1.  THE DIGESTIVE TRACT OF THE HORSE

2. THE CHARACTERISTICS OF DIGESTION

The digestive system of the horse, although conforming in general principles to that of all monogastric ungulates (one- stomached grass-eaters), has acquired certain specific characteristics as a result of the circumstances in which equine have evolved into one of the most successful classes of herbivorous animals. For a species to survive and to prosper in the competitive process of evolution, the animal must be able to secure sufficient nourishment from its surroundings and also to protect itself from its natural predators. If the latter do not already exist, they will soon appear in the evolutionary process to take advantage of any animal’s fortuitous defensive weakness. In many respects these two important factors in evolution tend to be in conflict with each other. The herbivorous animal must have mouth parts adapted to deal with the sort of food it eats, but the modifications needed to produce an effective defensive weapon, are liable to interfere with the efficiency with which the necessarily large quantities of bulky vegetable foods can be gathered and chewed. Some

animals, such as the wild pig, have evolved an enlarged tooth or tusk in the lower jaw which is a fairly effective weapon of defense and quite useful for other things as well, but which does not impair the food-gathering ability. In most herbivores the mouth parts have not been modified for defense but

instead, have developed methods, including camouflage, useful for concealment, armour plating, horns, or 27pecialized patterns of gregarious 27peciali.

The survival and biological success of the horse has resulted from its development of an outstanding ability to outpace its pursuing predators. A large body with a capacious gut capable of dealing with a large volume of bulky food, coupled with elongated legs with very light weight hooves capable of being rapidly accelerated and decelerated, produced an excellent solution to the problem of both eating enough and escaping from pursuers. The fact that this happy combination was so successfully achieved in the course of the horse’s development from the rather small and short-legged eohippus would suggest that for most of its evolutionary period, the horse must have been under constant pressure from predators; and these predators were also evolving the means of increasing the speed of pursuit, in a fifty million year race for survival. In order to move quicker the horse had to be larger, which meant that it had to eat more food, both to support the increased bulk and  to provide the energy for bursts of muscular activity at any moment.

Herein lay a considerable difficulty. Most animals are unable to exert themselves to violent activity if the stomach is full and distended after a heavy meal. This is partly due to the natural discomfort of trying to run fast in this condition but particularly because of the necessity of an increased supply of blood to the viscera required for the purposes of digestion. This naturally reduces the performance of the voluntary musculature, because of the temporary sub-optimal supply of sugar and oxygen. The horse has overcome this difficulty which otherwise could have been a defect fatal to the species’ survival, that of having a stomach which is small in relation to the body size and the volume of food which has to be digested. The situation is also relieved by the ability to pass the ingesta rapidly on to the small intestine without the stomach ever having become much more than about two thirds distended. This system, however, necessitates the food being taken in small quantities and since the total amount to be consumed is large, meals must be frequent. This sort of feeding regime in fact fitted in very well with the horse’s original environment on the grass plains of Central Asia.

Nevertheless it was a regime which also demanded that the horse’s digestive mechanism should be constantly ready to deal with food at short notice and frequently just as its body and muscles had to be ready for instant action. This is in fact the case and it is remarkable that, quite unlike other animals, gastric secretion is a continuous process, even during fasting and does not occur only at the sight of food or at the onset of a meal. A horse’s mouth does not water when it sees food – it waters all the time.

In compensation for the small size of the horse’s stomach, it is necessary for the remainder of the gut and especially the colon and caecum to be correspondingly larger in order to retain the contents for the time needed for digestion and absorption to occur.

The practical importance of these peculiarities of equine digestion cannot be over-stressed. First and foremost, it follows from the above observations that horse feeding should be in small sized meals at

frequent intervals. The stomach has only a limited capacity to regulate the flow of material (or 28peci as it is commonly called) once it has become about two thirds full. If the meal is too large or if it is eaten too fast, the 28peci may pass too rapidly into the small intestine, and digestive disorder or impaction of the intestine may result. Such is most unlikely to occur with horses of natural grazing habits. It is with large meals of palatable concentrates that the danger is most likely. Where the horse’s work load necessitates large daily rations, the number of feeds per day must be increased and not the size of each meal.

