IMPACT OF ESTIMATED GENETIC CORRELATIONS ON INTERNATIONAL EVALUATIONS TO PREDICT MILK TRAITS

Authors: POWELL, REX; SANDERS, ASHLEY; NORMAN, H

Submitted to: Journal of Dairy Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/13/2005
Publication Date: 10/1/2005
Citation: Powell, R.L., Sanders, A.H., Norman, H.D. 2005. Impact of estimated genetic correlations on international evaluations to predict milk traits. Journal of Dairy Science. 88(10):3679-3687.

Interpretive Summary: The International Bull Evaluation Service combines national genetic evaluations of dairy bulls by treating them as different, but correlated, traits. Applying estimated genetic correlations is meant to improve accuracy but also results in different rankings of bulls on each national scale. This study was to examine whether the confusion from different rankings is justified by improved accuracy of prediction for bulls lacking local daughters. Data for 10 countries for yield (6 for udder health) showed that prediction improved using genetic correlations estimated for each country pair versus unity, marginally for most countries but substantially for scales and bulls of Australia and New Zealand.

Technical Abstract: The Interbull procedure for combining dairy bull evaluations uses estimated genetic correlations between countries. It is important to know whether the resulting difficulties from differences in ranking in each country are justified by improved accuracy relative to a system assuming unity correlations. Data submitted for the May 2001 yield and somatic cell score (SCS) Interbull evaluations were processed both with the usual estimated genetic correlations (E01) and assuming these correlations to be essentially unity (0.995) (U01). The two sets of resulting evaluations were compared with August 2004 national evaluations (N04) for bulls not having local evaluations used in the 2001 evaluations. Thus, the examination was of Interbull evaluations from foreign data in predicting national evaluations. Countries in the study for yield were Australia, Canada, France, Germany, Great Britain, Ireland, Italy, The Netherlands, New Zealand, and the United States. Countries included for SCS were Canada, France, Germany, Great Britain, The Netherlands, and the United States. Standard deviations of differences between E01 or U01 and N04 were smaller for E01 by about 5 to 7% and correlations between E01 and N04 were higher by 0.01 or the same as for U01 and N04. While use of estimated correlations tended to improve prediction, the advantage was small. A previous study had concluded no difference in accuracy for yield but did not include Australia and New Zealand, countries with the lowest correlations with other countries. Excluding bulls from those countries produced results for the other 8 countries more like the previous study, but still favoring E01 slightly. Those two countries were not in the SCS data. Estimated genetic correlations improved the prediction of future national evaluations slightly in most countries but substantially for the bulls and scales of Australia and New Zealand.