212 FOOD REQUIRED [chap. 



during which the animal is under investigation, the 

 composition of the food is known, the composition of 

 the excreta is determined, and also the amount of 

 carbon dioxide and methane given off by the animal. 

 The difference between the carbon in the food and the 

 excreta will represent the carbon stored up in the body 

 as fat and lean meat, and as the amount of nitrogen 

 stored is similarly determined, we can further calculate 

 how much of the carbon has been stored in the form 

 of fat. From this kind of experiment it has been 

 determined that of i lb. of pure digested fat about \ lb. 

 is stored in the body, supposing that the fat is given in 

 addition to a maintenance ration and none of it is 

 needed by the animal for general purposes. The 

 proportion varies somewhat with the nature of the fat, 

 the range being from about 47 per cent to about 60 

 per cent, as a maximum. Of pure digested protein 

 about 23 per cent, could be stored as fat. Digested 

 starch and crude fibre can give rise to about 25 per cent, 

 of their weight as fat, while sugar gives rise to less than 

 19 per cent These proportions, however, refer only to 

 the pure food constituents in their digested form. In 

 many feeding stuffs the digestible constituents do not 

 attain these full values, a large and varying proportion 

 being spent in the work of digestion. Roughly speaking, 

 in the various cakes and meals the digestible constituents 

 possess their full value for making fat, such as is shown 

 by the pure constituents themselves. It is in the case 

 of the coarse fodders, such as bran, hay, and roots like 

 mangolds, that a deduction must be made from the full 

 value to express the value to the animal in producing 

 increase. We can, in fact, attach to each feeding stuff 

 which has been under investigation a factor showing the 

 percentage of their full value which the digestible 

 constituents will possess in that particular feeding stuff. 



