212 THE FEEDING OF ANIMALS 



297. Calculation of the energy value of a ration. — 

 Three methods may be adopted for determining the 

 energy expenditure by an animal eating a given ration. 

 The one of these most easily carried out is largely a 

 matter of mathematical calculation. By the use of 

 average digestion coefficients it is possible to ascertain 

 approximately the amounts of digestible protein, carbo- 

 hydrates, and fats contained in any ration which is 

 apparently accomplishing a desired resxilt. We have 

 learned from previous determinations what are the calorific 

 values of individual compounds such as albiunin, starch, 

 sugar, stearin, and olein and these compounds are assumed 

 to represent the energy value of the classes of nutrients to 

 which they belong. If, then, we multiply the calculated 

 quantities of digestible protein, carbohydrates, and fats by 

 their respective assumed energy factors, we get a number 

 which has been taken as an expression of the available 

 energy of the ration under consideration. This method 

 must now be regarded as greatly inaccurate, because 

 the metabolizable energy of the digestible material 

 of feeding-stuils is found to be much below the calorific 

 value of the pure nutrients to which energy measure- 

 ments have been applied. See Tables XXVIII and 

 XXXIII. The older theoretical method by computation 

 might give the relative, but not the actual, units of 

 metabolizable energy in the several feeding-stuffs, for the 

 results by means of combustion in a Zuntz calorimeter 

 of pure nutrients do not measure physiological results 

 with complex mixtures. 



298. Energy value of digested nutrients. — ^A second 

 method, which is probably a step in the direction of greater 

 accuracy, is to determine by the use of a calorimeter the 

 heat units of the ration and also of the urine and feces. 



