76 PRINCIPLES OF FEEDING FARM ANIMALS 
purposes. Under ordinary practical conditions, however, rations are 
composed of roughage‘and concentrates in about similar proportions, 
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 produc- 
tion, and are not to be followed blindly. Farm animals vary greatly 
in their productive capacity, as well as in their feed requirements 
and their 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 standards, several points must be considered 
besides the chemical composition and the digestibility of the feeding 
stuffs. 
The same feeds vary greatly in chemical composition and digesti- 
bility, as we have seen ; this fact renders it quite unnecessary to make 
a certain combination of feeds conform absolutely to the feeding 
standard, for we have 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 laborious 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 is quite useless to try to make a certain combination of 
feeds conform to a definite standard within a few hundredths or 
tenths of a pound. The standards are a valuable guide to the practi- 
cal feeder and the student of animal nutrition, but it would be a mis- 
take to look upon them as precepts that must be rigidly adhered to. 