We have already noted the ability of the horse to exert its full energy capacity in violent activity immediately after feeding and it is possibly the only animal which is able to do this. However, in this context, feeding means the horse’s natural food, which is grazing, consumed at a very slow rate. Horses which are hand fed with concentrates or bait are in a rather different situation, since  although the meals may be small and frequent as is convenient as possible, they will not simulate grazing and the rate at which the food is eaten will always be greater than the rate at which grass can be grazed. It is therefore advisable to allow a period of rest of about one hour after a hand-fed meal before the horse is put to hard work. There is no objection to taking the horse straight from grazing to work at any time.

The essential difference between food taken at grazing and that eaten (particular as concentrates) in the stable from a manger, is of ever greater importance in the necessity for delaying and restricting the watering and feeding of an exhausted, tired or over-heated horse, until its body temperature and blood circulation has returned to normal. Such, in fact, is the case with any animal but not many are put to such vigorous work as are horses. The sign that a horse’s temperature and circulation are normal may be seen when it has ceased to sweat and its coat has dried off.

The regularity with which feeds are given and the maintenance of their constant composition are important in securing digestive efficiency and avoiding colonic disturbance. Horses get used to a particular routine and in fact all animal 28peciali and involuntary bodily functions tend to fall into habit patterns determined by preceding environmental influences. This is particularly the case with a horse’s digestive system. If the established sequence and timing of watering and feeding is suddenly altered, the system tends to be thrown out of its customary gear and will fail to function with its normal efficiency until a new habitual sequence has been established. In some cases the effects can be more serious than just a loss of efficiency and can result in a dangerous attack of colic. If changes in the composition or timing of feeds are unavoidable, it is essential that the alterations should be in small steps, and spread over several days, to allow the whole organism to become adapted to the new regime.

5.  THE MECHANICS OF DIGESTION

There are three processes by which food is incorporated into the horse’s body:

  • In digestion the food is softened and converted into a soluble form which can pass into the aqueous body fluids. In the case of fats, these become split into minute globules which facilitate a further breakdown of the fat into its water soluble component parts (fatty acids and glycerol).
    • In absorption these soluble substances are taken up from the gut by the blood and lymph streams and carried throughout the body.
  • In assimilation they are deposited by the blood and lymph streams in the organs where they are 29peciali for growth, reproduction, repair of tissue or muscular energy.

DIGESTION

There are three stages of digestion which may be distinguished:

Salivary digestion: Begins as soon as the food enters the mouth and becomes mixed with the secretion of the salivary glands. In the saliva, an enzyme called ptyalin is found which actively changes the insoluble starch of carbohydrate foods into partly soluble sugars, a process which is completed later during digestion, by the enzymes in the small intestine. Ptyalin is only able to act in an alkaline medium and its action therefore ceases when the food has reached the acid gastric juices in the stomach. The saliva also has the important function of being incorporated with the dry and mealy types of food, by the action of chewing, and the formation of lubricated boli which can easily be swallowed.

Stomach Digestion: When the food enters the stomach it is acted upon by the gastric secretions of the glands in the stomach walls. This juice is strongly acid (hydrochloric and lactic acids) and contains, among others, the enzymes pepsin, which splits the protein constituents of the food into their component amino-acids. The stomach actively engages in a vigorous churning movement which has the effect of thoroughly mixing the gastric juices with the contained food. This also warms up the whole mass to body temperature which is the same as that at which the enzymes operate most effectively. At this stage the food is converted into a brownish coloured material of uniform consistency. The food remains in the small stomach until the latter is about two thirds full and is then hurried through to the small intestine to make room for further amounts entering the mouth. In this way the stomach may allow an amount of food, two or three times greater than its actual capacity,  to pass through it during a period of continuous feeding. When no more food is taken, the last third of the stomach contents are retained and passed through, progressively and slowly, to the small intestine. Even with prolonged fasting the stomach does not completely empty itself.

Intestinal Digestion: The softened and semi-fluid chime, when leaving the stomach, has a strong acid reaction. Shortly after entering the small intestine it is 29pecialized by the alkaline secretions of the liver (bile) and the pancreas. The former is partly composed of complex salts and pigments and partly of waste product derived from the blood or from the absorption by the liver of waste products which are of no use in the body economy. The function of bile is threefold; it aids the emulsification of fats, dividing large globules into almost colloidal size particles, which are more easily split into their component parts by other enzymes prior to absorption; it assists in keeping the intestinal contents at the right degree of fluidity to enable efficient peristaltic action, and it controls undue fermentation and putrifaction through its slight antiseptic action against bacteria. The latter function is extremely important in reducing the production of gas, which if it happens in large quantities from bacterial fermentation, can result in sometimes fatal colic. The pancreatic juice possesses at least three powerful enzymes, which are probably sufficient in themselves to ensure complete digestion of a food without other assistance.

