l8 MAINE AGRICULTURAL EXPERIMENT STATION. 



From this table the following constants are deduced : Aver- 

 cge number of hours from 



Discovery of heat to service ^ 6.357=!= .134 hrs. ■ 

 Standard deviation • = 5.737=+= .095 hrs. 



Coefficient of variation == 90.25 per cent. 



From the above data it appears that: 



1. Successful fecundation of the cow may occur as many 

 as 41 -]- X hours after the onset of heat. What the value of 

 .1- in this expression is, is not entirely clear, but the available 

 evidence indicates that it is small. 



2. While in an isolated instance successful service after 

 so long a time as 41 hours may occur, we see from Table 6 

 that, in general, the vast majority of successful services occur 

 at much shorter time intervals than this after the discovery 

 of heat. Thus over 79 per cent of these 834 successful ser- 

 vices occurred within 10 hours after the discovery of heat. 

 The inference from this table is plainly that if one washes to 

 be sure of getting a cow with calf it is not wise to postpone 

 service much beyond 10 hours after the cow is known to 

 be in heat. 



3. There appear to be no significant differences between 

 the distributions for the different breeds. 



Further work on this subject, in which successful and un- 

 successful services will be compared, is now in progress. 



2. THE LENGTH OF TIME; A BREE^DING COW IS DRY. 



One of the points on which the cooperative record blanks 

 furnish data is as to the time each cow is dry prior to calving. 

 Two important factors are involved here : one, the normal 

 duration of lactation, which varies from individual to indi- 

 vidual and is probably a matter of inheritance in large part ; 

 the other, the method of managing and feeding the animals. 

 Some progressive dair}'men aim to keep a cow continuously 

 milking, never letting her go dry, but keeping her in good phy- 

 sical condition by appropriate feeding. It is of interest to 

 examine statistically the facts regarding this matter from the 

 herds of something like 150 of the most progressive dairymen 

 in Maine. The data for 712 cows are given in Table 7. 



