Skip to main content
ARS Home » Research » Publications at this Location » Publication #174070
Title: ACCURACY AND STABILITY OF NATIONAL AND INTERNATIONAL SOMATIC CELL SCORE EVALUATIONS

Author
item POWELL, REX
item SANDERS, ASHLEY
item NORMAN, H

Submitted to: Journal of Dairy Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/28/2005
Publication Date: 7/1/2005
Citation: Powell, R.L., Sanders, A.H., Norman, H.D. 2005. Accuracy and stability of national and international somatic cell score evaluations. Journal of Dairy Science. 88(7):2624-2631.

Interpretive Summary: Health traits continue to gain importance and somatic cell score (SCS), an indicator of udder health, now has the highest relative value (9%) aside from yield and productive life in the US Net Merit index. Investigating the accuracy and stability of SCS evaluations is warranted, as these are key factors in successful genetic selection. Earlier evaluations tended to slightly overestimate merit, and SD of changes were higher than expectation based on reliabilities, but bias was small. Comparisons of US with multi-country evaluations showed increased accuracy from added foreign daughter information in predicting later evaluations. Evaluation systems for SCS are generally operating as they should.

Technical Abstract: Somatic cell score (SCS) evaluations have been published in the United States since 1994 and international evaluations have been available through Interbull (IB) since May 2001. The accumulated data provides an opportunity to investigate the accuracy and stability of SCS evaluations. US domestic evaluations from January 1995 through August 2004 for 21,500 Holstein bulls were considered over time, and sequentially within bull for changes to the November 2004 evaluation. On average, SCS evaluations increased (worsened) by 0.002 from earlier evaluations to May 2004. Although bias was small, evaluation changes were more than expected based on change in reliability (REL). Looked at sequentially, bulls' earlier evaluations were generally lower (i.e. merit was overestimated) relative to November 2004. Differences were small, and scores increased steadily with the addition of second crop daughters. All 8,001,717 pairs of bulls' evaluations were considered for PTA differences relative to change in REL. The SD of change were larger than expected, but agreement of observed and expected SD improved for larger changes in REL. November 2004 US and IB PTA were matched with US and IB PTA from May 2001 (US04, IB04, US01, and IB01, respectively) for 14,652 Holstein bulls. For bulls with only US daughters in IB01, correlations were similar for US04 with US01 and IB01, and IB01 with IB04 and corresponding regressions were all nearly 1.00. For bulls also having non-US daughters in IB01, correlations with yield deviations calculated for later daughters (used as source of independent data) were higher (0.747 vs. 0.714) for IB01 than for US01. For bulls with added US daughters, correlation with US04 was also better for IB01 than US01, showing that inclusion of foreign data improved predictive value of SCS evaluations.