Difference between Monogastric and Polygastric Digestive Systems.

 

The difference between monogastric and ruminants.

Digestive System	Monogastric	Polygastric
Definition	This type of digestive system includes a single-compartment stomach.	This type of digestive system includes a four-chambered stomach.
Stomach	Only one compartment or chamber.	Four compartments or chambers.
Cellulose Digestion	Monogastric herbivores show partial cellulose digestion.	Ruminants or polygastric animals show complete cellulose digestion.
Bacterial Action	Only the gut bacteria of monogastric herbivores can partially digest cellulose.	The bacteria present in ruminants play a vital role in breaking cellulose.
Rumination	They do not regurgitate and chew the food.	They regurgitate and chew the food to stimulate digestion.
Examples	Humans, horses, rabbits, cats, and so on.	Cows, sheep, goats, and so on.

 

Monogastric Animals

A monogastric animal is one with a single stomach compartment. Monogastric animals include humans, poultry, pigs, horses, rabbits, dogs, and cats. Most monogastric people are unable to digest large amounts of cellulose-rich foods like grasses. Horses and rabbits are examples of herbivores with a monogastric digestion system that can digest cellulose in their diets via gut microbes, albeit with less energy than ruminants. Human-edible grains and soybeans account for a sizable portion of the feed given to intensively raised monogastric animals.

To begin the monogastric digestive system, food is ingested into their mouths. The tongue and teeth gather and cut feed into smaller pieces to make it easier for the animal to digest. The oesophagus is a long tube that connects the mouth to the stomach, through which food travels. In the stomach, enzymes break down the feed components, which are then temporarily stored and digested before being absorbed into the bloodstream. Any undigested food enters the small intestine, where it is further processed. After the small intestine has eliminated all nutrients from the feed, the remaining substance is passed into the large intestine and finally expelled from the body via the rectum or anus.

 

Herbivores with monogastric digestion can digest cellulose in their diets by using symbiotic gut bacteria. However, when compared to ruminants, they are less effective at recovering energy from cellulose digestion. Herbivores consume cellulose via microbial fermentation.

 

Fermentation is a metabolic process that involves the action of enzymes to produce chemical changes in organic substances. It is narrowly defined in biochemistry as the extraction of energy from carbohydrates in the absence of oxygen.

 

Foregut fermenters are ruminants, whereas hindgut fermenters are monogastric herbivores that can digest cellulose almost as well as ruminants. The digestive organs are divided into two categories based on their size in comparison to the rest of the body:

• Caecal fermenters, which are smaller animals like rabbits and rodents.
• Colonic fermenters, which are larger species like horses and rhinos.

The fermentation of plant materials in great apes’ hindguts produces a large amount of phytanic acid.

 

Phytanic acid is a branched-chain fatty acid obtained by humans from dairy products, ruminant animal fats, and certain fish. The average Western diet contains 50 – 100 mg of phytanic acid per day.

 

When food enters the mouth, the monogastric digestive system begins to work. Saliva moistens the food and initiates digestion. (Note that horses have no (or negligible amounts of) amylase in their saliva). After swallowing, food travels from the oesophagus to the stomach where stomach acid and enzymes aid in digestion. The pancreas secretes enzymes and alkalis to neutralise stomach acid once the food leaves the stomach and enters the small intestine.

 

A. The Gastrointestinal Tract is the Key to Feeding Horses

Feeding of horses. 
 

We know horses are herbivores, and we know they graze for long periods each day if there is pasture available. You’ve probably heard the advice to feed ‘little and often’ and to base your horse’s diet on fibre. The reason you should do these things is because of how their gut is structured.

The horse is classified as a monogastric (or single-stomached) hindgut fermenter, with the mouth, stomach, small intestine, caecum, large colon, small colon, and rectum.

 
The gastrointestinal tract of a horse.

 

The muzzle is the area of the horse’s head containing the mouth, nostrils, chin, lips, and front of the nose. The muzzle is very sensitive and mobile.

