Submitted to: American Dairy Science Association Proceedings
Publication Type: Abstract Only
Publication Acceptance Date: July 26, 2000
Publication Date: N/A
Technical Abstract: The dairy industry has expressed concern that bull evaluations for yield traits decline too often. Although changes in evaluations with additional data are accepted, earlier evaluations are expected to be unbiased; i.e., individual evaluations may change, but means for any subset of bulls should remain the same. To examine evaluation stability, January 1995 genetic evaluations of yield traits for 540 Holstein bulls in active artificial- insemination (AI) service at that time were compared with their February 2000 evaluations. Mean changes in predicted transmitting ability (PTA) were small (-9 kg, milk; -1.4 kg, fat; -.5 kg, protein); mean reliability (REL) increased from 81.8 to 96.1%. Forty bulls with low (<70%) reliability in 1995 had much larger PTA changes (-63 kg, milk; -2.4 kg, fat; -1.6 kg, protein) than did 97 bulls with high (>90%) REL in 1995 (+11 kg, -.2 kg, +.3 kg). Bulls in the top decile for PTA milk in 1995 had the largest PTA declines (-50 kg, milk; -4.4 kg, fat; -2.5 kg, protein); mean PTA for bull in the lowest decile were nearly unchanged (+2 kg, -.4 kg, +.3 kg). Mean PTA of the 406 bulls from traditional AI organizations were fairly stable (-5 kg, milk; -1.3 kg, fat; -.4 kg, protein), but PTA declines for the other 134 bulls were larger (-23 kg, -1.8 kg, -.8 kg). Changes in PTA were correlated positively with initial REL and negatively with REL change over 5 years. Similar results were found for active AI bulls in February 1997, except that PTA tended to decline more often overall. Thus, active AI bulls tended to decline in PTA, particularly for bulls with lower REL, higher PTA, and from less traditional AI organizations. Overevaluation of such bulls will be reexamined after implementation of model and base changes in the national system for genetic evaluations in August 2000.