Optimization of Parental Selection as Well as Number and Size of Crosses in Hybrid Maize Breeding with Doubled Haploids.

Gain from selection and optimum allocation of test resources in breeding programs are strongly influenced by parental selection. We compared two commonly used hybrid maize breeding schemes with evaluation of testcross progenies in two consecutive stages: (a) breeding scheme DHTC, where doubled haploid (DH) lines were tested in both stages and (b) breeding scheme S₁TC-DHTC, where S₁ families were tested in the first stage and DH lines within superior S₁ families in the second stage. Our objectives were to (1) determine the optimum allocation concerning the number of crosses, S₁ families, DH lines, and test locations in both breeding schemes, (2) investigate the impact of parental selection on the optimum allocation and selection gain (ΔG) calculated by numerical integration, and (3) compare the maximum ΔG achievable with each breeding scheme. We made various assumptions for the budget and variance components representing the breeding situation in North America and Europe. Maximum ΔG was largely increased with parental selection in both breeding schemes, in comparison with randomly chosen crosses. With an increasing correlation between the mean phenotypic performance of the parents and the mean genotypic value of the testcross performance of their progenies, this superiority increased strongly, while the optimum number of crosses decreased in favor of an increased number of test candidates within crosses. Thus, for short-term success in advanced cycle breeding, the concentration on few crosses among the best parental lines within each heterotic group seems promising. Breeding scheme S₁TC-DHTC led to a larger ΔG but had a longer cycle-length than DHTC. Cycle-length of S₁TC-DHTC could be shortened with further improvements in the DH technique and the realization of more than two generations per year. Then, early testing of S₁ families prior to production of DH lines would be very attractive in hybrid maize breeding.