For ordinary purposes the horse-owner will be content to refer to what has already been stated for the purpose of deciding what article of diet will be most useful and economical, but the German and French investigators have not been content with this general knowledge, and we are indebted to them for a number of interesting and important experiments, the outcome of which is to enable the curious in such matters to calculate with almost mathematical accuracy the exact relation which the food bears to its digestibility and to the waste of the system.

It appears that the power of assimilation in different animals varies very considerably in reference to the various constituents of food; thus a horse will digest, out of every 100 parts of mixed diet, 69 parts of albuminoids, 59 of fatty matters, 68 of the carbohydrates (starch, sugar, and gum), and 33 of cellulose and fibre. The annexed table will show the difference in these respects of the digestive powers of the ox, cow, and sheep: -

A further analysis indicates that the above proximate principles are differently digested in different articles of food, as shown by the table on p. 97, which relates to the digestibility of the proximate principles of different kinds of food by horses.

Various circumstances appear to modify the digestibility of different articles of diet. Age and mode of growth and preparation are among the modifying influences. Young plants are more digestible than mature ones, and the digestibility of old hay is less than that of new. It is also stated by the authorities which have been referred to that the digestibility of food is not affected by the amount which the animal consumes, neither is it altered by the amount of labour which the animal performs. According to Wolff, however, the addition of one food will alter the digestibility of another. Thus starch or sugar added to a diet of hay or straw in a larger amount than 10 per cent lessens its digestibility, especially in regard to the albuminoids which the food contains.

Digestibility Of Food-Stuffs, Showing The Proportion Digested For 100 Supplied

Oil in small quantities appears to increase the digestibility of hay and straw, but a large amount causes loss of appetite. Potatoes, owing to the amount of starch they contain, prevent the digestion of hay. Roots have a less depressing effect, owing to the sugar which they contain.

Col. Smith remarks that in some of his own experiments the addition of 2 lb. of oats to 12 lb. of hay increased its digestibility by more than 9 per cent. He adds that, in calculating a diet to ascertain its suitability for an animal, it is necessary to obtain from the tables given a proportion of principles digested, and then to find out the nitrogenous, fatty, and complete nutritive ratios. In illustration of this method of calculation, it is assumed that a horse receives as a daily ration 12 lb. of hay of medium quality and 10 lb. of oats. It is required to know the ratio of this diet before and after digestion, and the proportion of proximate principles digested.

This information is conveyed in the following table, referring to the constituents of hay and oats. It will be seen that the fat in the hay is imperfectly digested, the fatty ratio falling from 1:4:1 to 1:12 after digestion. Otherwise the nutritive value of the food is raised, as the cellulose is excluded from the calculation.

In the above table it is clearly demonstrated that only a certain amount of the digestible matter is appropriated by the animal's system; thus the total diet contained: albuminoids 218 lb., fats .84 lb., carbohydrates 10.61 lb., cellulose 4.50 lb.; while the system of the horse only appropriated: albuminoids 1'6 lb., fat '5 lb., carbohydrates 6.9 lb., and cellulose 1.5 lb. In other words, only 10.5 lb. out of the total of 18.13 lb. was assimilated.

Comparing the feeding values of different foods, the method of establishing a nutritive equivalent is adopted. Taking one typical food as represented by 100, every food containing the same amount of nitrogen as the typical food is considered to be equal to it, i.e. 100. A food, however, which contains twice the amount of nitrogen would have for its nutritive equivalent 50, because half the amount would be equal to the typical food; but if it possessed half the amount of nitrogen, its equivalent would be 200, as it would take twice as much food to contain the amount of the typical food.

Hay is taken as the standard of comparison, and the following table by Boussingault gives the relative values of the different articles of food in comparison with hay: -

In reading the table, the numbers must be taken to indicate the number of parts of each article which will represent the 100 parts of hay, i.e. 45 parts of wheat, 30 of linseed, 350 of barley straw, and so forth, are equivalent to 100 parts of hay. However interesting this information may be, its true value can only be realized by taking it in connection with the fact that the animal's temperament and digestive capacity have a dominant concern in the appropriation of the different articles of diet. It is quite conceivable that the table might be found absolutely correct for a certain number of horses, while in an equal or possibly greater number of animals of different constitution, and under different circumstances, the nutritive equivalents given would have to be considerably modified.

All the elaborate experiments which have been performed by a large number of investigators have been mainly directed to the important object of discovering the influence of different foods in producing heat and force, or working power. The amount of heat evolved from the oxidation of certain foods is readily converted into its equivalent of mechanical energy. It was found by Frankland that when a dried food mixed with a powerful oxidizing substance, such as chlorate of potash, was placed in an apparatus surrounded with water, and burned, the heat developed raised the temperature of the water. 1 gramme (15.432 grains) when raised 1° C. (1'8° Fahr.) is called a heat unit. 1 lb. of albumen when oxidized raises the temperature of 4263 lb. of water 1.8° (Fahr.), while 1 lb. of fat raises the temperature of 9069 lb. of water to the same extent (1.8° Fahr.). Joule of Manchester demonstrated that the heat required to raise the temperature of 1 lb. of water 1° (Fahr.) was equivalent to the power required to raise the weight of 1 lb. 772 feet high. The foot-pound is, therefore, the unit of work, and 772 ft.-pounds is the mechanical equivalent of 1° (Fahr.). Knowing the number of heat units each substance is capable of producing, its potential or mechanical value may be calculated; and the author of Veterinary Hygiene suggests that the most simple way of doing this is to multiply the actual amount of food digested by certain numbers which represent the comparative heat-forming value of fat, albumen, and starch.