Abstract
AbstractGlyphosate-resistant (GR) Palmer amaranth is a troublesome weed that can emerge throughout the soybean growing season in Nebraska and several other regions of the United States. Late-emerging Palmer amaranth plants can produce seeds, thus replenishing the soil seedbank. The objectives of this study were to evaluate single or sequential applications of labeled POST herbicides such as acifluorfen, dicamba, a fomesafen and fluthiacet-methyl premix, glyphosate, and lactofen on GR Palmer amaranth control, density, biomass, seed production, and seed viability, as well as grain yield of dicamba- and glyphosate-resistant (DGR) soybean. Field experiments were conducted in a grower’s field infested with GR Palmer amaranth near Carleton, NE, in 2018 and 2019, with no PRE herbicide applied. Acifluorfen, dicamba, a premix of fomesafen and fluthiacet-methyl, glyphosate, or lactofen were applied POST in single or sequential applications between the V4 and R6 soybean growth stages, with timings based on product labels. Dicamba applied at V4 or in sequential applications at V4 followed by R1 or R3 controlled GR Palmer amaranth 91% to 100% at soybean harvest, reduced Palmer amaranth density to as low as 2 or fewer plants m−2, reduced seed production to 557 to 2,911 seeds per female plant, and resulted in the highest soybean yield during both years of the study. Sequential applications of acifluorfen, fomesafen and fluthiacet premix, or lactofen were not as effective as dicamba for GR Palmer amaranth control; however, they reduced seed production similar to dicamba. On the basis of the results of this study, we conclude that dicamba was effective for controlling GR Palmer amaranth and reduced density, biomass, and seed production without DGR soybean injury. Herbicides evaluated in this study had no effect on Palmer amaranth seed viability.
Highlights
Native to arid areas of southwestern United States and northern Mexico (Sauer 1957), Palmer amaranth was first listed as a problem weed in 1989 in South Carolina in a survey conducted by the Southern Weed Science Society (Webster and Coble 1997)
Biomass, height, seed production, and soybean yields were different between years; data are presented separately
No soybean injury was observed from dicamba or glyphosate applications; protoporphyrinogen oxidase (PPO) inhibitors resulted in 10%–20% soybean injury at 14 d after treatment (DAT) and no injury 28 DAT
Summary
Native to arid areas of southwestern United States and northern Mexico (Sauer 1957), Palmer amaranth was first listed as a problem weed in 1989 in South Carolina in a survey conducted by the Southern Weed Science Society (Webster and Coble 1997). By 2016, Palmer amaranth was ranked as the most troublesome weed in agronomic crops in the United States (WSSA 2016). Widespread occurrence of Palmer amaranth has been observed in the last 5–7 yr in Nebraska, in agronomic crop fields in south central, west central, and panhandle counties (Vieira et al 2018). Palmer amaranth has a greater plant volume, dry weight, and leaf area, as well as a 24%–62% higher growth rate than other pigweed species (Horak and Loughin 2000). Palmer amaranth is a competitive weed and can cause significant yield reductions in agronomic crops. Massinga et al (2001) reported yield losses of 11% and 91% with Palmer amaranth at densities of 0.5 and 8.0 plants m−2, respectively, in corn (Zea mays L.) in multiyear field
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