The hindgut
The hindgut, also called the large intestine, consists of five parts: the caecum, the large colon, the small colon, the rectum and the anus.
The hindgut is approximately 7m long and can hold a volume of 140 to 150ℓ.
Digestion in the hindgut is largely bacterial rather than enzymatic.
Symbiotic bacteria break down plant fibres and other starches and process them into simpler compounds called volatile fatty acids, which are later absorbed through the gut wall.
The caecum
The caecum is a fermentation chamber that forms part of the hindgut, approximately 1.2m in length, and can hold up to 36ℓ of feed and liquids.
Microbes break down feed that was not already digested in the small intestine.
The caecum is unique in that the entrance and exit of the chamber are both at the top of the organ, meaning that food enters at the top, is processed towards the bottom of the organ, and is then sent out again through the top.
If a horse eats a lot of dry feed and does not consume enough water, then compaction of food can occur here and this in turn results in colic.
The bacterial population in the caecum is a consequence of the feed the horse usually digests (meaning the bacterial population adapts to the diet of the horse).
It can take two to three weeks for the bacterial population to adjust to a new diet, which is why it’s important to gradually introduce new feed if you are changing your horse over onto something new.
Feed is processed in the caecum for approximately seven hours, allowing bacteria to slowly break it down through the process of fermentation.
The large colon
The large colon is made up of the right and left ventral colons and a dorsal colon.
Fermentation continues in the large colon and the majority of the nutrients made through bacterial digestion are absorbed here.
The ventral colons consist of a series of pouches, designed to digest large quantities of fibrous materials.
The design does, however, pose as a risk for colic, as the pouches can easily twist and fill with gas due to the fermentation.
Feed in the large colon will stay there for 48 to 65 hours before being passed through into the small colon.
The small colon
The main function of the small colon is to absorb moisture, as most of the nutrients have been digested by this stage.
From the small colon the remaining food is made into faecal balls, which are then passed out of the rectum and anus as manure.
Important
As so many horse owners know, the equine gut is extremely sensitive.
Sudden changes in diet can quickly upset the system, leading to colic – the leading cause of equine death.
The bacterial population and microflora can also be upset if a horse experiences stress, such as when traveling long distances or when ill.
Feeding your horse as close to a natural diet as possible (high roughage, low concentrates) will maximise the health of his digestive system.