NUTRITION OF POULTRY l8l 



off an inferior article, it does not always inform him even approxi- 

 mately of the nutritive value of the article. Many, especially of the 

 highly concentrated by-products, run very unevenly in composition, 

 and the manufacturers, to be on the safe side, place their guaranty 

 below the minimum (see " Beef scrap," Chapter XII). The bulletins 

 of the various state experiment stations giving analyses of foods of 

 this class offered for sale in the state afford the most trustworthy 

 information in regard to their composition. 



Neither nutrient ratio nor potential energy gives a generally 

 applicable standard for accurately measuring nutritive values. Both, 

 however, are serviceable in comparisons of food values, and com- 

 parison of either the nutrient ratios or the fuel values of two similar 

 articles often shows their relative feeding values. Judged by prac- 

 tical observation, a comparison which considers both may be even 

 more accurate. It might be so invariably if feeding value depended 

 solely on the quantities and proportions of the principal elements ; 

 but, as the description of foods in the following chapter will show, 

 there is sometimes a very great difference between the feeding 

 value of two articles as indicated by their chemical constitution and 

 as demonstrated in practice. 



Nutrients vary in digestibility. Creatures differ in digestive 

 power, and' the same creature digests a certain kind of food more 

 completely at one time than at another. Investigators of the 

 science of feeding have determined, by careful experiment, "diges- 

 tion coefficients " for most of the common food articles for the 

 larger animals, and in a few instances for poultry ; but, in the case 

 of poultry especially, the observations are too few and the results 

 too irregular to warrant the use of these coefficients in a study of 

 foods and feeding. 



Expression of nutritive values. Nutritive standards are com- 

 monly expressed in terms of nutrient ratio and fuel value. Although, 

 as has been said, neither of these measures is accurate, they give 

 the best basis that we have for the comparison of food values in 

 numerical terms. They are found for each kind or class of animals, 

 and for each purpose for which the animals are fed, by calculating 

 the chemical values of rations the actual feeding value of which has 

 been demonstrated in practice. What was said of the comparison 

 of values of different food articles on the basis of nutrient ratio 



