38 PROTEIN REQUIREMENTS OF CATTLE: MITCHELL 



ing? If the latter is true, then an examination of the nitrogen content 

 of the fatty tissues in the two groups of steers should reveal a corre- 

 sponding difference. 



Does fattening involve nitrogen deposition? — Although all of the 

 fatty tissues from the carcasses of these steers were not analyzed sep- 

 arately, separate analyses are reported for the offal fat, the kidney fat 

 and the fat from the round, loin and rib, except, apparently, when there 

 was no considerable amount of fat on the rib cut. In Table 20, the 

 weights of nitrogen in the analyzed fat samples are summarized and 

 totaled separately for the steers of each group. The steers whose results 

 are collected on the same line of this table were of approximately the 

 same age when slaughtered. 



The total nitrogen content of the adipose tissues of the steers of 

 Group I was always considerably greater than that of the steers of 

 Group II of approximately the same age, the same relation existing be- 

 tween the steers of Group II and those of Group III. Evidently the deposi- 

 tion of fat in adipose tissue involves cellular proliferation (or cellular 

 protoplasmic enlargement), and therefore fattening possesses its own 

 nitrogen requirement, distinct from that of growth. This difference in 

 adipose tissue nitrogen may account largely for the difference in nitrogen 

 content of Group I and Group II steers of like age, and for the more rapid 

 deposition of nitrogen in the Group I steers during the first two years, 

 although the differences in Table 20 are far too small to account for the 

 differences in Table 13. However, a greater nitrogen content of the 

 other adipose tissues, such as the subcutaneous, intermuscular, and 

 marrow connective tissues, in the steers of Group I over that of the 

 steers of Group II also must have occuiTed. 



A comparison of the nitrogen content of the lean samples for the 

 steers of Groups I and II slaughtered at ages of two years or more is of 

 interest in this connection. The nitrogen in the lean of the round for 

 the five oldest steers in Group II averaged 94 per cent of the nitrogen 

 in the same samples for the five oldest steers of Group I. For the lean of 

 the loin and the rib, however, this percentage was 78 and 83, respectively. 

 But the lean of the latter cuts was considerably fatter than the lean of 

 the round, and hence the greater nitrogen content for the Group I steers 

 over that for the Group II steers in these cases may have been due to a 

 considerable extent to the increased content of adipose tissue cells. 



Further light may be thrown upon this question by computing the 

 percentage composition of the added empty weight of Group I over 



