Interaction of dicamba, fluthiacet-methyl, and glyphosate for control of velvetleaf (Abutilon theophrasti) in dicamba/glyphosate-resistant soybean

Velvetleaf is an economically important weed in agronomic crops in Nebraska and the United States. Dicamba applied alone usually does not provide complete velvetleaf control, particularly when velvetleaf is taller than 15 cm. The objectives of this experiment were to evaluate the interaction of dicamba, fluthiacet-methyl, and glyphosate applied alone or in a mixture in two- or three-way combinations for velvetleaf control in dicamba/glyphosate-resistant (DGR) soybean and to evaluate whether velvetleaf height (<= 12 cm or <= 20 cm) at the time of herbicide application influences herbicide efficacy, velvetleaf density, biomass, and soybean yield. Field experiments were conducted near Clay Center, NE in 2019 and 2020. The experiment was arranged in a split-plot with velvetleaf height (<= 12 cm or <= 20 cm) as the main plot treatment and herbicides as subplot treatment. Fluthiacet provided >= 94% velvetleaf control 28 d after treatment (DAT) and >= 96% biomass reduction regardless of application rate or velvetleaf height. Velvetleaf control was 31% to 74% at 28 DAT when dicamba or glyphosate was applied alone to velvetleaf <= 20 cm tall compared with 47% to 100% control applied to <= 12-cm-tall plants. Dicamba applied alone to <= 20-cm-tall velvetleaf provided <75% control and <87% biomass reduction 28 DAT compared with >= 90% control with dicamba at 560 g ae ha(-1) + fluthiacet at 7.2 g ai ha(-1) or glyphosate at 1,260 g ae ha(-1). Dicamba at 280 g ae ha(-1) + glyphosate at 630 g ae ha(-1) applied to <= 20-cm-tall velvetleaf resulted in 86% control 28 DAT compared with the expected 99% control. The interaction of dicamba + fluthiacet + glyphosate was additive for velvetleaf control and biomass reduction regardless of application rate and velvetleaf height.