The Equine Digestive Tract

The Horse's Digestive System 

The horse has evolved as an animal that is grazing for the majority of the day (often around 14 hours), frequently on poor quality pasture. Therefore their digestive tract reflects this need to consume small amounts of food regularly.

Horses are herbivores, meaning that their diets consist only of grasses and other forages containing cellulose, which is an insoluble (complex) carbohydrate. Cellulase is the enzyme which is secreted to break down the cellulose, however mammals have not adapted to excrete it. In the horse this process is done in the hindgut by microorganisms.

It is essential for every horse owner to have a basic knowledge of their horse’s digestive tract to limit the risk of problems such as colic, which can result from improper feeding.  Let’s look at the horse’s digestive system from start to finish.


Equine digestive tract
 
1.    The Mouth

Incisors in the horse’s mouth are able to bite off food selectively and graze close to the ground on areas of poor pasture. Molars are used for grinding fibrous foods and all food must be ground down to a size of 1mm or less before swallowing, which takes longer with hay than with concentrate feeds. When eating concentrates the horse will need to chew around 800 – 1200 times to consume a kilo, however a kilo of hay needs to be chewed around 3000 times. Saliva production is promoted by this chewing action, and is essential to lubricate the food when passing through the digestive system and also to act as a buffer, or neutralise, in the acidic conditions in the stomach. An average horse produces 10 – 12 litres of saliva a day. Therefore to reduce the risk of conditions such as ulcers and impaction colic (blockages in the large intestine) it is essential that the horse consumes lots of forage based feeds, promoting chewing and therefore saliva to reduce the pH in the stomach and the easy passage of food.

2.    The Stomach

Food passes down the oesophagus and through the cardiac sphincter valve to enter the stomach (this is the valve that stops a horse being able to regurgitate food, or be sick!). An average 16hh horse has a stomach the size of a rugby ball and horses evolved with a stomach of this size as, in the wild, the horse would be a constant grazer; they would not consume large meals in one sitting. However, now the domesticated horse is frequently being given substantial sized bucket concentrate feeds, this means food is being pushed out of the stomach quicker and into the small intestine when only partially digested. The stomach does break down some fibrous material, but mainly the acid acts to digest proteins into polypeptide chains.

3.    The Small Intestine

Once food has passed through the stomach it enters the small intestine, which is the main site of enzymatic digestion of soluble carbohydrates, proteins and fats. It is also the site where key vitamins and minerals are absorbed into the blood stream. The wall of the small intestine is folded and covered in finger-like projections called villi which increase the surface area of the intestine and therefore the area available for absorption. The small intestine requires an alkaline environment, unlike the acid environment in the stomach.

4.    The Large Intestine

Fibrous foods and forages are not digested in the foregut (stomach and small intestine) of the horse. Instead they pass into the large intestine, where microbial fermentation of the insoluble carbohydrates occurs. As a horse’s diet should be primarily fibrous, this is a very important part of the digestive system. The hind gut is split into three sections: The caecum; the large colon; the small colon.
The caecum and large and small colons provide a hospitable environment for the microorganisms in the gut which digest cellulose and hemicellulose in forage. These microorganisms (bacteria and protozoa) ferment the cellulose and hemicellulose into volatile fatty acids, these are absorbed into the blood stream and are later used in the production of energy. 

When considering your horse’s diet, it is essential to maintain good digestive health. A horse needs a minimum of between 0.5 – 1% of its LW (live weight) per day fed as roughage. This ensures trickle feeding (when fed at intervals throughout the day), helps to maintain the gut pH, reduces gastric distension and also helps to relieve boredom, by mimicking the horse’s natural grazing pattern. A horse should receive no more than 0.5g of starch/kg of LW per meal, this prevents food passing too quickly through the digestive system when not properly digested and dumping into the large intestine, increasing the risk of colic.

It’s also essential that a horse receives a balanced ration of all essential vitamins and minerals. For horses on high fibre diets there are many supplements available that will provide the recommended daily amounts of important minerals. Horses on supplemented concentrate feeds may already be receiving adequate amounts. A pre and pro biotic is also essential for horses on a high fibre diet, to maintain the microbial health in the hind gut.

Always seek the advice of a nutritionist if you are unsure about your horse’s diet and how it may affect their digestive system.