82 MANUAL OF DAIRY CATTLE BREEDING 



That is if a cow has a record of 8000 pounds as a two-year-old 

 and we wish to get her probable record at 8 years we multiply 

 8000 X 1.417 or the probable record is 11,337 pounds. 



The class should determine these correction factors for different 

 ages entering their results on pages 8-87 and make a table similar to 

 that given in Maine Station Bulletin 311, page 17. 



The student will find a good deal of interest in making comparisons 

 like these. Murne Cowan, 19797, has three records for Advanced 

 Registry. Each record is for the year period. The first record 

 is for 16,729 pounds of milk at 6 years 3 months of age. The second 

 record is for 24,008 pounds of milk at 8 years 9 months of age. The 

 third record is for 17,384 pounds of milk at 11 years 1 month of 

 age. If we examine table 4 in Maine Station Bulletin 311 we note 

 that in the six year age row, at 17,000 pounds of milk (column 15) 

 the expected production of this cow at 8 years would be 17,682 

 pounds instead of the 24,008 pounds she actually produced. In 

 other words, her production was 6326 pounds more than was ex- 

 pected'. Before considering this difference, let us examine the 

 third record. Figure 3 of this bulletin shows that, on the average, 

 the cows at 11 years 1 month produce slightly more milk than those 

 at 6 years 3 months. The 6 year 3 month record of Murne Cowan 

 would consequently predict a record of about 17,000 pounds at 

 11 years 1 month old or a record closely similar to that actually 

 made. This illustration brings out a true biological fact concern- 

 ing milk records. The phenomenal record is made under such pres- 

 sure that the cow, herself, and every condition surrounding her 

 must be most favorable. Furthermore, the most favorable con- 

 ditions for one cow may not be favorable to another so that it is 

 extremely hard to duplicate the conditions which are favorable. 

 The high records tend to be made when the cows are in the pink 



variation of milk yield with age. The formula which determines this rela- 

 tion is milk yield at standard age (F s ) equals milk yield at x age (F x ) multi- 

 plied by standard deviation of the milk yield at standard age (o- s ) divided 

 by standard deviation at x age (<r x ), minus mean milk yield at x age (Yx) 

 multiplied by standard deviation at standard age (cr s ) divided by standard 

 deviation at x age (<t x ), plus average milk yield at standard age (F s ) or 



v _ ^ x °" s Fx q~s . — 



' 6 — — T Is 



fy <Xx 



where the overlined value represents mean values. 



