Testcross selection prior to further inbreeding in maize: Mean performance and realized genetic variance

Testcross evaluation of maize (Zea mays L.) families is usually done after, but not prior to, several generations of selfing. The objectives of this study were to (i) evaluate the usefulness of one cycle of testcross selection and recombination of F₃ families prior to further selfing and (ii) develop a method for estimating realized genetic variance. In each of two biparental maize populations (POP1 and POP2), five Cycle 0 F₃ families that had the highest yields when testcrossed to B73 were recombined to form Cycle 1. Testcrosses of Cycle 1, Cycle 0, and the five selected Cycle 0 F₃ families were evaluated at five locations in 1993. Selection differentials (S) for yield were 7 to 8% of the corresponding Cycle 0 means. Observed response to selection (R) expressed as a proportion of the Cycle 0 mean was 7% in POP1 and 5% in POP2. In each population, the five highest-yielding Cycle 1 testcrosses outyielded the five highest-yielding Cycle 0 testcrosses by 8 to 9%. Testcross selection would have added 2 yr to the time required for inbred development, but average gains per year for yield were 0.9% larger with than without selection and recombination of the best five F₃ families in Cycle 0. Realized genetic variance was estimated as (R)δt² = SR/i², where i was the standardized selection differential with truncation selection. There was no evidence that genetic variance decreased after one cycle of testcross selection. In experiments that compare several cycles of selection, realized genetic variance can be estimated with bulks of all families and bulks of the selected families instead of estimating genetic variance with large numbers of individual families in each cycle.