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

Abstract

Butterfat records of 454 6-record cows and of their 979 dams and daughters were studied (Hol.-Fri. Assoc. of America, H.I.R. data). The degree of repeatability of records of the same cow is indicated by the average values gross [tau] = 0.41, intra-herd [tau] [long dash]0.29. Between dam and daughter records the av. values found were gross [tau] = 0.19, intra-herd [tau] = 0.07. Records separated by 1 or more lactations were found to be less closely correlated than were adjacent records. The size of this "proximity" effect in the intra-herd repeatability study is indicated to be of the order of 0.03 to 0.09. Prediction value was found to increase materially for averages of 2 or 3 records as contrasted to the use of a single record. Formulae which describe the relative reliability for prediction purposes of averages based on different numbers of records and which. can be used to estimate a dairy cow''s real ability and her transmitting ability are presented.