382 NUTRITION OF FARM ANIMALS 



amino acid cannot be synthesized in the body but must be supplied 

 in the feed in order to permit the construction of the new protein 

 molecules in the tissue, while for maintenance (399) it appears to be 

 dispensable. Moreover, they have shown that the addition to in- 

 adequate proteins like gliadin of other proteins containing lysin per- 

 mits growth to take place and furthermore that the proportion of the 

 second protein which must be added in order to support normal 

 growth is less in proportion as it is richer in lysin. 



Osborne and Mendel's conclusions have been strikingly confirmed 

 by the results obtained by Buckner, Nollau and Kastle l from feeding 

 young chicks grain mixtures of high and low lysin content. 



It appears that the lack of lysin in a protein renders it in- 

 capable of supporting growth, although it may still be adequate 

 for maintenance (399), and that the proportion of lysin in those 

 proteins containing it constitutes a limiting factor for the 

 amount of growth which they can support. Tryptophan is 

 obviously another limiting factor in this respect, while it must 

 be regarded as altogether probable that other amino acids 

 belong in the same category and may become limiting factors 

 if the supply of them is deficient. In other words, the amount 

 of some particular amino acid which is available may become 

 the minimum factor which determines the rate of growth, just 

 as the minimum supply of potassium, for example, may deter- 

 mine the rate of growth of a crop. The unsatisfactory results 

 obtained in practice with maize as the sole feed for young 

 animals may well be due in large part to the poverty of the 

 mixed proteins of this grain in tryptophan and lysin, it having 

 been shown that as the sole source of protein they can support 

 but slow growth (783). 



Unfortunately the knowledge available on these points is as 

 yet chiefly qualitative in character and affords no sufficient 

 foundation on which to base a quantitative discussion of the 

 relative values of proteins in farm practice. Accordingly, in 

 the case of growth as in that of maintenance it appears neces- 

 sary for the present to consider questions regarding the protein 

 requirement upon the basis of total protein, largely irrespective 

 of its nature. (Compare Chapter XVII, 4.) 



466. Percentage retention of feed protein. In the mature 

 animal, the katabolism of protein substantially keeps pace 



1 Amer. Jour. Physiol., 39 (1915), 162. 



