- DIGESTIBILITY
In the last two lectures we covered the digestion and absorption of food inside the animal, both the animal with a simple stomach and the ruminant. You should understand quite clearly the difference between these two kinds of digestion, as they play a vital role in animal nutrition.
Now we want to consider the Digestibility of the various foods that are used in farming. We have seen how food is broken down and digested; now we will consider how much of the food is digested. In animals with a simple stomach, and remember that man has a simple stomach, most of the food that is eaten is digested, mainly because the food itself is highly digestible. Farm animals such as pigs are fed concentrate foods like cereals, and these foods are fed as a finely ground meal which helps the animal to digest the food. In the case of man, the food that he eats is refined food such as white sugar and white flour from which most of the indigestible roughage has been removed during the manufacturing process. Furthermore, unrefined foods like meat, potatoes and vegetables are cooked before being eaten and this softens and breaks down the fibre and cellulose making digestion much easier. In the case of man, the feeding value or energy value of a food is measured in calories. The requirement of a man is 3 000 calories a day and that of a woman is 2 200 calories. A calorie is the amount of heat or energy (remember that heat and energy are really the same thing) required to raise 1 gram of water through 1°C. The calorie system works well with man because the food he eats is highly digestible through being refined or cooked. Foods that are high in calories are said to be rich foods and those low in calories roughage.
When we come to consider the digestibility of foods in the ruminant animal, the situation is very different, because ruminants eat very bulky foods which are high in cellulose and lignin. Although the ruminant digestive system is designed to deal with bulky foods, the amount that is actually digested will depend on the food itself. Concentrate foods especially those that are ground up before feeding, are highly digestible; young grass is low in fibre and is easily digested, but veld grass in the winter is old and tough; it has a low digestibility. The animal will eat the grass and satisfy its appetite, but much of the fibre will not be broken down and digested: a lot of the grass will pass through the animal and be passed out as dung. Another point to remember when feeding animals is that the highly digestible concentrate foods are expensive and the roughage foods, although they have a lower digestibility, are much cheaper. This is why it is important to have a good knowledge of the digestibility of the different foods when deciding what to feed various kinds of livestock; in other words, when you are working out their rations.
- ANALYSIS OF FOODSTUFFS
How is the digestibility of a food worked out? Well, this is a complicated process and requires a lot of chemical analysis which has to be done in a laboratory, but the principle is fairly simple. The first step is to analyse the composition of the food to find out what it contains.
Any foodstuff, for instance maize, can be broken down into the following components or parts:
Suppose you sent a sample of maize grains to a laboratory for analysis. They would grind up the grains into a fine meal and carry out these tests:
· WATER:
A sample of the meal is put into a dish and weighed. The sample is then put into an oven and heated at 80°C for about 48 hours. After this time all the moisture will have been evaporated from the sample and only the dry matter will be present. The sample is then taken out of the oven, allowed to cool down and weighed. The difference between the first weight and the final weight will be the amount of moisture which has been lost from the sample, and this is given as a percentage of the original sample: For example:
Weight of sample before heating – weight of sample after heating x 100 / Weight of sample before heating
= % of water in the original sample.
Once you know the water, or moisture content of the grain, you can calculate the Dry Matter
of the grain.
Maize at harvesting will have 14% water so that the Dry Matter will be
100 – 14
= 86%.
After harvesting and before the maize is sold, it will dry down to 12% water or 88% Dry Matter.
- All further tests done on your maize will be carried out using the dried sample, and the results will be given on what is called the Dry Matter Basis; in other words, expressed as a percentage of the dry sample.
· MINERALS:
Part of the dry sample is weighed and then slowly burned in a dish using a small flame from a candle or Bunsen burner. The sample is then put into an oven at 500° C for 2 hours, taken out, cooled and weighed. All that is left of the sample is a white ash, because all the organic matter has been burned away. This ash contains all the minerals in the sample and from the difference in weights of the sample, the mineral content can be given as a percentage of the original dry sample. The ash can be further analysed to find out the amounts of the various minerals such as calcium, phosphorus, magnesium etc., and the results are given as a percentage for each mineral.
· CRUDE PROTEIN:
You may remember from the earlier lecture on protein, that protein always contains nitrogen, and in fact a fixed amount of nitrogen – 16%. It follows then that if you can find out the amount of nitrogen in a sample, you can calculate the amount of protein simply by multiplying the nitrogen by 100/16 or 6.25, which amounts to the same.
