Increased soybean seed cost has generated recent interest in reducing
seeding rates to improve economic returns. However, low seeding rates result
in reduced established plant stands with slower canopy development, and
canopy development is an important element of integrated weed management
(IWM). Field studies were conducted in 2012 and 2013 in Wisconsin to
determine the trade-off between reduced seeding rates and PRE residual
herbicide use for POST herbicide exposure. Soybean was planted in mid May in
38-cm-wide rows at five seeding rates ranging from 148,200 to 469,300 seeds
ha−1. A PRE application of metolachlor plus fomesafen was made
to half of the plots. One of two POST herbicide programs were sprayed at the
V4 soybean growth stage to determine whether blending herbicide-resistant
(HR) and non-HR soybean cultivars could be a practical alternative to reduce
soybean seed expenses while maintaining the potential benefit of weed
suppression before the POST herbicide application. An increase in seeding
rate did not reduce the density or size of weeds exposed to the POST
herbicide, and furthermore, end-of-season weed density and biomass were not
influenced. In contrast, the use of a PRE herbicide reduced total weed
density and biomass before POST application by 93 and 95%, respectively, in
both years. In 2012, the season was dry early and harvest stands of 161,100
and 264,100 plants ha−1 produced 95% of the maximum yield for the
PRE and no-PRE treatments, respectively. The difference was not repeated in
2013 with adequate early season rainfall. In conclusion, PRE herbicide use
produced maximum yield with fewer plants per hectare by limiting early
season weed competition and reduced weeds exposed to POST herbicide
application thus contributing to HR management (HRM). In contrast, higher
plant densities generated within the seeding rate range of this study did
little to improve IWM or HRM.