532 Report of the Department op Animal Hu►SBA^fDRY op the 



given ration approximates to a desired standard it is necessary to 

 determine the amounts and proportions of digestible nutrients 

 which the proposed mixture of foods contains. A knowledge of 

 the exact composition and digestibility of the feeding stuffs to be 

 used is not possible, usually, certainly not in ordinary practice, 

 and the computation of the amounts of digestible compounds 

 must be based upon the average composition and digestibility of 

 similar materials. It is assumed that figures reached in this way 

 are sufficiently accurate for all practical purposes. 



The compounding of rations from a variety of foods to cor- 

 respond to certain standards, as usually done, involves another 

 assumption, viz.: That a pound of digestible material, carbohy- 

 drates for instance, has a uniform value no matter what its 

 source. 



Probably well-informed agricultural chemists do not assent i.o 

 this statement, and doubtless they are agreed that this assump- 

 tion is possibly an element of weakness in the effort to compound 

 equivalent rations from greatly different mixtures of feeding 

 stuffs. 



Much of the doubt on this point pertains to the nitrogen-free 

 extract. Great uncertainty exists as tO' the relative nutritive 

 value of the sugars, starches, pentosans, cellulose and other com- 

 pounds, some of which belong to the hexose group, others to the 

 pentose, etc. We do know, however, that there is a marked 

 variation in the proportions in which these carbohydrates and 

 other compounds are found in the nitrogen-free extract of feeding 

 stuffs, especially when the coarse fodders are compared with the 

 cereal grains. 



Moreover, the protein of cattle foods is a collective name for a 

 mixture of nitrogenous compounds and there is good reason for 

 believing that N x 6.25 does not always represent the same nutri- 

 tive value, as for instance in roots, in green crops and in the oil 

 meals. 



It is proper to inquire, therefore, to what extent these possible 

 variations of composition and food values of different com- 



