156 THE CHEMISTRY OF CATTLE FEEDING 



Dynamic Value of Food. The energy which is expended 

 on internal work cannot be used over again to perform 

 external work. The static value, i.e. the amount of available 

 energy equivalent to unit mass of the food, therefore, affords 

 no true indication of the amount of external work that can 

 be derived from such food. It is only the balance or surplus 

 energy which remains after deducting what is consumed in 

 the work of digestion that can be used for the latter purpose. 

 This balance is called dynamic energy. The dynamic value 

 of a food is the number of kt. of dynamic energy that can be 

 obtained from unit mass (i Ib.) of the food. It is, in short, 

 the static value minus what is consumed in the work of 

 digestion. 



The dynamic value of linseed cake and all the more readily 

 digestible concentrated foods is practically the same as the 

 static value, because little or none is used in the work of 

 digestion. The dynamic value of hay is from 30 to 40 per 

 cent, less than the static value of that substance ; that of straw 

 is from 70 to 80 per cent. less. 



In the earlier parts of this chapter it was pointed out that 

 a certain amount of food is required for maintenance of the 

 animal, and that an additional quantity must be given sufficient 

 to provide the amount of available energy equivalent to the 

 work done. It will now be clear that this was intended to 

 signify energy available for work, i.e. dynamic energy. 



It has been said that all the energy expended in doing 

 work of any kind whatever is ultimately transformed into heat. 

 When the transformation takes place outside the animal's 

 body as it does when the work is external the heat pro- 

 duced is entirely lost to the animal. But the energy expended 

 in the performance of internal work is transformed into 

 heat within the body, and therefore helps to maintain its 

 temperature. 



The heat which is given off by radiation (p. 126) consists, 

 in part, of the energy which has been used for the work 

 of circulation, respiration, and other vital functions which 

 never cease during life. In short, the energy required for 

 these functions is included in the maintenance rations. 



