Abstract
AbstractField experiments were conducted in 2012 and 2013 across four locations for a total of 6 site-years in the midsouthern United States to determine the effect of growth stage at exposure on soybean sensitivity to sublethal rates of dicamba (8.8 g ae ha−1) and 2,4-D (140 g ae ha−1). Regression analysis revealed that soybean was most susceptible to injury from 2,4-D when exposed between 413 and 1,391 accumulated growing degree days (GDD) from planting, approximately between V1 and R2 growth stages. In terms of terminal plant height, soybean was most susceptible to 2,4-D between 448 and 1,719 GDD, or from V1 to R4. However, maximum susceptibility to 2,4-D was only between 624 and 1,001 GDD or from V3 to V5 for yield loss. As expected, soybean was sensitive to dicamba for longer spans of time, ranging from 0 to 1,162 GDD for visible injury or from emergence to R2. Likewise, soybean height was most affected when dicamba exposure occurred between 847 and 1,276 GDD or from V4 to R2. Regarding grain yield, soybean was most susceptible to dicamba between 820 and 1,339 GDD or from V4 to R2. Consequently, these data indicate that soybean response to 2,4-D and dicamba can be variable within vegetative or reproductive growth stages; therefore, specific growth stage at the time of exposure should be considered when evaluating injury from off-target movement. In addition, application of dicamba near susceptible soybean within the V4 to R2 growth stages should be avoided because this is the time of maximum susceptibility. Research regarding soybean sensitivity to 2,4-D and dicamba should focus on multiple exposure times and also avoid generalizing growth stages to vegetative or reproductive.
Highlights
Differential sensitivity of soybean to 2,4-D and dicamba has been well characterized
Of the 30 studies reviewed from the last seven decades, they concluded that dicamba was more likely to cause soybean yield loss when exposure occurred at reproductive growth stages rather than at vegetative growth stages; soybean response to 2,4-D was similar across all growth stages (Egan et al 2014)
Regression analysis revealed that soybean injury 7 d after treatment (DAT) increased with delayed exposure timing from planting until 413 growing degree days (GDD), at which time soybean was most sensitive to 2,4-D until exposure timing reached 1,262 GDD (Figure 1)
Summary
Differential sensitivity of soybean to 2,4-D and dicamba has been well characterized. On an equivalent-rate basis, soybean is far more sensitive to dicamba than to 2,4-D; sensitivity often depends on growth stage at the time of exposure (Andersen et al 2004; Egan et al 2014; Johnson et al 2012; Kelley et al 2005; Robinson et al 2013a, 2013b; Sciumbato et al 2004; Wax et al 1969). Growth stage at the time of soybean exposure to 2,4-D or dicamba and subsequent apical meristem death determines the extent of branching and subsequent reproductive growth at lower nodes (Carpenter and Board 1997; Kelley et al 2005; Robinson et al 2013b; Wax et al 1969). The objectives of this research were to determine sensitivity of soybean to sublethal rates of 2,4-D and dicamba across all growth stages and to predict the times of soybean maximum susceptibility to 2,4-D and dicamba
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