THE PRODUCTION VALUES OF FEEDING STUFFS 665 



Fries' experiments in which decreases of 8 to 10 per cent were observed 

 on rations varying in amount by from 42 to 186 per cent, although it 

 should be noted that in Armsby and Fries' later experiments only sub- 

 maintenance or moderate production rations were used. 



On the other hand, however, it was found in the latter experiments 

 that the losses of energy in the methane were distinctly greater on 

 the lighter rations so that the differences in the amount of feed con- 

 sumed, within the limits of these experiments, failed to show any 

 unmistakable effect upon the quantity of energy actually liberated 

 in the body from a unit weight of feed. Moreover, it must be borne 

 in mind that a considerable amount of the additional energy secured 

 by the more extensive fermentation of the lighter ration is liberated 

 in the digestive tract as heat of fermentation and does not enter into 

 the energy exchange of the body tissues, so that the difference in the 

 net nutritive effect is likely to be less than that in the metabolizable 

 energy as ordinarily denned. How far such a compensation would 

 occur in more liberal feeding is difficult at present to say. 



Influence on heat production. It is believed by some, however, 

 that, aside from differences in digestibility, etc., the metabolizable 

 energy actually derived from the feed is less efficiently utilized on 

 heavy than on light rations and by fat than by thin animals, i.e., 

 that a unit of metabolizable energy supplied yields less product. 

 This does not appear exactly probable, a priori. So far as the in- 

 creased heat production is due to mechanical work of digestion, it 

 would appear that it would be substantially proportional to the 

 amount of dry matter consumed, except possibly on extremely heavy 

 rations. So far as it is due to a stimulation of the body metabolism 

 by the digestive products resorbed (367 e) it would appear more 

 likely that, in accordance with the general laws of mass action, it 

 would be a diminishing function of the quantity present. Certain 

 authors, especially Grafe and Miiller, have, it is true, reported experi- 

 ments which are claimed to demonstrate a so-called "luxus consump- 

 tion" on heavy rations of carbohydrates, but their results scarcely 

 appear to the writer entirely conclusive. 



It has already been shown in Chapter X (450) that any heat 

 production arising from a synthesis of body substance, such as that 

 of fat from carbohydrates, for example, and which might be supposed 

 to result in a decreased efficiency of the feed energy on supermain- 

 tenance as compared with submaintenance rations, is apparently not 

 sufficient in amount as to materially affect the net energy values of 

 feeding stuffs. As regards cattle, the writer has elsewhere l discussed 

 the results of Kuhn's and Kellner's respiration experiments in their 

 bearing on this question, reaching the conclusion that their general 

 1 Principles of Animal Nutrition, pp. 466-471. 



