Digestion in grain-eating birds

The digestive system of the chicken, which is a representative of the world of birds, is comprised of organs lined up in straight line succession: a beak, mouth, gullet, craw, front stomach (with gland), gizzard, small intestine, caecum, large intestine, cloacae and anus. In addition, enzymes secreted from the pancreas, liver and gallbladder also have digestive duties. Ending in a sharp point, the beak was created especially to be able to pick up single pieces of feed. Chickens do not have assisting organs like the palate, cheek, tongue or teeth that are normally found in the mouth, the first door of digestion in most animals. Ruminating animals like cows, sheep, goats and camels use the tongue instead of a beak for taking food into their mouths, and then their teeth are used to begin the breakdown of food. In chickens, however, this breakdown is initiated with the bottom and top beak. In ruminants there are taste buds in different shapes on the tip, sides and middle of their tongues which also function as the organ for the sense of taste. Since such taste buds are not found in chickens, there is also no perception of taste like that experienced in ruminating animals.

The structure and function of the gullet, or food pipe, has similar characteristics in chickens and other animals. However, just as in all birds that eat single seeds and grains, there is another digestive organ in chickens in the shape of a bag opposite to where the gullet expands. Called a craw, this tiny pouch is a place for the storage, wetting and softening of feed, and it serves the purpose of bringing food to the consistency needed for further digestion; consequently, it lightens the burden of the stomach (just like when we leave food that is difficult to cook, like chick peas or beans, standing in water overnight before we cook it).

A chicken’s stomach is composed of two different sections, the front stomach and the gizzard. The feed the chicken eats passes from the gullet and the craw to the front stomach, where digestive secretions are made. The food is stored here briefly and mixed with the special fluids of the stomach. Found in this secretion are both the pepsin enzyme, which starts the digestion of proteins, and hydrochloric acid, which is secreted to generate the pH level that makes this enzyme effective (and which also helps in the dissolution of minerals). These two important secretions are produced by different glands in the stomach. Like the craw, the gizzard, which comes after the front stomach, is the place where mechanical digestion, which is particular to all birds that eat grains, takes place. The gizzard is called the stomach with muscles because it is comprised of a pair of thick and strong muscle layers. As a result of these muscles constricting strongly, feed is broken down mechanically and ground up. While gathering feed, the chicken usually also swallows small pieces of sand, stone and limestone as if it knows how its digestive system works. Although many of us think that chickens swallow stones because they cannot distinguish them from feed, it is obvious that there is a purpose why they are doing it. If these stones, which are like “mill stones” for the digestive tract, are not found in the gizzard, the feed is not fully ground; consequently, it will pass to the small intestine in a form that will not be fully beneficial to the body.

Depending on its characteristics, the feed eaten can stay in the gizzard for a few minutes or several hours. The small intestine, which comes after the stomach, is comprised of the duodenum (twelve-finger intestine), the jejunum and the ileum. The small intestine is similar in grain-eating birds to those of other animals. The duodenum empties the secretions coming from the pancreas and the bile coming from the gall bladder. The digestion and absorption of the feed actually takes place in the small intestine due to these secretions. Final digestion and the absorption of carbohydrates and protein are effected by bacteria found at the point where the small intestine ends, in the 10-15 centimeter-long, V-shaped caecum.

The large intestine and the cloacae are found in the advanced sections of the digestive system. The role of the large intestine, which is twice as large in diameter as the small intestine, is to temporarily store the waste from the digested food and to maintain the balance of water in the body. The cloacae is a small orifice with a structure formed by the widening of the large intestine towards the anus and where the digestive, defecation and reproduction tracts open up.

While giving grain-eating birds the above mechanisms in order to nourish them, God equipped ruminating animals with different oral, dental, gastric and intestinal structures as well as different digestive strategies.

Digestion in ruminating mammals

While secreting saliva is essential for ruminating animals, it is not necessary for chickens because of differences in their digestive system. As a result of the secretion of saliva the fodder of ruminants is softened and dryness of mouth is prevented. Ptyalin (or alpha amylase), which is found in saliva, has antibacterial properties that protect animals from infections in addition to playing a role in digestion. The mother cows constantly licking their newborn calves is both an expression of compassion and also of wisdom in protecting the newborn from probable infections (Similarly, animals like dogs and cats licking their wounds and thus speeding up healing is no coincidence, but a sign of universal wisdom and mercy). This wisdom shows that saliva, of which cows secrete 90-180 liters and sheep 5-8 liters per day, is not a waste.

