154 THE CHEMISTRY OF CATTLE FEEDING 



Whatever the amount of work may be, the amount of food 

 should be at least roughly proportional. The actual amount of 

 energy required for any given output of work is easily calculated 

 by the method described above, and a quantity of suitable food 

 sufficient to yield that amount of energy must be added to the 

 previously determined maintenance ration or the animal will 

 lose weight. 



Internal Work. Work such as trotting, hauling, etc., 

 which alone has so far been considered, is commonly referred 

 to as external work, to distinguish it from the work of circula- 

 tion, respiration, digestion, etc., which is done within the 

 animal's body. The distinction is of considerable importance 

 in its bearing upon the practical question of feeding the 

 animals. 



The amount of work done in the internal economy of the 

 animal has been the subject of numerous investigations. 

 That it is not inconsiderable may be gathered from the fact 

 that the work of circulation alone, i.e. the work done by the 

 heart in pumping the blood through the body, is estimated at 

 some 15,000 foot-pounds per hour in the case of men, and about 

 65,000 foot-pounds per hour equal to 3*4 kt. per day in 

 the case of oxen. The work of circulation and respiration is 

 practically constant for any given animal when at rest, but it 

 is increased when the animal does external work. In any 

 case it is quite independent of the nature of the food, pro- 

 vided the same is sufficient for all the vital functions. 



The work of mastication and digestion stands in a very 

 different relation. It is not directly affected by the conditions 

 so far as work or rest is concerned, but it depends upon the 

 quantity and quality of the food consumed. An animal when 

 doing hard work may consume twice as much food as it does 

 when at rest ; obviously, then, if the food is of the same kind, 

 the work of digestion will be doubled. Some kinds of foods 

 are much more easily digested than others. A great deal 

 more energy is expended by animals in digesting hard fibrous 

 materials such as hay and straw than is required in the case of 

 cakes, grains, and other more readily digestible substances. 

 This energy is, of course, derived from the food. This is 



