ANIMAL FOODS AND FEEDING 291 



experiment, by subtracting from the amount of food eaten the 

 amount present in the faecal excrement, as it is in this excrement 

 that the undigested food material remains. 



The digestibility of a food constituent will vary also according 

 to the condition in which it is fed, as, for example, whether it 

 is mixed with a large amount of other material, or, in the case 

 of human foods, whether it is raw or cooked. While this is 

 important, it does not alter the actual amount digested so 

 much as it does the rapidity or ease of digestion. In general, 

 the coefficients of digestibility refer to a food constituent in the 

 usual form in which it is eaten. For man therefore the coef- 

 ficient of digestibility of starch refers to cooked starch, while 

 for domestic animals it refers to raw starch as present in cereal 

 grains, etc. For cattle the average digestibility of starch in 

 corn or wheat is almost as high as for man, viz. 92-95 per cent, 

 while in other animal foods, such as oats and hay, the diges- 

 tibility of nitrogen-free extract falls as low as 62 per cent and 

 with horses and sheep from 47 to 62 per cent. 



We see, therefore, that the particular species of animal and 

 even the particular individual animal may possess a different 

 coefficient of digestibility. To obtain absolute figures therefore 

 for the coefficients of digestibility a great deal of experimenta- 

 tion is required. We can, however, take certain average re- 

 sults and use them in calculating the digestibility of foods and 

 their food value. 



Without going more into detail the data in the preceding table 

 represents the average results of experimental evidence on the 

 subject. 



Application of Coefficients of Digestibility. — From these 

 coefficients of digestibility we can readily calculate, from the 

 percentage composition of a food, the exact amount of each of 

 the food constituents which an animal actually digests. The 

 following examples will illustrate: 



