UTILIZATION OF FOOD 427 



Significance of the Pjoductive Value. We have seen that a 

 portion of the value of a food is lost in undigested material, a 

 part as marsh gas, or oxidized in fermentation, a part in the in- 

 completely oxidized material of the urine, and a portion is used 

 for digestion and other processes fitting it for the use of animal. 

 What remains of the food after these losses are deducted, may be 

 used for maintenance, work, fat, flesh, milk, etc. It represents 

 the net value of the food to the animal. It also corresponds to 

 what we elsewhere termed the kinetic energy of the food. 



The productive value of a food is the best measure so far 

 devised for the net value of a food. Rations have heretofore 

 been calculated on the assumption that all digestible nutrients of 

 the same group have the same value to the animal, regardless of 

 the origin of the material. We now know, however, that the net 

 value of a food may vary widely from its value based entirely on 

 digestible nutrients, so that the value of a food for the purpose 

 of producing energy is best measured by its productive value. 



It is quite possible that the kinetic energy of different feeds 

 undergo somewhat different losses when transformed into fat, 

 so that the quantity of fat produced may not be the most exact 

 possible measure of the net values of feeds. The energy used 

 in digestion and given off as heat may also prove useful under 

 certain circumstances, such as with an animal on a maintenance 

 ration in cold weather. 



While the fat values of feeding-stuffs probably represent their 

 comparative values for fattening purposes, and perhaps for milk, 

 it does not follow that they represent the values of the feeds for 

 productive work and for maintenance of the animal. The con- 

 version of proteids, etc., into fat undoubtedly consumes energy, 

 and a greater quantity of energy may be required to convert the 

 proteids of one feeding-stuff into fat, than those of another; 

 whereas if the kinetic energy is used directly for work or main- 

 tenance, these proteids might be equal in value for these pur- 

 poses. We have seen that a feeding-stuff possesses both kinetic 

 and thermal energy, and that the thermal energy may be used to 

 keep the animal warm. While the thermal energy fed to an 



