Understanding Equine Digestion

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     Since its first domestication (about 5000bc), the horse has been used for many purposes; initially for meat, then for transport and for use in battle, leading us to its modern use, for pleasure. The horse is suited to a high fiber diet due to the continual microbial fermentation within its caecum and colon. It is its domestication that leads the horse to use and therefore need more energy than can be provided by their usual high fiber natural diet of grass. It is these requirements that have lead to the inclusion of grains and supplements to the horse's diet. In many ways, this being possible has lead to a lot of benefits, but it also has the potention to result in many problems.

Feeding horses is a combination of science and art. By using information relating to the scientific processes of digestion, metabolism and nutritional requirements, the owner can put this factual knowledge into practice for each individual horse.

The first part of the horse's digestive system is relatively simple; it is the later part that requires us to carefully consider how we feed our horses. Horses do not ruminate like cattle and therefore need to take in small amounts of food, chew and swallow; this is where the term 'trickle feeders' stems from. The nature of the feed that the horse eats will greatly influence the chewing rate and speed of ingestion. The average 500kg horse will chew at a rate of 40 minutes per kilogram of hay. The same 500kg horse would eat oats at a rate of 10 minutes for each kilogram! This can have knock on effects. The less time eating, the more chance behavioural problems can occur out of boredom. The ability to eat food so quickly also prevents the horse from chewing and breaking down the particle size of food before swallowing, significantly reducing the nutritional value that is gained from the feed ingested. Saliva is produced in response to chewing, rather than in anticipatin of food. The more food is chewed the more saliva the horse produces. It has been suggested that 12 liters of saliva is produced a day on a hay and concentrate diet. On a diet of dry hay up to 100 litres can be produced. This helps to lubricate the bolus and prevent choke.

The horse's stomach volume is between 9-15 litres. It is relatively inelastic and ingested food is retained for 20 minutes although there is always some food retained for several hours following a meal. The larger the meal the quicker the horse's stomach empties. This also means that food empties into the small intestine more quickly. The dryer the bolus that passes though the horses digestive system, the slower the digestion and mixing of the feed with the gastric juices, reducing the effective fermenting that needs to take place.

Most digestion occurs in the small and large intestines, rather than the stomach. Naturaly the horse will graze for 16-20 hours a day and they have evolved without a gall bladder. Bile is secreted continuously as food passes through the gastrointestinal tract. It is this that helps alkalise the digesta and is required to emulsify and digest lipids. A horse's dietery energy is provided to the horse by 4 sources
?Starch
?Cellulose (found in a primarily fiberous diet)
?Fats (usually less than 3% of total feed intake and any supplementary oils should be introduced gradually)
?Proteins (not a prefered energy source due to their inefficiency for this purpose)

Adult horses secrete over 100 litres of fluid per day into this pre-caecal part of the gastrointestinal tract (GIT). Large pelleted or grain filled meals fed infrequently can result in a low blood volume due to to meal stimulated secretions in the upper GIT. These changes can be avoided when chaff is included in the feed to slow the horse down as it eats or by frequently feeding smaller meals. It is also important to remember this when wanting to exercise after feeding. Greedy or fast eating horses will already be in a state of relative dehydration. There are other factors that need to be considered in relation to exercising after feeding;
?Glucose reaches its peak between 1-3 hours after feeding. The brain can only use glucose as a fuel and if the horse is exercising at this time would result in a larger drop in glucose levels. The horse would have to rely even more on stored glycogen potentially resulting in a quicker onset of fatigue.
?Having a full GIT after eating would restrict the amount of space available for lung expansion.
?Blood flow is diverted to the gut to enable digestion effectively. This would reduce the blood flow to muscles used when in work, again quickly bringing on fatigue.

Food travels quickly through this upper section of the GIT and most of it will reach the Caecum / Colon within 45 minutes. This does vary depending on the feed fed and the way in which it is fed. Fiberous food moves the slowest though the horses GIT. It is in the Caecum and colon that the majority of the nutritional value of the feed is utilised and the time it spends in here determines the effectiveness of the processes. Digestion and absorbtion of carbohydrates relies on microbial action and absorbtion in this section of the GIT, there is very little enzyme activity here. The fact that the microbial fermentation occurs after the 'monogastric' first part of the horses GIT is very important to the way we feed our horses. It is this microbrial breakdown that can rid the large intestine of undigested protiens and urea, resulting in urea as a byproduct, the majority of which is reutilised by the bacteria, actually stimulating important bacterial growth. Any microbial protein synthesised here can not be used by the horse. Horses requiring higher protein levels need to have dietary protein supplements which can be broken down before the large intestine. Most of the water soluble vitamins are synthesised in the large intestine, so no additional feeding of these are needed. It is in the large intestine that water is stored and absorbed.

The upper part of the digestive tract has a small capacity and the horse has digestive and metabolic limitations to high grain, highly soluble carbohydrate diets. Due to the requirements we place on our horses to exercise for prolonged, frequent or intensive periods of exercise, it it vital that we remember that they have not evolved to cope with large quantities of grain feed. Whilst giving the horse the energy to perform as we require, the horses stomach can easily be overwhelmed and its digestive capability can be reduced. This can lead to rapid fermentation of the grain carbohydrate in the hindgut, causing a decrease in the pH. This in turn will cause a negative chain reaction including a change in the microflora within the GIT, changing the action of these, possibly allowing the release of endotoxins that can cause damage to the large intestine. It is good to remember that a horse on a roughage based diet would not result in decreases in the GIT pH and therefore none of the potential problems associated. This is one of the reasons that supplementary oils and fats have become popular as a nonstarch energy source.

Good nutrition can not improve the core ability of a horse but poor nutrition can place limitations on its performance.