The digestibility of a food is its capability of being absorbed in the system.

According to Muntz and Grandeau, the respective percentages of digestible material contained in the undermentioned foods is as follow : -

Despite the theoretical correctness of the above table, we know from experience that oats, especially in large quantities, agree with a horse much better than barley or beans, both of which are put down as being more digestible than oats.

This apparent anomaly is explained by the fact that the wholesomeness of a food for horses is largely dependent on the respective percentages of fibre (p. 65) and nitrogenous matter (p. 63) contained in it. Oats are rich in fibre, which, though practically indigestible, greatly aids the digestion of the nutritive part of that grain, and prevents digestive disturbance. Also, oats do not contain, like beans, too high a percentage of nitrogenous matter, which, when consumed in excess, has a poisonous effect on the system (p. 122). Thus we find that when the quantity is not limited, a horse can obtain a larger amount of nutritious material from oats, without his digestion becoming upset, than from either barley or beans. We may therefore see that digestibility and wholesomeness, as applied to foods, are not convertible terms; and that a comparatively indigestible food may be an eminently wholesome one. What a horse, like every other animal, requires, is a wholesome food which contains a sufficiency, but not an excess, of easily digestible nutritive matter, the respective constituents of which are in proportions suitable to his requirements.

As the wholesomeness of a food cannot be exactly measured, we have in estimating it to rely more on practical observation, than on chemical experiment. In this respect, the state of the dung is a valuable guide. In ordinary conditions of health, the dung, without being hard, will be fairly formed, brittle, though moderately damp and free from foul odour and adherent slime, the presence of which will indicate irritation of the intestines. A loose and unformed state of the dung will show either that the mechanical nature of the food is unsuitable (being irritating or too watery), or that the absorption of some of its constituents had produced a purgative effect. During constipation, the dung of a horse becomes hard and dry from its being kept too long in the lower part of the large intestine, and it assumes a consistency somewhat resembling that of the dung of goats. Also, from undue retention, the residue of the food which is in the' intestines is liable to become decomposed, with the result that the animal's health suffers more or less from the absorption of deleterious products. The bad effect of diarrhoea has already been discussed.

It is interesting to note that the condition of the dung is by no means uniform in all animals. Thus, as compared to horses, dogs are in a constant state of constipation; and horned cattle, in one of diarrhoea.