Accuracy and stability of national and international somatic cell score evaluations.

Somatic cell score (SCS) evaluations have been published in the United States since 1994 and international evaluations have been available through Interbull since May 2001. The accumulated data provides an opportunity to investigate the accuracy and stability of SCS evaluations. United States 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, predicted transmitting ability (PTA) SCS increased (worsened) by 0.002 from earlier evaluations to November 2004. Although bias was small, PTA changes were more than expected based on change in reliability. When looked at sequentially, bulls' earlier evaluations were generally lower (i.e., merit was overestimated) relative to November 2004. Differences were small, and PTA SCS increased steadily with the addition of second-crop daughters. All 524,081 evaluations were considered pairwise providing over 8,000,000 pairs of bulls' evaluations for analysis of PTA differences relative to change in reliability. Agreement of observed and expected SD improved for larger changes in reliability. The November 2004 US and Interbull PTA were matched with US and Interbull PTA from May 2001 (US04, IB04, US01, and IB01, respectively) for 14,652 Holstein bulls. For bulls having only US daughters in IB01, correlations were similar for US01 and IB01 with US04, and IB01 with IB04. Corresponding regressions were all nearly 1.00. For bulls also having nonUS 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 higher for IB01 than US01, showing that inclusion of foreign data improved predictive value of SCS evaluations.