- The amount of Nitrogen in the dry sample is found by a complicated method called the Kjeldahl method, using special equipment. The sample is boiled up with concentrated sulphuric acid (a very dangerous chemical), which converts all the nitrogen in the sample to ammonia gas which is collected and measured. From this the amount of nitrogen is calculated and the crude protein worked out:
- Crude Protein = Nitrogen x 6.25
- The crude protein, or C.P., is then given as a percentage of the original dry sample.
· FATS AND OILS:
The fat and oil content of the sample is found by a process called ether extract. The sample is mixed with a chemical called petroleum ether and this dissolves all the fat in the sample. The amount is calculated from the increase in weight of the petroleum ether. This is a dangerous experiment which needs special equipment.
· FIBRE:
The sample is boiled up with a weak solution of sulphuric acid which dissolves away all the non-fibrous material. What is left is the fibre, which is known as the Crude Fibre.
- SOLUBLE CARBOHYDRATE:
The technical name for this is the Nitrogen Free Extract and it includes all the starch, sugars, some of the cellulose and some of the lignin. The Nitrogen Free Extract, or N.F.E., is calculated by adding all the other results together and taking the total away from 100. For example:
- N.F.E. = 100 – (Ash and Crude Protein and Crude Fibre and Fat), and it is expressed as a percentage of the original dry sample.
Below is a sample of maize that has been analysed, and the results would be as follows:
| Moisture Content 13% | Dry Matter 87% | |
| 1.5% | Minerals or Ash | |
| 10.0% | Crude Protein | |
| 2.0% | Crude Fibre | |
| 4.5% | Ether Extract or Fat | |
| 69.0% | N.F.E. or Soluble Carbohydrates |
The mineral or Ash content is 1.5% and this would be further broken down into the following:
- Minerals or Ash 1.50%
- Calcium 0.01%
- Phosphorus 0.36%
- Potassium 0.33%
- Chlorine 0.07%
- Other Minerals 0.73%
Going back to the diagram of the breakdown of a food, we can fill in the figures for maize:
Similar figures have been worked out for all the common foodstuffs used in agriculture all over the world and they are published in various books and tables for reference. The figures for all the common feeds used in stock feeds are given in the next few lectures and are summarised in a table at the end of lecture 12. However, you must remember that these figures are averages of the results obtained by analysing many hundreds of samples. There are variations between individual samples, because crops vary from farm to farm and from year to year, depending on the weather, fertilizer applications, cultural practices etc. For example the Crude Protein in maize and other cereals is affected by the amount of fertilizer nitrogen applied to the crop and by the time it is applied. Heavy top dressing of nitrogen given at a late stage in the growth of the crop will increase the Crude Protein of the grain. Due to these variations, it always pays for the farmer who is using a lot of home-grown feed for his cattle to send samples for analysing. Samples of maize, soybeans, hay, silage, etc. should be analysed so that the farmer has the exact figures when he is working out the rations for his stock.
3. CALCULATING DIGESTIBILITY
Having worked out the analysis of any foodstuff, it is possible to find out the digestibility of that food for any type of animal.
Suppose we want to know the digestibility of maize when it is eaten by rats. This would be done by putting a rat in a cage arranged so that the dung and urine from the rat could be collected each day. The rat would be fed maize meal that had been analysed, and the dung and urine would be analysed in the same way. At the end of a week you would know how much of the maize fed to the rat has been passed out as dung and urine – i.e. had not been digested. The remainder, the part that had stayed inside the rat had obviously been digested. In this way, it is possible to calculate the amount of maize digested by the rat, and also the amount of each part of the maize that has been digested. This allows us to calculate the digestibility of maize and also the digestibility of the Protein, Fat, Fibre, Minerals and N.F.E. in the maize.
By analysing the maize meal going into the rat, and the dung and urine coming out, and by measuring the meal going in and the dung and urine coming out, we can work out how much of each part of the food has been digested. By expressing this as a percentage of the original feed, we can get a percentage figure which we can call a Digestibility Coefficient. Using these, we can work out the digestible protein, fat etc. for maize meal when fed to rats.
The digestibility of the common foods has been worked out for all the farm animals. In the case of ruminants, sheep have been used for experiments as digestibility for sheep and cattle are very nearly the same. Digestibility is important in animal feeding. By analysing the food, we know how much protein, fat, etc., that specific food contains. By using the digestibility coefficient for each part of that food, we can work out how much protein, fat, etc., from that specific food will be used by the animal. For instance, the Crude Protein of maize is 10%, but the Digestible Crude Protein (D.C.P.) is 7%, therefore only 70% of the protein in maize meal is used by the animal. The other 30% is passed out of the body undigested. Most food tables used in rationing give figures for Crude Protein (C.P.), and Digestible Crude Protein (D.C.P.).
