Wilson — The Variations of Milk Yield with the Cotv^ Age. 103 



Had Mr. Gavin continued his observations over a longer time, he would probably 

 have been able to indicate hov*f yield varies with length of lactation. This can be 

 done, however, with the Ayrshire reports. The records in those for 1913, 1919, and 

 1920 — the war period reports contained less material data — have first been divided 

 into separate groups according to the cows' ages ; then each of these age groups has 

 been subdivided into groups according to the lengths of the lactation periods in 

 calendar months. Groups have been formed for all ages of cows between two 

 years and twelve and for all lengths of lactations between nine months and eighteen. 

 But, as the numbers in the remaining groups are few, we shall use only those con- 

 taining the yields of three- to eight-year-old cows in eleven-, twelve-, thirteen-, 

 fourteen-, and fifteen-month lactation periods. 



In some groups there are yields which are abnormally large and others which 

 are abnormally small. They may be correct, but they are still abnormal and 

 distort the averages. A two-year-old heifer may have given 1050 gallons of milk 

 in a thirteen months' lactation period ; but if her yield be added to those of 

 thirty otlier heifers whose average is nearer 600 gallons, the average for the group 

 is increased by fifteen. Such yields must be neglected, and this has been done 

 by eliminating all which were more than twenty-five per cent, above or below 

 the average of their group as it stood when the extraction from the reports was 

 completed. This method does not give absolute accuracy — no method does — 

 for groups which were loaded originally with an unusual number of high or 

 low figures may still have an unfairly high or low average when the elimination 

 has been completed, but the unfairness is not great. 



The following table, which is constructed from the data in the Ayrshire Report 

 for 1920, shows the average number of weeks cows of different ages were in milk 

 during lactation periods of different lengths. The numbers of cows are enclosed 

 within brackets : — 



Mr. Gavin found that the yields of cows which are in calf begin to fall below 

 those of cows which are not in calf about twenty-four weeks before the next calves 

 are born (40 weeks in gestation, minus 16, during which the yield is unaffected, 

 equal 24). The foregoing table indicates how long cows continue in milk in 

 lactation periods of different lengths. If, then, as in the diagram, the points on 

 the thick line where the yields of cows having lactation periods of different length 

 begin to fall below those which are not in calf be joined with the points on the 



' This figure is omitted in the average. 



