PROTEIN REQUIREMENTS OF CATTLE: MITCHELL 47 



The daily amounts of nitrogen retained by calves of the four breeds 

 are offered as measures of the nitrogen requirements for growth. They 

 are, of course, only tentative in nature, being subject to revision as more 

 data accumulate. The nitrogen retention of young calves in particular 

 needs further study, and the estimated values of Table 23 for 200 and 

 to a less extent for 300 pound calves cannot be considered accurate or even 

 satisfactory. The gro\rth curve at the younger ages and in particular 

 the change in composition of young calves needs further investigation. It 

 may further be questioned whether data on steer calves should be applied 

 to heifer calves. It is known that heifer calves fatten more rapidly, 

 especially at the younger ages, than do steer calves. This difference un- 

 doubtedly vitiates to some extent the method of calculation employed in 

 estimating the values given in Table 23. It appears that this effect will 

 lead to an over-estimate of the nitrogen requirement of the heifer calves. 



The questions here raised call for further study, but until more data 

 become available the estimates presented may be used tentatively, with no 

 probability of great error, in computing the nitrogen (protein) require- 

 ments of cattle for growth. 



THE PROTEIN REQUIREMENT FOR FATTENING 



Armsby has concluded (2' p- ^^^)from a study of available slaughter and 

 respiration experiments on cattle that the laying on of fat, although 

 accomplished largely by an increase in the fat content of existing cells, 

 also involves cellular proliferation. Hence, the fattening even of mature 

 animals increases the protein requirement of the animal. The Missouri 

 data confirm this conclusion in a very decisive fashion. 



It is a difficult matter, however, to assess quantitatively the increased 

 nitrogenous needs of fattening. The composition of adipose tissue affords 

 an uncertain basis for estimation, since it varies with age and with ana- 

 tomical location. A study of the Missouri data shows that the younger 

 tlie animal, the less fat and the greater protein its adipose tissues contain, 

 and that at all ages the kidney and offal fatty tissue are higher in fat and 

 lower in protein than the loin, rib, and particularly the round fatty 

 tissue. These differences are very largely explained by the fatty satura- 

 tion of such tissues, since on the fat-free, or " protoplasmic " basis, the 

 different adipose tissues are not greatly different in composition. 



No rational method of separating the requirements of growth from 

 those of fattening suggests itself, since no qualitative differences appear 

 to exist between the two processes. Normal growth does not occur without 

 a considerable deposition of fat, and fattening is just as surely accom- 

 panied by a deposition of protein. For steers growing and fattening 



