Cymraeg

Body Systems
Session 3 - The Digestive System

This resource is suitable to support Level 2 Agriculture courses.

The Digestive Systems of Farm Animals

The digestive system breaks down feed into small molecules that can be absorbed by the digestive tract into the blood and lymphatic systems.

Cow eating

The Digestive Processes

The three main processes of digestion are:

  • Physical break down – e.g. chewing the food
  • Chemical break down – e.g. hydrochloric acid released by the stomach wall; and Enzymes which are proteins that speed up chemical reactions without getting changed themselves.
  • Microbial - e.g. fermentation of food in the rumen (first stomach of a cow or sheep)
Image of sheep chewing

Physical Break Down of Food

The animals grazes or eats out of a feeder and chews the food with their teeth.

  • In the case of poultry they grind the food in the crop.
    • Image of cows chewing

    Chemical Breakdown of Food

    Examples of chemical break down:

  • Food passes into the stomach region and hydrochloric acid is released by the stomach wall which begins to break down proteins.
  • Enzymes which are proteins that speed up chemical reactions without getting changed themselves are present in saliva. They start to break down carbohydrates.
    • Diagram of stomach and intestines

    The Digestive System of a Monogastric

    Digestive systems have evolved over time to suit the natural diet of the particular animal.

  • The alimentary canal is the term used for the digestive tract.
  • A monogastric is also called a single stomached animal. An example is a pig.
    • Image of pig

    Parts of the Digestive System of a Pig


    The Mouth

    The teeth are used for collecting and chewing food.

    The types of teeth are:

  • Incisors – for cutting food.
  • Canines – for tearing flesh.
  • Molars and premolars – for crushing and grinding foods.

    • The teeth are adapted to the diet. For example, pigs are omnivores (they eat a wide range of feeds from plant and animal sources), their teeth have smooth enamel and they have incisors on both the upper and lower jaw.

    Image of pig

    The Tongue

    The tongue is made up of muscles that manipulate feed into a bolus (ball) ready for swallowing. It contains taste buds that allow animals to taste the feed.

  • The ability to taste feed allows animals to detect poisons which have a bitter or unpleasant taste and avoid eating them.
  • The animal swallows food when it has been moved to the back of the mouth; a flap called the epiglottis closes off the windpipe (trachea) to prevent choking.
  • Saliva contains water, electrolytes, mucus and enzymes to break down starch. It moistens feed to make it easier to swallow.

    • The Oesophagus

    This is the tube that connects the mouth to the stomach.

  • Food is pushed along by muscular contractions- this is called peristalsis.

    • The Stomach of the Monogastric

    This is where the majority of protein breakdown occurs through the activity of an enzyme called pepsin.

  • To make enzymes work effectively, hydrochloric acid is released by the stomach wall to make the contents of the stomach acidic. The muscles in the stomach wall mix the feed before it is gradually released to the next stage of digestion in the small intestine.

    • The Small Intestine of the Monogastric

    Chemicals to complete the breakdown of food are released into the small intestine from:

    • The wall of the intestine which releases enzymes.
    • The pancreas which releases pancreatic juice which contains enzymes.
    • The gall bladder which stores and releases bile which is made by the liver.
    • Bile is produced by the liver, it is stored in the gall bladder and released into the duodenum (the 1st part of the small intestine).
    graphic of small intestine

    Functions of the Digestive Chemicals

  • Bile - Neutralises the pH so that the relevant enzymes can work better.
    Breaks down (emulsifies) the fats and oils into small droplets so that they have a larger surface area for the enzymes to work on.
  • Amylase - Breaks down carbohydrates to sugar.
  • Trypsin - Breaks down proteins to amino acids.
  • Lipase - Breaks down fats to fatty acids and glycerol.

    • The Small Intestine

    When the food particles are of molecular size they are small enough to pass through the wall of the intestine into the blood or lymphatic system.

    • To speed up this process the small intestine it is covered in villi which create a large surface area.
    Image of small intestine

    The Villi of the Small Intestine Showing the Blood Supply

    Diagram of of villi
    Diagram of of villi

    What is the Role of the Liver?

    Once the absorbed feed particles have entered the circulatory system they will pass through the liver to remove toxic particles.

    Diagram of of liver

    What does the Circulatory System Do to Help the Digestive System?

    The circulatory system will move the products of digestion around the body so the digested feed particles can be taken to where they are required e.g. sugar to muscles to provide energy.

    The Large Intestine

    This is where the process of digestion and absorption is completed and water is reabsorbed to the body from the digestive tract.

  • The waste material is held in the rectum until it is passed out through the anus.
    • Diagram of large intestine

    The Ruminant Digestive System

    The digestive system of the ruminant is specially adapted to a diet which contains high levels of plant fibre that is difficult to break down.

  • This has led to the development of a structure that has four stomachs; including a final chamber which works in the same way as the stomach of a pig.
    • Image of cows in a field

    The Mouth of the Ruminant

    The teeth of a ruminant are used for collecting and chewing food.

  • Cattle and sheep have no teeth on the upper jaw at the front of their mouth, rather they have a hard pad of bone and skin.
  • As cattle and sheep get older they will change their first set of teeth (milk teeth) for their adult teeth in a set pattern.
    • Picture of cows face

    Ruminant Molars and Premolars

    Ruminant molars and premolars on both upper and lower jaws at the back of the mouth have a rough surface to physically break down plant fibre.

