558 



THE NATURE OF THE GROWTH RATE 



appear to have been carefully made and at regular intervals. They 

 therefore afford good material for study. Eckles and Swett used 

 various criteria for measuring the heifers. They concluded that 

 height at the withers is the most reliable measure, though for compara- 

 tive studies weight is also useful. 

 w^ In computing the growth rate of these animals I have used a slightly 



dx . , . 

 different equation; viz., ~vj=k{a—x) which upon integration becomes 



a: = a(l-0- 



500 



a- 



3 



250 



10 



20 



Time in months 



30 



36 



Fig. 5.^ Growth rate of Jersey heifers represented by mean weight. The curve 

 shows the values obtained from the equation :e = 850 (1 — e~^°^^'); points within 

 circles represent observed weights at intervals of 2 months. 



The rate as thus expressed is directly proportional to the amount of 

 growth yet to be made. It is therefore more rapid at the outset and 

 slower near maturity than the formula used for computing the growth 

 rate of maize in Table V. In the case of the heifers, a, the weight at 

 3 years of age, was found by a series of approximations to be 850 

 pounds, and k, the growth constant, was 0.065. The equation used for 

 determining the calculated values of x was therefore x = 850(1 —e~°"^^0 

 (Fig. 5). 



