562 NUTRITION OF FARM ANIMALS 



669. Ash requirements. As pointed out in previous chap- 

 ters, the ash requirements of an animal deserve greater con- 

 sideration than they generally receive, while in the case of 

 growth, at least, the presence of certain accessory substances 

 in the ration is necessary. So far as the working horse is con- 

 cerned, however, no sufficient data seem available for a dis- 

 cussion of these topics. 



The energy requirement 



670. Economic, or over-all efficiency. In the preceding 

 section certain comparisons were made between the efficiency of 

 the animal body as a prime motor and that of artificial engines. 

 The animal body, however, is not only a prime motor but in- 

 cludes also the furnace in which the fuel is burned and resembles 

 in this respect a complete power plant, such as a locomotive, 

 for example, rather than an engine. 



Just as the energy of the fuel of a locomotive is subject to 

 certain losses due to incomplete combustion and to radiation of 

 heat before the steam reaches the cylinders, so portions of the 

 energy of the feed escape in the excreta or are expended in the 

 various processes incident to the formation in the body of those 

 substances whose katabolism yields the energy for a muscular 

 contraction. 



Since these losses and expenditures are largely unavoidable, 

 they constitute part of the energy requirements of the work 

 animal, and from the economic point of view the efficiency of 

 the animal is measured by a comparison of the total feed energy 

 with the work done. This might be called the economic effi- 

 ciency, comparable to the over-all efficiency of a steam plant as 

 computed by a comparison of coal consumption with the brake 

 horse power obtained. Any such comparisons, of course, 

 must take account of the maintenance requirement of the 

 animal when doing no work and must therefore be made on the 

 basis of the 24-hour output of work (650). 



Few satisfactory direct determinations of the economic effi- 

 ciency of work animals in the foregoing sense, i.e., of the re- 

 lation of the work done to the feed (or total feed energy) required 

 for its performance, have yet been reported. 



