426 PRINCIPLES OF AGRICULTURAL CHEMISTRY 



The following is an example of the method of calculating the 

 fat value of a roughage : 



JOHNSON GRASS HAY 100 POUNDS. 



Digestible protein 3-3 X 0.235 = 0.78 



Digestible fat 0.7 X 0.474 = 0.33 



Digestible crude fiber 22.6 



Digestible nitrogen-free extract ... 28. i 



~50.7 X 0.25 = 12.42 



Total 13.53 



Total crude fiber 38.0X0.14= 5.32 



Productive value 8.21 pounds fat 



This means that 100 pounds of the Johnson grass hay added to 

 a ration already sufficient to maintain the animal, should produce 

 8.21 pounds fat. The fat value is the productive value for fatten- 

 ing, when the feed is used for fat and for no other purpose. 



Kellner expresses the productive value of feeds and rations in 

 terms of starch; that is, the quantity of starch which is capable 

 of producing the same quantity of fat as 100 pounds of the feed 

 is capable of producing. Armsby 1 expresses the productive value 

 in terms of heat units (therms). There are two objections to 

 this manner of statement. The first is, that the heat units are 

 used for the total heat value of the feed, for the available heat 

 value, and for the productive value, introducing some confusion in 

 distinguishing between them. The second objection is, that the 

 energy equivalent to the quantity of fat produced is not the pro- 

 ductive energy of the feed, since no doubt some of the energy 

 must be consumed in the process of transforming the nutrients 

 into fat. The productive energy may be proportional to the fat 

 produced, but is not identical with it. 



We therefore prefer to express the productive value of feed 

 in terms of the actual experimental basis, namely, of the fat 

 which they were found to produce. 



The productive value of a feed may be defined as the quantity 

 of fat which the feed will produce, when it is fed in addition to a 

 ration already sufficient to supply the needs of the animal for 

 maintenance. 



1 Farmers Bulletin No. 346. 



