52 PROTEIN REQUIREMENTS OF CATTLE: MITCHELL 



calves, 55 pounds for Jersey calves, 71 pounds for Guernsey calves, 72 

 pounds for Ayrshire calves, and 73 pounds for Dairy Shorthorn calves/ 

 Sex differences have not been considered here, since the percentage 

 error in the estimate is probably greater than the percentage difference 

 in growth rate between the sexes. 



It is evident from these estimates that the nitrogen requirement of 

 pregnancy is inconsiderable for the first four or five months, but that it 

 rapidly increases from then on, until in the last two months it is equiva- 

 lent to a deposition of 0.17 to 0.26 pound of protein daily for the Hoi- 

 stein cow. 



TABLE 27 



The Estimated Daily Retention of Nitrogen ix the Growth of the Bovine 



Fetus and Membranes * 



Daily retention of nitrogen 



* Exclusive of the nitroRen retained in the amniotic fluid. At full term, Eckles estimates ( ^^ ) that 

 the amniotic fluid contains 1.07 pounds of protein, or 77.7 gms. of nitrogen. In so far as this is of 

 fetal urinary origin, it pos.'^esses no significance with reference to nitrogen requirements during 

 intra-uterine growth. 



In a calf weighing 75 pounds at birth, Popov (^^) found 6 kgms. of pro- 

 tein. Assuming that this was all laid down in the last 100 days of ges- 

 tation and that the digestible protein possessed a biological value of 70, 

 Popov computes that the pregnant cow would require an average of 85 

 gms. of digestible protein daily for the formation of her fetus. This is 

 equivalent to a daily requirement of 9.6 gms. of digestible nitrogen 

 with a biological value of 100. This estimate is comparable with the 

 values of Table 27 for the 7th, 8th and 9th months for the Guernsey, 

 Ayrshire and Dairy Sliorthorn breeds. 



THE PROTEIN REQUIREMENT FOR MILK PRODUCTION 



According to the general scheme of expressing the protein requirements 

 of animals pursued in this paper, the protein requirement for milk pro- 



^ The latter three breeds have been grouped together for an average birth 

 weight of 72 pounds. 



