Proteins are the chemical foundation of all living matter; in other words, of all life. Proteins, together with water are the material from which plant and animal protoplasm is made. Furthermore, the enzymes, chromosomes, digestive juices and many other important chemicals in animals and plants are protein or protein-like substances. Some years ago, an interesting experiment was carried out by a group of scientists who were trying to find out how life started on Earth. They calculated what gases would make up the atmosphere as the Earth was cooling down, and they made up a mixture of the gases in a vacuum container. They then passed a strong electric spark, such as might have been provided by lightning through the gas mixture, and they found that protein molecules had been formed. This suggests that the whole of life on Earth today may have developed from similar protein molecules.

Proteins are made up of carbon, oxygen, hydrogen and nitrogen atoms which combine together to make very large, very complicated molecules of various proteins. The difference between proteins and carbohydrates is that proteins contain nitrogen and carbohydrates do not. An interesting and useful fact is that all proteins contain 16% nitrogen and this is used in the analysis of feeding stuffs. If a chemist wants to find out the amount of protein in, say, maize meal, he measures the amount of nitrogen in the sample of meal. This he can do fairly easily in a laboratory. As he knows that the amount of nitrogen is 16% of the total protein in the sample, he uses the following simple calculation:

Protein = Total Nitrogen x 100/16

In fact, 100/16 is equal to 6.25, and this is the figure that is used.

Due to proteins being such large, complicated molecules, they are built up from other molecules (which also contain C, H, O and N) called AMINO ACIDS. These are the building blocks of proteins, much the same as a wall is built up of bricks. Amino Acids are chemical substances which are crystalline, colorless, and usually soluble in water. They have a complicated chemical structure. Those students who have done some organic chemistry will remember that the formula for a fairly simple amino acid is C2 H5 O2 N1, arranged as CH2(NH2)COCH.


As we have said, proteins are built up from amino acids and usually contain from 20 to 25 different amino acids. A typical food protein contains 22 amino acids. The question you might ask is, where do amino acids come from? The answer is that plants can manufacture amino acids and also proteins, from simple chemical substances which contain nitrogen.

Animals cannot do this and must be fed the amino acids in order to build up their protein. We shall look at this in more detail.


One of the most important systems in the growing of crops is called the Nitrogen Cycle. This is discussed in the Soil Science Course, but we will go over it again. When organic matter, plant residues, weeds, animal dung and urine, dead insects, bacteria etc., are ploughed back into the soil, it is attacked by bacteria and fungi, and decomposed or rotted down. The protein part of the organic matter is broken down into amino acids and then further broken down to ammonia gas (chemical formula NH3).

Some of the ammonia escapes from the soil because it is a gas, but the rest is used by certain bacteria in the soil, and built up into chemicals called nitrites. These are then converted by other bacteria into nitrates, which dissolve in the water in the soil. Plants that are growing in the soil take up the soil water and the nitrates, and once inside the plant, the nitrates are built up into amino acids and then into protein. The cycle of breakdown to ammonia gas and the build-up to plant protein is called the Nitrogen Cycle.

The farmer can affect the Nitrogen Cycle by adding further nitrates to the soil in the form of artificial fertilizers such as Sulphate of Ammonia and Ammonium Nitrate.

The protein level of crops, cereals and grasses can be increased by the amount and timing of applications of nitrogen fertilizers. A top dressing of ammonium nitrate applied late to a crop of maize or barley can increase the level of protein in the grain.

The important fact to remember is that plants can take a simple chemical like ammonium nitrate and build this up in the plant in very complex amino acids and then into proteins. They can manufacture proteins.


Animals in general cannot build up protein from simple nitrogen compounds. There is one exception to this and that is in the case of ruminants and urea which will be discussed later on in this lecture. However, in all other cases, animals are dependent for their needs on plant or animal proteins. What animals can do is take in plant or animal proteins and break them down into their many amino acids and then build up different proteins from these amino acids.

Plant) Break BuildFresh
 )Protein Amino Acids Animal
Animal) Down UpProtein

Amino Acids that are unused by the animal are broken down further and excreted. They cannot be stored in the way fat can be stored in an animal, so, as protein foods are expensive, it is wasteful for a farmer to feed too much protein to his stock.


During the process of digestion and absorption, animals can change some amino acids into others, but some amino acids have to be fed to the animal. The ones that have to be fed are called the Essential Amino Acids and these vary from one species to another. To give you an example, the following 10 amino acids are essential for Pigs and must be included in their ration in the amounts given below:

  • ARGININE                     0.20%
    • HISTIDINE                     0.20%
    • ISOLEUCINE                 0.55%
    • LEUCINE                        0.60%
    • LYSINE                            0.75%
    • METHIONINE               0.55%
    • PHENYLALANINE        0.50%
    • THREONINE                  0.40%
    • TRYPTOPHAN               0.23%
    • VALINE                           0.50%

Of these, the most important in pig nutrition is lysine, and there is a high proportion of this in the protein of white fish meal and soybean meal. In fact, lysine is so important in pig nutrition that plant breeders are trying to produce varieties of maize and barley with a high lysine percentage in their protein.

