Progress in Experimental Methods 193 



particular ration is the measurement of the digested 

 nutrients and the determination of the gain or loss of 

 nitrogen. This is accomplished, as heretofore stated, by 

 ascertaining the quantity of various compounds eaten 

 and the amount of the same in the feces, the differ- 

 ence being the digested portion. The urine is also 

 collected, and if the nitrogen in it is less or more 

 than that in the digested protein, then the animal is 

 either gaining or losing nitrogenous body substance, 

 unless the measurement is with a milch cow, when the 

 nitrogen in the milk must be taken into account. By 

 an experiment conducted in this way, with careful and 

 continued weighings of the experimental animal, it is 

 possible to secure a probable relation between a unit 

 of digested dry matter and a unit of production. Such 

 a method has been used to determine what is a main- 

 tenance ration for animals of several classes, and in 

 those cases where the experiments have been continued 

 for a sufficient length of time and have shown on 

 repetition a reasonable agreement, we are justified in 

 accepting the results as a close approximation to fact. 

 When a ration keeps an animal in nitrogen equilibrium 

 for one or more months and no material gain or loss 

 of weight occurs, we may safely regard it as approxi- 

 mately a maintenance ration under the conditions in- 

 volved. Experiments of the same kind are equally 

 useful in testing the productive power of various food 

 combinations, and whenever by such continued tests 

 one ration shows no superiority over another, it is 

 safe to assume that no differences exist which would 

 be especially important to the farmer's pocketbook. 



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