Effectiveness of Progeny Test Index Selection for Field Performance of Strain-Cross Layers: II. Predicted and Realized Responses

Abstract

Selection for egg production and egg quality traits in two sets (TX1 and TX2) of pedigreed strain-cross White Leghorns was based on strain-cross progeny performance averaged over field environments. Retrospect index weightings, selection intensities, and genetic parameters were used to predict genetic responses in TX1 and TX2 for a 5-year period. Predicted genetic responses were compared with realized genetic responses.Retrospect index weightings for 13 component traits showed less emphasis on early mortality (EMORT) and adult mortality (AMRT) and more emphasis on Haugh units and early shell texture than was intended. Selection intensity was about 1.5 σ for males in male lines and about 1.0 σ for females and males in female lines and for females in male lines. Average intensity of selection was 20% greater for retrospect than for intended selection index. Average intensity of selection among 3-year-old parents was no greater than that among 2-year-old parents, because 3-year-old parents had lower egg production, reduced fertility, and greater mortality.Predicted yearly change was in the desired direction for all traits except for a slightly negative predicted change in specific gravity. Predicted change in economic value (ECON) was $.09 for TX1 and $.12 for TX2. Most predicted change in ECON was due to traits affecting egg numbers (EGGS). Predicted changes in EGGS were 1.83 for TX1 and 2.33 for TX2.Estimated genetic changes were $.21 and 4.95 eggs for TX1 and $.21 and 5.00 eggs for TX2. Significant or near significant differences between predicted and estimated genetic changes indicated that EMRT, AMRT, egg weight, EGGS, and ECON changed more favorably than predicted and that excitability and blood spots changed less favorably than predicted.