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 Digestive Tract: Large Intestine

The large intestine is the terminal part of the alimentary canal; it consists of the caecum (blind sac), colon, and rectum. The size of the large intestine varies considerably in farm animals. In horses it constitutes 46% of the total volume of the digestive tract. In ruminants, it represents only 10% of the total digestive tract capacity. The large intestine serves as a major site for the absorption of water, sodium, and chloride. Other salts such as potassium, phosphorus, and magnesium can also be absorbed from the large intestine. The large intestine has only mucous secretions and no enzymatic activity.
Undigested materials which escape the digestive processes in the stomach and small intestine are subjected to microbial digestion in the caecum. In monogastric animals, particularly horses and rabbits, the caecum serves as a major site for digestion of fibrous materials. This is accompanied by a variety of microflora. However, microbial digestion and absorption of end products in the caecum is not as efficient as it is in the rumen, and therefore does not nutritionally benefit the animal as much. The microbial protein synthesized in the caecum is mostly excreted in the faeces. The colon is part of the large intestine extending from caecum to rectum. The rectum is a dilatable tube serving as a storage place for feces until it is excreted.
Anus: 
The anus is the opening at the posterior end of the alimentary canal under the root of the animal's tail. The anal canal or lumen is normally closed by the contraction of sphincter muscles. During defecation, this canal is opened and undigested material (excreta) is passed through the anus.
Note: Next post on this blog will cover, "Practical Implications of Comparative Digestion in Farm Animals"

Functions of various parts of the digestive tract: Accessory Organs

Pancreas: The pancreas is a lobulated organ located in the deodenal loop. It has both endocrine and exocrine functions. Exocrine tissue produces pancreatic juice, which is composed of the digestive enzymes amylase, lipase, and trypsin and sodium bicarbonate. Other salts including potassium, chloride, calcium and magnesium are also present in relatively small quantities. The endocrine system consists of a vascular area called the Islet of Langerhans which secretes the hormones insulin and glucagon. Exocrine secretions drain directly into the duodenum through the pancreatic duct, while endocrine secretions are absorbed into the blood and circulated. Pancreatic enzymes require an optimum pH of 6.9 for their action in the small intestine, which is provided by pancreatic juice. Compared to monogastrics, the pancreatic juice of ruminants has a relatively low bicarbonate content, and therefore the pH of the acidic digesta entering the duodenum increases only slowly. Hence, the initiation of proeolytic activity does not take place immediately in the small intestine of ruminants.
Pancreatic enzymes are classified into three major groups: proteases, amylases and lipases. Among these, proteases are the major enzymes, representing about 72% of the total enzyme secretion. Pancreatic proteases are responsible for protein digestion, so they are also known as protelytic enzymes. Most of the proteolytic enzymes secreted from the pancreas are in inactive form and need to be activated by other enzymes present in the small intestine.
Pancreatic lipase causes hydrolysis of fats into carboxylic acids and glycerols, This action is facilitated by the prior emulsification of fat by the action of bilel salts. In other words, complete hydrolysis of fats in the small intestine is accomplished by the combined action of bile salts and pancreatic lipases. In adult animals, the activity of pancreatic llipase is increased when the animal consumes a high-grain diet. Pancreatic amylase activity is relatively low in ruminants compared to non-ruminants, because most of the starch is fermented in the rumen and little escapes to the small intestine. Pancreastic amylases convert starch into dextrin and maltose (disaccharides) and are of greater sifnificance in starch digestion than the salivary amylases.
Liver and gall bladder: Bile is continuously secreted by the liver. All domestic animals except horses have a gall bladder which serves as a storage organ for bile, which is spoured into the duodenum through the bile duct. In most farm animals, bile and pancreatic secretions are carried into the duodenum through a common duct. In cattle and buffaloes, pancreatic juice and bile are discharged into the duodenum through separate ducts.
Fats entering the small intestine are mostly in the dorm of large globules which cannot be hydrolysed by pancreatic lipases until emulsified by bile salts. Bile contains calcium and potassium salts of flycocholic and taurocholic acids which are required for maintaining alkaline pH and emulsification of fats. In addition to salts, bile also contains pigments (bilirubin and biliverdin), cholesterol, and mucin.
Note: Next Post will Cover The "Large Intestine" of domestic animals.

Functions of various parts of the digestive tract: Small Intestine

The small intestine is a long coiled tube divided into three parts; duodenum, jejunum, and ileum. The duodenum is the upper part of the small intestine which starts from the stomach/abomasum. The bile duct from the liver and the pancreatic duct from the pancreas open into the duodenum. The second, middle part of the small intestine is the jejunum. The jejunum is a coiled tube. Along with the ileum, a straight tube, the jejunum opens into the caecum through the ileo-caecal orifice. The inner lining of the small intestine is folded and has figerlike projections called villi. These structures are richly supplied with blood and lymph for absorption of digested nutrients. The muscular layer of the small intestine causes contraction and expansion movements called peristalsis which mixes and moves the digesta toward the large intestine.
The small intestine is the principal site of absorption of sugars, amino acids, fatty acids, vitamins and minerals. In ruminants, most of the carbohydrates are digested in the rumen; while in non-ruminants, the primary digestion site for carbohydrates is the small intestine. The semi-digested feed flowing from the stomach/abomasum is further digested and absorbed in the small intestine through the action of various enzymes secreted by the intestinal glands and supplied by pancreatic juice. Amylase, maltase, lactase, and sucrase are the major enzymes involved in the digestion of carbohydrates. Trypsin, chymotrypsin, and peptidases take part in the digestion of protein. A summary of various digestive enzymes is given in following table;

