AGE OF COW IN RELATION TO MILK YIELD 83 



of condition for making high records, the low records when the 

 conditions are unfavorable. There is a tendency for high and low- 

 record cows when retested to have their records regress toward the 

 average of the breed, due to the change in conditions surrounding 

 the cow. The correction on the basis of the mean curve lake- do 

 cognizance of the environmental conditions but assumes that the 

 conditions remain as they were in the first test. The method of 

 correlations as given elsewhere takes the changing conditions into 

 account. Both methods have their uses. 



In view of these facts, as illustrated by the records of Murne 

 Cowan, what is the probable error within the limits of which we 

 might expect the record of any one cow? The standard deviation 

 of milk yield offers a means of determining this range. The stand- 

 ard deviation of the milk yields of 8-year-old cows is 2335 or tha 

 approximate probable error is 0.67449 X 2335 or 1575, or for any 

 determination found in table 4 of Bulletin 311 of the Maine Station 

 it is about an even chance that the actual milk yield of the cow will 

 be within 1600 pounds either side of the figure shown. Thus for 

 Murne Cowan's 6-year-old milk yield the probable 8-year production 

 was 17,682 ± 1575 and the 11 year old milk yield predicted 17,695 

 =fc 1575 as the probable 8-year-old production. In her phenomenal 

 record this cow produced much over this mark. In fact, it may be 

 shown that there is only one chance in over 25,000 that a cow would 

 make such a record as she actually did. 



The following references should be read: 



REFERENCES 



M. S., Chapter IV. 



Pearl, Raymond, and Patterson, S. W. 1917. The change of milk flow with 

 age, as determined from the seven-day records of Jersey cattle, Annual 

 Report of the Maine Agricultural Experiment Station, Bulletin 2(Y2, 

 pp. 145-152. 



Pearl, Raymond, and Miner, John Rice. 1919. The variation of Ayrshire 

 cows in the quantity and fat content of their milk, Jour. Agricultural 

 Research, vol. XVII, pp. 285-322. 



Pearl, Raymond, Gowen, John W., and Miner, John Rice. 1919. Studies 

 in milk secretion. VII. Transmitting qualities of Jersey sins for milk 

 yield, butter-fat percentage, Annual report of the Maine Agricultural 

 Experiment Station for 1919, Bulletin 281, pp. S9-2G4. 



Gowen, John W. 1920. Studies in milk secretion VIII. On the infim 

 age on milk yield and butter-fat percentage, as determined from tin 