The stomachs of ruminants are comprised of four sections; the first three are the rumen, the reticulum and the omasum. These three sections are called the front stomach. The section that does the real work is called the abomasum. Because the nutritional value of grass is very low in comparison with meat, it is obvious that a large bodied ruminant will need a lot of grass. Consequently, the capacity of rumen being 150 liters is a wise and fitting design. In addition, the cellulase enzyme that digests the cellulose walls in plant cells is not produced in the tissues of any mammals. The possessor of infinite knowledge and power, God placed high concentrations of bacteria, protozoa, yeast and fungus-all of which can produce the cellulase enzyme needed to break down cellulose-in the rumen of ruminating mammals. The rumen acts as a fermentation factory because of these microorganisms. Breaking down the cellulose-rich food consumed by ruminants, these microorganisms both fulfill their own nutritional needs and help to meet the host’s energy, protein and vitamin (especially B12) needs within a symbiotic relationship. I wonder where the microorganisms in the rumen learned about this synthesis and assistance mechanisms!

What a magnificent engineering wonder is the “sulcus esophagus,” a semi-duct structure in the shape of a canoe that extends from the end of the food pipe in newborn ruminants to the abomasum where the essential stage of digestion takes place. Due to this structure, milk drunk reaches the last stage of the stomach without passing through the rumen, for there is no matter like cellulose in the content of milk that needs to be broken down. Consequently, there is no need for the milk to go to the rumen. If it were not like this, the milk would go directly to the rumen where it would be ruined by microorganisms, lose its nutritional quality and be like a bribe given in vain to the microorganisms in the rumen.

And surely in the cattle (feeding on the pastures of the revived earth) there is a lesson for you: We give you from that which is within their bodies, (marvelously distinguished from) between the waste and blood, milk that is pure and palatable to those who drink. (Nahl 16:66)

Ruminants, probably the most abundant of the herbivores such as cattle, sheep and goats, are foregut fermenters with a four-chambered stomach (rumen, reticulum, omasum and abomasum) and are an essential component of utilizing marginal land in the world in a sustainable way.

In the verse above from the Qur’an, the Creator and the Sustainer of the universe draws our attention to many of the benefits we get from domestic animals. While the main message of the verse is easily understandable to the general reader, it also contains some concise hints, even descriptions about the physiological details of milk production in ruminants that would be fully understood and explained by science only centuries after the Qur’an was revealed. The purpose of this article is to explain this process in a general sense and milk production in some detail.

Mammals are generally categorized according to the dietary habits into three classes-flesh-eating (carnivore), plant-eating (herbivore) and both flesh and plant-eating (omnivore). In a sense, humans (omnivores) and carnivores depend on herbivores for their nutrition. Generally speaking, all humans and animals in the world have directly or indirectly benefited from the plants. The ability of herbivores to utilize plants as their main energy source is dependent on symbiotic microorganisms which live at various sites within their gastrointestinal tract. The animal provides the microorganisms with food and habitat for growth and the microorganisms provide the animal with fermentation acids and microbial protein.

Herbivores are divided into two types, those with post-gastric (hindgut) fermentation and those with pre-gastric (foregut) fermentation. Fermentation is a chemical process during which microorganisms obtain energy from organic products. Ruminants, probably the most abundant of the herbivores such as cattle, sheep and goats, are foregut fermenters with a four-chambered stomach (rumen, reticulum, omasum and abomasum) and are an essential component of utilizing marginal land in the world in a sustainable way. Rumen and reticulum contain millions of microorganisms, which form about 3 to 10 percent of rumen fluid.

