Practical implications of comparative digestion in farm animals

Monogastric (non-ruminant) animals cannot digest fibrous feed (cellulose, hemicellulose) and depend on high quality, low-fiber concentrate diets. Among the monogastrics, horses and rabbits, due to their well developed caecum is less efficient than in the rumen. Therefore, horses should be fed good quality forages. On the other hand, ruminants, because of efficient microbial fermentation in the rumen, have acquired the ability to digest a wide range of roughages, including poor quality cereal straws and other crop residues. Ruminants can also utilize non protein nitrogen from sources such as urea as a dietary crude protein and convert it to a protein of high biological value (microbial protein).
The unique digestive features of ruminants enable them to utilize agro-industrial by products and wastes which cannot otherwise be used by monogastrics, including man. As ruminants depend largely on the fermentation products of the rumen (VFA's and microbial protein), conditions in the rumen should be optimal at all times to support efficient fermentation. This can easily be achieved by taking care of the nutrient requirements of rumen microbes. Recent developments in ruminants nutrition have emphasized that these animals should be fed in such a way that their capacity to utilize roughages and non-protein nitrogen is exploited to the greatest extent possible, and at the same time high quality feeds (such as good quality feeds and starches) should be protected from rumen fermentation so that they escape from the rumen and are efficiently digested and abosrbed in the small intestine. Unlike ruminants, monogastrics cannot synthesize amino acids and vitamins B and K. Therefore they depend on feed resources for these nutrients.

Functions of various parts of the digestive tract: Stomach - 2

Earlier Post: Functions of various parts of the digestive tract: Stomach
Omasum: The third compartment of the ruminant stomach is known as the omasum. It constitutes about 12.6% of the total volume of the stomach in cattle, buffaloes, sheep and goats. The inner lining of the omasum is raised into longitudinal folds are pillars, which greatly increase the surface area. The principal function of the omasum is to absorb water and reduce size of the feed particles. The contents of the omasum are much drier than those of the rumen and abomasum. Because of the small size of the omasum, its contents are not retained for long and are passed on to the abomasum.
Abomasum: This is the fourth and last chamber of the ruminant stomach. It resembles the simple stomach of the monogastric animals and is the only glandular part of the ruminant stomach. The inner lining of the abomasum has folds which are more prominent in the glandular region. It secretes hydrochloric acid and pepsin. The acidic environment of the abomasum causes destruction of microbial cells which are subsequently digested by the pepsin secreted into the abomasum. The rate of abomasal secretion is influenced by the volume and composition of ingesta entering the abomasum. The digesta leaving the abomasum (in a ruminant) and the simple stomach (in a non-ruminant) is called chyme.
Development of the stomach in young ruminants: In ruminants, the rumen is small and non - functional at birth. The abomasum, on the other hand is comparatively large and well developed. As the animal grows, the rumen gradually develops and attains a size which is 10 times greater than the abomasum in adult animals. Since the rumen does not play any role in digestion during early life, the milk suckled by the calf bypasses the rumen and goes directly in to abomasum through a special mechanism, the oesophageal groove. In the abomasum an enzyme, rennin, is secreted which has a strong coagulating action on milk. This helps in retaining the milk for longer time in abomasum for further digestion. The oesophageal groove is a tube like structure with a slit. Suckling and swallowing of milk and other liquids causes closure of the oesophageal groove, thus directing the liquid food to the abdomasum. Ingestion of solid food causes the oesophageal groove to open, allowing the feed to drop in to the rumen. As the intake of growing calf gradually changes from milk to solid feed, the stomach compartments, particularly the rumen, increase in volume, formation of papillae on the internal surface, and establishment of rumen micro-flora. Early development of the rumen can be encouraged by offering good quality solid feed after two weeks of age. This can also help in early weaning of  young calves.
B. Non - Ruminants (Mono Gastrics): In these animals, the stomach is a "V" - Shaped muscular sac. Muscular contractions of the stomach wall help in breakin gdown the feed particles, and at the same time cause mixing of the stomach contents. About two thirds of the inner surface of the stomach wall has glands which secrete gastric juice containing mainly hydrochloric acid and the enzyme pepsin. The pepsin digests protein into peptides and amino acids and requires acidic medium for its action. Hydrocholoric acid helps in maintaining acidic pH in the stomach. The feed is exposed to gastric enzymes ofr some time, and the semi-digested feed is passed to the small intestine through an opening controlled by sphincter muscles.

