154 Essay on the Interpretation of Milk Records. 



writer, working on the records kept for the last twenty-four 

 years on Lord Rayleigh's dairy farms, has endeavoured to 

 deal with the preliminary questions that will arise in the 

 interpretation of these records. 



The first difficulty in endeavoviring to trace the inheritance 

 of milk yield in cows is to assign a definite numerical value to 

 their inherent milking capability. 



Breeders generally rely on such figures as total yield per 

 calf, total yield per calendar year, average per week, &c., but 

 the enormous fluctuations found in the same animal show 

 these to be subject to a variety of outside influences. To 

 make them of value it is necessary to enumerate in every 

 instance the particular circumstances in which the cow in 

 question has been placed during the period taken. 



In any statistical study of the inheritance of milk yield, or 

 indeed in any systematised breeding where more than a few 

 cows are dealt with, such a procedure is of course impossible, 

 and it becomes necessary to define a cow's milking capability 

 as accurately as possible by a single and unqualified figure. 

 By milking capability is meant of course the cow's individual 

 somatic power of giving milk. No reference is intended to 

 her genetic qualifications in this respect. 



The chief of these exterior circumstances that affect the 

 milk yield seem to be : — 



1. Age of cow, 



2. No. of weeks in milk. 



3. „ „ ,, rest before calving (i.e., since the end of 

 the previous lactation period). 



4. Interval between calving and subsequent service. 



5. Time of year of calving. 



6. Food, weather, and general treatment. 



It is therefore necessary : — 



(a) To select a figure affected by the minimum number 



of these influences, and 

 (J) To estimate as accurately as is possible the effect of 



those influences under which it does fall. 



(a) Selection of a figure. 

 After a preliminary examination of the material it was 

 decided to see whether either of the two following figures 

 would give a satisfactory basis of comparison, namely : — 



1. Average yield per day from the fifth to twelfth week 



after calving. 



2. Maximum yield on any one day. 



It was subsequently thought that a single maximum, that 

 is, the highest daily yield that appeared once during a cow's 



