THE PRODUCTION VALUES OF FEEDING STUFFS 645 



750. Significance of metabolizable energy. By metab- 

 olizable energy, as already explained, is meant simply the 

 energy capable of transformation in the body, with no impli- 

 cation as to the proportion of the energy thus transformed 

 which can be utilized by the organism. The heat evolved 

 during the methane fermentation, for example, constitutes part 

 of the metabolizable energy as thus denned, although it does 

 not enter into the tissue metabolism. 



The metabolizable energy of a feeding stuff does not meas- 

 ure its production value, since it takes account of only one of 

 the two classes of losses to which its chemical energy is sub- 

 ject. Obviously, however, it is an essential factor in fixing 

 that value, since frequently from one-fourth to one-half or more 

 of the feed energy is thus rejected unused. The determination 

 or estimation of the metabolizable energy of a feeding stuff is, 

 therefore, an important step in ascertaining its production 

 value as regards energy, and constitutes an advance over the 

 simple determination of digestibility, since it takes account of 

 the losses in urine and methane as well as of those in the feces. 



751. Real and apparent metabolizable energy. The metab- 

 olizable energy of a feeding stuff as determined experimentally 

 in the manner illustrated in a preceding paragraph (747) is the 

 aggregate effect as regards energy of all the influences which the 

 feeding stuff exerts on the digestive processes. 



For example, in one of Kellner's experiments' beet molasses 

 added to a basal ration diminished the amount of energy car- 

 ried off in the methane by 135.8 Cals., while at the same time 

 it so depressed the digestibility of the basal ration that the 

 amounts lost in the feces and urine were increased by 1865.9 

 Cals. and 272.3 Cals. respectively. By the method of com- 

 putation here used, the algebraic sum of these amounts is vir- 

 tually regarded as representing the losses of energy from the 

 molasses and is subtracted from the gross energy of the latter 

 to obtain its metabolizable energy. The metabolizable energy 

 as thus computed expresses the net increase in the amount of 

 energy available for conversion in the body and may be called 

 the apparent metabolizable energy. 



On the other hand, the results for the metabolizable energy 

 of the digestible nutrients recorded in the next paragraph in- 

 clude corrections for these secondary effects. They aim to show 



