I' ROT BIN REQUIREMENTS OF CATTLE: MITCHELL 71 



more readily consumed than the low-protein rations. The results of the 

 experiments were not considered to be sufficiently significant and con- 

 cordant to warrant positive statements concerning either the adequacy 

 of the low-protein ration for a normal growth rate, or the superiority of 

 the high-protein over the low-protein ration in growth-promoting value. 



In the metabolism experiments that were undertaken upon a minority 

 of the calves, the high-protein calves in general seemed to be storing 

 nitrogen at a considerably greater rate than the low-protein calves, though 

 the significance of the nitrogen balances obtained is seriously questioned 

 by the investigators themselves, either because the collection periods or 

 the periods of preliminary feeding were too short, or because the bal- 

 ances were quite inconsistent with the live weight increases. 



On the basis of gains in live weight, differences between high-protein 

 and low-protein calves were not consistent, and in a large proportion of 

 comparisons the low-protein calf gained faster than its mate on the high- 

 protein ration. 



However, significant comparisons cannot readily be made between 

 high-protein and low-protein calves, since, either intentionally or in- 

 advertently, the net energy intakes of the paired calves were not equal- 

 ized, the result being that differences in growth cannot be interpreted 

 with reference to differences in protein intake only. It was very clearly 

 shown in these investigations that the planning of experimental rations 

 and the control of the consumption of food by experimental animals can- 

 not safely be based upon the use of average analyses of feeds and average 

 digestion coefficients. 



The performance of the low-protein calves is of particular interest to 

 this discussion, since their intake of crude protein was of the same order 

 of magnitude in most cases as the estimates of this report summarized in 

 Table 31. The data on these calves pertinent to the question at issue are 

 given in Table 32.^ The computed normal growth of the calves was ob- 

 tained from the growth equations given above on the basis of the initial 

 weight of the calf, rather than the initial age. Values for true protein 

 have been converted into values for crude protein on the basis of the find- 

 ings in the digestion trials relative to the ratio between digestible true 

 protein and digestible crude protein. 



The markedly subnormal growth of the first four Massachusetts calves, 

 receiving slightly less than the estimated requirements of protein is not 

 surprising in view of the large underestimation of the energy intake. 



^ Two of the low-protein calves in the North Dakota experiments are not included 

 in this table, since their intake of digestible crude protein was considerably greater 

 than the requirements as estimated in this report. 



