Il8 NUTRITION OF FARM ANIMALS 



total nitrogen of the feed represents, of course, the amount of 

 nitrogenous matter which has been dissolved and which, there- 

 fore, is regarded as digestible. 



Comparisons by Kellner, 1 Pfeiffer, 2 G. Kiihn 3 and others between 

 the natural and artificial digestion of protein have shown that the 

 former method gives lower results on account of the presence in the 

 feces of nitrogenous excretory products (154, 158), but that when a 

 correction is made for the latter in the manner indicated on a subse- 

 quent page (166) the results of the two methods show a substan- 

 tial agreement. In other words, the method of artificial digestion 

 shows with a good degree of accuracy the true as compared with the 

 apparent digestibility (163, 167) of the protein. 



163. Influence of excretory products on apparent digesti- 

 bility. Since the digestion experiment as ordinarily conducted 

 ignores the presence in the feces of excretory products, the re- 

 sults obtained by its use will necessarily be too low, since sub- 

 stances are reckoned as undigested ingredients which really are 

 not such. Obviously, the ingredients most affected by this 

 error will be those which, on the one hand, are contained in 

 the feed in the smallest proportion and which, on the other 

 hand, are relatively most abundant among the excretory 

 products in the feces. These ingredients are, when the or- 

 dinary scheme of feeding stuffs analysis is followed, ash, ether 

 extract and 'nitrogenous substances. As regards the crude 

 fiber, on the other hand, this error is absent, since obviously 

 no crude fiber is included among the excretory products, and it 

 seems probable that substantially the same thing is true of 

 the nitrogen-free extract. 



164. Digestibility of ash ingredients. Certain ash in- 

 gredients, particularly iron, calcium, magnesium and phos- 

 phorus, are largely or wholly excreted from the body in the 

 feces (199). Furthermore, the resorption of the ash ingredients 

 of the digestive juices may not be complete and these residues 

 may be added to the ash content of the feces. The ordinary 

 digestion experiment, therefore, affords little information as to 

 the extent to which the ash ingredients of the feed are actually 



1 Centbl. Agr. Chem., 9 (1880), 763. 



2 Jour. Landw., 33 (1885), 149; 34 (1886), 425. 

 "Landw. Vers. Stat., 44 (1894), 188. 



