Natural selection for grain yield in dry bean populations bred by the bulk method

Three dry bean harvests are possible in some regions of Brazil that differ significantly in temperature, rainfall and day length conditions. Thus in breeding programs, generation advance using the bulk method can be carried out in these three periods. It is questionable whether under conditions such as these the action of natural selection would contribute to an increased frequency of individuals considered superior by breeders. The present study assessed six segregating populations, for up to 17 successive generations to check the effects of natural selection during inbreeding. Six populations were generated from a partial diallel: one group consisting of two early maturity cultivars, ESAL 686 and Manteigão Fosco was crossed to a second group of three cultivars with medium maturity, Carioca MG, Milionario and Ouro. The six populations plus the five parents were assessed in experiments using a randomized complete block design, in three locations: Lavras, where in breeding started from the F₂ generation; Lambari and Patos de Minas, from the F₃ generation. A linear regression equation was fitted to the parents and hybrids mean data in each location, considering grain yield as the dependent variable (y) and generations (F₂ = 1, F₃ = 2, etc.) as the independent variable. Genetic progress was estimated from the differences between the linear regression coefficients of the hybrids (b_i)and the mean of the b coefficient of their respective parents (b_j). The (b_i-b_j) were positive in all cases, showing that natural selection acted in all the segregating populations and contributed to an average increase in grain yield of 2.5% per generation over the mean of the parents.