FOOD REQUIREMENTS 379 



In calculating the ration " true protein " is taken as 

 " digestible protein." Separate tables of food values are 

 given for ruminants, horses, and swine. 



Armsby's system is less open to objection than Kellner's. 

 If the net energy values which he assigns to the different 

 feeding stuffs be used only in conjunction with his feeding 

 standards no error is likely to arise, since the requirements 

 are estimated in accordance with, and stated in terms of, 

 these values. 



While a unit of productive value enables the feeder 

 to compare different food - stuffs on a common basis, 

 its use is apt to focus attention too exclusively on 

 the food to the neglect of other factors that affect the 

 productive value of rations. Two of these may be briefly 

 indicated. 



(1) For maintenance, the energy value for food depends 

 upon the energy that can be liberated by the animal, and, 

 consequently, foods can be compared according to their 

 caloric values (p. 257). For productive purposes, however, 

 the energy value of different foods cannot be compared on 

 the basis of any common unit. For example, in fattening, 

 after the maintenance requirements are satisfied, of the surplus 

 food absorbed, about 87-5 per cent, of the energy of the 

 starch can be transformed to energy in deposited fat, while 

 only a doubtful proportion, certainly less than 50 per cent, 

 of the energy of the surplus protein, can be so transformed. 

 On the other hand, if protein tissue be the form of produc- 

 tion, as in growth, nearly 100 per cent, of the energy of the 

 surplus protein may be deposited in the growing tissue, 

 whereas for tissue formation starch has only an indirect 

 value as a protein saver. 



(2) In both Kellner's and Armsby's systems, in estimating 

 the productive value, the heat liberated in increased meta- 

 bolism due to taking food is regarded as waste and deducted. 

 Whether or not it is waste depends upon the external tem- 

 perature. Below the critical temperature (p. 271), it serves to 

 maintain body temperature, and so saves food of an equiva- 



