102 



MAINTENANCE RATIONS OF FARM ANIMALS. 



proteins the more economically it could be utilized for the building up or repair 

 of protein tissues, and that thus there might be very considerable differences in, 

 nutritive value between different proteins. 



EXPERIMENTAL METHODS. 



Considerations like the foregoing have been advanced by numerous authors, 

 but as yet little satisfactory experimental work upon the relative values of the 

 proteins has been reported. Indeed the problem is far from being an easy one. 

 Aside from technical difficulties, it is, of course, a simple matter to substitute 

 one protein for another in the ration; the difficulty lies in finding a satis- 

 factory measure of the effects. The most obvious thing, of course, is a deter- 

 mination of the balance of income and outgo of nitrogen, which, when extended 

 over reasonably long periods, affords an approximate measure of the relative 

 gain or loss of protein. As has been clearly shown on preceding pages, however, 

 the nitrogen balance, especially in a mature animal, is a more or less fluctuat- 

 ing thing, being materially affected by various factors besides the momentary 

 protein supply. Especially important are the influence of the previous pro- 

 tein supply upon the general level of protein nutrition, the influence of the 

 store of body fat carried by the animal, and the supply of available energy 

 in the feed. Only after these influences have been eliminated as completely 

 as possible can differences in the nitrogen balance be ascribed to differences 

 in the nature of the proteins consumed. On this account, experiments in which 

 additions of protein are made to a ration already containing a considerable 

 supply and in which gains of nitrogen in different periods are made the basis 

 of comparison are quite unsatisfactory, as Magnus-Levy 1 has pointed out. 

 A more satisfactory basis of comparison is the amounts of the different pro- 

 teins required to maintain nitrogen equilibrium under conditions otherwise 

 comparable. Furthermore, the protein supply must not be too liberal. Protein 

 supplied in excess of the minimum requirement is utilized largely as fuel ma- 

 terial. Under such circumstances, it is easily conceivable that proteins differ- 

 ing widely in constitution may furnish enough of each of the essential cleavage 

 products to meet the relatively small demand for the maintenance of tissue and 

 that thus differences really existing may be masked by the excess of protein 

 supplied. 



These considerations clearly indicate that the most promising method of 

 investigation is to compare the minimum amounts of the different proteins 

 required, along with an abundance of nonnitrogenous nutrients, to maintain 

 nitrogen equilibrium on as low a plane of protein nutrition as practicable in 

 the same animal in like bodily states and under identical conditions, so far 

 as it is possible to insure these. Any consistent differences appearing in a 

 considerable number of trials may then, it would seem, be safely ascribed to 

 differences in the nature of the proteins. 



Thus far but three investigations, according to the general method just out- 

 lined, have been published, all of them appearing within the year 1909. For 

 the present purpose it seems superfluous to review the older investigations, 

 made by less satisfactory methods and in many cases from a different point 

 of view. 



MICHAUD'S INVESTIGATIONS. 



Michaud 2 experimented on three dogs by substantially the method just out- 

 lined. Reasoning that any loss in transforming feed protein into body protein 



1 Von Noorden's Handbuch dor Pathologie des Stoffwcchsels, vol. 1, p. 78. 



2 Zeitschrift fur Physiologischo Chemie, vol. 59, p. 405. 



