PROTEIN REQUIREMENTS OF CATTLE: MITCHELL 75 



Since normal growth was never obtained on these low intakes of protein, 

 the Missouri data afford no support for the estimates of the protein re- 

 quirements of growing calves developed in this paper. It may be per- 

 missible, however, to go behind the summary tables and to examine the 

 original data and the observations upon individual calves so lavishly 

 recorded. A great number of disturbing factors immediately become evi- 

 dent. Besides the probable progressively increasing overestimation of the 

 digestible protein intakes from the higher to the lower protein levels, and 

 the changes in the source of protein, quite possibly further handicapping 

 the low-protein rations, there are indications, mentioned by the authors 

 themselves, that the low-protein rations were inadequate in vitamins or 

 minerals. These indications are based not only upon a study of the 

 rations themselves, but upon the pathological symptoms (stiffness, irreg- 

 ular appetite, ill-defined sickness, bhndness) not infrequently observed in 

 the calves while subsisting upon them. These periods of sickness and 

 refusal of feed apparently have not been eliminated from the experimental 

 data relating to the low-protein periods. 



The change from one ration to another was made abruptly and the 

 data obtained during the periods of adjustment to the new ration are 

 included in the summaries. The change from a prolonged regime on an 

 inadequate diet that has induced a stunted and unthrifty condition, to 

 "an adequate ration, will tend to occasion a rapid resumption of growth 

 that, for a time at least, will exaggerate the nutritive value of the second 

 ration. It would appear that such transitional periods should not be con- 

 sidered in the comparative evaluation of experimental rations. 



Most of the calves in the Missouri experiments were bred during the 

 period of experimental observation and calved shortly after the termina- 

 tion of the experiment. The disturbances in body weight thus produced 

 must have been considerable and their effects in all cases wall complicate 

 the interpretation of the weight data secured for the last few periods. 



These irregularities and disturbances in the Missouri experiments may 

 be fairly considered as detracting from the significance of the summarized 

 data upon which the conclusions are based. A number of them will oper- 

 ate definitely in detracting from the apparent value of the low-protein 

 rations, and all of them will interfere with effective comparisons between 

 rations. The conclusions apparently based so securely upon the various 

 summaries of data, must be discounted accordingly, so that the disagree- 

 ment between these summaries and the estimates of protein requirements 

 given in Table 31 may be more apparent than real. 



Uncertainties in the use of factored requirements. — In conclusion, it 

 should be pointed out that there are uncertainties in the method of 



