In the previous lecture, we discussed how any foodstuff, maize, cottonseed, grass, hay, silage etc., can be analysed in a laboratory and broken down into different parts or constituents.
These parts are:
- Dry Matter
- Ash or Minerals
- Soluble Carbohydrate or N.F.E.
Each one is expressed as a percentage of the total dry matter of the food. We also know, from lectures 5 and 6 on the Digestive Systems that an animal with a simple stomach obtains its energy for maintenance, growth and production from fats and soluble carbohydrates, and a ruminant obtains its energy from fats, soluble carbohydrates and fibre in the form of cellulose. Both types of animal obtain their protein requirements from the protein in the foodstuff. Using the laboratory analysis of foods, we can classify them into different categories based on their energy and protein value to the animal.
- High Energy, Low Protein foods such as maize, other cereals and hay.
- High Energy, High Protein foods such as oil and legume seeds.
- Succulent foods such as silage and grass. These foods have a low dry matter and high water content. Most of these foods are high energy, low protein foods.
- PROTEIN VALUES
This is a fairly simple matter in animal feeding. As you know from the last lecture, any food can be analysed to find out the amount of nitrogen that it contains, and this is multiplied by 6.25 to give the amount of total protein, or crude protein in the food. The symbol for this is C.F. and it includes the true protein manufactured from amino acids, together with any simple nitrogen compounds called the amides. The amides can be used by ruminants but not by the simple-stomach animals.
Crude protein (C.P.) is the total amount of protein in a foodstuff, and the other term which you should remember is Digestible Crude Protein, or which is the amount of protein in a foodstuff actually used by the animal. The tables which give the analysis of foodstuffs usually give a figure for both C.P. and D.C.P. If only C.P. is given, you can calculate the D.C.P. using the formula which you were given in the last lecture. When working out rations for stock, some people use C.P. and others use D.C.P.
- Maize, which has a D.C.P. of 6.5 to 7, is a low protein food.
- Cottonseed which has a D.C.P. of 17 is a medium protein food.
- Soybean cake which has a D.C.P. of 37, is a high protein food.
2. ENERGY VALUES
To ascertain the energy value of a foodstuff is a little more complicated. As you know, energy comes from the soluble carbohydrates (N.F.E.), the fat or oil, and some of the fibre in the foodstuff as well as all the compounds which contain carbon. The source of energy is the carbon in the food, in exactly the same way as the source of energy (or heat) is the carbon in coal or wood when they are burned.
However, proteins contain carbon as well as nitrogen, in fact quite a lot of carbon, so that the proteins in a foodstuff supply not only protein to the animal but also energy. Energy is supplied by every part of a foodstuff except the minerals and water.
An early attempt to work out the energy value of foodstuffs was begun in 1867 by a man named Gustave Kuhn, who was the director of an experimental station in Germany. His work was carried on by Kellner in 1892, and Kellner was quite successful. He used a special chamber in which he kept a steer, and by measuring the amount of carbon and nitrogen which went into the animal as food, and the amounts that came out as urine, dung, carbon dioxide and methane gas, he was able to calculate the amount of carbon and nitrogen required by the steer for maintenance. In other words, the amount needed to keep the animal in a stable condition and not putting on any weight. He then fed pure starch over and above the maintenance ration and found that 1kg of pure starch caused the animal to put on 0,25kg of fat. His next step was to feed other foods to the animal and compare their ‘fat forming’ powers with that of pure starch. Using the results of his feeding experiments, he worked out what he called the Starch Equivalents for a number of foodstuffs. The Starch Equivalent of a food is the number of kilograms of that food which will produce the same amount of fat (or energy) as 100 kilograms of pure starch. To give an example, the Starch Equivalent of maize is 77 and that of barley is 71 so that 77kg of maize and 71kg of barley will produce the same amount of fat or energy as 100kg of pure starch. Following Kellner’s experiments, the Starch Equivalents were worked out for all the common farm foods, and the system was used until very recently for the calculation of rations for cattle in the United Kingdom.
TOTAL DIGESTIBLE NUTRIENTS
The problem with the Starch Equivalent System is that it is a comparative system. The Starch Equivalent of food is not the actual energy value of that food, but the energy value compared to that of pure starch.
Because of this, the system can be inaccurate. The system of Total Digestible Nutrients, or T.D.N. was developed in America and is a direct measure of the energy value of a foodstuff. This is the system used in Southern Africa, America and also in the United Kingdom today.
To find out the T.D.N. of a foodstuff, a sample is analysed in the laboratory, where the amounts of protein, N.F.E., fibre and fat are determined. This is a straight forward feeding stuff analysis. The fat percentage is multiplied by 2.25 because fat has just over twice the energy value of the other carbohydrates. The figures for all the constituents are then multiplied by a Digestibility Coefficient for each constituent. These Digestibility Coefficients have been worked out from digestibility experiments carried out with different animals and can be obtained from published tables – different tables for different types of animal.
The following example shows how the T.D.N. is calculated for a sample of dried grass:
|Chemical Analysis||Digestibility Coefficient||DigestibleNutrients|
|Crude Protein 20.11% |
Fat 3.34 x2.25
33.03 = 64.13
The T.D.N. for the sample of dried grass = 64.13%. In other words, for every 100kg of the dried grass fed to cattle, the amount digested by the animal and converted into energy is 64.13kg. This is a fairly accurate measurement of the energy values of foods and is widely used in animal rationing. All the common foodstuffs have been analysed and their T.D.N. values worked out, and these are published in books and tables. The T.D.N. for maize is 80, indicating that maize is a high energy food.
METABOLISABLE ENERGY OR M.E
This is a measure of the energy values of foods being used in animal nutrition and all poultry feeds are expressed in M.E. It is a very precise measure of energy values. The gross energy value of a food is the number of heat units, or calories, produced when the food is completely burned up. The metabolisable energy is the gross energy of the food, less the value of energy lost in the form of dung, urine and the gases carbon dioxide and methane.
Metabolisable energy, expressed in kilojoules per kg (or calories) has largely replaced TDN as a measure of the energy value of feedstuffs.
Gross energy, digestible energy, metabolisable energy and net energy are all used in expressing energy values of feeds. These can be explained as follows:
3. NUTRIENT VALUES OF SOME COMMON FOODS
The following list gives the nutrient values of some of the common foodstuffs used in Southern Africa. The thing to remember when using these figures is that foods vary from sample to sample and these figures are averages. However, they are used when calculating rations for farm animals.
|Average Analysis of Feedstuffs|
|DM %||T.D.N. %||D.C.P. %||Ca %||P %|
Corn and Cob Meal
Maize whole plant
|Oil Seeds and Cakes|
|Cotton Seed |
Cotton Seed Meal Decorticated
Cotton Seed Meal Un-decorticated
Groundnut Cake Decorticated
Groundnut Cake Un-Decorticated
Soybeans Oil Cake
Sunflower Seed Cake
|Animal Protein Feeds|
|Fish Meal |
Meat and Bone Meal
|Lucerne Green |
|Average Analysis of Feedstuffs|
|DM %||T.D.N. %||D.C.P. %||Ca %||P %|
|Lucerne Meal |
|Maize Silage (Good) |
Maize Silage (Average)
Maize Silage (Poor)
Maize / Legume silage
|Grass Early Flower||90||50||3.5||0.5||0.2|
Veld Hay Av.
|Ca = |
|Urea||46% N||CP 287%|
|Chicken Litter||4% N||CP 25%|