The natural food of the wild horse is the fresh moist succulent grasses and cereals of the temperate zones. These include many species of the Graminaceae, and plants like the mallow, rumex, and ranunculus from other orders. In the domesticated animals, however, whilst grass forms part of the food for part of the year, many working horses in large towns live on the dried stems, flowers, and fruit of the grasses, with some addition from the seeds of the leguminous plants.
The food of an average-sized horse performing a moderate amount of work may be taken in England at 12 lbs. of hay, 10 lbs. of oats, 2 lbs. of crushed maize, and 8 lbs. of chaff, with five or six gallons of water, according to the requirements of the animal.1 In considering the changes wrought in the food during the process of digestion, it is to be borne in mind that the main constituents of the various plants eaten by the horse, whether in the open field or in the form of hay, corn, and beans, are reducible to a few groups which are named proteids, farinaceous and saccharine compounds, oils, vegetable acids, water, and salts.
The proteids constitute the most important of the food constituents because they contain nitrogen, sulphur, and phosphorus. They are represented by the gluten of wheat, the legumin of beans and peas, and the aleuron grains of many seeds. Chemical analysis shows that they contain, in every 100 parts, 52 of carbon, 7 of hydrogen, 16 of nitrogen, and 24 of oxygen, with about 1 per cent of sulphur. In the animal they exist in two conditions - in the fluid and soluble, as in white of egg, the fibrin and albumen of blood, and the casein of milk; and in the solid and insoluble form, as in the substance of muscle, connective tissue, and the protoplasm of various cells.
1In France, in 1885, the omnibus companies supplied each horse with 10 lbs. of oats, 8 lbs. of bruised maize, and about 2 lbs. of feverole, a sort of small bean, the product of Vlcia faba.
The starches, sugars, and gum have been classed together under the general term of the carbohydrates, since one of their constituents, carbon, is combined with oxygen and hydrogen united in the proportion to form water. The compositions of several varieties is represented by the formula C12, H20, O10, of others by C12, H24, O12. The starches are very widely distributed in plants, appearing as the first evident products of assimilation, and being formed by the decomposition of the carbon-dioxide contained in the air and water they absorb. They are sometimes found in solution, as in the case of various sugars, but more commonly in the form of grains occupying the spaces in the interior of cells, and constituting a reserve of nutriment which is drawn upon in the course of the growth and development of the plant, but which is also a valuable aliment capable of being-assimilated by animals. Starch is abundant in many fruits, as in the banana and fig; in seeds, as in those of all the cereals; in rhizomes, as in the arrow-root; in tubers, as in the potato; and in stems, as in that of the sago palm. The process of the ripening of fruit consists in large measure in the change of starch into pectin, dextrin, and sugar, under the influence of light and heat. The particular form of sugar, as cane, grape, beet, maple, eucalyptus, or mushroom, depends on the special activities of the plant, and the several forms differ inter se in their solubility, crystalline form, chemical composition, action on polarized light, and other characters.
The oils found in plants are divisible into two groups, the fixed and the volatile. The fixed oils are compounds formed of glycerine united with the fatty acids; thus, palmitin is composed of glycerine and palmitic acid, stearin of stearic acid and glycerine, olein of oleic acid and glycerine. The process of emulsification is simply that of reducing them to fine globules by shaking them up with any glairy fluid. Such emulsions may last unchanged for a considerable period, but as a rule the mist of oil runs together into droplets and these again into drops, which collect together, and the original condition of a layer of oil is recovered. There is no chemical alteration in emulsification. The process of saponification is, on the contrary, attended with a profound change in the chemical composition of the oil or fat. It is effected when the oily substance is intimately mixed with an alkaline solution, as of soda or potash. The alkali displaces glycerine and combines with the acid. Soaps, therefore, are oleates, palmitates, margarates, or stearates of soda, or potash, or lime. These salts, as they may be called, are capable of traversing animal membranes, which is not readily accomplished by the oils in their natural state.
The volatile oils are very numerous, and give to different plants their peculiar and characteristic odours. Thus the smell of new-mown hay is due to the volatile oil (cumarin) which is contained in the Anthoxanthum odoratum, the odour of mint to the oil of mint, and so on; the quantity being usually small, in mint, for example, not exceeding 1 or 2 per cent of the dried plant.
