544 NUTRITION OF FARM ANIMALS 



work. Aside from the small quantities of fat and glycogen con- 

 tained in the meat the energy for work under these conditions 

 could have been derived only from the proteins or their cleavage 

 products. These results show clearly that protein may be used 

 to a large extent as a source of muscular energy, but it is never- 

 theless true that under ordinary conditions, and particularly 

 with farm animals, the main supply of energy is, as already 

 stated, through the non-nitrogenous ingredients of the feed. 



It is by no means impossible, however, that a certain amount 

 of protein katabolism may be necessary in a muscular contrac- 

 tion. Such a contraction is a function of the protoplasm of 

 the muscle fibers and it is conceivable that a portion of the 

 energy arising from the katabolism of the proteins and nucleo- 

 proteins of the muscle and ordinarily appearing as heat in the 

 resting muscle may be switched off, so to speak, to aid in pro- 

 ducing the contraction. In other words, it is possible that a 

 certain level of protein metabolism may be necessary in order 

 to maintain the most favorable conditions for transforming 

 the potential energy of non-nitrogenous materials into work. 1 

 Such a fact would, of course, have an important bearing upon 

 the amount of protein required for a working animal, but at pres- 

 ent the matter belongs in the realm of speculation. 



2. THE EFFICIENCY OF THE BODY AS A MOTOR 



General results 



646. Body substance is immediate source of energy. 



While the energy expended in work production is of course de- 

 rived ultimately from the feed consumed, its immediate source, 

 as stated in i (630), is the katabolism of body substance, and 

 an animal may perform a considerable amount of labor in the 

 fasting state at the expense of stored-up material. It will aid 

 in the discussion of the somewhat complicated question of the 

 efficiency of the animal as a prime motor to consider first the 

 efficiency with which the body utilizes this stored-up energy, 

 i.e., to inquire what percentage of the total energy of the body 

 material katabolized for work production is recovered in the 



1 Compare Armsby, Principles of Animal Nutrition, pp. 207-209. 