Horses use their mobile upper lip and incisors to select and shear forage from their base, as well as to select and pick up hay or concentrate feeds. Horses are extremely skilled with their lips and teeth, and they can choose carefully which feeds and forages they want to eat or not (much to the chagrin of their owners). After entering the mouth, the tongue pushes the feed to the molars at the back of the mouth, where it is thoroughly chewed. The particle size of the feed is reduced to aid digestion further down the gastrointestinal tract, in addition to being mixed with saliva during chewing. Saliva, in addition to lubricating the food and making it easier to swallow (preventing choking), acts as a buffer, insulating the upper stomach from the acid production of the stomach’s lower regions.

After sufficiently chewing, the feed travels down the oesophagus and into the stomach.

Horses have teeth that range from 36 – 44. They have different shaped teeth for different tasks.

 
The horse’s dental anatomy.
 

The horse’s stomach is a “J” shaped organ with a capacity of approximately 5 – 15 litres, implying that the stomach accounts for approximately 10% of the total volume of the horse’s digestive tract. The stomach serves two major purposes, including:

1. Feed storage and controlled release into the small intestine.
2. The start of protein digestion.

The horse’s stomach differs from that of other monogastric animals such as dogs and humans (both meal feeders) for two reasons: the first is that acidic gastric juices are constantly secreted into the stomach (dogs and humans only secrete gastric juices when they see or begin to eat food), and the second is that feed passes quickly through the stomach (unlike in carnivorous monogastric animals such as dogs, where food spends a long time in the stomach).

While both of these characteristics are well suited to the grazing horse consuming a high-fibre diet, they are also likely to contribute to the occurrence of gastric ulcers in horses. Once feed is released from the stomach it enters the small intestine.

 

B. Understanding Poultry Digestion

“How does the chicken chew its feed without any teeth?”

Chickens have one of the most efficient digestive systems in the animal kingdom, even without teeth.

Food is taken in with the beak, which is ideal for pecking crumbled or pelleted feed, small grains, grass, or insects. Chickens are omnivores, which means they can eat meat (grubs, worms, and the occasional mouse) as well as vegetation (grass, weeds, and other plants) in addition to commercial feed. As food passes from the mouth into the oesophagus, a small amount of saliva and digestive enzymes are added. Food moves from the oesophagus to the crop, an expandable storage compartment at the base of the chicken’s neck where it can stay for up to 12 hours. The crop’s food trickles into the bird’s stomach (proventriculus and gizzard), where digestive enzymes are added, and physical grinding occurs.

The gizzard is why chickens do not need teeth. It is a muscular part of the stomach and uses grit (small, hard particles of pebbles or sand) to grind grains and fibre into smaller, more digestible, particles.

Food passes from the gizzard into the small intestine, where nutrients are absorbed. The residue is then passed through the caeca, a blind sack along the lower intestinal tract where bacteria aid in the digestion of undigested food. Food moves from the caeca into the large intestine, which absorbs water and dries out indigestible foods. This remaining residue passes through the cloaca, where it is mixed with the waste by the chicken’s urine (the white in chicken droppings). Both leave the chicken through the vent, which is the cloaca’s external opening.

And don’t think of chicken manure as “waste” to be discarded; it’s an excellent fertiliser. Because it is high in nitrogen, it is best to age it in a compost pile before using it as fertiliser.

The Poultry Digestive System:

• Beak– A beak is the mouth of a chicken. There are no teeth inside the beak. Tiny barbs instead cover a small, triangular tongue. As a result, chickens must always have access to water because they use it to soften and swallow feed.
• Oesophagus (Gullet): Transports food from the mouth to the stomach.
• The crop is a component of the chicken digestive system. Food is transported from the beak to the crop. Food is stored in the crop until it is transported to the stomach. The crop may appear as a bulging mass on the right side of a hen’s breast if it has recently eaten.
• Stomach (Proventriculus and “Gizzard”): The organ where food is broken down into smaller units. It is divided into two sections: the proventriculus for storage and the gizzard. The gizzard is a muscular part of the stomach that grinds grains and fibre into smaller particles using grit.
• Small intestine: Aids in digestion and nutrient absorption. Composed of the duodenum, jejunum, and ileum.
• Liver: The largest glandular organ in the body. Aids in the metabolism of carbohydrates, fats, and proteins.
• Caeca: Bacterial action in the caeca helps break down undigested food passing through the intestine. The caeca transition into the large intestine, which connects with the cloaca.
• Large intestine: Functions primarily to absorb water, dry out indigestible foods and eliminate waste products.
• Cloaca: Where the digestive, urinary, and reproductive systems meet.
• Vent: The external opening of the cloaca that passes waste to the outside.

