120 NUTRITION OF FARM ANIMALS 



pepsin-insoluble nitrogen of the feeds appears quantitatively 

 in the feces, where it may be regarded as representing indigesti- 

 ble feed protein, while the pepsin-soluble nitrogen of the feces 

 is contained in the excretory products, part of which are protein 

 (mucus, epithelium, etc.) and part non-protein (residues of 

 digestive fluids, etc.). An approximate correction for the 

 amount of nitrogenous excretory products may also be com- 

 puted by the use of Pfeiffer's factor of 0.4 gram nitrogen per 

 100 grams digested dry matter. 



167. Apparent digestibility. When the results of the or- 

 dinary digestion experiment are corrected, in the manner just 

 outlined, for the nitrogenous excretory products in the feces 

 we get an approximation to the true percentage digestibility 

 of the protein, while, as regards the carbohydrates, the error, 

 as has been shown, is probably not serious, at least for herbivora. 



There is another way of looking at the matter, however. 

 The intestinal products found in the feces are, in effect, part 

 of the cost of digesting the feed. They represent the " wear 

 and tear " of the digestive organs. The difference, then, be- 

 tween feed and feces will show the net gain to the animal from 

 the digestion of the feed, that is, it will show how much more 

 proteins, carbohydrates, etc., the body has at its disposal than 

 it would have had if the feed had not been given. From this 

 point of view, we may speak of the digestibility as ordinarily 

 determined as the apparent digestibility, and regard it as a 

 measure (approximately at least) of the matter gained by the 

 body from the consumption of the feed. For many purposes, 

 therefore, the apparent digestibility gives a better basis for com- 

 paring the values of feeding stuffs than does the real digestibility. 

 It was from this point of view that Atwater 1 proposed the use 

 of the term availability as the equivalent of what is here called 

 apparent digestibility. 



168. Composition of digested crude fiber. The crude 

 fiber (109) consists of the cellulose of the plant together with 

 varying amounts of pentosans and of lignin and other incrusting 

 substances, the ratio of which to the cellulose increases with 

 the maturity of the plant. Cellulose itself seems to be attacked 

 and dissolved with comparative ease by the organisms of the 

 rumen and the coecum, and the same is probably true of the 



1 Rpt. Conn. (Storrs) Expt. Sta., 1897, p. 156. 



