46 PROTEIN REQUIREMENTS OF CATTLE: MITCHELL 



For the Jersey heifer, t* = 8.9 months and for the Hereford-Shorthorn 

 steer t* = 8.3 months. Hence, a heifer, at 30 months after conception, is 

 equivalent in age to a steer of a conceptional age of [(20 — 8.9) xl.78] 

 + 8.3 = 28.1 months. 



Computing equivalent ages by this method, and assuming that the 

 nitrogen retention of calves per day per unit of weight is the same at 

 equivalent ages, the data given in Table 23 are obtained, relating to 

 regular intervals of body weight. The Hereford-Shorthorn data are taken 

 from Table 17. The calculation of the data for the other breeds may be 

 best explained by a specific illustration. 



A 600-pound Holstein-Friesian heifer, according to Eckles' data, 

 would possess an average age of 13,6 months (from birth). Using 

 Brody's method as just explained, such a calf is equivalent in age to a 

 Hereford-Shorthorn steer of 21.2 months from birth or 30.6 months 

 from conception. From the differential equation giving the rate of nitro- 

 gen deposition of the Group II steers at any age (see footnote to page 

 30), it may be computed that a calf at 30.6 months (from conception) 

 will retain daily 9.72 grams of nitrogen. But such a calf weighs 785 

 pounds (computed from Equation (3)). Hence, on the assumption that 

 calves of equivalent age will retain equal amounts of nitrogen per unit 

 of body weight, the daily nitrogen retention of the 600 pound Holstein- 

 Friesian heifer would be 9,72 x-r-— =7.43 gms., the value given in 



785 *^ 



Table 23, 