A major reason why human beings keep ruminants is their ability to convert food which humans find inedible-or at least unpalatable-to food (meat, milk) which humans can eat. They play an important role in the livelihood of farmers throughout the world, providing sustenance such as milk and meat, manure for crop production, cash income from sales of their products and a safety net of capital assets to face risks and misfortune in harsh environments. Currently, humans obtain about fifty percent of the meat and most of the milk they consume from ruminants. Scientists who have conducted studied on ruminants have developed cow breeds, which have higher milk and meat production than traditional cow breeds, and thus supplied an important development to meet the nutritional requirements of humans.

Pre-gastric fermentation provides three important nutritional advantages to the host animal.

First, cellulose and other plant polysaccharides are brought into solution and become available as energy sources. Cellulose is the most abundant natural carbohydrate polymer in nature, but mammals do not produce enzymes that can degrade it. Ruminant animals utilize cellulose via a symbiotic relationship with ruminal cellulolytic microorganisms. During ruminal fermentation, microorganisms ferment the carbohydrates to produce energy, gases (methane and carbon dioxide), heat, and volatile fatty acids (VFA) in the rumen. Effective digestion of plants requires a means of dealing with cellulose, the most important structural material of plants, which is extremely insoluble and remarkably resistant to a chemical attack. Cellulose digesting enzymes that are called cellulases and produced by microorganisms are also present in the intestinal tract of several invertebrates that feed on wood and similar plant products. Rumen harbors the different functional groups of the microbial population, which is responsible for about seventy percent of total digestion in ruminants, and the ability to digest cellulose has been ascribed to a large number of bacterial, fungal and protozoal species isolated from the rumen.

The energy content of plants is low, and the herbivore must consume a large quantity in order to satisfy its energy requirements. Therefore, herbivores spend a lot of time eating; eight or more hours per day may be spent eating.

Secondly, the rumen microorganisms can utilize non-protein nitrogen for growth, converting it into microbial protein which becomes available to the host. Proteins provide the amino acids needed for maintenance of vital functions, reproduction, growth and lactation. Non-ruminant animals need pre-formed amino acids in their diets, but ruminants can utilize many other nitrogen sources because of their rare ability to synthesize amino acids and protein from non-protein nitrogen sources via a symbiotic relationship with ruminal microflora.

Ruminants possess a rumeno-hepatic nitrogen circulation mechanism, which does not exist in non-ruminant animals, in order to save nitrogen. By this mechanism, ruminants can be fed non-protein nitrogen sources such as urea and nitrate when nutrients are in short supply to obtain high quality milk protein. Feed proteins are degraded by microorganisms in the rumen via amino acids into ammonia. Ammonia is used by bacteria to build their proteins and any excess of it is absorbed through the rumen wall into the blood and then converted to urea in the liver. When a diet is low in nitrogen, large amounts of urea (which is normally excreted in the urine) return to the rumen where it can be used by the microbes. In non-ruminants, urea is always entirely lost in the urine. If ammonia levels in the rumen are too low there will be a nitrogen shortage for bacteria and feed digestibility will be reduced. Too much ammonia in the rumen leads to wastage, ammonia toxicity, and in extreme cases, death of the animal.

Thirdly, vitamin synthesis by the microbial population makes the ruminant animal virtually independent of dietary sources of all vitamins, except for vitamins A and D.

However, rumen fermentation also brings some disadvantages. First of all, rumen metabolism causes environmental pollution. Methane is produced as a natural consequence of the anaerobic fermentation; it is a potent greenhouse gas. Dairy farming is the largest agricultural source of methane, one of the greenhouse gases. Furthermore, the major environmental concern associated with the animal industry is ammonia volatilization, which increases atmospheric acid deposition because of the impact of nitrogen-rich excreta on the environment. Therefore, worldwide, scientific research projects have been carried out to find sustainable strategies for reducing emissions of the greenhouse gas methane and ammonia volatilization from domestic ruminants to the environment.

In conclusion, the symbiotic relationship between ruminants and ruminal microorganisms plays an important role in the recycling of nutrients between humans and plants. This relationship also contributes to human life by converting low quality nutrients (grass and hay) to high quality food (meat and milk). Humans will benefit more from ruminants as scientific knowledge about relationships between ruminants and rumen microorganisms advances.

Zubeyir Altuntas has a PhD in Molecular Medicine. He is a research associate in Immunology Department of The Cleveland Clinic.