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Duration of the reproductive phase (DRP) is critical for soybean [Glycine max (L.) Merr.] yield. Manipulation of this phase may benefit breeding for higher yield. The soybean E-gene series control time to flowering and maturity through a photoperiod-mediated response. It is possible that E-genes and photoperiod may control the DRP. Two 2-yr studies were conducted with the objective to assess whether the DRP is influenced by either or both E-gene alleles and post-flowering photoperiod. In a planting-date experiment, the main plots were two planting dates, and sub plots were 15 E-gene near-isogenic lines (NILs) in two genetic backgrounds. In a daylength-extension experiment, seven E-gene NILs were planted to synchronize flowering date. Two photoperiods treatments were imposed after flowering: (i) ambient and (ii) ambient plus 3-h incandescent daylength extension. Early planting and daylength extension exposed plants to a longer post-flowering photoperiod, and resulted in an increase in the DRP. When exposed to the same post-flowering photoperiod, the DRP was positively related to the number of dominant E-gene alleles. Both E-genes and post-flowering photoperiod controlled the DRP and this was in addition to the known impact of E-genes on time to flowering. Hence, E-genes control time to maturity through both their effect on time to flowering and through their effect on the DRP. The results of this study underline the importance of time of flowering for determination of the DRP.