Gastrointestinal Physiology

The Key to Understanding Horse Nutrition

Equines in their natural environment will spend up to 75% of their time grazing and eat a large variety of grasses, herbs and shrubs.  In order for horses to survive on this high fibre diet, the equine gastrointestinal tract has developed into a highly specialised structure that allows them to extract energy and protein from the often low quality diet they consume.

Given that our domesticated horses often don't have access to the grazing conditions and variety of forages that their wild counterparts do, plus have extra demands placed on them by through riding, training and competition, we must manage their diets carefully to ensure we are meeting their nutritional requirements.  To do this effectively, without causing any harm, we need to have a sound understanding of how their unique gastro-intestinal tract functions.  The following article aims to give you a basic understanding of the structure and function of your horse's gastrointestinal tract. It is essential that you understand your horses gastrointestinal tract, as it is the key to understanding how to feed your horse correctly.

The horse can be classified as a monogastric or single stomached animal whose gastrointestinal tract consists of the mouth, stomach, small intestine, caecum, large colon, small colon and rectum (Figure 1).

Figure 1: The structure of the gastrointestinal tract of the horse (Drawing L. Ferguson)

The Mouth

Horses use their mobile upper lip and incisors to select and shear forage from its base or to select and pick up hay or concentrate feeds they are fed.  Once the feed is in the mouth it is pushed by the tongue to the molars at the back of the mouth where it is chewed extensively.  During chewing, the particle size of the feed is reduced to facilitate digestion further down the gastrointestinal tract and it is mixed with saliva.  The saliva lubricates the feed, making it easy to swallow, and also provides some buffering to protect the upper regions of the stomach from the acids produced in the lower regions of the stomach.  Once the feed is chewed sufficiently it is swallowed and travels down the oesophagus into the stomach.

The Stomach

The stomach of the horse is a "J" shaped organ with a capacity of approximately 5 - 15 litres meaning the stomach comprises around only 10% of the total volume of the horse's digestive tract. The stomach has two major functions. These are:

  1. the mixing, storage and controlled release of feed into the small intestine; and
  2. the initiation of protein digestion.

The stomach of the horse is unique in comparison to other monogastrics for two reasons: the first is that acidic gastric juices are constantly secreted into the stomach (other monogastrics generally only secrete gastric juices when feeding is initiated); and the second is that feed tends to pass rapidly through the stomach (unlike in carnivorous monogastrics where food spends a large amount of time in the stomach).   While both of these features are well suited to the grazing horse consuming a high fibre diet, it is likely that these same design features play a role in the incidence of gastric ulcers in horses that are confined to stables and fed a concentrate diet.

Once feed is released from the stomach it enters the small intestine.

The Small Intestine

The small intestine is where a majority of protein, fats and non-structural carbohydrates (starch and simple sugars) are digested and absorbed. The small intestine of a 500 kg horse is approximately 20 to 27 meters long and has a capacity of 55 to 70 litres.

When feeds enter the small intestine they are mixed with digestive enzymes. These enzymes include proteases that digest protein, lipases that digest fats and glycanases, that digest non-structural carbohydrates.   These enzymes act much like scissors, cutting the large protein, fat and carbohydrate molecules into very small pieces that can be absorbed from the small intestine into the horse's bloodstream or lymph for transportation around the body.

The passage rate of feed through the small intestine is relatively rapid, with feed passing through the entire small intestine in as little as 45 minutes.   Given that the feed can move so quickly it is very important that everything entering the small intestine is easy to digest.   Horses have a natural ability to extensively digest fats and proteins, however starch from raw cereal grains is very difficult for horses to digest in the small intestine.   For this reason, cereal grains must be cooked via a process such as micronising to improve its digestibility in the small intestine.

At the end of the small intestine all of the fat, simple carbohydrate and a majority of the protein components should have been digested, leaving only the structural carbohydrate or fibrous components to continue on and enter the caecum.

The Hindgut

In the horse the caecum, large colon, small colon and rectum are collectively referred to as the 'hindgut'.  In other monogastrics these same structures are more commonly referred to as the large intestine. The hindgut is a specialised structure the horse has developed to enable them to digest high fibre forages.

Monogastric animals do not possess the enzymes necessary to digest fibre.  In order to extract the energy from fibrous feeds the horse houses billions of bacteria in its hindgut.  These bacteria do posses the enzymes necessary for fibre digestion and they digest the fibre that enters the horse's hindgut in a process known as fermentation.  As the bacteria ferment fibre in the horse's hindgut they produce volatile fatty acids.  These volatile fatty acids are absorbed and used by the horse as a source of energy.  In fact, for horses consuming pasture or hay as the major component of their diet, these volatile fatty acids are where they get nearly all of their energy from.

The fermentation of structural carbohydrates is a lengthy process.  Consistent with the clever design of the horse's gastrointestinal tract, passage rate of feed through the horses hindgut is slowed dramatically, with feed taking from 50 hours to 10 days to travel from the end of the small intestine to the rectum where it is excreted as faeces, ensuring there is plenty of time for extensive fermentation to take place.

Any starch that is left undigested as it passes through the small intestine is also fermented in the hindgut.  However, unlike the steady fermentation of structural carbohydrates, the fermentation of starch is a rapid process.  During this rapid fermentation volatile fatty acids are produced in such large amounts that the ability of the horse to absorb them is overwhelmed.  Lactic acid is also produced in large quantities and the accumulation of these acids in the horse's hindgut causes a condition known as hindgut acidosis.  Hindgut acidosis can cause serious diseases including laminitis and colic.  Preventing starch from entering the hindgut must be a priority when feeding all horses.

The hindguts second important function is to reabsorb and conserve electrolytes and water that have been secreted from the body into the gastrointestinal tract during the digestion process in order to prevent dehydration and electrolyte deficiency.

The Gut is Our Key to Feeding

So we can see from the way a horse's gut is designed that they are best suited to diets that are high in fibre and available continuously through the day and night.  Because it isn't always possible to satisfy a working horses nutrient requirements using a high fibre diet nor practical to feed a stabled or yarded horse several times a day, we must design diets that stray away from their natural diet, but that are still safe and healthy for the horse.

Understanding how the horse's gastrointestinal tract functions is our best guide to working out how and what to feed our horses.  Over the coming months as we cover many horse nutrition topics we will frequently refer back to the structure and function of the horse's gastrointestinal tract, so keep this article handy as you may need to refer back to it.

Supplied by Dr. Nerida Richards.