In regard to the salts, they may be divided into two groups - those formed by the so-called mineral acids, such as the sulphuric, nitric, phosphoric, hydrochloric, and silicic acids, in combination with the bases, sodium, potassium, magnesium, lime, and others; and those formed by the organic acids, which are combined with the same bases, and of which the chief are oxalic acid, found in such plants as the oxalis, dock, rhubarb, and spinach, malic acid in apples, tartaric acid in the grape, formic acid in the nettle, acetic acid in chamomile, propionic acid in milfoil, butyric acid in the pansy, and many others.
If we now consider the composition of the ordinary food of the horse in the light of these preliminary remarks, we shall find that whilst such substances as oats and maize can be analysed with great exactness, it is almost impossible to give even an approximate account of the composition of hay, since it varies with the soil, the species forming the herbage, and even with the period of growth of the plants of which it is composed: the saccharine principles being most abundant at the period of inflorescence, mucilage during the period of the maturation of the seed, and the proteids, saline and bitter substances in the aftermath or later crop. The analyses of M. Boussingault give as the average percentage composition of ordinary hay - Water, from 13 to 16 parts; proteids, 7 to 13 - 5 ; sugars and starch, 44; woody tissue and cellulose, 24; fats, 4; ashes, 5 to 8 -parts. A large proportion of the ashes consists of silex, and the remainder is nearly all composed of the salts of calcium, potassium, and sodium. The composition of oats is - Water, 12; proteids, 10 to 14; fats, 5 to 7; starches, gum, and sugar, 50 to 55; woody fibre, 10; and salts, 3. Of maize - Water, 13; proteids, 10 to 15; fat, 4.5 ; farinaceous compounds, 68.5; woody fibre, 2.5; ashes, 1.5. It may just be added that the proportion of nitrogen to carbohydrate in the different cereals is as follows: - wheat, 2.29 : 78.64; rye, 2.17:78-81; barley, 2.06 :75.29; oats, 1.90:65.93; maize, 1.81 :78.74; rice, 1.45 : 88.01; millet, 1.95 : 76.09. In green food the quantity of water in 100 parts is very considerable, but when it is dried the total quantity of nitrogen is tolerably uniform, being in peas, 4.69; vetches, 5.57; turnips, 4.76; carrots, 2. It must be remembered that a considerable proportion of nitrogen is contained in other constituents of plants which are not proteids, and that they are not all capable of undergoing digestion. Thus, for example, more than half the nitrogen of lettuces, water-cress, and spinach exists in the form of nitrates, which are useless as flesh-formers; and the same holds with the amides of beet-root, potatoes, and unripe pulse.
In giving a connected account of the process of digestion in the horse of its ordinary food, hay and oats, the first point to be noted is, that in the mouth these substances are subjected to mastication and insalivation. The large and uneven surfaces of the molar teeth bruise and break down the stems and fruit of cereals, and the seeds of leguminous plants, enabling them to be more readily acted on by the several fluids of the alimentary canal. Simultaneously with this crushing process, the contraction of the muscles moving the jaws, and the stimulus of the sense of taste, causes an abundant secretion of saliva, the quantity having been ascertained by experiment to amount to between 80 and 90 lbs. per diem, most of which is reabsorbed in its further course down the alimentary canal. The saliva softens the food, enables it to be rolled into a coherent mass, and facilitates its deglutition. In addition, it exerts a powerful chemical action on the starchy compounds, for it contains a ferment named ptyalin which has the power of converting starch, through several intermediate stages, first into dextrin and then into a form of sugar named maltose. This action is termed diastatic or amylolytic, and has for its result the chemical union of water with starch, thus changing it from an insoluble substance into a soluble one; from starch, that is to say, which will not pass through an animal membrane and cannot therefore be absorbed, into sugar, which readily permeates the walls of the blood-vessels and lacteals. The ferment is most abundant, or most active, in the saliva winch flows from the parotid gland. It has not indeed been isolated, but it is known to act far more energetically on boiled than on raw starch. The presence of dextrin, maltose and a little grape-sugar, after the addition of saliva to boiled starch, is easily demonstrable after a minute or two; whilst with raw starch, especially in the case of wheat starch and potato starch, a much longer time is required. The proteids and oils of the food undergo no change in the mouth.