Aside from an apparent slight inferiority of the scrub steer 

 as regards the mixed grain in the experiments of 1907, which arises 

 from an exceptionally low result in one of the trials, the foregoing 

 figures fail to indicate any material difference in the percentage 

 of the total energy of the feed which the two animals were able to 

 metabolize. 



Availability of Metabolizable Energy. 



The metabolizable energy of a feeding stuff is that portion of 

 its total potential energy which is capable of conversion into the 

 kinetic form in the body. It corresponds to the conception of 

 "Physiologischer Warmewert". or "Physiologischer Nutzeffeckt" in- 

 troduced by Rubner* in connection with his experiments upon the 

 replacement values of nutrients, or to the fuel values of Atwater**. 

 Upon the results of his earlier experiments, Rubner based his 

 celebrated law of isodynamic replacement. This law is in effect 

 that, when fed in limited quantities, the several nutrients may be 

 substituted as sources of energy for body substance previously 

 katabolized, or may replace each other, in amounts inversely pro- 

 portional to their physiological heat values, i. e., their metabolizable 

 energy. The validity of this law has been generally accepted and 

 not only have the relative values of single nutrients and of human 

 dietaries been estimated upon the basis of their metabolizable en- 

 ergy, but it has been attempted also to apply Rubner's factors to 

 the digestible protein, carbohydrates and fats, so-called, of stock 

 feeds and extensive tables of the fuel values of the latter computed 

 in this way have been published. Moreover, there has been a natural 

 tendency to overlook the limitation which Rubner set to his law 

 and to apply it to productive as well as to maintenance rations. 



It has been fully demonstrated, however, that only under 

 special conditions is the metabolizable energy, or fuel value, of a 

 nutrient or feeding stuff the measure of its value to the organism. 

 This was shown clearly by the early experiments of Zuntz and 

 his associates, which antedate Rubner's results, while later investi- 

 gations have fully confirmed the earlier ones, the difference being 

 especially marked in the case of herbivorous animals. Zuntz & 

 Hagemannt have demonstrated that this is the case with the horse. 



* Zeitschrift fur Biologie; 21, 250 and 337. 



**U. S. Dept. of Agr., Office of Experiment Stations, Bui. 21; Conn. 

 (Storrs) Experiment Station, 12th Report (1899), p. 71. 

 t T.andw. Jahrb., 18, 1; 23, 125; 27, Erganzbd. III. 



(8) 