  • Saliva – large amounts of saliva are produced by ruminants. The saliva is slightly alkaline to prevent rumen fluid from becoming too acidic.
    • Picture of ruminant teeth

    Grazing

    Sheep will tend to graze by biting grass with their incisors cutting up against the hard pad on the upper jaw, cattle tend to use their tongues to collect and pull at the pasture before biting.

    Image of cow grazing

    Chewing the Cud

    Ruminants will swallow the food before chewing it as thoroughly as they can, regurgitate, and send it back to the mouth to finish chewing later.

  • Ruminant molars and premolars on both upper and lower jaws at the back of the mouth have a rough surface to physically break down plant fibre.
  • Saliva – large amounts of saliva are produced by ruminants. The saliva is slightly alkaline to prevent rumen fluid from becoming too acidic.

    • Chewing the Cud

    The Oesophagus

    This is the tube that connects the mouth to the rumen. The first three parts of the stomach of the ruminant have developed from the oesophagus; they do not produce any digestive chemicals.

  • Cows do not have 4 separate stomachs but 4 parts to a large stomach.

    • The 4 Parts to the Stomach of a Ruminant

    The stomachs are:

  • Rumen
  • Reticulum
  • Omasum
  • Abomasum (or true stomach)

    • What is the Size of the Rumen?

    The rumen of a cow has a capacity of around 120 litres but varies according to body size and breed.

  • The sheep rumen has a capacity of 15-20 litres.
    • Cow picture

    What Happens in the Rumen?

    Feed in the rumen is broken down by microbes, mainly bacteria and protozoa.

  • Bacteria use some of the food to reproduce and increase numbers. Bacteria are moved down the digestive system with the rest of the feed and get digested. This is referred to as microbial protein.
  • Microbes also add nitrogen to molecules to produce protein. Microbes will release waste products, some of these can be used by the cow e.g. volatile fatty acids. Waste gases e.g. methane produced by microbes are removed by belching. Cattle and sheep must get rid of the waste gases to prevent bloat where trapped gases expand the rumen, cause discomfort and put pressure on internal organs. Severe cases can result in death.
  • If too much highly fermentable feed (e.g. wheat) is given then too much acid will be produced; this will upset the digestive system and cause acidosis.

    • Rumen Microbes

    Rumen microbes are adapted for specific feed.

  • Diet changes have to be gradually introduced to give the rumen population time to adjust.
    • Diagram of rumen microbes

    The Rumen

    The muscles of the rumen move the food and gases; this helps to remove the gases and mix the food and bacteria.

  • Some chemicals can be absorbed through the wall of the rumen.
    • Image of rumen

    The Reticulum

    The reticulum is much smaller than the rumen. Food passes into the reticulum from the rumen.

  • Large food particles are sent back to the mouth so that they can be chewed again. When the particles in the reticulum are small enough they pass into the next stomach, the omasum.
    • Reticulum

    The Omasum

    The omasum is the third stomach.

  • This stomach has folds of tough tissue which can grind fibrous material.

    • The Abomasum

    The abomasum is the fourth stomach.

  • This stomach works in the same way as the single stomach of the pig where hydrochloric acid and pepsin break down feeds.
  • It is known as the ‘true stomach’ or ‘milk stomach’.

    • Video on Ruminant Digestion

    How do Rumen Flora and Fauna Work?

    Rumen flora and fauna break down feeds to their constituent parts for absorption in the lower gut. Some feeds, particularly high quality fat and proteins, are already in a form that can be broken down and absorbed in the lower gut therefore it is a waste of energy for such compounds to be broken down in the rumen.

  • Due to the cost of high quality fat and protein, it is usual that only highly productive animals have rations designed to contain ‘protected’ sources of fat and protein that should not be broken down by rumen microbes.

    • The Small and Large Intestine

    Digestion in the ruminant and absorption in the intestines is similar to the process in the single stomached animal.

    Diagram of intestines

    Development of the Ruminant Digestive System in Young Animals.

    At birth, the first three stomachs are not well developed and young stock need to get milk into the abomasum for it to be digested.

  • When young ruminants drinks milk a gathering of tissue called the oesophageal groove diverts milk through the rumen, reticulum and omasum directly to the abomasum.

    • From Milk to Hard Feed

    Poor feeding practice e.g. having the buckets at the wrong height, can prevent the oesophageal groove from forming properly and animals lose milk to the rumen, reticulum and omasum which results in digestive problems such as scouring.

    Weaning

    As the animal eats more feed, especially feed which is high in fibre, the first three stomachs will develop.

  • It is important that the young animal is eating enough solid feed before it is weaned (no longer fed on milk).

    • The Importance of Colostrum in the New Born Lamb and Calf

    During the first six hours of life lambs and calves are able to absorb large compounds through the wall of the digestive tract.

  • This allows them to absorb antibodies from colostrum (the first milk). The digestive system alters very rapidly thereafter and the ability to absorb antibodies is lost after about six hours.
  • The antibodies provide passive immunity from disease.
    • Calf and cow picture
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