Il6 NUTRITION OF FARM ANIMALS 



The digestible matter contained in the total ration, computed 

 exactly as in the previous example, was as shown in the first part 

 of Table 21. If, now, it be assumed that the digestibility of the 

 clover hay was unaltered by the addition of the maize meal, it is 

 possible to compute how much of each kind of digestible matter (pro- 

 tein, crude fiber, nitrogen-free extract, etc.) in the total ration was 

 derived from the hay; the remainder, therefore, must have come 

 from the maize meal and by comparison with the total amounts 

 present in the latter the percentage digestibility is computed. 



It is evident that the determination of the digestibility of a con- 

 centrate in this way is less accurate than that of a feed which can be 

 given by itself. The assumption that the digestibility of the roughage 

 is not changed is unproved and probably not strictly correct. More- 

 over, any errors arising from this source and likewise all the errors in 

 weighing and analysis are, by the method of calculation, assigned to 

 the concentrate. The writer has shown 1 that the range of uncer- 

 tainty thus introduced may be very wide. It will evidently be 

 greatest when the proportion of concentrate to roughage is least and 

 will affect most those ingredients of the concentrate which are present 

 in the smallest proportion, such as crude fiber and often ether extract. 

 In extreme cases, absurd results are sometimes obtained, such as 

 negative digestibility or a digestibility greater than 100 per cent. 



162. Laboratory determination of digestibility. Actual 

 digestion experiments upon animals according to the method 

 just outlined, while simple in principle, require special facilities 

 and a considerable expenditure of time. It would obviously 

 be very desirable to possess methods by which the action of the 

 digestive fluids of the body could be imitated in the laboratory 

 and the digestibility of feeds thus determined in a simpler 

 and more expeditious manner. 



Numerous attempts have been made to solve this problem, 

 but as yet a satisfactory method has been worked out only 

 for protein, while attempts to devise similar methods for the 

 non-nitrogenous ingredients of feeding stuffs have not yet 

 proven successful. The method for protein is based upon 

 suggestions made long ago by Stockhardt and by Hofmeister, 

 but was first put into practical form by Stutzer. 2 It consists 

 in treating the feed with a solution of pepsin and hydrochloric 

 acid under specified conditions and determining the undissolved 

 nitrogen in the residue. The difference between this and the 



1 Amer. Jour. Sci., 29 (1885), 35-5- 2 Jour. Landw., 28 (1880), 195 and 435. 



