MAINE AGRICULTURAL EXPERIMENT STATION. 



19 



TABLE 7. 



Showing the Time {in days) during ivhich Cozvs were Dry 

 before Calving. 



Days Dry. 



Frequency. 



Percentage 

 Frequency. 





0-39 



205 



28.79 





40-79 



408 



57.30 





80-119 



65 



9.14 





120-159 



28 



3.93 





160-199 



4 



.56 





200-239 



2 



.28 





Total 



712 



100.00 



From this table we have the following constants : 

 Average number of days dry r= 56.404d= .y^ 



Standard deviation in numiber of days dry := 29.410^; .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. 2), the milk production of a cow changes with 

 age in a definite way. This means that the profitalble 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 

 comprehensive 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 hi^ 

 progeny have reached an age where their performance as 

 milkers, for example, can be measured, then clearly this guid- 



