The future release of ‘Balance GT’ soybean, which is resistant to isoxaflutole and glyphosate, opens up the possibility for control of glyphosate-resistant (GR) Canada fleabane using HPPD-inhibiting herbicides (Group 27) in soybean. Field trials were conducted over two years to evaluate the dose response of an isoxaflutole plus metribuzin tank mix, as well as each chemical applied alone, to assess their response using Flint's adaptation of Colby's equation. Factorial experiments were performed in growth room and greenhouse environments to assess isoxaflutole versus glyphosate, isoxaflutole versus metribuzin, and isoxaflutole plus metribuzin versus glyphosate. Tank mixes of isoxaflutole plus metribuzin in a 1:4 ratio provided 80% control of GR Canada fleabane at a dose range between 420 (84 336) and 611 (122 489) g a.i. ha^-1 at 8 WAA (weeks after application). Tank mixes achieved an 80% reduction in biomass at a dose range between 498 and 738 g a.i. ha^-1, while 80% reduction in density was obtained with doses from 96 to 423 g a.i. ha^-1, 8 WAA. With glyphosate as a constant tank partner, field treatments of isoxaflutole plus metribuzin were mostly synergistic with some analyses showing an additive response. When tested in the growth room, isoxaflutole plus glyphosate tank mixes indicate additivity in the majority of treatments on glyphosate-susceptible (GS) fleabane.