Why do plant breeders focus more on selecting between families by the F5 generation?

In the context of plant breeding, breeders focus on selecting between families by the F5 generation because, by this stage, the hybrid condition has largely given way to homozygosity (the pure-breeding condition) as a result of successive generations of natural self-pollination.1

Because the plants have become extensively pure-breeding, the families derived from original F2 plants begin to display their unique, stable characteristics. This shift in emphasis allows breeders to evaluate these families for quantitative traits—such as yield or quality—more effectively. At this stage, each selected family is typically harvested in mass to obtain the larger quantities of seed required for precise performance evaluations in plots that simulate commercial planting practices.1