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Factors Affecting Stability of Modified Contemporary Comparison Sire Evaluations for Milk Yield¹

Department of Dairy Science, Virginia Polytechnic Institute and State University, Blacksburg 24061

ABSTRACT

Characteristics of early Predicted Differences for milk from Modified Contemporary Comparison were related by regression analysis to stability of evaluations over time as measured by change of Predicted Difference. Change was considered both as actual difference and as absolute difference. Bulls were required to add at least 25 daughters between early and last available evaluations. Characteristics of early evaluations included number and production of daughters and contemporaries; effective daughters per herd; number, deviation milk, and repeatability of contemporary sires; percent daughters registered and percent culled in first lactation; percent records in progress; pedigree value of the bull; and Predicted Difference for type of the bull, his sire, and his maternal grandsire. Regression models accounted for relatively little of the variation of change of Predicted Difference, suggesting that random sampling was a major cause of change. Adding type information explained much variation, but measures of type were not associated with large differences of stability. Production by daughters and contemporaries significantly influenced changes and could aid decisions by studs. Artificial insemination and natural service bulls that entered artificial insemination service had similar absolute changes and were less stable than natural service bulls remaining in natural service. The three groups did not differ for actual change of Predicted Difference.

¹ Supported by a grant from Eastern Artificial Insemination Cooperative Inc., Ithaca, NY.