The first of these is trypsin which is concerned with the further splitting up of protein substances which have been partly acted upon by pepsin in the stomach. After proteins have been acted on for a period by trypsin, the resultant amino-acids are capable of being absorbed and made use of in the

body. The second pancreatic enzyme is amylopsin, which acts on the carbohydrate constituents in the food, splitting them up into sugars and other substances but not carrying the process far enough to allow complete absorption. Amylopsin has an action similar to that of ptyalin, but is more powerful, and completes the conversion of starch into absorbable sugars which was begun by the latter in the mouth. The third enzyme is steapsin, a lipase of fat-splitting agent, which acts on the fats in the food after they have become emulsified by mixing with the bile. The resultant constituents, glycerol and a variety of fatty acids, depending on the nature of the fat, are then capable of being absorbed.

The processes which have been described so far are never 100% efficient and completely different carbohydrates and proteins have a widely varying resistance to decomposition by enzymic action, some requiring a very long time. The larger particles of food also naturally tend to escape action by the juices, except on their surfaces, to an extent depending on the length of time allowed for the reaction to occur. Thus further digestion of the more resistant proteins and carbohydrates is achieved by action of the intestinal secretions which are very complex. Their chief constituent enzymes are erepsin and enterokinase which are concerned with protein splitting (peptidases) and maltase, lactase and invertase which convert the relevant un-absorbable sugars into absorbable glucose. The intestinal secretions main bulk consists of particular mucus which lubricates the passage of the food (or chyle as it is now called at this stage) and which is resistant to the action of the digestive process upon its own constitution.

Bacteria also have an important digestive function in the intestines, especially the large intestine and caecum of the horse. Their action is similar in nature to that which occurs in the first stomach of ruminants and accomplishes the breaking down of cellulose (which is by far the major part of the carbohydrate portion of the horse’s food) into volatile fatty acids which can readily be absorbed. This process is thought to occur to a very considerable extent in the horse’s caecum which is much larger than in other animals, having a capacity of about 35 litres, and accounts for the efficiency with which horses are able to extract nutrient energy from cellulose, almost (but not quite), equal to the ability of ruminants. (1) Some of the emulsified fat particles which are a little more than the size of large molecules can be absorbed as fat by the villi of the small intestine and converted into small globules in the lacteals within the villi before entering the lymph stream.

ABSORPTION

Water passes through the stomach and into the intestines very quickly, reaching the large intestine in the course of a few seconds. It is rapidly absorbed and is, in fact, the medium by which the soluble constituents of the food, and the soluble products of digestion, are taken into the body. (2) Absorption occurs throughout the length of the stomach to the end of the large intestine, but more particularly in the small intestine. Both are covered on their interior surfaces with small finger-like structures called villi. These organs, of which there are millions, project into the lumen of the intestine and are therefore continually in contact with the chyle passing through it. Each villus contains a lymph vessel at its centre where fat globules are reformed and ultimately passed into the lymph and blood streams. Glucose, salts and soluble amino-acids and fatty acids pass directly into  the small blood vessels in the walls of the intestines. They are then carried to the liver and so enter the general circulation.

The food is passed onwards through the various folds and coils of the intestines, each particular part removing some useful portions of the food; the residual and un-absorbable residue is eventually discharged through the rectum and anus as defecation

E(2) Except for emulsified fat as mentioned in note (1).

ASSIMILATION

After the products of digestion have been absorbed into the blood and lymph streams they are carried around the body, ultimately reaching every organ and tissue. The body cells extract from the lymph and blood in the capillaries whatever nutritive substances they may require for growth, repair of worn-out or damaged tissue or production of physical energy; each 31pecialized cell in the body is programmed to seize exactly the right quantities of what it needs from the complex mixture of sugars, salts, amino-acids, alcohols and a host of other things including the most important, oxygen.

When the supply of materials is in excess of immediate requirements, the surplus potential energy is stored either as glycogen in the liver or muscles, or as fat around the viscera, under the skin or between the muscle fibres. In times of shortage it then enters the blood stream again to compensate for the reduced supply of food entering the horse’s mouth.