no longer recognisable as such, but is now a semi-fluid which is called chyme, and this passes slowly through the sphincter from the stomach into the small intestine in a series of spurts. An important point to remember is that no food is absorbed in the stomach. The only things which are absorbed through the wall of the stomach are alcohol and some drugs, but these cannot be classed as food.

At this stage the food that was originally placed in the mouth has undergone the following processes:

  • It has been chewed up and ground into fine particles, mixed with saliva and gastric juice and converted into chime;
  • Some starch has been converted into sugar by the enzyme ptyalin in the mouth;
  • Some protein has been broken down by the enzyme pepsin in the stomach;
  • Any milk has been coagulated by the enzyme rennin in the stomach; and
  • The pH has been lowered and further dissolving has taken place by the action of the hydrochloric acid in the stomach.

The stomach acts as a container, a reservoir which holds up the passage of the food for some time allowing it to collect and be acted upon by the gastric juice. Once the chyme enters the small intestine,

it is moved along continuously by the muscular contractions of the wall of the intestine. This action is called peristalsis and is the same as the action which moves the food bolus down the oesophagus. Once in the small intestine, the chyme is acted upon by the following juices which are added to it as it passes along the length of the intestine:

  • Intestinal juice or succus entericus as it is called.
    • Pancreatic juice from the pancreas.
    • Bile from the liver, which has been stored in the gall bladder.

The most active digestion takes place in the small intestine, as the carbohydrates and proteins have been only partly digested, and the fats have hardly been affected at all. The succus entericus is produced by glands in the lining of the intestine, and it contains three enzymes, maltase, sucrase and lactase which break down complex sugars into the simple sugar glucose; another enzyme, peptidase, continues with the breakdown of the proteins.

The pancreatic juice contains one enzyme, amylase, which breaks down sugars into glucose, three enzymes, chymotrypsin, peptidase and trypsin which break down proteins and lipase which acts on the lipids or fats. In addition, the pancreas produces the two regulatory hormones, insulin and glucagon, which control the levels of glucose in the blood; these were discussed in the last lecture.

      Regulatory: serving or intended to regulate something.   Emulsion: a fine dispersion of minute droplets of one liquid in another in which it is not soluble or miscible.

The bile from the liver acts on the fats in the chyme, breaking them down into very fine droplets which mix with the watery chyme. Normally oil and water do not mix, but if the oil is in fine droplets and is mixed vigorously with the water they form a mixture called an emulsion.

The very fine drops of fat are then broken down by the action of the enzyme lipase, and they are broken down into a substance called glycerol and fatty acids – this will be discussed when we consider digestion in the ruminant. The other function of bile is to reduce the acidity of the chyme which has come from the stomach. The chyme in the small intestine becomes alkaline.

Having been acted upon by all these different substances, the chyme passes into the caecum, which acts as a small reservoir and holds up the passage of the chyme for a while. In most animals with simple stomachs such as the pig, the caecum is a small organ, and in the case of humans it is not used at all and

can be removed (the appendix). However, in the case of the horse, the caecum is large and plays an important role in the breakdown of fibrous, grassy material. Much of the digestion which takes place in the rumen of ruminants is done in the caecum of the horse. From the caecum, the chyme passes into the colon which, together with the caecum, makes up the large intestine. At this stage the original food has been broken down completely, the carbohydrates to the simple sugar glucose, the proteins to amino acids, and the fats to glycerol and fatty acids, most having been absorbed through the wall of the intestine. What is left passes into the rectum and out through the anus. The main function performed in the colon is the removal of water from the chyme, and this water is absorbed into the body and re-used by the animal. This is a most important function, as without it the animal would have to consume vast quantities of water, most of which would be wasted by passing straight through the body.


You may have noticed that, on previous lectures, one of the examples of a carbohydrate was a substance called cellulose. Cellulose is the material which makes up the cell walls of plant cells. As the plant matures the cellulose toughens, and turns into a very tough material called lignin. Maize stover and wheat straw is made mainly of lignin, and so are the stalks of mature veld grasses. Cellulose comprises the fibre in foodstuffs, and roughage such as grass, hay and straw are high in fibre. Concentrate feeds such as maize meal are low in fibre.

      Lignin: a complex organic polymer deposited in the cell walls of many plants, making them rigid and woody.

In the process of digestion, the cellulose or fibre cannot be broken by the actions of enzymes or chemicals. This means that most animals with simple stomachs, the non-ruminants, cannot digest fibre and cannot thrive on fibrous foods such as grass. The best example is the pig which has to be fed on concentrates to get the best performance out of it. Humans can also not eat grass, and have to assist the digestion of vegetables and other foods by cooking them before eating. The non-ruminant which is the exception to this rule is the horse. It eats a lot of grass, but has a special caecum which aids digestion whereas the other non-ruminants have very small caecums.

The only way that cellulose can be broken down is by the action of micro-organisms which are mainly bacteria. The problem with cellulose is that it makes up the cell wall, and until this has been broken down, the nutrients inside the cell are unavailable. We shall be discussing the breakdown of cellulose by the action of bacteria when we cover ruminant digestion. In non-ruminants such as the pig, a small amount of bacterial breakdown takes place in the caecum, and a very large amount in the caecum of the horse.

A summary of the various agents of breakdown and the end products of carbohydrates, protein and fats is shown below. This may look rather complicated and is for general interest only, although you will come across some of the names later on in the course.

Figure 1: Breakdown of Carbohydrates and Proteins