The same four groups of substances found in animal bodies, viz.: water, ash, fat and protein, are also found in the food they consume and, in addition, the food of herbivorous animals contains a class called carbohydrates.
All foodstuffs, no matter how dry they may seem, contain a considerable amount of water. In grains and dried fodders it ranges from 8 to 15 per cent of the material, in green forage and silage it is about 80 per cent, while in some roots it amounts to 90 per cent. While water is essential to animal life and the water in the food fulfils the same function as that drunk by the animal, we do not value food materials for the water they contain, and computations are based upon the water-free or dry matter.
When a foodstuff is burned till the organic matter is all driven off, the residue is the ash. It is composed largely of lime, magnesia, potash, soda, iron, chlorine, and carbonic, sulfuric and phosphoric acids. The ash of the food is the source of the mineral matter of the animal body, and as such is of great importance. Ordinary combinations of feeding stuffs, however, contain an abundant supply of mineral matter for the use of the animal, so it is not a matter of practical concern except as it has a bearing on the mineral elements of fertility in the manure.
This group embraces the materials which may be dissolved from a feeding stuff by ether. It includes, besides the true fats, wax and coloring matter. Fat in the food may be either stored in the body as fat, or burned to produce heat and energy.
This term includes two groups, nitrogen-free extract, such as starch, sugar, gum, etc., and fiber, or the woody parts of plants. The former are quite freely digested, the latter much less so, though fulfilling the same function to the extent it is digested. The carbohydrates constitute the largest part of vegetable foods. They are not stored in the animal body as such, but are converted into fat or used (burned) to produce heat and energy.
Since the carbohydrates and fat serve nearly the same purpose in the animal economy, they may, for convenience, be grouped together. Experiments, however, have shown that fat is about 2 1/4 times as effective as a food as are the carbohydrates. Hence it is customary to multiply the amount of fat by 2 1/4 to reduce it to a "starch equivalent" before adding it to the amount of the carbohydrates.
The protein of foods, like that of the animal body, is characterized by containing nitrogen. It, therefore, is frequently termed "nitrogenous matter." The term albumenoids is sometimes used to designate this group, though it more correctly implies a certain class of protein substances. The function of protein in the food is, first of all, to build up and repair the working machinery of the body, and to supply protein for the production of milk, wool, etc. No other food constituent can fulfil this function.
The importance of a sufficient supply of protein in the ration, is, therefore, apparent. If in excess of the amount required to build up and repair the waste of the body, the protein may be converted into fat and deposited as such or used to produce heat and energy. Its efficiency for these purposes is about the same as the carbohydrates, but as it is usually far more expensive to supply than the carbohydrates, economy would dictate that only so much should be supplied to the animal as will suffice to repair the wastes of the animal machinery and build up new growth in case of growing animals, or for the production of milk, wool, etc.