596 NUTRITION OF FARM ANIMALS 



minimum of digestible protein." 1 As Henry and Morrison have 

 pointed out, " The feed unit system has been evolved in a com- 

 paratively small region where similar crops are grown on the 

 different farms and the price of purchased feeds does not vary 

 widely throughout the district." 



704. Comparison of feed units and net energy values. 

 The writer is not able to agree with those who would introduce 

 the feed unit system in this country with its wide variety 

 of feeding stuffs and conditions. The applicability of the feed 

 units, as just pointed out, is conditioned upon the presence 

 of sufficient protein in the rations. As thus limited, however, 

 they practically attempt to measure the relative values as sources 

 of energy, and for this purpose the use of the net energy values 

 to be considered in the next chapter is just as simple arithmeti- 

 cally and equally accurate, while it has two immense advantages. 

 First, the net energy values are rational and not empirical values. 

 They are based on physiological investigations and their very 

 imperfections tend to stimulate further investigation which may 

 lead to their great improvement or to the discovery of new and 

 still better methods of comparison. The feed unit, on the other 

 hand, constitutes a dead end so far as investigation is concerned, 

 leading to nothing beyond some increase in numerical accuracy, 

 while it is far inferior in pedagogic value. Second, the feed 

 units are purely relative values, based on direct comparisons of 

 the results with different materials with no attempt to discover 

 the causes of the observed differences. They show to what 

 extent one feeding stuff is better or worse than others, but es- 

 tablish no relation between feed and product. Energy values, on 

 the other hand, aim to show the amount of product which may 

 be expected from a unit weight of the feeding stuff -r- i.e., the 

 amount of energy which it can contribute to the maintenance 

 of the body or to the building up of new tissue. Thus, if aver- 

 age maize meal, for example, has an energy value of 85 Therms 

 per hundred pounds, this means that one hundred pounds of it, 

 fed as part of a maintenance ration, would conserve in the body 

 of the animal an amount of fat and protein having an energy 

 value of 85 Therms, which would otherwise be burned up to 

 support the vital activities. Furthermore, it means that, if 

 added to the maintenance ration, the maize will furnish ma- 



1 Woll, loc. cit. p. 13. 



