THE COMPUTATION OF RATIONS . 703 



By proceeding in this manner, with a little patience we can 

 usually get a ration corresponding as closely as is necessary to 

 the requirement, provided the feeds available admit of it. With 

 a little experience one very soon learns to guess pretty closely, 

 and with some practice the computations become very easy. 

 An exact agreement with the requirement need not be sought 

 for, since in practice the composition of the feeds will probably 

 vary more or less from the average of the tables. 



803. The choice of feeding stuffs. When, as in the last 

 example, feeding stuffs must be purchased in order to get the 

 desired relation between the protein and the energy of the 

 ration, it is evident that often a wide range of choice may be 

 offered. In such a case the question at once arises which of 

 the various feeds available is it most economical to purchase, it 

 being evident, of course, that this is not necessarily the one 

 offered at the lowest price. 



No simple method of determining this point is possible, be- 

 cause, as we have seen, the food serves two entirely distinct 

 purposes in the body. Sometimes the supply of protein is the 

 specially important point, and in other cases what is needed 

 is a supply of energy without special reference to whether its 

 source be protein or non-nitrogenous material. Consequently, 

 the relative values of two feeding stuffs may vary under differ- 

 ent circumstances. Some writers have based their compari- 

 sons of the values of by-product feeds solely upon their con- 

 tent of protein, for the reason that such feeds are often bought 

 especially to supply this ingredient while the fats and es- 

 pecially the carbohydrates are usually produced in abundance 

 upon the farm. They regard that purchased feeding stuff as 

 the most economical which furnishes a pound of digestible pro- 

 tein at the lowest cost, ignoring any value in the other ingre- 

 dients. It is obvious, however, that this is a one-sided view. 

 The other ingredients have a value, and this is especially true 

 in the case of a feeder who buys a considerable part of his grain 

 supply and depends upon it as a source of energy as well as of 

 protein. The method of comparison illustrated in the following 

 pages is based primarily upon the cost per unit of energy because 

 this is on the whole the most important function of the feed, 

 but the method takes account also of the amount of protein 

 present. 



