Refinement of Feeding Experiments for Milk Production 233 

 TABLE 10. Individual Variation 



Individual production, two-weeks period 

 (pounds) 



Average 



number of 



weeks 



after 



calving 



Number of 

 individuals 



Coefficient of 

 variability 



200-249 . 

 250-299. 

 300-349 . 



350-399 ■ 

 400-449 . 

 450-499 . 

 500-549 . 



550-599 • 

 600-649 . 

 650-699 . 

 700-749 . 



750-799 

 800-849 , 



14 

 14 

 12 

 12 

 II 

 13 

 9 

 8 



7 

 6 



5 

 7 

 4 



Total . . . 

 Average . 



5 

 17 

 13 



27 



24 



27 



30 

 16 



13 

 9 

 5 

 6 



7 



199 



08 

 16 

 84 

 41 

 94 

 34 

 50 

 8.00 

 15.60 

 II .07 

 11.88 

 10.18 

 8.19 



30 



15 

 II 

 10 



13 

 10 

 II 



±6.42 

 ±1-75 

 ±1-57 

 ±0.96 



± 1.36 



±0.95 

 ± 1 .00 



0.95 

 2.06 

 1.76 



2.53 

 1.98 

 1.48 



± 

 ± 

 ± 



12. II ±0.43 



The data in this table are less consistent than those given in table i 

 (page 225) for animals in a uniform stage of lactation. A wider range 

 of coefficients is shown in table 10, due to the value found for the first 

 class. The large probable error attached to this and other coefficients 

 indicates that little reliance can be placed on them. The average coefficient 

 is practically identical with the one found for individuals in the same 

 stage of lactation. 



It would naturally be expected that cows in various stages of lactation 

 would be more variable than animals uniform in this respect. The 

 reduction in milk yield during the lactation period is one factor that 

 should cause greater variation in the former group. However, if this 

 reduction were represented by a straight line, this factor would not be 

 operative under the conditions of selection used in this study. Woll 

 (191 2) has shown that the monthly decrease is practically constant until 

 the eighth month, when a more rapid fall begins. In the present study, 

 practically all the cows were • less than eight months advanced in the 

 stage of lactation at the end of the twenty- weeks period. Therefore this 

 factor of an accelerated rate of decrease with advancing lactation would 

 not be operative under the method of selection used. 



Another factor that becomes operative with varying stages of lactation 

 is the decrease in milk flow due to the development of a fetus. The 

 results of Gavin's work have already been cited showing that this decrease 

 is not constant. However, this factor may be as largely operative in 



343 



