As I mentioned in my Feeding blog a few weeks ago, horses' digestive systems are designed for them to eat grass. To start to understand this in more detail knowing about the digestive system and how it works will help.
The food travels through a system called the 'alimentary canal' from the lips to the anus.
1. The horses mobile lips are adapted for grasping and the incisors (biting teeth) for biting off the grass. The lips will also sort and identify food.
2. The tongue then moves the food from the front of the mouth to the back where the molars (grinding teeth) chew the food (masticate). The upper surface of the tongue is covered in taste buds.
3. As the molars chew and grind the food it is mixed with saliva before the tongue forms a portion into a ball (bolus). Little digestion occurs in the mouth.
4. The bolus is passed into the pharynx (throat) which operates the epiglottis to ensure the food enters the oesophagus and not the trachea.
5. A process called peristalsis moves the food down the oesophagus. This is when the muscles of the oesophagus contract and relax to squeeze the food down.
6. The bolus then moves into the stomach. There is a muscle, the cardiac sphincter muscle, which acts as a non-return valve. This ensures that food cannot pass back out of the stomach, this means that horses cannot be sick! A horses stomach is relatively small as it is only the size of a rugby football, able to contain 9 - 18 litres. The size of the stomach is what makes the horse a 'trickle' feeder - needing to eat little and often.
7. The food will stay in the stomach for between 30 minutes and 3 hours. Gastric juice which is secreted by glands in the stomach wall begin breaking down the food.
8. Passing through the pyloric sphincter muscle the food moves into the small intestine. The small intestine is made up of the duodenum, jejunum and the ileum. Secretions from the liver and pancreas flow into the duodenum to aid digestion, this is where the real food breakdown begins. The small intestine is nearly 20 metres long and is held in loose coils. It is through the walls that the vitamins, minerals, glucose and amino acids (protein) are absorbed.
9. The food passes from here into the large intestine. The large intestine is made up of the caecum, large colon, small colon, rectum and anus. Here the food continues to be broken down by the bacteria in the gut. This is where the cellulose (fibre) is digested and each type of bacteria break down a certain type of food. This explains why changes to a horses diet should be made gradually - the types of bacteria need to change to digest the new food properly. If this is not done the horse could develop colic. The breakdown of the fibre can take several days.
10. Waste (what is left after all the nutrients have been absorbed) then passes into the rectum where it is formed into balls of dung to be passed out through the anus.
On average food takes 3 - 4 days to pass through the horse, although some foods eg: grain, will pass through more quickly.
I hope the pictures give you an idea of the alimentary canal, finding one which accurately shows the process was not easy!
In some ways the horses' digestive system works in a similar way to ours but there are key differences as a result of their diet of grass. If you keeping an eye on the colour and consistency of your horse's dung you will notice changes between the seasons and if you change their food. Compare your horse's dung in the winter to the summer - the difference will be noticeable even if your horse is grass-kept all year round!
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Until next time!