Among the many important results obtained in the experiments at Rothamsted, the data furnished for estimating the relative value of barn-yard manures are of particular interest. The essential constituents of barn-yard manure, or those having a commercial value, are nitrogen, potash, and phosphoric acid, which are of course derived from the feed consumed by the animals making the manure. The composition of the food being known, the percentage of its several constituents that are voided in the manure may be estimated from the data obtained in these experiments, so that the relative value of the manure produced from different articles of food can be determined with sufficient accuracy for all practical purposes. From the results of the Rothamsted feeding experiments, we can not avoid the conclusion that the cereal grains and the highly nitrogenous linseed and cottonseed cakes have essentially the same value as fattening foods, and that there is but little, if any, difference in the feeding value of timothy and clover hay. When the production of manure is concerned, however, the clover has a much higher value than timothy, and the linseed and cotton-seed cakes are worth very much more than the cereal grains. That is to say, the digestible and available non-nitrogenous constituents of the food determine its nutritive value, provided always the moderate required supply of nitrogenous materials is present, and the comparative manurial value is determined by the nitrogenous constituents. A variety in the ration would undoubtedly be desirable for nutritive purposes, as the best results can not be obtained with any single article of diet.
From the facts already presented it appears that a large proportion of the increase of fattening animals, in many cases more than two thirds, is fat. It was formerly supposed that the fat of animals was derived from the fatty materials in the food, but this source was found to be entirely inadequate. The non-nitrogenous constituents of the food—the carbo-hydrates—were then quite naturally looked upon as the source from which the fat was elaborated; but afterward Professors Voit and Pettenkofer insisted that the fat of animals was almost exclusively formed from the nitrogenous constituents of the food. In experiments with pigs, which are evidently the most suitable animals for experiments relating to the formation of fat, Drs. Lawes and Gilbert conclusively show that, with foods in which the ratio of the nitrogenous to the non-nitrogenous constituents was a suitable one for fattening purposes—as in Indian corn and barley—a large proportion of the fat in the stored-up increase must have been produced at the expense of the non-nitrogenous constituents. There was also evidence that the nitrogenous constituents of the food, when in excess, might replace the carbo-hydrates, to some extent, in the formation of fat.
The analysis of the entire bodies and parts of the ten animals made at Rothamsted furnish some interesting data in regard to the compo-