The urinary system consists of two kidneys and two ureters. The kidneys are located in the pelvic bones. They filter waste from the blood and pass it through the ureter to the outside via the cloaca or vent.

 
The digestive system of chickens.
 

Every day, we rely on our immune systems to keep us safe from viruses, bacteria, toxins, and fungi. When our body’s network of organs, cells, and proteins defeats an invader, it creates a record of how to defend against the invader. When the body is confronted with this specific attacker again, the immune system will be able to defeat it quickly and efficiently.

The immune system of the chicken works similarly. It is also complicated. Its primary defence is lymphoid organs, which produce, store, and transport infection-fighting cells. The thymus and the bursa of Fabricius are the primary lymphoid organs in chickens:

• Thymus – this series of lymphatic lobes runs almost the whole length of the neck. It is similar to the thymus in humans.
• Bursa of Fabricius – this organ is unique to birds and is located on top of the rectum. It forms a kind of pocket (the word bursa means “purse”) that contains folds of lymphoid tissue.

T-cells are produced by the thymus, while B-cells are produced by the bursa of Fabricius. These immune cells spread throughout the body, including the Harderian gland, spleen, and bone marrow. More than 60% of these immune cells, however, migrate to and reside in the digestive tract, including the caecal tonsils and Peyer’s patches. They defend the body from these locations against intruders.

The digestive system contains other important components that support the chicken’s immune system in addition to hosting immune cells. Throughout the digestive tract, beneficial microflora (bacteria and yeast) exists. They perform critical functions such as preventing harmful bacteria from colonising the intestine walls. Scientists believe that the beneficial microflora keeps the bird’s body on high alert for disease-causing organisms.

So, the immune system of a chicken includes both immune cells and beneficial microflora that are located in the digestive tract.

 

C. The Digestive Tract of the Pig

The digestive system of the pig is classified as monogastric, or nonruminant. This digestive system is also found in humans. They only have one stomach (mono means one and gastric means stomach). The monogastric digestive system differs from the polygastric, or ruminant, digestive system found in cattle and sheep. These creatures have a single stomach divided into four compartments. Cattle and pigs have different digestive systems, so they can consume different types of feed. Cattle and sheep can live on hay and pasture, whereas pigs must eat grains that are easier to digest.

The digestive tract of the pig has five main parts: the mouth, oesophagus, stomach, and small and large intestines.

 
A digestive tract of swine.
 

Food enters the digestive tract through the mouth, and the mechanical breakdown of food begins. Food is chewed and ground into smaller pieces by the teeth. Saliva, which is produced in the mouth, softens and moistens the small food particles. Saliva also contains an enzyme that initiates starch digestion. The tongue aids digestion by pushing food towards the oesophagus.

 
The functional parts of the mouth of swine.

 

The pig is born with 8 deciduous teeth, which grow to 32 as it grows older. The full set of 44 permanent teeth is usually not acquired until the piglet is 18 months old. Permanent teeth are made up of three pairs of incisors, which are used for rooting, grasping, and shearing food, one pair of canines, four pairs of premolars, which coarsely grind the food, and three pairs of molars, which crush the food into smaller particles.

