602 NUTRITION OF FARM ANIMALS 



tor in determining the actual production values discussed in the 

 next chapter, since, as there shown (742), the excretion in the 

 feces constituted the greatest, although not the only, loss of 

 chemical energy suffered by the feed. 



The percentage digestibility of a feeding stuff or of its several 

 constituents, however, has not a fixed and invariable value, 

 analogous to the solubility of a chemical compound, but may be 

 affected more or less by a variety of conditions, although to a 

 less extent than is frequently supposed. This arises from the 

 fact noted in Chapter III (155) that portions of ingredients 

 capable per se of solution and resorption in the digestive tract 

 actually escape digestion for various reasons and reappear in 

 the feces. Any conditions which influence the digestibility in 

 this way, however, necessarily affect the value of the feeding 

 stuff by whichever method determined, and the more impor- 

 tant of them may be conveniently considered in this connection. 



The conditions which affect, or which are supposed to affect, 

 the degree of completeness with which the potentially digestible 

 ingredients of a feeding stuff are actually digested may be di- 

 vided into those relating to the animal itself and those relating 

 to the feed. 



Conditions relating to the animal 



712. Variation at different times. An important fact, 

 which must be borne in mind in studying the influences of 

 various factors upon digestion, is that the percentage digesti- 

 bility of the same feeding stuff by the same individual has been 

 found to vary more or less at different times. 



This has been shown especially by G. Kiihn. 1 In experiments 

 upon the digestibility of meadow hay by cattle the variations in the 

 percentage digestibility of the dry matter, which is the one least sub- 

 ject to error, ranged from 0.6 to 2.1, averaging 1.3, and the digestibility 

 of the organic matter showed about the same variations. That for 

 the nitrogen-free extract averaged 1.8, while in the case of the crude 

 fiber, protein and ether extract it reached 3.3. These variations were 

 shown to be materially larger than the possible errors of experiment. 

 Similar, although relatively somewhat smaller, variations were ob- 

 served on rations of hay and bran. Moreover, Kiihn points out that 

 the maximum differences were found in those cases in which the larger 



1 Landw. Vers. Stat, 29 (1883), 129, 147 and 153- 



