138 THE CHEMISTRY OF CATTLE FEEDING 



will be dealt with more particularly as they arise, but the 

 more important difficulties may be briefly summarised before- 

 hand. 



For a variety of reasons the foods used specially for fat- 

 tening, work, and milk production, i.e. as additions to the 

 maintenance rations, are generally of a different more con- 

 centrated and more expensive character than those used 

 merely for maintenance ; and the nutrients of two such 

 different classes of feeding stuffs cannot properly be added 

 together without correction, for they have very different 

 values. If the necessary corrections are made, the process 

 must be reversed when the actual rations are deduced from 

 the feeding standards. 



The amounts of nutrients required for maintenance of 

 each kind of animal depend upon the size; the amounts 

 required for other purposes depend upon the amounts of 

 work, milk, or increase produced. There are, therefore, two 

 factors to be considered the food for maintenance and the 

 additional food for special purposes ; and in each case these 

 two factors vary independently of the other. It is generally 

 advisable, therefore, to keep the two things theoretically 

 distinct, and to calculate the portions of the rations for each 

 purpose separately. 



The feeding standards are purely empirical ; they generally 

 represent a compromise, and can, at best, only give approxi- 

 mate results. The system is, however, in general use, not 

 only in this country, but also throughout Europe and in 

 America. The feeding standards commonly employed are, 

 therefore, given for comparison with the formulae. 



It will be found that, in most cases, the data correspond 

 with those calculated by means of the formulae given in this 

 and later chapters, so far as the conditions assumed by the 

 feeding standard are strictly adhered to. Discrepancies of more 

 or less considerable importance, which arise when the circum- 

 stances are varied, may be attributed, in most cases, to the 

 fact that the feeding standards are not applicable in the new 

 conditions. 



The maintenance requirements of oxen, calculated by 



