SELECTION INDEXES FOR CANADIAN BEEF-PRODUCTION USING SPECIALIZED SIREAND DAM LINES

Relative economic values and literature estimates of genetic and pheno typic variances and covariances were used to develop selection indices for specialized maternal and terminal strains of beef cattle. The env isioned commercial production setting used three maternal strains mate d in rotation. Yearling heifers were mated to a terminal sire strain ( M3) to minimize dystocia and neonatal mortality without closely monito red calving. Mature cows were bred to a second terminal strain (TX) to produce market progeny. Accuracies for all indices were low: 0.22, 0. 22 and 0.19 for the maternal strains, M3 and TX, respectively. Predict ed correlated responses to index selection were not economically favor able for all traits within strain but were offsetting between strains. At the production system level, genetic improvement in profitability resulted from all traits except feed conversion. Fertility and calf su rvival were improved in all strains. Direct genetic potential for grow th was reduced in the maternal strains but was increased in TX and to a lesser degree in M3. There was a tradeoff between maternal and termi nal strains in responses of carcass traits. Maternal strains increased in dressing percentage and decreased in cutability, while M3 and TX d ecreased in dressing percentage and increased in cutability. Arduously slow improvement in profitability is anticipated: $1.07 per breeding female per unit of selection response.