232 AGRICULTURE OF MAINE. 



From this table the following constants are deduced: 



Average 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 X 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 services 

 occurred within ten hours after the discovery of heat. The in- 

 ference from this table is plainly that if one wishes to be sure 

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

 much beyond ten hours after the cow is known to be in heat. 



3. There appear to be no significant dififerences 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 OE TIME A BREEDING 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 individual 

 and is probably a matter of inheritance in large part; the other, 

 the method of managing and feeding the animals. Some pro- 

 gressive dairymen aim to keep a cow continuously milking, 

 never letting her go dry, but keeping her in good physical condi- 

 tion by appropriate feeding. It is of interest to examine statis- 

 tically the facts regarding this matter from the herds of some- 

 thing like 150 of the most progressive dairymen in Maine. The 

 data for 712 cows are given in Table 7. 



