Refinement of Feeding Experiments for Milk Production 239 



The value found for the coefficient of variability for Jerseys in groups 

 of six made up of unselected individuals is similar to the figure found for 

 Holsteins. The calculated value on the basis of the average coefficient 

 for individual variability shown in table 15 was found to be 4.68. This 

 checks closely with the observed figures shown in table 16. 



EFFECT OF AGE ON THE PRODUCTION OF DAIRY COWS 



The herd records used in the foregoing investigations afforded an oppor- 

 tunity for some additional study on the effect of age on the production 

 of dairy cows. In discussing the variability of mature and of immature 

 cows, the question arises as to when a cow is mature. In order to answer 

 this question, and also to trace the increase of production from the first 

 lactation to the lactation of maximum production, the following study 

 was made. 



All the cows of all breeds were used for which records of production 

 were available for at least four lactations, beginning with the first. Records 

 of the production of seventy-nine individuals were obtained, ranging in 

 length from four to eleven lactations. Production for each lactation 

 was taken, rather than production per year, since a calendar year begin- 

 ning in any month would include parts of two lactations for a large num- 

 ber of cows. Altho the lactations of individuals vary considerably in 

 length, the average lengths of the various lactation periods of numbers of 

 cows agree closely. 



In getting the average production of cows for each of the first four 

 lactations, the records of all cows were used. This gave the records of 

 seventy-nine cows for four lactations. Likewise, in getting the average 

 production of cows for each of the first five lactations, the records of all 

 cows were used which had completed five lactations. There were sixty 

 of these cows. In a similar way the average production per lactation 

 for six, seven, eight, nine, and ten lactations was obtained. Only four 

 cows had completed ten lactations. 



The relation of age to milk production is shown graphically in figure 59 

 (page 241). The maximum milk production of these cows was not reached 

 until the eighth lactation. Only fourteen cows had completed eight 

 lactations, and hence the data are not conclusive. The consistency 

 of the longer curves of fewer cows with the shorter curves of a larger 

 nimiber of cows — as far as the latter go — tends to substantiate the 

 above conclusion. Cows five years of age are ordinarily considered to be 

 mature. The average production of sixty cows for five lactations shows 

 that their maximum production was not reached before the fifth lactation, 

 corresponding to an age of from six to seven years. 



349 



