H.D. Norman,1 J.R. Wright, R.H. Miller
Animal Improvement Programs Laboratory, Agricultural Research Service, USDA, Beltsville, MD 20705-2350
2009 J. Dairy Sci. (?)
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Four U.S. genetic-economic indexes for dairy cattle were retrofitted to compare progress that could have been made for currently evaluated traits if selection had been based on those indexes across 2 generations. Holstein AI bulls (106,471) were categorized by quintile for each index, and 25 cow groups were formed based on sire and maternal grandsire quintiles. Data included records from 1,756,805 cows in 26,106 herds for yield traits, productive life, pregnancy rate, and somatic cell score; 692,656 cows in 9,967 herds for calving difficulty; and 270,564 cows in 4,534 herds for stillbirths. For each index, least-square differences between the 25 cow groups were examined for 8 first-parity traits (milk, fat, and protein yields; productive life; somatic cell score; pregnancy rate; calving difficulty; and stillbirth) that had been standardized to mature equivalence. Analysis was on a within-herd basis with cow birth year in the model. Seven of the cow groups were recombined into 3 groups based on selection intensity for their male ancestors (low, medium and high). The cow group from high selection intensity on the 2006 net merit index produced more milk (219 kg), fat (21 kg), and protein (11 kg) and had longer productive life (6.3 mo), lower somatic cell score (0.21), higher pregnancy rate (1.2 percentage units), fewer difficult births in heifers (3.8 percentage units), and lower stillbirth rate (4.6 percentage units) than did the group from low selection intensity. Yield advantages based on 197l (milk and fat) and 1977 (milk, fat, and protein) indexes were much larger but were less favorable and sometimes even unfavorable for the other traits. The 1994 net merit index generally produced differences that were intermediate to those of the 1976 and 2006 indexes. Selection based on the 2006 net merit index should provide phenotypic improvement for all traits included in the index and result in more efficient and healthier dairy population.
(Key words: genetic-economic index, selection, yield, fitness)