174 The Feeding of Animals 



variation for the same class is seen not to be large, but 

 whether the animal derives energy in similar propor- 

 tions must be decided by experimental evidence. 



In recent j^ears much attention has been given ex- 

 perimentally to the physiological values of the nutrients. 

 Among the most painstaking and extensive investiga- 

 tions of this sort are those conducted at Mockern by 

 Kellner and his associates. This work includes forty- 

 four metabolism experiments, each of fourteen days' 

 duration, and one hundred and eighty -four respiration 

 experiments, each of twenty-four hours' duration. In 

 order to secure the desired data, there was added to a 

 basal ration gluten, oil, potato starch, extracted straw 

 (mostly cellulose freed from incrusting and accom- 

 panying compounds), meadow hay, oat straw, and well- 

 ripened wheat straw. From the results obtained, 

 through exact measurements of the ingested food, the 

 excreta and the products of respiration, — thus making 

 it possible to determine the relation of each substance to 

 the maintenance of the animal and to the storage of 

 flesh and fat, — Kellner worked out both the energy and 

 the production values of the experimental materials. 

 While the figures given should not be regarded as 

 final, they have behind them so much careful and severe 

 investigation that they must be accepted as having 

 great weight. They at least correctly record what 

 happened with particular animals. 



In presenting these results a distinction is made 

 between available energj^ value and production or net 

 value. It is the former which interests us at this point, 

 and it is this which is shown in the following figures: 



