76 PRINCIPLES OP FEEDING FARM ANIMALS 



and assimilation. Rations composed of feeds that supply similar 

 amounts of digestible matter might, therefore, differ greatly in the 

 amounts of net energy that they supply, and would in that case 

 have a different feeding value for maintenance or productive pur- 

 poses. Under ordinary practical conditions, however, rations are 

 composed of roughage and concentrates in about similar propor- 

 tions, and no great error is therefore introduced by the use of these 

 standards. They have been simplified in this book by combining 

 digestible fat with the digestible carbohydrates, according to its fuel 

 value, so that only dry matter, digestible protein, and digestible 

 carbohydrates and fat are now considered, making the necessary 

 calculations very simple. The fact that these standards are based 

 on the vast amount of work done during the last half century or 

 more, in the lines of chemical analysis, digestion trials, and feeding 

 experiments with all kinds of farm animals, renders them especially 

 valuable to both farmers and students of feeding problems, and they 

 may safely be taken as aids to rational feeding, even though they 

 cannot be considered infallible guides. 



Limitations of Feeding Standards. — Feeding standards are 

 intended to be used only as gauges by which the farmer may estimate 

 the quantities of nutrients required by his stock for a certain pro- 

 duction, and are not to be followed blindly. Farm animals vary 

 greatly in their productive capacity, as well as in feed requirements 

 and capacity to make economical use of their feed; hence feed- 

 ing standards can apply only to average conditions, a point which 

 should always be kept in mind in using them. In constructing 

 rations according to the standard, several points must be consid- 

 ered besides the chemical composition and the digestibility of 

 the feeds. 



We have seen that the same feeds vary greatly in chemical 

 composition and digestibility; this fact renders it unnecessary to 

 make a certain combination of feeds conform absolutely to the feed- 

 ing standard, as there is no assurance that the particular feeds avail- 

 able will closely correspond to the average figures for the digestible 

 components given in tables of composition of feeding stuffs ; in fact, 

 the chances are that they will vary more or less from the average 

 data given in the tables. Therefore, unless samples of the feeds on 

 hand are analyzed by a chemist, and digestion trials conducted with 

 each feed — both of which are lengthy and labdrious tasks — we can 

 know only in a general way what, the actual values of the available 

 feeds are. In view of this uncertainty as to the exact composition of 

 the feeds, it would seem useless to try to (make a certain combination 

 of feeds conform to a definite standard within a few hundredths or 

 tenths of a pound. 



