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Animal Improvement Programs Laboratory, Agricultural Research Service, USDA, Beltsville, MD 20705-2350
Corresponding author: H. D. Norman; e-mail: dnorman{at}aipl.arsusda.gov .
If genetic evaluations are calculated with a single-trait
repeatability model, evaluation changes may be attributed in part to
bulls that have daughters that deviate considerably from the
typical response to aging. Differences in maturity rate of bull
daughters were examined to determine whether they influence change
in bull evaluations. Standardized milk records for Holsteins
that first calved between 1960 and 1998 were used to calculate
12 tailored predicted transmitting abilities (PTA) for each
bull. Predicted transmitting abilities were tailored from
combinations of 4 annual cut-off dates and 3 parities. Date screening
selected cows first calving before January of 1996, 1997, 1998, or
1999. Parity screening selected milk records from the first 1,
2, or 3 parities. Therefore, 4 evaluations (PTA1) included
only first-parity records available for daughters and
contemporaries prior to the respective years designated. Four more
evaluations (PTA1,2) included the records from the first 2
parities for cows first calving prior to those same year cutoffs;
likewise, the last 4 evaluations (PTA1,2,3) included
records from the first 3 parities. Stability of bull evaluations
(standard deviations of differences as well as correlations between
bull evaluations) across time was compared. Bulls born after 1984
with 500 daughters
were of interest because of the high precision of evaluations
and recent activity. Tailored PTA of those bulls had more uniformity
across years in mean records per daughter than did official
USDA PTA. Standard deviation of differences in PTA1,
PTA1,2, and PTA1,2,3 for milk between
evaluation years 1996 and 1997 were 28, 28, and 27 kg compared with
63 kg for official evaluations; similarly, between 1996 and 1999, SD
were 36, 32, and 32 kg compared with 80 kg. Results suggested that a
modification to the current evaluation model to account for maturity
rate should reduce fluctuations in individual bull PTA across time
and may improve accuracy of evaluations.
Key Words: genetic evaluation · maturity rate · milk yield · parity
Abbreviations: PTA1 = PTA based on first-parity records · PTA1,2 = PTA based on first- and second-parity records · PTA1,2,3 = PTA based on first-, second-, and third-parity records · PTA2 = PTA for second-parity lactation · PTA3 = PTA for third-parity lactation.
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