Retrofitting genetic-economic indexes to demonstrate responses to selection across 2 generations of Holsteins

Authors: Norman, H; Wright, Janice; Miller, Robert

Submitted to: International Committee on Animal Recording(ICAR)
Publication Type: Proceedings
Publication Acceptance Date: 6/16/2008
Publication Date: 1/12/2009
Citation: Norman, H.D., Wright, J.R., Miller, R.H. 2009. Retrofitting genetic-economic indexes to demonstrate responses to selection across 2 generations of Holsteins. International Committee on Animal Recording (ICAR). ICAR Tech Ser. 13:181–185. 2009.

Interpretive Summary: First-parity least-squares means for cows that had sires and MGS selected based on NM06 demonstrate that selection based on the current USDA genetic-economic index for dairy cattle should provide phenotypic improvement for all traits included in the index. Such selection will result in a dairy population that performs more satisfactorily for a number of health and fitness traits. Some of the expected improvements are large enough that they will be noticeable to producers in a single generation (e.g., increases in productive life and pregnancy rate and declines in somatic cell score and stillbirths). Future concern by consumers about animal welfare issues should be reduced substantially through the use of a comprehensive composite index that includes health and fitness traits even though progress for yield traits will be slowed.

Technical Abstract: Three U.S. genetic-economic indexes for dairy cattle were retrofitted to demonstrate the progress that would have been made for currently evaluated traits if selection had been based on those indexes across 2 generations. Holstein bulls 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 ease; 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. When cow groups were combined by selection intensity (low, medium and high), the highly selected group based on the 2006 net merit index had an advantage of 219 kg more milk, 21 kg more fat, 11 kg more protein, 6.3 mo longer productive life, 0.21 lower somatic cell score, 1.2 percentage units higher pregnancy rate, 0.19 lower calving difficulty score and 5 percentage units lower stillbirth rate over the group with low selection intensity. Corresponding differences for the 1976 index, which included only milk, fat and protein yields, were larger for yield traits but were less favorable for other traits. The 1994 net merit index had differences intermediate to 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 a dairy population that performs more satisfactorily for a number of health and fitness traits.