202 NUTRITION OF FARM ANIMALS 



3. THE BALANCE or MATTER 

 The gain or loss of protein 



287. The nitrogen balance. Feed protein which fails to 

 be stored up in the body is not excreted as protein but in the 

 form of the various products of its katabolism. The gain or 

 loss of protein, therefore, cannot be determined by a direct 

 comparison of its income and outgo because there is no outgo 

 of protein as such. Since, however, the protein of the schematic 

 body (280) is equivalent to total nitrogenous matter, the gain or 

 loss of protein may be inferred from that of its characteristic 

 element, nitrogen, and this is readily ascertained by a com- 

 parison of the total nitrogen of the feed with the total nitrogen 

 of the excreta, i.e., by a determination of the nitrogen balance. 



288. Free nitrogen not excreted. In Chapter V (228) it 

 was stated that all the nitrogen of the protein katabolized is 

 found in the urea and other organic compounds which are ex- 

 creted in the urine. Obviously this is a point of fundamental 

 importance. If nitrogen leaves the body only as combined 

 nitrogen in the urine and in the feed residues and nitrogenous 

 excretory products found in the feces, it is a comparatively 

 simple matter to compare the income and outgo. If, however, 

 the metabolic processes or the fermentations of the feed in 

 the digestive tract yield also gaseous nitrogen, then the nitrogen 

 of the respiratory products must also be determined, a task of 

 no small difficulty. 



The question of the excretion of gaseous nitrogen has been 

 the subject of a vast amount of investigation and controversy. 

 Substantially two general methods of experimentation have been 

 followed, viz., a comparison of the income and outgo of com- 

 bined nitrogen and direct investigation of the respiratory 

 products, and the results of both have been in substantial 

 accord. The cumulative force of the great number of experi- 

 ments in which substantial equality between income and 

 outgo of combined nitrogen has been observed under condi- 

 tions which precluded the possibility of any considerable gain or 

 loss of body protein, together with the fact that the very careful 

 and accurate investigations of recent years upon the respiratory 

 excretion of free nitrogen have given negative results, amount to 



