Genetic Gain x Management Interactions in Soybean: I. Planting Date.

Planting date is a commonly manipulated management practice in soybean [Glycine max(L.) Merr.] production. A majority of U.S. growers now plant soybean before 15 May, weeks ahead of historical planting dates. The impacts of past and ongoing agronomic improvements, such as earlier planting, on genetic yield improvement and associated changes in seed protein and oil have not been evaluated. In addition, genetic improvement in physiological and phenological traits with respect to planting date also required examination. The objectives of this research project were to determine if a 30-day difference in planting date affected measured rates of genetic improvement in (i) yield, (ii) seed mass, and (iii) seed protein and oil, (iv) total dry matter (TDM) production, (v) harvest index (HI), and (vi) growth stage duration in the North Central U.S.A. Research was conducted at Arlington, WI, Urbana, IL, and Lafayette, IN during the 2010 and 2011 growing seasons, using 59 maturity group (MG) II cultivars (released 1928-2008) at WI, and 57 MG III cultivars (released 1923-2007) at IL and IN, with targeted planting dates of 1 May and 1 June. A mixed-effect regression analysis was used to model the genetic improvement in yield and cultivar traits as impacted by planting date.

Earlier planting provided increased cultivar mean yields in both MGs, but had an inverse effect on seed protein (decreased) and seed oil (increased) concentrations. An interaction between planting date and cultivar year of release suggested that the trend toward earlier planting is one of the agronomic improvements that, when coupled with genetic improvement, has provided a synergistic increase in on-farm soybean yields in the Midwestern U.S. Soybean breeding efforts have improved TDM(R7), HI, seed-fill duration (SFD), and the duration of reproductive growth over time, while decreasing the duration of vegetative growth. Early planting resulted in increased TDM(R7) and decreased TDM(R1), but had no effect on TDM(R4), HI, or SFD. A synergistic planting date by cultivar year of release interaction existed for TDM(R7), but not for HI or SFD, suggesting that the higher yields in newer, early planted cultivars have resulted from greater TDM production, not improved HI or SFD.

Considering the greater yield response of newer cultivars to early planting, it may prove beneficial for breeders to employ strategies in their breeding programs to exploit this synergistic interaction. Yield evaluation and selection under early planting conditions could enable progress that would otherwise go unnoticed in breeding settings where later planting to avoid spring frost, among other environmental uncertainties, is a standard practice. Successfully identifying and exploiting other synergistic agronomic by genetic yield gain interactions may provide soybean breeders and agronomists with tools that can facilitate greater yield improvement in the future.