48 PROTEIN REQUIREMENTS OF CATTLE: MITCHELL 



simultaneously, the results from the Missouri cattle of Group I (Tables 

 16 and 17) would be more applicable than those for Group II, involving 

 what may be considered as growth alone. The differences in the rate of 

 nitrogen retention between these two groups of cattle appear to be due 

 to a considerable extent to the difference in the rate of fattening. 



For the fattening of mature cattle, use may be made of the average 

 values computed by Armsby (*• p- ^^*) of the composition of the organic 

 matter gained. Protein appears to make up about one-eighth of the 

 organic matter deposited in the tissues of mature fattening cattle. If 

 the moisture content of this increase is taken roughly as 25 per cent, 

 then each pound of gain contains 0.096 pound of protein or a little less 

 than 7 grams of nitrogen. These figures represent, therefore, the protein 

 or nitrogen requirement for each pound of increase in the fattening of 

 mature animals. It is interesting to note that the Group I steers at 2 

 years of age, and at an average weight of 1320 pounds, were retaining 6.4 

 gms. of nitrogen per pound gain in weight (Table 16) . However, much of 

 this nitrogen increase, as well as that represented in Armsby's calcula- 

 tions, represents probably a growth requirement, since feeder cattle are 

 not mature, in any strict sense of the term. 



THE PROTEIN REQUIREMENT FOR PREGNANCY 



The requirement for nitrogen or protein peculiar to the gestating 

 female is measured by the rate of nitrogen deposition in the developing 

 embryo or fetus and in the maternal nourishing and protecting tissues 

 and fluids. Intra-uterine growth thus presents an experimental problem 

 similar in all respects to that of extra-uterine growth. 



Unfortunately, the published data on intra-uterine growth in the bovine 

 species are few, and appear to be limited to the weights and analyses of 

 three Jersey fetuses at the Missouri Agricultural Experiment Sta- 

 tion (^*>). However, in this work the maternal tissues and fluids appar- 

 ently^ were not included in the chemical samples. Eckles has shown (^^) 

 that at full term the placenta and amniotic fluid contain 18 per cent 

 as much protein as the fetus (Holstein) ; at younger ages the pro- 

 portion may be much greater. The neglect of these tissues would thus 

 introduce a considerable error in estimating the daily protein require- 

 ments of pregnancy. 



Illinois investigations on the bovine fetus. — During the last year. 

 Professor W. W. Yapp of the Department of Dairy Husbandry, Uni- 

 versity of Illinois, has secured some valuable data on the composition 



' The description of the preparation of the fetuses for analysis given in the 

 bulletin is not sufficiently complete to warrant a definite statement on this point. 



