EXPERIMENT STATION BULLETINS. 511 



the above is true, and further that the amount of energy expended in chewing 

 and preparing food material for solution in the body is much different with dif- 

 ferent foods. It must also be clear that such factors as crude fibre in a ration 

 have much influence on this point.' 



A food then is of value to the animal, not in proportion to the gross amount 

 of nutrients contained therein, but to the energy available after deducting from 

 the digested portion the amount which has been expended in the various physio- 

 chemical processes involved in making that food an intergral part of the animal 

 body. It must be true also that a digestion experiment, in which the difference 

 between the gross amounts of nutrients in the food and in the dried feces is 

 taken as a measure of the value of that food, is not wholly reliable because the 

 expense to the animal of the digestion and absorption of that food is not clearly 

 shown.- 



It has been the custom, heretofore, to take the difference between the amount 

 of nutrients in the food and the dry feces as being the amount of nutrients in 

 the food digested and utilized by the body. This method left out of considera- 

 tion two quite important factors. A not inconsiderable amount of nitrogen is lost 

 by even the air drying of the feces and hence the computation of digestibility on 

 the basis of what nutrients remain in the dry feces places the figures for digesti- 

 bility too high. Again, there has been shown to be a considerable amount of 

 nitrogenous matter in the feces remaining after drying and bearing but an indirect 

 relation to the food consumed. This nitrogen, together with the nitrogen lost 

 on drying, may be considered a waste product from the juices and linings of 

 the intestinal canal, and from the various body fluids, and represents in part the 

 cost of the digestion of that particular food. This is called nitrogen of metabolism. 

 The effect on the figures of digestibility of the nitrogen of metabolism in the dry 

 matter of the feces is to lower the apparent per cent of digestible nitrogen in the 

 food. The fact, however, that the nitrogen of metabolism in the dry matter of 

 the feces has been taken into consideration in establishing our ordinary coeflBcients 

 of digestibility while the nitrogen lost on drying has been almost entirely ne- 

 glected, makes clear that the factors of digestibility as formerly determined are 

 too high by just the amount of metabolic nitrogen lost in the drying of the feces. » 



"We take note, then, of net available energy, or that remaining after subtract- 

 ing from the total energy of the digested food the energy due to metabolism or 

 that which constitutes in part the cost of the digestion of the given food to the 

 animal, plus the energy of the real undigested food. It is somewhat evident, 

 it is true, that while the separation of the nitrogen of the feces into nitrogen 

 of metabolism and nitrogen of undigested food gives a clearer index to the actual 

 per cent of digestibility of the food, at the same time where no such separation 

 is made the figures of digestibility may be said to represent in part the net 

 availability of that food, provided the nitrogen lost on drying of the feces is 

 taken into account.* However, this permits of no comparison of different foods, 

 for even though they apparently contain the same amount of digestible matter the 

 cost of the digestion of one may be considerable more than of the other, and 

 there is no indication in such results that would permit of the economic com- 

 parison of the two diiferent feeds. 



It has been demonstrateds that the burning of tissue in the body and that the 

 liberation of energy to promote the work of digestion and assimilation varies in 

 different foods, and foods dlfflcuU of digestion require the consumption of more 

 body tissue in the effort to digest them. If it be true that an increase of cell 

 oxidation is accompanied in food digestion by an increase in the metabolic products 

 in the feces, then a knowledge of the amount of these metabolic products in the 

 feces is an indication of the real value of the food not otherwise indicated. It 



1. It is a -well known fact that foods rich in cnide 5ber or roughage material are subjected to more prolonged action 

 in the gastro-intestinal canal of ruminants than are the concentrates with a small amount of crude fiber. In fact ruminat- 

 ing animals, if fed wlioUy on concentrates lose the power of iniminating the food which, of course, indicates that the period of 

 contact with the body fluids is shortened. 



2. Nearly all the present digestion coefficients are based on the dry matter of the feces without attempt to separate 

 tte various products present in the feces a considerable amount of which be;ir no direct relation to the food consumed. 



3. See Penna. Report, July 1, 1900— June 30. 1901. W. W. Cooke, pp 273—274. It seems quite evident from our 

 experiments that the amount of metabolic nitrogen excreted does not depend upon the dry matter of the food but upon 

 other factors in the food. 



4. Ibid, page 274. It is assuredly unsafe to assume that the nitrogen lost on the drying of the feces is sufficient to 

 balance or counteract ttie influence of the nitrogen of met;ibolism remaining after drying, it does not vary with the latter 

 and therefore if we desire to establish the approximate net availability of a food the results are too high unless we take into 

 consideration the nitrogen lost on drying which is sometimes considerable and again may be of little note. 



5. See Armsby's Principals of Animal Nutrition, "comparison of hay and grain." 



