234 



Bulletin 397 



table I as it is in the case of the results here under discussion, for the 

 time of service bears no definite relation to the time of previous calving. 



In view of the above discussion, it is not very surprising that, under 

 the conditions of selection used, the coefficient of variability of cows 

 in various stages of lactation is not greater than the value for animals 

 uniform in this respect. If the twenty- weeks period included records 

 for the last three or four months of the lactation period, the results would 

 doubtless be quite different. 



The colimm Average number of weeks after calving is placed in the table to 

 show that the lowest producers are the ones most advanced in the stage of 

 lactation, and to show the decrease in the number of weeks with increasing 

 production. This is what would be expected, other conditions being equal. 



Fixed limits versus percentage limits in selecting individuals on the basis 

 of equal production. — Brief mention has been made of the possible effect 

 of a fixed range in class limits thruout the series, in selecting individuals 

 of similar production. In order to determine the possible effect of this 

 factor, the limits for selecting cows of equal production were fixed on 

 a percentage basis. The range of each group is 10 per cent of the lower 

 limit of the group. The range of limits thus fixed increases as the lower 

 limit increases. Beginning at 200 the class limits are thus: 200-219, 

 220-241, 242-265, 266-292, and so on. The variation of individuals of 

 similar production selected on this basis is shown in table 1 1 : 



TABLE II. Individual Variation; Percentage Limits Used for Classes of 



Individuals of Equal Production 



Individual production, two- weeks period 

 (pounds) 



220-241 . 

 242-265. 

 266-292 . 



293-321 . 

 322-353 . 

 354-388 . 

 389-426 . 

 427-469 

 470-516. 

 517-568 . 

 569-626 . 

 627-689 . 

 690-758 



759-834 

 835-918. 



Total . . . 

 Average . 



Average 



number of 



weeks 



after 



calving 



16 

 12 



14 

 II 



14 

 12 



13 

 15 



ID 



10 



6 



5 

 6 



5 

 4 



Number of 

 individuals 



4 



4 



9 



9 



10 



22 



13 



23 



29 



27 

 15 

 12 

 II 

 6 

 4 



198 



Coefficient of 

 variability 



30.07 



23-37 

 8.22 



13-51 

 9-52 

 10.35 

 10.58 

 10.37 

 12.31 

 10.83 



8-95 



17-25 



13-31 



7-43 



9-23 



± 

 ± 

 ± 



± 

 ± 



7.17 



5-57 



I-31 



2-15 



I 44 

 1.05 



1 .40 



1.03 



1 .09 



± 0.99 



± 

 ± 

 ± 

 ± 



1 . 10 

 2.38 

 1. 91 



1-45 

 2.20 



11.75 ±0.42 



344 



