30 SCIENTIFIC FEEDING OF ANIMALS 



sary for calculating the digestibility of both of the 

 foods are obtained. 



Investigations of this kind are known as avail- 

 ability determinations and are very important, 

 because they are the only means by which a know- 

 ledge of the way any given food is utilised by the 

 animal body can be obtained. All the efforts 

 which have been made to separate the digestible 

 portion of a food from the indigestible by chemical 

 methods have only been successful in the case of 

 crude protein. This has been done by artificial 

 digestion in acid gastric juice, and has served to 

 determine the total quantity of crude protein 

 which is digestible. No method, however, has yet 

 been devised for estimating the digestibility of 

 other constituents of the food nitrogen-free ex- 

 tract, crude fibre, fat, etc. Attempts have also 

 been made to calculate from the chemical com- 

 position what proportion of a food-stuff would be 

 digested, but without any great success. In the 

 case of fodder plants, harvested at the time of 

 flowering, and also with straw of various kinds, it has 

 been found that the total amount of nitrogen-free 

 extract in the food is in many cases equal to the 

 crude fibre and nitrogen-free extract substances 

 which have been digested. With young plants 

 this does not, however, hold good, for in that case 

 the figure obtained for the undigested portion is 

 too low. 



