50 FEEDS AND FEEDING 



productive purposes, such as the formation of flesh or fat, or the pro- 

 duction of external work or of milk. The energy used up in the ' ' work 

 of digestion" all takes the form of heat and is wasted so far as such 

 production is concerned, but this heat helps to keep the body warm. 

 Therefore, either the available energy, which includes the energy thus 

 spent, or else the total digestible nutrients, are better measures of the 

 relative value of different feeds for maintaining animals, at least when 

 there is need of a large amount of heat to warm the body, as during 

 winter in the northern states. (90) 



All these considerations emphasize the importance, in feeding any class 

 of stock economically, of studying the actual results secured when 

 the different feeds have been fed to that class of stock in extensive 

 practical feeding trials, as reported exhaustively in Part III of this 

 book. It will be found that conclusions based upon such trials are the 

 safest guide in feeding operations, for they take into consideration 

 other factors, such as palatability, quality of proteins, amount and 

 kind of mineral matter, and perhaps the presence or absence of 

 vitamines, all of which factors, as we shall see in later chapters, may be 

 just as important as the amount of net energy or the amount of 

 digestible nutrients. (56, 94, 95-101, 104-5) 



III. FACTORS INFLUENCING THE NUTRITIVE VALUE OP FEEDS 



81. Differences in composition of feeding stuffs. The figures given in 

 Appendix Table I for the composition of any feed are in most instances 

 averages of all analyses of normal samples of that feed which have been 

 reported by the various stations. It is obviously important to learn 

 what variations from these averages may be expected in the case of 

 samples of a given feed originating in different sections of the country, 

 grown in different years, or when gathered at different stages of matu- 

 rity. Lack of space prohibits any detailed consideration of this question. 

 However, from the mass of data compiled in Appendix Table I, includ- 

 ing over 53,000 analyses in all, from all parts of the country, the 

 following notes will give a fair idea of the range in the chemical 

 composition of typical feeding stuffs. 



It has been found that the composition of a crop may be influenced to 

 a limited extent by the amount of available plant food in the soil on 

 which the crop is grown. Climatic environment and stage of maturity 

 are, however, the most important factors in determining the composition 

 of a given feed. Indeed, with some feeds they influence the content of 

 nutrients to such a degree that an average of analyses from all sections 

 of the country or at all stages of maturity is of little value for any pur- 

 pose. Of the cereals, wheat is the most variable in composition, being 

 profoundly influenced by climate, especially in its protein content. The 

 analyses for this grain from different sections of the country are there- 

 fore given separately in Table I. It is there shown that while the aver- 



