Submitted to: Journal of Dairy Science
Publication Type: Peer reviewed journal
Publication Acceptance Date: 5/31/2005
Publication Date: 9/1/2005
Citation: Norman, H.D., Wright, J.R., Powell, R.L., Van Raden, P.M. 2005. Impact of maturity rate of daughters on genetic ranking of Holstein bulls. Journal of Dairy Science. 88(9):3337-3345. Interpretive Summary: Bull evaluations were calculated using only the first one, two, or three lactation milk yields for daughters first calving through 1995, 1996, 1997, and 1998. Results showed bulls appear to differ in rate of maturity of their daughters as reflected by their milk yields. Tailored evaluations for bulls with >=500 daughters were more stable across time, e.g., had < 25% of the residual variance of those in the official runs. A modification to the evaluation model to account for rate of maturity might be helpful in improving the accuracy.
Technical Abstract: If genetic evaluations are calculated with a single-trait 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 bulls' daughters were examined to determine whether they impact 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. Tailored PTA were derived from combinations of 3 parities and 4 annual cut-off dates. Date screening included cows first calving prior to January of 1996, 1997, 1998, or 1999. Parity screening included milk records from the first one, two, or three parities. Therefore, four evaluations included only first parity records (PTA1) available for daughters and contemporaries prior to the respective years designated. Four more included the first two parity records (PTA1,2) on cows first calving prior to these same year cutoffs; likewise, the last four included the first three parity records (PTA1,2,3). Stability of bull evaluations (standard deviations of differences, and correlations between bull evaluations) across time was compared. Bulls born after 1984 having >=500 daughters were of interest because of their high precision and recent activity. Their tailored PTA had more uniformity across years in mean records/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 to 63 kg for the official evaluations. Similarly, SD were 36, 32, 32, and 80 kg, respectively between 1996 and 1999. PTA 1,2 were usually intermediate to PTA1 and PTA1,2,3 (but closer to PTA1,2,3) for bulls having large differences between PTA1 and PTA1,2,3. There was little evidence that bulls' daughters deviate from a normal biological progression for maturity across parities. Results suggest that a modification to the current evaluation model to account for rate of maturity should reduce fluctuations in individual bull PTA across time, as well as improve the accuracy of evaluations.