Enzyme	Origin	Place of Action	Substrate	Products
Amylase	Salivary Gland Pancreas	Mouth Duodenum	Starch Starch	Maltose Maltose
Maltase	Salivary Glands Intestinal Glands	Mouth Small Intestine	Maltose Maltose	Glucose Glucose
Pepsin	Gastric Mucosa	Abomasum	Protein	Polypeptide
Rennin	Gastric Mucosa	Abomasum	Milk Protein	Coagulates Milk Protein
Lipase	Gastric mucosa Pancreas	Stomach/Abomasum Small Intestine	Lipids Lipids	Faddy Acid Glycerol
Trypsin	Pancreas	Small Intestine	Protein	Peptides/Amino Acids
Peptidases	Intestinal Glands	Small Intestine	Peptides	Amino Acids
Sucrase	Intestinal Glands	Small Intestine	Sucrose	Glucose, Fructose
Lactase	Intestinal Glands	Small Intestine	Lactose	Glucose, Galactose

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.

Functions of various parts of the digestive tract: Mouth, Oesophagus

Mouth:

The primary function of the mouth include prehension (grasping), mastication (chewing), and mixing of feed with saliva as preparatory steps for subsequent digestion in the remaining parts of the alimentary canal. The lips, tongue, and teeth are used to pick and hold feed while it is in the mouth. The teeth grind the feed, and the tongue mixes it with saliva. The lips of sheep, goats, and horses are flexible, while those of cattle and buffaloes are relatively stiff and immobile. In horses, the upper lip is very sensitive and mobile and is used to feel the feed and direct it towards the moouth. Horses also have both upper and lower incisors which cut herbage like scissors and help in prehension. The mastication of feed with grinding molars is also very efficient in horses. In cattle, buffaloes, and sheep, the upper incisors are absent. Instead, they have a hard pad called the dental pad. During grazing, the tongue is used to guide feed into the mouth, and plants are grasped between the dental pad and lower incisors and torn away.

Saliva is a watery solution containing small amounts of mucin, inorganic salts and enzymes. It is secreted into the mouth from three pairs of salivary glands (paratoid, sub-maxillary, and sub-lingual). Secretion of saliva increases during mastication of feed. Cattle and buffaloes produce 130 - 200 litres, sheep and goats 2 - 3 litres, and horses 10 - 12 litres of saliva per day. Saliva lubricates the ingested feed and thus facilitates swallowing. It also helps in maintaining optimum pH in the rumen and provides a mechanism for recycling urea in ruminants.

Oesophagus:

This is a muscular tube extending from the back of the mouth (pharynx) to the stomach. Ingested feed is forced through the oesophagus by its wavelike muscular contractions.

(Note: In next Post, find about Function of stomach in digestive system of farm animals)

Parts of digestive tract

The digestive tract, also known as the alimentary canal, is a tube like structure extending from the mouth to anus. The different parts of the digestive tracts of ruminants and non-ruminants are illustrated in figures. The alimentary canal is comprised of the following parts,

1. Mouth (lips, tongue, gums and teeth) and pharynx
2. Oesophagus
3. Stomach (In ruminants, the stomach includes the rumen, reticulum, omasum, and abomasum)
4. Small intestine (duodenum, jejunum, and ileum)
5. Large intestine (caecum, colon, and rectum)
6. Anus

In addition to the above, the following accessory organs/glands also take part in the process of digestion: salivary glands, liver, gall bladder, and pancreas.

Among domestic animals, the proportion of the digestive tract to total body weight is higher in ruminants than in non-ruminants, as shown in table. Horses, for example, have a large caecum (15.6% of the total alimentary tract) where extensive microbial fermentation of plant materials takes place. Similarly, rabbits also have a well developed caecum. In ruminants, the stomach compartments (rumen, reticulum, omasum, and abomasum) occupy the major part (62%) of the digestive tract. Large variations in the proportion of different parts of the digestive tract in farm animals reflect differences in their feeding habits. These variations have evolved over time, and the animals have gradually adapted to various feed resources.

	Cattle	Buffaloes	Sheep/Goats	Horses	Dogs
Body Weight (Kg)	450	600	50	450	20
	Volume in liters, and proportion of body weight (percent)
Stomach
Reticulo-rumen	97.6 (48)	130.3 (48)	10.7 (56.7)	-	-
Omasum	15.8 (7.6)	20.9 (7.6)	0.6 (3.3)	-	-
Abomasum	11.8 (5.8)	15.7 (5.8)	1.3 (6.7)	8.0 (8.9)	1.0 (63)
Small Intestine	50.9 (25)	67.7 (25)	3.8 (20)	27.0 (30.0)	0.37 (23.0)
Caecum	7.7 (3.8)	10.3 (3.8)	0.6 (3.3)	14.0 (15.6)	0.02 (1.0)
Large Intestine	19.7 (9.5)	26.0 (9.5)	1.9 (10.0)	41.0 (45.6)	0.21 (13.0)
Total	203.5	270.9	18.9	90.0	1.6

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.