Reduced rates of sulfentrazone plus chlorimuron and glyphosate in no-till, narrow-row, glyphosate-resistant Glycine max

Field studies were conducted in 1998 and 1999 to evaluate crop response, weed control, Glycine max yield, and economic returns of labeled (1×) and one-half labeled (½×) rates of early preplant (EPP) sulfentrazone plus chlorimuron and postemergence glyphosate, compared to glyphosate-alone systems in no-till, narrow-row, glyphosate-resistant G. max. Treatments containing a 1× or ½× rate of EPP sulfentrazone plus chlorimuron with glyphosate followed by (fb) a postemergence treatment of glyphosate provided 80 to 100% control of Xanthium strumarium, Ambrosia artemisiifolia, and Polygonum pensylvanicum and 82 to 100% control of Setaria faberi and Amaranthus rudis if glyphosate was applied mid-postemergence (MPOST) or late postemergence (LPOST). Glyphosate alone EPP fb glyphosate postemergence or sequential postemergence treatments of glyphosate provided 77 to 100% control of S. faberi, A. artemisiifolia, and P. pensylvanicum. Glycine max yield did not significantly differ between treatments that contained 1× or ½× rates of sulfentrazone plus chlorimuron EPP with postemergence glyphosate or sequential glyphosate. Residual herbicides fb glyphosate reduced overall weed control variability but did not reduce the overall yield variability compared to glyphosate alone. Greater weed control, G. max yield, net incomes, and lower coefficient of variation (CV) of net incomes were generally associated with treatments that included both EPP and postemergence treatments vs. single herbicide applications. A greenhouse study was conducted to determine the optimal spray additive to maximize the foliar activity of sulfentrazone on three annual weeds. Sulfentrazone alone and in combination with a nonionic surfactant (NIS), methylated seed oil (MSO), crop oil concentrate (COC), and a silicone-based surfactant (SBS), with and without ammonium sulfate (AMS), were applied on two sizes of Abutilon theophrasti, P. pensylvanicum, and S. faberi. AMS provided little additional efficacy of sulfentrazone on S. faberi, but improved efficacy on A. theophrasti and P. pensylvanicum. SBS or MSO plus AMS with sulfentrazone generally provided the greatest efficacy on all species.Nomenclature: Chlorimuron; glyphosate; sulfentrazone; Xanthium strumarium L. XANST, common cocklebur; Ambrosia artemisiifolia L. AMBEL, common ragweed; Amaranthus rudis AMATA, common waterhemp; Setaria faberi Herrm. SETFA, giant foxtail; Polygonum pensylvanicum L. POLPY, Pennsylvania smartweed; Abutilon theophrasti (L.) Medik. ABUTH, velvetleaf; Glycine max (L.) Merr. ‘Asgrow 3601’, soybean.