Page:Popular Science Monthly Volume 22.djvu/403

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will of course affect the percentage of total dry substance which is the sum of the figures given in the three preceding columns.

We may now consider the relations of the constituents of the food to the constituents stored up in the increase. In the experiments to determine the amount of food and of its several constituents, consumed by an animal of given weight within a given time, and required to produce a given increase in live weight, the foods presented a wide range of variation in composition, and the rations were so planned that the animals had a supply of ad libitum food containing more or less nitrogenous substance, that enabled them practically to fix for themselves the relative proportions of the nitrogenous and non-nitrogenous constituents consumed. In all of the feeding experiments it was found that the amount of food consumed by a given live weight of the animal, within a given time, and also the increase in live weight obtained from it, depended more upon the non-nitrogenous constituents, or even on the total dry substance, than upon the nitrogenous constituents, which had been generally assumed to be the true measure of nutritive value. In experiments with animals expending their energies in the form of work, the same demand for the non-nitrogenous constituents of the food was observed as in the case of fattening animals. A certain moderate amount of nitrogenous substances was evidently needed in the food, but any increase beyond this required quantity had no influence upon the returns obtained for food consumed, either in the form of muscular force in working animals, or in increase in live weight in those that were fattened, or even on the amount of nitrogen contained in such increase. The nitrogen discharged in the urine, in the form of urea, was found to have no relation to the activity of the muscles, but it was directly increased by an increment of nitrogenous materials in the food. The age and habits of the animals themselves, when growing or fattening, seemed, however, to determine, to some extent, the proportions of nitrogenous materials in the stored-up increase.

The average results show that oxen supplied liberally with food of good quality, containing a moderate proportion of grain or other concentrated food, would consume at the rate of from twelve to thirteen pounds of dry substance[1] per week for each hundred pounds of their weight, and that one pound of increase in live weight would be returned for it. Sheep, of several different breeds, consumed, on the average, about fifteen or sixteen pounds of dry substance of slightly better food per week for each hundred pounds of live weight, and returned about one pound of increase in weight for each nine pounds of dry substance in their food. Pigs, with food composed largely of

  1. Cattle-foods differ widely in the amount of contained water; the average being in hay from ten to fifteen per cent, in grain from eight to fifteen per cent, and in roots from seventy-five to ninety per cent. The strictly "dry substance," excluding this variable element of water contents, is therefore taken as a basis in estimating nutritive values.