Note: Next post on this blog will be comprised of details about "Small Intestine".

Functions of various parts of the digestive tract: Stomach

The stomach serves as a reservoir for feed where active digestion begins. The structure and function of the stomach vary greatly in ruminants and non-ruminants.

Ruminants (Polygastrics): Compared to non-ruminants, ruminants have three additioonal stomach compartments and are therefore known as compund stomach animals. The different compartments of the uminant stomach are the rumen, reticulum, omasum and abomasum.

Rumen and Reticulum:  The first two parts are closely associated and sometimes reffereed to as the reticulo-rumen. The reticulo-rumen and omasum develop from the distal part of the oesophagus; thus they are also called the "forestomach".

The rumen serves as a big fermentation vessel divided into dorsal and ventral sacs. The reticulo-rumen in adult animals occupies about 50% of the total capacity of the digestive tract and 75% of the abdominal cavity. The internal surface of the reticulo-rumen is non glandular and raised into folds. The reticulum has a honeycomb like structure, while the rumen is internally covered with papillae of varying shapes and sizes. Reticulo-rumen contractions, which increases from 47 - 80 at resting and 79 - 100 during eating, are responsible for the breakdown and mixing of feed particles.

Rumen Contents: The rumen contents have a large amount of water (850 - 930 g/Kg). The proportion of dry matter in the rumen contents varies from 7 - 15% depending on the nature of the diet. The rumen never empties, but the contents become more watery with fasting. In regularly fed animals, the rumen contents can be divided into a liquid phase and particulate phase. The liquid phase occupies the ventral sac, while the particulate phase, having suspended feed particles, is limited to the dorsal sac.

Rumination: As stated earlier, the mixing and breakdown of rumen contents is accomplished by strong ruminal movements. These movements force part of the rumen contents through the oesophagus and back into the mouth. This is called regurgitation. The regurgitated matter in the form of boluses is re-masticated and swallowed, and this process is called rumination. Rumination involves regurgitation, re-mastication, re-salivation, and re-swallowing of ingested feed. It enables the animal to take its feed at one time and chew it slowly later on while resting. Rumination stimulates saliva secretion which is important for proper rumen function. Normally, cattle and buffaloes spend up to one-third of their time (about eight hours) ruminating through out the day. It has been estimateed that one rumination cycle takes about a minute, of which four seconds are used for regurgitating and re-swallowing, and the remaining time is used for re-masticating. The coarseness of feed affects rumination time. Animals with a diet of concentrate have a shorter rumination time (two to three hours) compared to those with a roughage-based diet (more than eight hours).

Rumen micro-organisms: Fermentation in the rumen is a continuous process taking place under anaerobic conditions. Ruminant digestion does not require enzymes produced by the animals but occurrs a result of the combined action of bacteria, protozoa, and fungi. The population density of bacteria inhabiting the rumen is about 10(9) per ml of rumen contents. More than 60 species of rumen bacteria have been identified to date. They can be broadly classified in to cellulolytic (cellulose-digesting), amylolytic (starch-digesting), and proteolytic (protein-digesting). The important rumen bacterial species are listed in Table. These bacteria secrete various enzymes for digestion of feed in the rumen. The relative population of these bacterial species largely depends on the composition of feed. For example, amylolytic bacterial will predominate when animals are consuming a large quantity of cereal grains in the diet. On the other hand, a high roughage diet will support a cellulolytic bacterial population in the rumen. Rumen bacteria are very sensitive to dietary changes, therefore frequent and sudden changes in the diet may disturb rumen functions.

Table

Rumen bacteria, under suitable conditions, multiply very rapidly. They may be present either in the liquid phase (free floating bacteria) or attached to feed particles (adherent bacteria). Free floating bacteria generally live on soluble carbohydrates and protein and can be quickly washed out of the rumen with liquid flowing to the small intestine. In contrast, adherent bacteria are retained in the rumen for a longer time. Microbial cell lysis and multiplications in the rumen is continuous process which maintains their population in a steady state.

Rumen microorganisms serve two main purposes:

1. They digest feed and convert it to end products, which are easily assimilated by the host animal, and
2. They serve as a source of high quality protein which leaves the rumen and is digested in the small intestine. 

