Digestive Systems and
Digestion of Feeds

Feeds are digested and the nutrients in it are then absorbed by the animal. The digestive systems of the different species of livestock are very different, and this affects the feeds that they are able to eat and obtain any benefit from, as well as how often they need to feed. The simplest division of the digestive system is between those with simple and complex stomachs - monogastric livestock and the ruminants.

Monogastric livestock Pigs Poultry Ruminants Sheep and Goats References and Further Reading

The pig has a gut very similar in structure to that of a human.

In the pig, there is a small, blind sac (called a fundus) at one end of the stomach that is protected from the very acid conditions in the rest of the stomach. In the fundus, some micro-organisms can live, and so there is some microbial breakdown of feed here as well. This allows some of the fibre in the diet to be digested. Without the assistance of gut micro-organisms, no fibre would be digested at all, as birds and mammals do not produce the enzymes needed to digest fibre. All birds and animals are therefore incapable of digesting fibre, unless they support a population of micro-organisms somewhere in their gut to digest the fibre for them. Ruminant animals support a much larger microbial population in the rumen than pigs do in their fundus, and so ruminants can digest much more fibre than can pigs (or poultry).

Small intestine

When the food passes out of the stomach, it moves to the small intestine. Here conditions rapidly become much less acidic, and large amounts of enzymes are added to the half-digested feed. This is where most of the digestion takes place, and also where most of the nutrients are absorbed from the gut. The lining of the gut is covered with small projections (villi) to increase the surface area of the gut, and therefore increase the chances of nutrients being absorbed. These villi shrink when the animal is ill, particularly if it is suffering from a gut infection. This reduces the amount of nutrients that can be absorbed from the gut. Since feed intake is usually lower when an animal is sick as well, this has the effect of rapidly weakening the animal as few if any nutrients are getting into its body.

Large intestine

Digesta then moves from the small intestine to the large intestine. There is a large microbial population in the large intestine, and these micro-organisms ferment most of the remaining feed. Some fibre digestion can take place here, therefore. The fibre will be fermented to acids, which are absorbed from the gut and can be used as a source of energy by the animal. However, this fermentation is much less extensive than that which takes place in the rumen of sheep and goats. The micro-organisms in the gut grow and multiply in response to the feed that they have been provided with, producing microbial protein. However, it is too late in the gut for this microbial protein to be digested and absorbed by the pig, and so this potentially valuable source of protein is excreted in the faeces.

The large intestine is the main site from which water is absorbed, and it is important that this part of the gut remains healthy so that the animal does not lose too much water in its dung (as happens when the animal has diarrhoea).

Unlike pigs and poultry, sheep and goats are ruminant animals and so are particularly well adapted to eating fibrous feeds. They maintain a large microbial population in the rumen and reticulum, which are the first chambers that a feed enters after it has been eaten.

Mouth

When food is eaten it is passed quickly into the rumen, but it is then brought up again for much more extensive chewing (cudding) later. Feed will be continually regurgitated for further chewing until, combined with the digestive actions in the rumen and reticulum, it is small enough to pass down the rest of the tract. Large amounts of saliva are produced during cudding, and this helps neutralize the acid that is produced as a result of fermentation in the rumen and reticulum. Although much of the water needed for saliva production is recycled within the animal, the animal needs to consume enough water to meet this high demand.

Rumen and reticulum

The rumen and reticulum have a very similar function and are often considered the same organ. Together they constitute about 85% of the total stomach, and fill most of the abdominal cavity in the adult animal.

There is a huge microbial population in the rumen, in which microbial digestion and fermentation of the feed takes place. Fibre, as well as any starch and sugar that may be in the feed, is broken down then fermented to produce acids and gas. The acids are absorbed by the animal and used as a source of energy. The gas is mostly belched out by the animal. If the animal is fed too much starchy or sugary feed (e.g. cereals or molasses), particularly if it has not had a chance to adapt to a diet with a high starch or sugar content, then the fermentation will proceed too fast and the animal will not be able to get rid of the gas it has produced. This is one of the causes of bloat, where the rumen swells because of the accumulation of gas. The animal will also not be able to cope with the large amount of acid produced by the fermentation of the starch or sugar. The acidosis this causes makes the animal go off its feed, it is ill and in severe cases it will die.

If too much fibrous feed is fed, on the other hand, the microbial population will not be able to flourish because of a shortage of protein, and of readily fermentable material to encourage fermentation. The fibre then stays in the rumen and prevents the animal from eating until it has been digested.

If the animal eats a lot of forage from legumes, then it may also develop bloat by being unable to belch out the gas that it produces. Some legumes, when they get digested, produce something that makes a stable foam in the rumen. This collects round the entrance to the rumen and gas can not get through it to be belched out. Drenching the animal with an oil can break down the foam and relieve the bloat.

