Impact of Different Strategies and Amounts of Preferential Treatment on Various Methods of Bull-Dam Selection

Abstract

Three records of milk yield, fat yield, and type were simulated for each cow in 20 herds of 200 cows over 13 yr. Preferential treatment or bias was simulated by increasing milk and fat yields by an average of 0, 16, and 32% for separate copies of the simulation. The bias was given to a limited number of cows from the original herds based on four strategies. Five methods of bull-dam selection that used an index with a 2:2:1 ratio of milk to fat to type to select the top 2% of cows were compared: ETA using first lactation, using all lactations, after phenotypic minima were required, after preselection on three-generation pedigree index, and on pedigree index alone. Selection on ETA for first or all lactations gave the highest average of true breeding values at 0 and 16% for all strategies studied. In general, selection on pedigree index alone or after phenotypic minima were required gave poor results and should not be considered to be viable. Preselection of bull-dams on pedigree index proved to be extremely useful for biased and unbiased data. The optimal policy was to preselect the top 12% of the population before reranking and selecting on ETA for all lactations. © 1994, American Dairy Science Association. All rights reserved.