Saliva (a watery, mucoid secretion of the salivary glands) is primarily secreted in the mouth by three large glands: the parotid glands, the mandibular (also known as submaxillary) glands, and the sublingual glands. The saliva is transported to the oral cavity via major ducts from the parotid and mandibular glands, while the sublingual glands have multiple openings beneath the tongue. In addition, there are several small glands with excretory ducts in the mouth. The parotid glands are mostly made up of serous cells, whereas the mandibular and sublingual glands are made up of both serous and mucus cells. Amylase is only found in the sublingual glands. Serous saliva is thin and watery, whereas mucus saliva is thicker (viscous) and contains glycoprotein-rich mucins.

The stomach is the next part of the digestive tract. It is a reaction chamber where chemicals are added to the food. Certain cells along the stomach wall secrete hydrochloric acid and enzymes. These chemicals help break down food into small particles of carbohydrates (the main nutrient that supplies energy to the body (starch and cellulose). Protein (the nutrients that supply the building materials) from which body tissue and many body regulators are made) and fats (energy nutrients which supply 2.25 times as much energy as carbohydrates.) Some particles are absorbed from the stomach into the bloodstream. Other particles that the stomach cannot absorb pass on to the small intestine through the pyloric valve.

 
Illustration of the stomach of swine.
 

The small intestine is a complex tube which lies in a spiral, allowing it to fit in a small space. Its wall has many tiny finger-like projections known as villi.

 

Villi are tiny finger-like projections located along the wall of the small intestine that aid in food absorption which increases the absorptive area of the intestine.

 

The cells along the small intestine’s wall produce enzymes that aid digestion and absorb digested foods.

The pancreatic duct transports digestive juices from the pancreas to the small intestine. These secretions contain enzymes that are essential for fat, carbohydrate, and protein digestion. The jejunum and ileum, the second and third parts of the small intestine, absorb the majority of food nutrients. The ileocecal valve transports undigested nutrients and secretions to the large intestine. The cecum (or “blind gut”) is located at the beginning of the large intestine. In most animals, the cecum serves no purpose. The cecum, on the other hand, is very important in the digestion of fibrous feeds in animals such as horses and rabbits.

 
The small and large intestines of pigs.
 

This was a general discussion of the digestive tract of the pig. The tract acts to digest and absorb nutrients necessary for the maintenance of cells and growth.

The Ruminant Digestive System

Sheep, cattle, and goats are ruminants, which means they have a ruminant digestive system and process their food differently than other animals. Let’s take a closer look at a ruminant’s digestive system to see how it works, why they chew the cud, and how their unique stomachs aid in the processing of their fibrous diet.

 

Dietary fibre, also known as roughage, is the portion of plant-derived food that cannot be fully broken down by human digestive enzymes. Dietary fibres vary in chemical composition and can be classified broadly based on their solubility, viscosity, and fermentability, all of which influence how fibres are processed in the body.

 
The main components of the rumen system.

What is a ruminant?

Ruminants are hoofed mammals with a unique digestive system that allows them to better utilise the energy found in fibrous plant material. A ruminant’s digestive system can ferment the food the animal consumes and provide the animal with energy precursors. Sheep, cattle, and goats are examples of livestock animals with ruminant digestive systems.

Ruminants use their mouths (oral cavities) and tongues while grazing and eating harvested foods. When you look inside a ruminant’s mouth, you’ll notice that the roof has both a hard and soft palate and that the upper jaw lacks incisors.

 
The Teeth of sheep (top) and cattle (bottom).

 

When the animal chews, the lower jaw’s incisors grind against the hard dental pad. On both the upper and lower jaws, the molars and premolars are the same. Plant material is crushed and ground by the ruminant’s teeth while chewing and rumination.

When feed or forage mixes with saliva, a bolus forms. The oesophagus, a tube-shaped passage, is used to carry the bolus from the mouth to the ruminant’s reticulum. These substances pass through the oesophagus and into the reticulum due to variations in pressure and muscle contractions. A ruminant typically consumes food very quickly, frequently swallowing large portions of it whole before swallowing. An animal with a ruminant digestive system has an oesophagus that is unique from those of other animals in that it can move in both directions.

