630 RATE OF GROWTH OF DAIRY COW 



value of A can be estimated still more easily by changing equation 



X oc 

 (2) log = Kt to y = Kt when y stands for log Since 



A — X A — X 



X 



y = Kt represents a straight line therefore log , if plotted 



A. — X 



against /, should also give a straight line for the proper value of A. 



X 



A few values of A are chosen and a few values of log are plotted 



A — X 



conveniently with the aid of semilogarithmic paper or still better by 

 the aid of Robertson's tables.^ The value of A giving the straightest 

 line is the one chosen for computation. The last graphic method of 

 verifying the correctness of the estimated value of A was called to 

 our attention by Dr. E. R. Hedrick of this University, and it has 

 proved very satisfactory in this work. 



The first extrauterine cycle, A , should, in accordance with the tri- 

 cyclic theory of growth, be preceded by another cycle m Jitero; that 

 is, w in equation (3) should have some value other than zero. Since, 

 however, we have no data on intrauterine growth of the dairy cow 

 and since the neglect of the relatively small gain in the intrauterine 

 cycle cannot affect the results very seriously, we shall for the present 

 assume za = and the weight gained up to h is ^A. Assuming the 

 value of h for the Holstein cow to be 4.5 months when the animal 

 weighs 275 pounds (see Fig. 3), we get the value for A, 21 S X 2 = 

 550 pounds, and equation (2) for the Holstein cow takes the form 



"^""^^^^ = 0.158 (/- 4.5) 

 550 — X 



which should enable computation of comparable values of the animal 

 to at least 9 months (2 X 4.5) of age which in fact is the case as 

 shown in Table II. 



After 9 months of age the observed values are greater for the 

 probable reasons previously explained. 



By the same method the equation for the Jersey cow is found to be 



^""^TTT— = 0.169 (/- 5^ 



444 — X 



^Robertson, T. B., Univ. California Pub. Physiol., 1915, vi, 211. 



