242 THE PRINCIPLES OF AGRICULTURE 



insufficient it will be used for the first four pur- 

 poses, approximately in the order named ; and 

 only after the needs of the animal are fully 

 supplied in these respects will food be used 

 for the last purpose. The food used for the 

 first four purposes is called food of support or 

 food of maintenance ; that used for the last 

 purpose is food of production. 



433. Not all of the food taken into the body 

 can be used by the animal. The digestive fluids 

 fail to act upon a part of the food, and this passes 

 out through the intestines as undigested solid 

 excrement. It is only the food which is di- 

 gested that is of use to the animal. 



434. The proportion of food digested varies 

 with the animal. One animal may digest 80 per 

 cent of the food eaten ; another, standing by its 

 side, equally healthy and equally vigorous and 

 of similar age, may digest less than 40 per 

 cent. 



435. The amount digested varies with the food 



and with the different constituents in the food. 

 Some foods are almost wholly digested; of others 

 less than one-fourth is digested. In any given 

 fodder, one constituent may be readily and 

 largely digestible, while another is digested only 

 with difficulty and in a small amount. In general, 

 of the food eaten only from one-half to two- 

 thirds is digested. 



