Control of Glyphosate-Resistant Giant Ragweed (<i>Ambrosia trifida</i> L.) with Isoxaflutole and Metribuzin Tankmix

Abstract

Five field trials were conducted over a two-year period (2013, 2014) to determine the control of glyphosate-resistant (GR) giant ragweed with isoxaflutole (IFT) and metribuzin (MTZ) applied alone and in combination. Treatments were designed to assess the dose response of an IFT plus MTZ tank-mix as well as each chemical applied alone to classify the response using Flint’s adaptation of Colby’s equation. Two factor factorial experiments were performed in the growth room to ascertain the response of IFT versus glyphosate, IFT versus MTZ, and IFT plus MTZ versus glyphosate on single plants. Field experiments evaluated the control of GR giant ragweed with IFT plus MTZ in tank-mix in a 1:4 ratio. The rate of IFT plus MTZ for 80% control of GR giant ragweed at 4 and 8 weeks after application (WAA) was 518 (104 g a.i. ha -1 IFT + 414 g a.i. ha -1 MTZ) and 631 g a.i. ha -1 (126 g a.i. ha -1 IFT + 505 g a.i. ha -1 MTZ), respectively. A rate of 668 and 467 g a.i. ha -1 was required to reduce GR giant ragweed density and biomass by 80%, respectively. Field experiments evaluating the control of GR giant ragweed with tank-mixes of IFT plus MTZ, where glyphosate was a constant tank-mix partner, were mostly synergistic. However, the low tank-mix rate (52.5 + 210 g a.i. ha -1 ) had an additive response for GR giant ragweed biomass reduction. When tested in the greenhouse and growth room, glyphosate susceptible (GS) giant ragweed showed some antagonism with glyphosate and isoxaflutole tank-mixes at rates less than commercial field rates. GR giant ragweed showed an additive response across all treatments in the growth room. Greenhouse experiments evaluating IFT versus MTZ and IFT plus MTZ versus glyphosate revealed all tank-mix treatments to be synergistic at 2 WAA.