The digestibility of foods is an important consideration in feeding, as with some kinds more is absorbed into the system than others. With scarcely any of them is digestion complete throughout, a portion always being thrown out of the body undigested, no matter what amount of preparation it may have undergone.
With regard to this point, it may be remarked that age has an influence on the digestibility of plants and herbage. Thus, hay cut as soon as ripe is digested more easily than at a later stage of growth, and it is the same with clover. When young, plants contain more albuminoids, and less woody fibre or cellulose, than when old. Roots, however, such as carrots, turnips, potatoes, and mangolds, have their nutritive value increased by age, since the production of the carbo-hydrates - as sugar and starch - increases with growth.
Food containing a large amount of nutriment in small bulk is usually digested better than hay or straw. This is particularly the case with the albuminoids and fats contained in them; 85 per cent. of beans and 80 per cent. of linseed are digested as easily as 64 per cent. of hay and 45 per cent. of straw. The more nitrogenous the hay and straw, the better it is digested. Only 20 per cent. of wheat straw is digested, against 76 of lucerne hay. What is called "cellulose" is usually fairly well digested. Hay and straw of leguminous plants, as peas and beans, are not so easily digested as that of the cereals, because more woody matter, which is indigestible, is contained in the former than the latter. With wheat-straw chaff, it has been stated that about 22 per cent. of the total organic matter in it is digested.
It would appear that only a certain amount of each substance can be digested from a given quantity of food, and rest or work will not cause an animal to digest more, though it may happen that two animals of the same breed will digest different quantities of the same food.
The digestibility of one food may be increased by the addition of a second or third different kind, and a decrease in digestibility may be effected in the same way. Small quantities of oil added to a diet of hay and straw will slightly increase their digestibility; but the addition of sugar or starch, if it exceeds 10 per cent. of the diet (both being dry), diminishes the digestibility, the albuminoids and cellulose not being digested to the proper amounts. An excess of starch is more to be feared than an undue proportion of sugar in the digestion of hay.
More or less preparation of food will increase facility of digestion, so long as it does not alter the character of its constituents or impair the digestive process. Crushing grain which is covered by an insoluble husk, or is so dense that the gastric juice acts slowly on it, is to ensure its easier digestion in the stomach, should it escape crushing by the teeth in the act of mastication. Therefore it is that oats and barley are more nutritious crushed than whole; as if a portion misses the crushing action of the molars, it is almost certain to be voided unaltered from the animal. So it is, also, that the seeds of maize and beans are better for feeding when broken into pieces than when given whole. Even hay and straw may be rendered more digestible when cut up or "chopped" than when allowed whole, as it is then more easily mixed with the saliva and the gastric juice, and the animal is spared much of the fatigue of mastication. Some legumes even require to be boiled to render them digestible; and potatoes are better boiled or steamed than given raw, as in the latter state they are likely to give rise to intestinal irritation.
The condition of the horse has also a good deal to do with the digestion of food. A healthy, vigorous animal can digest much more food, and quicker, than a weakly one; and a sick horse may have its digestive powers seriously enfeebled, while hardship or fatigue may have a similar effect.