Six field experiments were conducted to investigate any interaction between pyroxasulfone and flumioxazin on soybean tolerance and control of multiple-herbicide-resistant (MHR) waterhemp in soybean during 2016 and 2017 in Ontario, Canada. There was a synergistic increase in soybean injury with the co-application of pyroxasulfone and flumioxazin at all rates evaluated at 2 weeks after emergence (WAE), the two highest rates evaluated (134/106 and 268/211 g ai ha−1) at 4 WAE, and the highest rate (268/211 g ai ha−1) evaluated at 8 WAE. Soybean injury with all pyroxasulfone and flumioxazin treatments was transient and had no adverse effect on soybean grain yield. Pyroxasulfone applied preemergence (PRE) at 45, 89, 134, and 268 g ai ha−1 controlled MHR waterhemp up to 72, 89, 92, and 95%, respectively. Flumioxazin applied PRE at 35, 70, 106, and 211 g ai ha−1 controlled MHR waterhemp up to 78, 90, 93, and 96%, respectively. Pyroxasulfone/flumioxazin applied PRE at 45/35, 89/70, 134/106, and 268/211 g ai ha−1 controlled MHR waterhemp up to 92, 96, 98 and 100%, respectively. There were no significant antagonistic or synergistic interactions for the control of MHR waterhemp with pyroxasulfone/flumioxazin at rates evaluated except at 268/211 g ai ha−1 which provided a synergistic increase in MHR waterhemp control at 4 WAE. The MHR waterhemp biomass and density reductions followed a similar trend as visible control. Pyroxasulfone/flumioxazin at 268/211 g ai ha−1 caused a synergistic response in biomass reduction (9% difference). Based on these results, there is an additive increase in MHR waterhemp control and potential for synergistic soybean injury with the co-application of pyroxasulfone plus flumioxazin.

Two studies were conducted to ascertain the biologically-effective-dose (BED) of flumioxazin and pyroxasulfone for multiple-herbicide-resistant (MHR) waterhemp [Amaranthus tuberculatus (Moq.) Sauer] control in soybean [Glycine max (L.) Merr.] in southwestern Ontario, Canada, during 2016 and 2017. In the flumioxazin study, the predicted flumioxazin doses for 50, 80 and 90% MHR A. tuberculatus control were 19, 37, and 59 g ai ha−1 at 2 weeks after application (WAA) and 31, 83, and 151 g ai ha−1, respectively, at 12 WAA. The predicted flumioxazin doses to cause 5 and 10% soybean injury were 129 and 404 g ai ha−1, respectively at 2 weeks after emergence (WAE), and the predicted flumioxazin doses to obtain 50, 80, and 95% of the weed-free control plot’s yield were determined to be 3, 14, and 65 g ai ha−1, respectively. In the pyroxasulfone study, the predicted pyroxasulfone doses that provided 50, 80, and 90% MHR A. tuberculatus visual control were 25, 50, and 88 g ai ha−1 at 2 WAA and 41, 109, and 274 g ai ha−1 at 12 WAA, respectively. 117 g ai ha−1 was the dose of pyroxasulfone predicted for 80% reduction in MHR A. tuberculatus density and the predicted doses of pyroxasulfone for 80 and 90% reduction in A. tuberculatus biomass were 204 and 382 g ai ha−1, respectively. The predicted doses of pyroxasulfone that caused 5 and 10% injury in soybean at 2 WAE were 585 and 698 g ai ha−1, respectively. 6, 24 and 112 g ai ha−1 was the predicted doses of pyroxasulfone required to obtain 50, 80, and 95% yield relative to the weed-free plots, respectively. Flumioxazin and pyroxasulfone applied PRE at the appropriate doses provided early-season MHR A. tuberculatus control in soybean.