Breeding soybean (Glycine max. L. Merr) varieties for single- versus double-crop production systems

Soybeans [Glycine max. (L.) Merr.] are grown throughout the southeastern United States as both a single crop and as a second crop following small grains. Varieties currently grown in both systems were developed under conventional, mono-crop conditions. Concern over the development of current varieties has prompted researchers to question if these varieties are best suited for double-crop production or if new varieties should be developed which are specifically adapted for double-cropping. Twenty-five determinate and 25 indeterminate F₄-derived breeding lines were evaluated for seed yield in conventional (tilled seedbed, optimum planting date, wide rows) and double-crop (wheat stubble seedbed, mid-June planting date, narrow rows) nursery environments to determine: 1) if relative yield of lines was similar in the two systems, and 2) if indeterminate lines were higher yielding than determinate lines under double-crop conditions. The tests were conducted at 26 location/year combinations in 1982-1986. The 50 lines were separated into two groups based on overall means from the 26 combinations: 1) a superior group consisting of 17 lines which yielded above the overall mean in both conventional and double-crop nursery environments, and 2) a non-superior group consisting of all other lines. Genotype X nursery environment interactions were significant for both the superior and non-superior groups, but the magnitude of interaction was twice as great for the non-superior group. Stability analysis showed that the superior group had significantly higher mean regression values than the non-superior group in conventional tests, but there was no difference in double-crop tests. Selection of the top lines based on means from combinations of one, two, three, and four conventional tests in 1985, and combinations of one, two, three, and four double-crop tests in 1985, each produced up to 65% of the superior lines. The best breeding line was selected in every case. Mean yield differences were not significant between growth types in conventional tests, but determinates were significantly higher yielding than indeterminates in double-crop tests. The results from this study indicate there is no immediate need to maintain separate selection nurseries to enhance the development of soybean varieties adapted for double-crop production systems.