Submitted to: Journal of Dairy Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: January 30, 2002
Publication Date: N/A
Interpretive Summary: Expression of genetic evaluations of dairy bulls across national borders is the priority of the International Bull Evaluation Service (Interbull). The Interbull system uses pedigree information and bulls evaluated in more than one country to provide ties between countries. Estimates of genetic correlation between each pair of countries (a measure of similarity in ranking) are used to calculate evaluations for all bulls on the national scale of each member-country. Differences in bull rankings across country scales make it difficult for non-member countries to interpret results, and for sire marketers to provide information that can be clearly understood and used in a multi-national market. The objective was to examine methods for combining Interbull evaluations into a common global evaluation or subglobal evaluations and to compare the relationships between the developed scales and existing national scales. Interbull evaluations of Holsteins from May 2001, expressed on 27 national scales were combined and expressed on a global scale. Overall, bull rankings on national scales were more similar to rankings on a global scale than to rankings on other national scales. The number of top 100 bulls for protein yield in common between national and global scales was significantly related tp the mean genetic correlation between a country and the other 26 countries. Alternative weighting procedures gave very similar results. Subglobal scales were established by grouping highly related countries. For countries with atypical conditions, these may be more useful. A global scale, or a limited number of subglobal scales would likely be beneficial for most non-participating countries and in global advertising and would simplify sire selection with little loss of accuracy.
Genetic evaluations on a global scale were calculated for Holstein bulls using the May 2001 International Bull Evaluation Service (Interbull) evaluations expressed on each of 27 national scales. National scale data were weighted by the country's proportion of total daughters from all bulls (population size) to represent market share. Correlations within birth year between Interbull evaluations on national scales and evaluations on a global scale ranged from 0.961 to 0.998 (mean of 0.988). Number of top 100 bulls for protein yield that were in common between national and global scales ranged from 54 to 94 and was related significantly to mean genetic correlation between a country and the other 26 countries. Weighting of evaluations on national scales by population size, inverse of population size weight, or equal weight produced practically the same group of top bulls and correlations within birth year among the three global scales were 0.999. Thus, the method for combining Interbull evaluations expressed on national scales had only minor impact and was much less important than use of all data. Subglobal scales were established by a clustering technique that gave 2 to 5 groups. For grazing countries or other atypical systems, a subglobal scale may provide better guidance, although a scale representing three grazing countries did not provide the expected improvement over a global scale in the relationship with the three country scales. If conditions in non-participating countries are generally represented by participating countries, most needs are met by a global scale.