VALUATION OF FEEDING STUFFS 223 



is it stored up as body fat, like that in rations for increase. 

 Any excess of protein, beyond what is required for maintenance 

 and what appears as such in the milk, may be converted into 

 fat and carbohydrates. For this purpose it is approximately 

 equivalent to an equal weight of starch. The whole of the 

 protein, however, is included in the starch equivalent on this 

 basis, whereas the larger part of it is disposed of in other 

 ways. The requirements of the animals for milk production, 

 in terms of starch equivalent, are, therefore, purely empirical 

 numbers. 



The disadvantages of the system of starch equivalents are, 

 perhaps, most conspicuous in connection with its application 

 to the case of maintenance rations. The value of a food 

 for this purpose bears no direct relation to its fattening 

 power. It depends upon the total amount of thermic energy 

 obtainable from the food. The starch equivalent, however, 

 does not represent the whole of this energy. The amount 

 expended on the work of digestion is deducted because it 

 is not available for fattening. But it is all available for 

 maintenance ; it is ultimately transformed into heat within 

 the animal's body and helps to maintain its temperature. 

 Thus, the maintenance ration for oxen is given as 6 Ibs. starch 

 equivalent per 1000 Ibs. live weight. It was previously shown 

 that about 35 kt. of thermic energy are required. There is 

 an apparent discrepancy between the two statements, for 6 Ibs. 

 of pure starch yield, on oxidation, only 2 2 '6 kt. ; 9*3 Ibs. 

 would be required to produce 35 kt. The difference (3-3 Ibs. 

 of starch) represents the amount which, it is estimated, is 

 spent on the work of digestion. 



Since a certain amount is deducted in calculating the 

 starch equivalent from the rations, the same amount must 

 be added again when the process is reversed. In practice 

 the difficulty is overcome by artificially reducing the estimate 

 of the animal's requirements in terms of starch equivalent 

 as shown above. Thus the starch value of hay is 31 per 

 cent., i.e. 0*31 per lb., and as the maintenance requirements 

 are given 6 Ibs. of starch equivalent, the amount of hay 

 required is [6-^0-31=] 19*3 Ibs. (cf. p. 139). The result, 