Protein is also attacked by the rumen micro-organisms and digested. Unlike the pig or chicken, digestion by rumen micro-organisms degrades protein to ammonia (instead of just amino acids). The ammonia is then used by the micro-organisms to make microbial protein. To do this, though, they need energy, which they get from the fermentation of carbohydrates (fibre, starch or sugar). It is therefore important that the supply of degradable protein and fermentable energy is balanced- when feeding a sheep or goat, you first have to feed the microbes. If there is insufficient energy for the micro-organisms, then they cannot make the microbial protein from ammonia. The ammonia is absorbed by the animal (and in large quantities this is toxic), and will then be excreted in the urine. If however microbial protein is made from the degraded protein, then this microbial protein, together with undigested feed, passes on into the omasum for digestion by the animal.

Fats inhibit microbial digestion and fermentation, because they form a coating over materials (like fibre) that could be digested by micro-organisms. The concentration of fat in the diet of a sheep or goat therefore needs to be kept fairly low (below 40 g/kg).

Omasum

Once the feed has been broken down into sufficiently small particles, it passes (with the microbial protein that has been produced in the rumen) into the omasum. Many minerals are absorbed from this site, but there is little digestive activity.

Abomasum

The abomasum is equivalent to the stomach of the pig and other monogastric animals. Here, acid and protein-digesting enzymes are produced to begin the process of digesting feed protein that has escaped digestion in the rumen, as well as microbial protein that flows with the feed from the rumen. In contrast to the stomach of non-ruminants, the abomasum secretes lysozyme, an enzyme that efficiently breaks down bacterial cell walls. Microbial protein from the rumen constitutes over half (often all) of the protein that is digested and absorbed by the animal.

Small intestine

Protein digestion continues in the small intestine, together with any starch that escaped digestion and fermentation in the rumen. Fat will also be digested here. Apart from the fermentation acids that were produced in (and absorbed from) the rumen, this is also the main site of absorption of nutrients from the gut into the animal.

Large intestine

As in the pig, the large intestine in the sheep and goat is another site of microbial degradation and fermentation of any material that has passed undigested through the rest of the gut. However, there is much less microbial activity in the large intestine of the sheep or goat compared with the pig, because there is much less material to ferment, and the more fermentable material will already have been fermented in the rumen. Fermentation acids that are produced from fermentation occurring in the large intestine will, however, be absorbed and contribute to the amount of energy supplied by the diet to the animal.

Digestion in Young Ruminants

The young ruminant's natural diet is based on milk supplied by the parent animal. Milk is a high quality product designed to provide all of the nutritional requirements of newborn and growing animals. (For more see the Nutritional Value of Milk). This material does not need to be digested by the microbial flora of the rumen and consequently the young of ruminants are not born with large rumens. In the young ruminant, the abomasum (the direct equivalent of the stomach in non-ruminant animals) forms about 70% of the volume of the four-compartments - the omasum, abomasum, rumen and reticulum. Moreover, the young ruminant has no functioning rumen with bacteria and protozoa.

When the young suckles, the milk bypasses the reticulum and rumen and is channeled through the esophageal groove. During the suckling process, impulses from the brain are sent to the esophageal groove, causing the sides to curve upward and form a tube. This allows a direct flow of milk into the abomasum. An enzyme (rennin) is secreted by the abomasum, causing the milk to coagulate. This slows the passage of milk through the abomasum and allows time for digestion.

When the young ruminants begin consuming various solid feeds, a microbial population becomes established in the rumen and reticulum, and its rumen gradually develops and increases in size and digestive function. As animals begin to mature and solids begin to constitute a greater proportion of the diet, they loose the ability to close the groove.

Rumen development

Protein and Non-protein Nitrogen Utilization in the Rumen

Proteins and non-protein Nitrogen are broken down by the microbial system in the rumen and converted to ammonia, organic acids, amino acids, and other products. Many rumen micro-organisms require ammonia for growth and synthesis of microbial protein. The extent of this conversion depends on a variety of factors including solubility of the protein, their resistance to breakdown, the rate of passage of feed through the rumen. Other factors include, for example, the presence of materials such as tannins which may inhibit protein digestion (although they also have important anthelmintic properties).

The rumen microbes convert the ammonia and organic acids into amino acids that then used to produce more microbial protein. Excess ammonia is mostly absorbed from the rumen into the blood stream, but small amounts may pass into the lower digestive tract and be absorbed. Any protein that escapes breakdown in the rumen together with the microbial protein passes to the abomasum and small intestine for digestion and absorption.

Ammonia also may be provided from non-protein Nitrogen sources such as urea - supplied in the form of Urea-molasses Blocks (UMB).

Digestion of Carbohydrates in the rumen

Digestion of Fats

Most of the digestion and absorption of fat occurs in the small intestine. The rumen bacteria and protozoa change unsaturated fatty acids to saturated acids through the addition of hydrogen molecules. Thus, more saturated fat is absorbed by cows than by simple-stomach animals. However, feeding large quantities of unsaturated fatty acids can be toxic to rumen bacteria, will reduce the digestion of fibre, and will lower rumen pH.

Vitamins

The rumen bacteria and protozoa manufacture the B vitamins and vitamin K, and therefore vitamin synthesis in the rumen is normally sufficient for growth and maintenance. Under most conditions, ruminant livestock do not require vitamin B or K supplements.