dairy, seed improvement, stock breeders meetings. 233 



Table; 7. 



Showing the Time {in days) During ivhich Cotvs were Dry 



before Calving. 



From this table we have the following constants: 

 Average number of days dry := 56.404^1 .744 



Standard deviation in number of days dry = 2().^io± .526 

 Coefficient of variation = 52.14 per cent. 



It thus appears that on the average these cows were dry about 

 4 days short of two months. Only about 14 per cent of them 

 were dry more than 79 days. On the other hand about 29 per 

 cent were dry less than 40 days. Cows dry four, five, and six 

 months can scarcely be profitable. There must be a very strong 

 reason for desiring a calf from a particular cow to warrant 

 carrying her dry in the herd for a long period of time. 



3. THE AGE OF BREEDING STOCK. 



The age of the animal is an important factor in many prob- 

 lems of cattle breeding. For example, as has been pointed out 

 already (p. 216), the milk production of a cow changes with 

 age in a definite way. This means that the profitable limits of 

 age of a cow as a milker can be determined with precision. What 

 is the composition of the average herd with reference to the 

 age of the cows composing it? Is it in major part made up of 

 cows at their most productive ages? 



Again the question of age is of direct importance in any com- 

 prehensive scheme of herd improvement by breeding. No 

 principle of genetic science appears to be more solidly grounded 

 than that progeny performance is the only sure test of breeding 

 worth. But if a herd bull is disposed of before any of his pro- 

 geny have reached an age where their performance as milkers, 

 for example, can be measured, then clearly this guiding principle 



