680 NUTRITION OF FARM ANIMALS 



of vegetable proteins derived from seeds is known, those con- 

 tained in roughages and in roots have not yet been investigated, 

 although a beginning has been made 1 in determining the pro- 

 portions of the different groups of amino acids which are yielded 

 by the total nitrogenous matter (crude protein) of various feed- 

 ing stuffs. 



Second, as has appeared in previous chapters, such informa- 

 tion as is available respecting the protein requirements of farm 

 animals has been derived from experiments in which only the 

 total protein supplied was considered without reference to its 

 kind. Practically no knowledge is available as to the amino acid 

 requirements of the various farm, animals for different purposes. 



Third, even were the production values of the various single 

 proteins known, it would not be possible to estimate from them 

 the production values of the mixed proteins of feeding stuffs, 

 since a deficiency in one protein might be compensated by a 

 surplus in another and the mixture show a much higher pro- 

 duction value than either of its ingredients separately. Thus, 

 as already noted, the value of wheat gliadin, which lacks lysin, 

 is practically zero for growth, while as part of a mixture with 

 other proteins supplying lysin it may have a high value, the 

 replacement of 25 per cent of it by lactalbumin, for example, 

 rendering the mixture fully adequate to support normal growth. 

 Each particular mixture of proteins would have its own pro- 

 duction value, which might differ widely from the mean of the 

 values for the individual constituents. 



The qualitative differences in proteins are doubtless of much 

 significance, and the researches in progress can hardly fail 

 ultimately to lead to a more rational method of valuation than 

 that now in use, but as yet they do not afford an adequate 

 basis for expressing the values of feeding stuffs in general as 

 sources of protein. For the purposes of the stock feeder, there- 

 fore, it still seems necessary to adhere to the older method which 

 regards the digestible protein of a feeding stuff as expressing 

 approximately its production value in this respect, thus vir- 

 tually assuming that in ordinary mixed rations the protein 

 deficiencies of the different ingredients will largely balance each 

 other, and this method has been followed in the tables of the 



1 Grindley, Joseph and Slater; Jour. Amer. Chera. Soc., 37 (1915), 1778 and 

 2762: Nollau; Jour. Biol. Chem., 21 (1915), 611. 



