The birds’ digestive system is the producer’s gateway to transforming a whole range of feed ingredients into money. As a result, the digestive tract is arguably the most important organ of the chicken’s body. This is reflected in how hard the intestines work. Although it makes up only 5% of total bodyweight, it uses up 30% of the birds total oxygen consumption and protein turnover.
A simple tube – with specialist functions
The digestive tract is basically a simple tube lined by specialist cells that are an extension of the epithelial cells covering the skin. As a result, the tract is open to the external environment and all the dangers there – bugs, toxins and extremes of pH. The cells lining the gut are the bird’s last line of defence against these challenges in the same way as a surface skin wound or damage to the respiratory tract.
The tract breaks down the feed the birds eat into its smallest components, which are then capable of being absorbed straight into the bloodstream. The bird then uses these ingredients as building blocks for its needs, be it body growth, a functioning immune system, maintenance and repair of tissues or the production of eggs.
But how does the bird undertake this miracle of chemistry?
To understand this, it is useful to follow the bird’s food on its torturous journey through the body, to see how the bird deals with it and some of the problems that can occur.
The beak and mouth
Unlike humans, birds do not produce much saliva or use their tongue to soften and break down food and of course the hen has no teeth! The beak is a very sensitive utensil allowing feed particles to be rapidly picked up and swallowed down the oesophagus (gullet) for storage in the crop.
The crop
Basically this is a pouch at the bottom of the oesophagus which acts as a food reservoir, but also moistens and softens the food. This is only a temporary stopping point and the rate of emptying into the proventriculus (glandular stomach) is influenced by a number of factors. These include how much food there is already in the crop or further down the system, as well as characteristics of the food swallowed – meal or crumb tends to have a shorter dwell time than pellets.
Proventriculus (glandular stomach)
This is where the real action starts. This fleshy organ secretes hydrochloric acid and enzymes capable of digesting protein onto the food. Food does not spend too much time here (from several minutes up to about 1 hour) before it moves into the gizzard.
The gizzard
This round and very muscular organ has a rough, tough internal lining. The function of the gizzard is mainly to grind the already moistened and softened food mixed with the acid and enzymes from the proventriculus.
Whole grain and fibrous material is given a real pounding here and grit added to the diet adds to the efficiency of the grinding action. The end result is the passing of a soup of partially digested sloppy food into the small intestine which the body can act on to capture the necessary nutrients.
The intestines
The first loop of the intestine, the duodenum, is lined with ducts or tubes which add more fluids, enzymes and salts to the soup. These attack the soup and break it down into building blocks which can be absorbed through the lining of the gut into the bloodstream.
There is a long list of helpful material squirted in from the liver, pancreas and gall bladder. This includes water to further soften the feed, digestive enzymes to break down protein and starch, bile salts to break down fats and mucus to coat the lining of the intestine protecting it against these powerful enzymes as well as lubricating the movement of food along the intestine.
The intestines also have a well developed population of different bacteria, some less harmful than others. This ‘gut flora’ is established in the young chick and the stable balance gives a population of bacteria that can protect against overgrowth of harmful bacteria as well as aiding digestion of certain feed constituents and even the manufacture of certain vitamins.
The intestine is long (about five times the length of the body when unravelled) and the lining is made up of a large number of finger like villi which massively increase the surface area for efficient absorption of the products of digestion into the bloodstream. The intestines obviously have a very rich blood supply. Waves of muscular contractions squeeze the food along like toothpaste in a tube into the large intestine.
The large intestine
This portion of the intestine is less important for digestion in chickens than for true grass eaters (herbivores). However, it serves a crucial function in reclaiming water that has been added by all the earlier fluid secretions and hence helps maintain fluid balance in the bird. It is estimated that for every 1 gram of feed eaten the body may produce 2 grams of water to aid digestion and absorption of nutrients. A large proportion of this water needs to be ‘captured’ before it leaves the body.
The caecae
These are two blind ending sacs similar to our single appendix where a bacterial soup aids the last chance of digestion and a significant rescue of water. As a result, if all goes well, dry faeces from which most of the available nutrients and water have been removed are ready to be excreted by the bird.
The end!
The end product of digestion is voided via the cloaca, consisting of the larger volume of dark dry faeces from the intestine, mixed with the white uric acid from the kidneys (the end product of protein digestion) and less frequent brown fluid caecal droppings.
When things go well, food taken in at one end is dealt with incredible efficiency extracting every last useful morsel of nutrients.
But what happens when things go wrong?
The old adage of ‘rubbish in, rubbish out’ applies well in nutrition. Efficient digestion depends on the bird having access to high quality feed. We are very fortunate in the UK in having access to technology and expertise from feed compounders which provides your birds with feed of high and consistent digestibility and quality. The rest is then up to you and your birds. Correct provision of fresh feed, correctly stored, via equipment that allows free access without undue competition or physical damage to the feed allows your birds to efficiently use that feed.
As in many areas of egg production, your birds perform best when they have a predictable routine. Feeding is no exception, with birds preferring to feed as a ‘meal’ allowing the crop to fill and then acting as a reservoir for food moving on into the gizzard in a regulated fashion.
Poor storage of feed allowing mould growth and problems further down the digestive system mean that poor quality feed can remain static in the crop. This can lead to sour crop or thrush. Whereas sometimes the foul smelling contents can be ‘milked’ from the crop, often affected birds must be culled.
The time feeds spends in the proventriculus and gizzard governs the muscular development there and efficiency of digestion. Provision of feed of a larger but uniform grist (particle) size as crumb, pellet or addition of whole or cracked grain can delay transit time and aid digestion, as long as there is adequate grit availability.
In a similar fashion, access of birds to long lush spring grass can cause problems in the upper intestinal tract. The long fibre can cause an indigestible plug or impaction in the gizzard if there is no grit available to aid mechanical grinding down of the fibre. This can block further passage of feed along the intestine, and birds can simply waste away.
In the intestine, the consistency or viscosity (stickiness) of the soup of digesting food material can be altered by certain feed constituents or their quality, and this can affect gut transit time.
A variety of factors can upset the calm balance of bacteria in the gut and allow proliferation of certain more
