R. L. Powell and H. D. Noman
Animal Improvement Programs Laboratory, Agricultural Research Service,
USDA, Beltsville, MD 20705-2350
ABSTRACT Holstein bull predicted transmitting abilities (PTA) for milk, fat, and protein yields were compared for January 1995 and May 2000, the beginning and end of the 1995 genetic base definition. Overall, PTA for the 14,012 artificial insemination (AI) bulls born in 1980 or later were stable for mean but varied slightly more than expected based on increase in data. As expected, changes were larger for bulls with lower initial reliability (accuracy) and greater increases in reliability over the 5.3 yr. For the 540 bulls in active AI service during 1995, mean PTA changes were near 0 (8 kg for milk, 1.3 kg for fat, and 0.5 kg for protein). Bulls in the top decile for PTA milk, fat, and protein declined 47, 2.2, and 2.2 kg, respectively, but that bias was not evident for bulls that had been progeny tested by major AI organizations (sampling code S). The standard deviation for change in PTA milk for top decile bulls was 191 kg; expected standard deviation was 140 kg. Consideration of only bulls with a sampling code of S removed the excess variation, which indicates that bulls from other sampling programs are the source of bias and lack of stability. Bulls with a sampling code other than S appeared to have an initial PTA that was biased, largely because of an inflated estimate of daughter performance. The hypothesis that inflated means of parent PTA produce inflated early PTA of bulls was not corroborated. Stability of the genetic base and unbiasedness of PTA was demonstrated for bulls with a sampling code of S, but concern continues for bulls with other sampling codes. Because much of the bias in PTA for those bulls is expected to be the result of preferential management of early daughters, improvement of the evaluation system to remove that bias will be difficult.
(Key words: genetic evaluation, bias, sampling, stability)
2001 J. Dairy Sci. 84:1913
© 2001, by the American Dairy Science Association. All rights reserved.