Rumen protozoa are larger in size but fewer in number (10(6)/ml rumen content) than rumen bacteria. They can be divided into two groups:

1. Holitrichs, covered with cilia
2. Entodinomorphs, having no cilia on their body.

Protozoa have the ability to engulf small food particles and rumen bacteria. Their numbers also increase rapidly; however, unlike bacteria, they usually adhere to feed particles and the rumen wall (sequestering), and so do not flow rapidly to small intestine. Although predation of bacteria by protozoa is considered a harmful effect on the bacterial population, protozoa have been shown to complement feed digestion in the rumen. The interaction of bacteria and protozoa is considered a harmful effect on the bacteria population, protozoa have been shown to complement feed digestion in the rumen. The interaction of bacteria and protozoa in the rumen is very complex and a subject of great interest these days. The number and species of protozoa in the rumen depend on the availability of the substrate and conditions prevailing in the rumen. Low rumen pH associated with high grain feeding considerably reduces protozoal population. 

The role of anaerobic fungi in rumen fermentation is not fully understood and is the subject of recent interest. it has been suggested that they play a complementary role in fibre digestion and are therefore considered beneficial in roughage-based diets, particularly tropical forages.

Substrate and fermentation products: Different substrates available for microbial fermentation in the rumen include dietary carbohydrates, proteins and lipids. Carbohydrate sources in ruminant diets include forages (cellulose and hemicellulose), cereal grains and their by-products (starch), and molases (soluble sugars). Leguminous forages (berseem, lucerne, and cow pea, etc.) also serve as a good source of protein. Oilseed cake and oilseed meal are commonly used as protein supplement and in addition, these feeds may contain a substantial amount of lipids. Compared to monogastrics, ruminants have a limited ability to digest lipids, and therefore feeding of large amounts of fats often adversely affect rumen function, particularly fiber digestion. The major end product of microbial fermentation of these substrates in the rumen are volatile fatty acids (VFA's), gases, ammonia, peptides, amino acids and microbial proteins.

Volatile fatty acids and gases: The principle VFA's produced in the rumen are acetics, propionic, and butyric acids. The proportions and concentrations of these VFA's depend on the type of substrate and microbial species in the rumen. Propionic acid, a glycogenic VFA is predominant in forage based diets. the concentration of butyric acid is usually high with readily soluble carbohydrates such as molasses. rumen gases include carbon dioxide, methane and hydrogen. The VFA's and a part of the gases are absorbed through the rumen wall and metabolized. A large part of the gases is lost through eructation.

Ammonia: Ammonia in the rumen is largely produced from protein fermentation. Protein in the diet is hydrolysed by rumen microorganisms via peptides to free amino acids which are further deaminated to produce ammonia. The concentration of ammonia in the rumen depends on the quantity and quality of dietary protein. Highly soluble protein results in a high concentration of ammonia in the rumen. Non-protein nitrogen such as in urea is almost instantly dissolved and converted to ammonia by bacterial ureases. Ammonia is one of the most important nutrients required by rumen bacteria for growth and multiplication. Deficiency of ammonia in the rumen generally depresses microbial growth and activity. Under conditions of ammonia excess, the surplus is lost from the system. However, very high concentrations of ammonia in the rumen may cause ammonia toxicity. Ruminants have the unique capability of recycling a part of the absorbed ammonia through saliva and the rumen wall. This nitrogen conservation system is more efficient when diets are low in protein.

Note: In next post, details about Omasum and Abomasum will be described.

Introduction to Digestion in Farm Animals

Digestion in animals involves mechanical, enzymatic, and microbial processes in the gastrointestinal tract, which convert large feed particles to a size which can be absorbed, transported, and used by the animals. The digestive systems of all farm animals are not the same, rendering them less competitive and able to adapt to a wide range of available feed resource niches. Based on the structure and functions of the digestive tract, farm animals are divided into two major groups:  ruminants or polygastrics, e.g. cattle, buffaloes, sheep, and goats; and non-ruminants or monogastrics, e.g. horses, donkeys, rabbits, dogs and cats. The key difference between these two groups of animals lies in the structure of the stomach. Ruminants, as opposed to non ruminants, have three additional stomach compartments where digestion takes place in different environments. In general, for farm animals the processes associated with digestion include prehension, ingestion, grinding or mastication, digestion of feed, absorption of nutrients, and excretion of waste products.