Residual herbicides affect critical time of Palmer amaranth removal in soybean

Glyphosate-resistant (GR) Palmer amaranth (Amaranthus palmeri S. Watson) is one of the most difficult to control weeds in soybean [Glycine max (L.) Merr.] production fields. Residual pre-emergence (PRE) herbicide applied at planting is one of the recommendations for management of herbicide-resistant Palmer amaranth; however, information is not available about the effect of residual herbicides on critical time of Palmer amaranth removal (CTPAR) to prevent an unacceptable yield loss in soybean. The objective of this study was to determine the CTPAR in soybean affected by residual PRE herbicides compared with the no PRE herbicide in southcentral Nebraska. Field experiments were conducted in 2018 and 2019 in a grower's field infested with GR Palmer amaranth near Carleton, NE. The treatments were arranged in a split-plot design with PRE herbicides (no PRE herbicide, flumioxazin, and a premix of flumioxazin/metribuzin/pyroxasulfone) as the main plot and Palmer amaranth removal timings as subplot treatments (a weed-free control; a nontreated control; and Palmer amaranth removal timing at the V1, V3, V6, R2, and R5 soybean growth stages). In the absence of a PRE herbicide, the CTPAR at 5% soybean yield loss occurred at V1 and V6 soybean growth stages, equivalent to 194 and 480 Celsius growing degrees days (GDDc) in 2018 and 2019, respectively. When flumioxazin was applied alone, the CTPAR was delayed until the V3 and V6 soybean growth stages, or 341 and 501 GDDc. When flumioxazin/metribuzin/pyroxasulfone premix was applied, the CTPAR was delayed until the V2 and R1 soybean growth stages, corresponding to 255 and 546 GDDc, in 2018 and 2019, respectively.