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Reliability of Averages of Different Numbers of Lactation Records for Comparing Dairy Cows¹

University of British Columbia

ABSTRACT

Formulae which describe the relative reliability for prediction purposes of averages based on different numbers of records have been presented.

The major increase in reliability occurs when a second record becomes available to use along with the first. The addition of a third record adds considerably to the reliability of the estimate. Records beyond the third do contribute more information but so little that they scarcely appear to be worth waiting for before estimating the consequences of keeping or culling the cow herself or her offspring.

It is concluded that cows can be compared fairly, without bias because of differences in number of records, by use of the following prediction equations:

1. Real producing ability (W) = herd average
   times (cow's average - herd average).
   
2. Transmitting ability or breeding value = herd average
   times (cow's average - herd average).
   

In these equations n is the number of records in the cow's average, r is the average intra-herd repeatability of records of the same cow (usually of the order of 0.3 to 0.5), q is the average intra-herd correlation between dam and daughter records (probably not far from 0.1 generally), and w², which is that part of r left after the effects of proximity are removed, is believed to have a value of approximately 0.03 to 0.09 less than r.

¹ Journal paper No. J-971 from the Iowa Agricultural Experiment Station. Project No. 317.

² This study was made while the author was Research Fellow at Iowa State College. He is indebted to Dr. J. L. Lush and to Dr. C. Y. Cannon for guidance during the conduct of this study and for valuable criticism of the manuscript.