44 PROTEIN REQUIREMENTS OF CATTLE: MITCHELL 



Comparing the percentage composition of the Minnesota steers at 

 different weights (Table xxiv of Minn. Bull. 193) with the composition 

 of the Missouri steers of Group II (Table 15 of this paper), the per- 

 centage of fat is quite similar up to 700 pounds in weight, but beyond 

 this point the Minnesota steers fattened much more rapidly. Thus at 

 900 pounds the Minnesota cattle contained over 20 per cent of fat, more 

 than the Missouri cattle contained at 1200 pounds. 



The rapid fattening of the Minnesota cattle in the latter half of the 

 weight range appears to account for the abnormal shape of the age- 

 weight curve and indicates that a much heavier plane of feeding was in 

 vogue at that time. The data thus are not homogeneous. The composition 

 of the cattle at the higher weights is not comparable with that at the 

 lower rates, since it is the result of a different system of feeding, and 

 the heterogeneity thus prevailing precludes an exact interpretation of 

 the experiment as a whole, and rules out the possibility of obtaining such 

 a simple and satisfactory mathematical description as the Missouri data 

 have yielded. 



As far as the specific question of nitrogen deposition is concerned, 

 the Minnesota data do not include direct protein determinations 

 (N'x6.25), the reported protein values being obtained by difference.^ 



Estimated daily nitrogen I'equirements for different hreeds of cattle. — 

 It appears, therefore, that the Missouri data are the only data available 

 from which the nitrogen requirements of growing cattle may be esti- 

 mated. The daily nitrogen retention figures for the steers of Group II 

 at different weights, given in Table 17, may be considered as measures 

 of the nitrogen requirements of Hereford-Shorthorn unsexed male 

 cattle. Is there any reasonable method by which similar estimates may 

 be made for other breeds of cattle? 



Upon certain approximate assumptions it appears possible to make 

 such estimates for those breeds of cattle for which satisfactory growth 

 data are available. For certain breeds of dairy cattle, Eckles and his 

 associates have secured fairly satisfactory information concerning the 

 age-weight relation, although the numbers of animals involved, particu- 

 larly at the later ages, are not sufficiently large to give the averages any 

 degree of finality. However, the data are not markedly irregular and have 

 been showTi by Brody(*^) to be satisfactorily describable by his growth 



^ In thus failing to use the Minnesota data for the purpose of studying the 

 growth of steers, there is no intention to detract from the vakie of this work for 

 other purposes. If the method of feeding these steers conforms to accepted 

 practices, the results secured may be of great practical value. 