The most important parts of the ruminant digestive system are the tongue, mouth, oesophagus, salivary glands (which create saliva that help to buffer rumen pH), a stomach with four compartments (the rumen, reticulum, omasum, and abomasum), gall bladder, pancreas, small intestine (which includes the duodenum, jejunum, and ileum), and large intestine (which includes the cecum, colon, and rectum).

Rumination

The act of chewing the animal’s cud is known as rumination. The food that has been regurgitated is chewed more thoroughly during rumination. Then it is again swallowed by the animal, whereupon it enters the reticulum.

 
A ewe chewing cud.
 

The food’s solid component then slowly makes its way into the rumen, where fermentation takes place. The majority of the liquid component will move quickly from the reticulorumen into the omasum. Following that, it enters the abomasum.

A substantial portion will remain in the rumen, where it typically stays for up to 48 hours. It changes into a dense mass where bacteria can consume fibrous foods. This is what produces the energy precursors.

The reticulorumen is home to a large number of microorganisms, also referred to as “rumen bugs” or “microbes”. Some of these include bacteria, fungi, and protozoa. To ferment and break down plant cell walls, separate them into various types of carbohydrates, and produce volatile fatty acids (including butyrate, propionate, and acetate, each of which is used to produce glucose, fat, or both), these “rumen bugs” are necessary.

The rumen, also known as the “paunch,” has a lining of papillae that is necessary for nutrient absorption. Muscular pillars that divide the rumen into several sections include:

• Dorsal.
• Ventral.
• Caudo-dorsal.
• Caudoventral sacs.

The rumen breaks down between 50% and 60% of the soluble sugar and starch. The rumen’s primarily bacterial microbes help break down the complex starch and cellulose found in plant cell walls. They also produce protein from nitrogen derived from sources other than proteins, as well as vitamin K and vitamin B.

The pH of the rumen usually ranges from 6.5 – 6.8. It exists in an anaerobic environment, which means that oxygen is absent. In the rumen, gases like methane, carbon dioxide, and hydrogen sulphide are produced. The gas will rise to the top of the rumen, sitting above the liquid.

 
The digestive path of harvested food through the rumen.

 

The reticulum is occasionally referred to as the “honeycomb.” This is because its lining resembles that of a honeycomb. The reticulum is located beneath and to the front of the rumen. It is against the diaphragm of the animal.

A short tunnel connects the omasum and reticulum. The omasum is spherical and is sometimes called the “many piles” because it has so many folds. These folds expand the surface area and provide more area for the absorption of nutrients. Absorption of water happens in the omasum.

The ruminant’s abomasum is known as the “true stomach.” It is the compartment most similar to the stomach of a nonruminant. The abomasum produces digestive enzymes and hydrochloric acid. The abomasum also transports digestive enzymes from the pancreas. These include pancreatic lipase (which is required for fat breakdown). These secretions are required to prepare proteins for absorption processes in the intestines. The pH of the abomasum is usually between 3.5 and 4. The mucous secreted by the abomasum’s chief cells protects the abomasal wall from acid damage.

The small and large intestines are also sites of nutrient absorption. The small intestine, which is shaped like a tube, can be 45 m long. The capacity of a mature cow is 76 l. When digests enter the small intestine and combine with secretions from the liver and pancreas, the pH rises from 2.5 to around 7 or 8. The pH of the small intestine must rise for the enzymes to function properly.

Bile that comes from the gall bladder comes into the first part of the small intestine (called the duodenum) to help with digestion. The small intestine is responsible for active nutrient absorption. This includes rumen bypass protein absorption. There are many villi (projections that look like fingers) on the intestinal wall. The intestinal surface area helps with nutrient absorption. Contractions in muscles help to mix the digesta and make it travel to the next section. The large intestine will also take water from the material that goes through it. It then moves the rest of the material to the rectum as faeces.

 

Animal Feed and the Food Supply Chain

Every step from primary production to final consumption, that is, from farm to fork, makes up the food chain. Feed production plays a significant role in the production of food of animal origin, and it is, therefore, a critical aspect of the food chain. Therefore, all key actors on every node of the food chain are responsible for the production of safe, healthy, and nutritious feeds.

 
The food chain.