Environment, production practices, and genetic differences are known to influence the number of days between developmental stages of soybeans [Glycine max (L.) Merr.], but the yield response to differences in days between various stages is poorly understood. Twice weekly observations were made on 10 central Iowa replicated field trials over a 3‐year period to determine the date that each plot reached several specific stages of plant development. These dates and final seed yields were utilized to investigate whether number of days between specific stages of development were associated with final seed yield. All sites were mapped as Webster silt loams (a member of the fine‐loamy, mixed noncalcareous, mesic family of typic haplaquolls). Variations among the 119 cultivars represented in the study accounted for more of the differences in days between developmental stages than did sites or years. There was no association between seed yield and number of days between any two vegetative stages of development. The length of the seed development period [days from stage R4 (pods 2 cm long at top four nodes) to R7 (pods yellowing)] was more highly correlated with seed yield than was the length of any other developmental period. Plots which initiated flowering later and matured later tended to be the highest yielding plots. Longer periods of pod development [days from stage R2 (flowers at top node) to R4] tended to be associated with lower yields. The three locations which also included fertility variables showed that differences in the P and K fertility levels utilized did not influence the number of days between any two developmental stages. These findings provide breeders with another potential selection criterion and production researchers with a better understanding of how environmental and production practice differences influence soybean yields.

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Evidence suggests that photosynthate production may be yield‐limiting in soybeans [Glycine max (L.) Merr.]. This study was designed to estimate potential for selection of CO2‐exchange rate (CER), specific leaf weight (SLW), leaf thickness (LT), and leaf density (LD) in early generations of soybeans.Two crosses (‘Amsoy’ ✕ ‘Ford;’ ‘Corsoy’ ✕ ‘Hawkeye’) involving adapted cultivars high (Amsoy, Corsoy) and low in leaf CER were used. CER, SLW, LT, and LD were measured in parents, F1s and F2s. CER was measured in selected F3 and F4 lines of one cross, whereas the other variables were measured in all F3 and F4 lines.All traits showed quantitative inheritance without dominance effects. Broad‐sense heritability, based on parental, F1 and F2 variances, was low to moderate for CER (0.36, 0.56); it was low to very low for the other variables. Selection for either high or low CER in the F2 or F3 was ineffective. Gain from hypothetical selection for SLW was about 596 in the F2 and 5‐11% in the F2. Gain from hypothetical F3 selection in LT was about 5%. LD showed poor gain.Use of pure lines from advanced generations may give an entirely different picture of the potential for direct selection for increased photosynthesis than that shown by this study.