Food proteins that contain all the essential amino acids are said to have a High Biological Value, and these are mainly animal proteins such as the protein in milk, eggs, meat and fish. Food proteins that contain only a few of the essential amino acids are said to have a low biological value. These are the vegetable proteins, such as cereals and nut protein.

The biological value of protein will depend on the species of animal and the needs of the animal. The requirement of the amino acids will be different for pigs, sheep, cattle and poultry, and will also vary according to the growth, maintenance, milk production, etc., of the animal. It is not enough to feed the correct amount of protein to an animal; you must feed the correct type of protein.

Due to the varying biological value of proteins, grazing animals and also human vegetarians must obtain their protein from a variety of sources in order to get all the essential amino acids. Meat- eating species get the essential amino acids, and in some cases an animal protein can increase the biological value of the plant protein if they are both fed together. An example of this is the feeding of pigs with a cereal meal and skim milk. The skim milk increases the value of the cereal protein.


At the beginning of this lecture we said that the amount of protein in a food was calculated by estimating the amount of nitrogen in the food and multiplying this by 6.25. This gives us what is called the crude protein in the food, and this term includes the true protein together with some other nitrogen compounds which are called amides. Amides are nitrates, ammonium salts, some amino acids and peptides. Animals with a simple stomach use the true protein only and are unable to use the amides. Ruminants can use both true protein and amides in the same way as they can use urea.

Young and immature crops generally are rich in amides. This applies to young grass in spring, and

particularly to pasture grass. The amides cause the scouring which you see in animals which have been turned onto fresh spring grass.

In animal feeding, two terms are commonly used when referring to protein; these are crude and digestible crude protein. Crude protein is the total protein in a food. For example, the crude protein in maize is 10%. The digestible crude protein or digestible protein as it is sometimes called is the amount of that protein digested by the animal. The way this is worked out is covered earlier on in this course, but as a rough guide, the amount of digestible crude protein (or D.C.P.) can be calculated as follows:

D.C.P. = (CP x 0.9) – 3. Therefore for maize

D.C.P. = (10 x 0.9) – 3

= 9 – 3

= 6

The Digestible Crude Protein of Maize is 6%.


Protein is used by animals for the following functions:

  • The building up of muscle which makes up the flesh or meat of the animal. Meat is a very high protein product;
  • The production of milk and eggs. The solids-not-fat in milk is high in protein and egg white is a pure protein called albumin;
  • The manufacture of protective tissues such as hair, horns, wool and feathers; and
  • The formation of digestive juices, enzymes, chromosomes and other internal secretions of the animal.

A deficiency of protein is a very serious matter, particularly in young animals, and will affect both growth and the production of milk, meat, eggs, wool etc. However, it is very wasteful to overfeed the animals with protein, as any excess cannot be stored. It is excreted by the animal.


Urea is a simple chemical which contains nitrogen. It is not a protein. The chemical formula for urea is (NH2) 2CO and pure urea contains 46% nitrogen. It is a white crystalline substance, which is used both as a fertilizer and a cattle feed.

When urea is fed to ruminants, the bacteria in the rumen of the animal break the urea down to ammonia gas (NH3) as it does in the nitrogen cycle. The ammonia gas is then used by the bacteria to form their own protein. When the bacteria die inside the rumen they are digested by the ruminant animal and used as a source of protein. The ruminant does not build up protein from urea but the bacteria do so for the animal. This happens also in the case of the amides. A ruminant can make use of amides whereas the animal with a simple stomach cannot.

The bacteria that turn urea into true protein require a supply of energy which they get from the

contents of the animal’s rumen. Due to this requirement, urea should be fed together with a good supply of available carbohydrate, either cereal or molasses. Other points to remember when feeding urea to cattle are:

  • Use urea in rations that are low in protein otherwise protein will be wasted. However, urea has a low biological value and should be fed together with some other sources of protein.
  • Urea has a very high crude protein value and only small quantities are needed in a ration:
  • Crude Protein = 46 x 6.25 = 287
  • Urea contains no minerals whereas animal and vegetable proteins are high in minerals. When changing a ration from normal protein to urea, extra minerals should be included.
  • When introducing urea into a ration it should be done slowly and by using small quantities. As it is broken down to ammonia in the rumen, too much urea can produce too much ammonia causing poisoning and death. An animal which is suffering from urea poisoning should be dosed with vinegar which is a dilute a solution of acetic acid to neutralise the ammonia.
  • Urea should be introduced into a ration over a period of 14 days to allow the bacteria in the rumen to adapt to the new source of protein.
  • Urea should not exceed 3% of a concentrate ration, or 1% of the daily dry matter intake of an animal.
  • Never feed urea to non-ruminants or young calves whose rumens have not developed. It will kill them.