The trend toward earlier soybean [Glycine max (L.) Merr.] planting in the midwestern United States has interacted synergistically with genetic yield gain to provide improvement in on-farm yields. However, the impacts of earlier planting dates and their interaction with genetic gain in physiological and phenological traits remain unclear. The objectives of this study were to determine if a 30-d difference in planting date affected measured rates of genetic improvement in (i) total dry matter (TDM) production, (ii) harvest index (HI), and (iii) growth-stage duration in the north-central United States. Research was conducted at Arlington, WI, Urbana, IL, and Lafayette, IN during 2010 and 2011 using 59 Maturity Group (MG) II cultivars (released 1928-2008) at Wisconsin, and 57 MG III cultivars (released 1923-2007) at Illinois and Indiana, with targeted planting dates of 1 May and 1 June. A mixed-effect regression analysis was used to model genetic change in TDM, HI, and growth stage duration as impacted by planting date. Breeding efforts have increased TDM(R7), HI, seed-fill duration (SFD), and reproductive growth duration over time, as vegetative growth duration has been reduced. Early planting provided increased TDM(R7) and longer reproductive growth duration, but had no effect on HI or SFD. A synergistic planting date × year of release interaction existed for TDM(R7) in both maturity groups, but not for HI or SFD, suggesting that the higher yields in newer, early-planted cultivars resulted from greater TDM production, not improved HI or SFD.
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