Equine Digestion

Evolution

Eohippus, a fox sized animal was the first ancestor of the modern horse; he appeared around 55 million years ago. A rabbit like animal he lived in swampy forests and his diet consisted high fibre stems and leaves. As the climate changed to a cooler, drier environment, plant species adapted by growing seasonally and therefore produced more fibrous feedstuff, which resulted in the early horse having to adapt to this diet by being able to ferment this cellulose rich vegetation. This developed in the colon or hind gut, in contrast to ruminants, which evolved at the same time, whose fermentation takes place in the fore-stomach.

As well as his environment changing, so did his predators, they became taller and faster, which meant that in order to stay ahead the horse had to develop a digestive system that could cope with eating and processing of large quantities of forage and have a flight response, which allowed him to run swiftly and accurately from his enemies. The horse evolved into a very successful herbivore more than capable of fleeing from any threatening situation.

Evolved feeding behaviour

The horse, if left to his own devices, will forage for food for up to sixteen hours a day or round about 60% of his time. The table below show's a recent study into the occupation of free living Camargue horses.

Confining horses to paddocks or stables does not alter the way in which they allocate their time. He will have the same time budget as the wild horse and if the feeding time is reduced then he will fill that time with other behaviour, not all of which we welcome.

The Digestive Tract

The equine digestive tract is split into 4 main parts, the mouth, the stomach, the small intestine & the large intestine.

Mouth & Teeth

As we look at the horse we notice that he has a long neck and a relatively large head. The long neck has developed so that the horse can eat off the ground; the large head houses the teeth which is the first part of the digestive system. The horse is a very selective feeder; he will use his upper lip to pick out suitable feed and then the incisors will cut the food, this will then be moved to the back of the mouth for crushing by the molars. At the same time saliva is mixed in to act as both lubrication and as a buffer to the gastric acid in the stomach. The saliva of the horse has no digestive enzyme activity.

The feed is reduced to lengths of around 1.6mm which depending on the type of feed offered can take between 800 chews per kg for a concentrate diet or to up to 5000 chews per kg for long fibre. The more a horse is encouraged to chew the better it is for him for the following reasons.

• Correct chewing helps keep the teeth in good shape
• By extending chew time the more saliva is produced-thus buffering the gastric acid
• By extending chew time we are taking up more of a horse's time and so he will be satisfying his natural instinct

The Stomach

The stomach of the horse is relatively small, around the size of a football, it has evolved to be this size as the horse is a trickle feeder, so therefore feed would be passing through the stomach most of the time. If the stomach is empty then the gastric acid will attack the lining of the stomach and ulceration will occur. It this therefore vital to ensure that this doesn't happen through correct management and feeding practices. The stomach works best when it ¾ full which equates to around 1½ kg of feed so it is essential that when giving bucket feed the amount should not exceed this size.

In the stomach feed is mixed with an enzyme called pepsin, which digests protein, and the gastric acid which breaks down solids.

The rate of passage of the stomach is directly related to the rate of input, so that in the horse that is fed large 'meals' the rate will be quicker than that of a horse that is fed in a more trickle fashion. Therefore large meals will spend less time in the stomach, and so will be less digested when entering the small intestine.

The Small Intestine

After the stomach feed passes in to the small intestines, which on average are around 20m long and 10cm in diameter. This is split into 3 parts the duodenum which is attached to the stomach, the jejunum and finally the ileum.

The small intestine is the only part of the digestive system that can digest and absorb carbohydrate, sugars, dietary proteins and fats. This is done through a number of processes, carbohydrases digest sugars and starch, and proteases break down proteins into amino acids. Bile breaks down or emulsifies fat, which can then be suspended in water. Indeed Bile is constantly flowing into the small intestine from the liver as the horse has no gall bladder in which to store it. This again shows use that a horse has evolved to be trickle feeding as he is not adapted to having an empty digestive tract.

After the food has been digested it is absorbed through the intestine wall and carried away in the blood. Most vitamins and minerals are also absorbed in the Small Intestine.

Again the rate of passage through this part of the digestive tract is regulated by the amount of feed entering the system, so in a horse fed large meals the speed will be high and the digestive activity low as the processes will have less time to act on the feed.

The Large Intestine

The large intestine is home to billions of micro-organisms, which the horse relies on to break down any feed that remains undigested, which should be mainly fibre. These bacteria break down the complex molecules of cellulose, hemicelluloses and pectin, with lignin being the only part of fibre remaining undigested. This process is slow in comparison to the digestion of starch and protein in the small intestine and feed can be in the large intestine for up to 48 hours.

The products of this microbial fermentation are volatile fatty acids (VFA) and B group vitamins. The VFA's, acetic, butyric and propionic acids, are absorbed and stored as an energy source for future use by the horse. This energy is what is quite often called slow release energy.

Water, B group vitamins, some minerals including phosphorous are all absorbed in the large intestine. Anything that is not digested is then passed out in the droppings along with the dead bacteria, indeed up to half the weight of dung can be made up from micro-organisms.

The large intestine is a very sensitive part of the horse's digestive tract and it is vital that good management practices are maintained to ensure good hind gut health. The over feeding of starch in particular should be avoided as this will harm the delicate balance within the hind gut and can lead to problems such as, laminitis and tying up. When Starch enters the hind gut and is fermented by the bacteria the end product of which is lactic acid which in turn decreases the pH of the hind gut thus causing the death of the microbes, which releases toxins into the bloodstream.

Summary

The horse has evolved as a trickle feeder that is physiologically adapted to a high fibre diet. When managing domesticated equines we owe it to them to ensure that we have a feeding and management system that takes this into account by ensuring:

• That forage is available for a large proportion of the day
• Clean fresh water is constantly available
• Bucket feeds are small
• Bucket feeds should be what he has evolved to eat-high fibre and low in starch