Photosystem II (PS II)-inhibitor herbicide resistance in Ontario waterhemp [Amaranthus tuberculatus (Moq.) Sauer] population is conferred via target-site resistance (TSR) and non-target-site resistance (NTSR) mechanisms. Metribuzin-resistant (MR) A. tuberculatus is due to TSR,. Conversely, in other populations of PS II-resistant A. tuberculatus, plants are resistant to atrazine but metribuzin-sensitive (MS). The objective of this study was to determine the biologically-effective-dose of metribuzin applied PRE and POST for the control of MS and MR A. tuberculatus. Ten field experiments were conducted in 2019 and 2020 to determine the effective doses of metribuzin for 50, 80, and 95% control of MS and MR A. tuberculatus. Metribuzin applied PRE at the calculated doses of 133, 350, and 1070 g ai ha-1 controlled MS A. tuberculatus 50, 80, and 95%, respectively, whereas the calculated doses of 7868 and 17533 g ai ha-1 controlled MR A. tuberculatus 50 and 80%, respectively at 12 WAA. Metribuzin applied POST at the calculated doses of 245 and 1480 g ai ha-1 controlled MS A. tuberculatus 50 and 80%, respectively; the calculated dose for 50% MR A. tuberculatus control was greater than the highest dose (17920 g ai ha-1) included in this study. Metribuzin at 560 and 1120 g ha-1 and pyroxasulfone/flumioxazin (240 g ai ha-1) applied PRE controlled MS A. tuberculatus 88, 95, and 98%, respectively at 12 WAA. The aforementioned treatments controlled MR A. tuberculatus 0, 4, and 93%, respectively at 12 WAA. Metribuzin at 560 and 1120 g ha-1 and fomesafen (240 g ai ha-1) applied POST controlled MS A. tuberculatus 65, 70, and 78%, and MR A. tuberculatus 0, 1, and 49%, respectively at 12 WAA. Based on these results, NTSR PS II-resistant A. tuberculatus (enhanced metabolism) is controlled with metribuzin applied PRE and POST, in contrast TSR PS II-resistant A. tuberculatus (glycine264serine altered target site) is not controlled with metribuzin.

Glyphosate resistance in weed species has presented immense challenges for farmers in Ontario. The co-application of burndown plus residual herbicides provides control of glyphosate-resistant (GR) horseweed control in soybean. Pyraflufen-ethyl/2,4-D is a premixed herbicide formulation sold under the tradename Blackhawk®. Five field experiments were conducted over a two-year period (2019, 2020) in fields in southwestern Ontario to ascertain the biologically-effective-dose of pyraflufen-ethyl/2,4-D, applied alone, or mixed with metribuzin, for GR horseweed control when applied pre-plant to soybean. Soybean visible injury for all treatments was <15%. At 8 wk after application (WAA), the calculated doses of pyraflufen-ethyl/2,4-D for 50, 80, and 95% GR horseweed control were 390, 1148, and >2108 g ha−1, respectively. The addition of metribuzin to pyraflufen-ethyl/2,4-D reduced the doses of pyraflufen-ethyl/2,4-D for 50, 80, and 95% GR horseweed control to 19, 46 and 201 g ha−1, respectively. Pyraflufen-ethyl/2,4-D + metribuzin controlled GR horseweed 97% which is comparable to the current industry standards. Based on these results, pyraflufen-ethyl/2,4-D + metribuzin (527 + 400 g ha−1) applied preplant can be used for GR horseweed control in soybean.

Glyphosate-resistant (GR) horseweed was first confirmed in Ontario in 2010. GR horseweed interference can reduce soybean yield up to 97%. Bromoxynil is a photosystem II-inhibiting herbicide that is primarily used for annual broadleaf weed control in monocot crops. The objective of this study was to determine the biologically-effective-dose (BED) of bromoxynil applied alone and when mixed with metribuzin applied preplant (PP) for control of GR horseweed in soybean in Ontario. Five field experiments were conducted over a two-year period (2019-2020) to determine the predicted dose of bromoxynil +/- metribuzin that would control GR horseweed 50, 80, and 95%. No soybean injury was observed. The predicted doses of bromoxynil for 50 and 80% GR horseweed control were 98 and 277 g ai ha-1, respectively; at 8 weeks after application (WAA). When mixed with metribuzin (400 g ai ha-1), the predicted doses of bromoxynil for 50, 80 and 95% GR horseweed control were 10, 25, and 54 g ai ha-1, respectively. Bromoxynil (280 g ai ha-1) plus metribuzin (400 g ai ha-1) controlled GR horseweed 97% which was similar to the industry standards of saflufenacil + metribuzin (99%) and glyphosate/dicamba + saflufenacil (100%) at 8 WAA control. This study concludes that bromoxynil + metribuzin applied PP provides excellent control of GR horseweed in soybean.

Tiafenacil is a recently developed protoporphyrinogen IX oxidase (PPO)-inhibiting herbicide from the pyrimidinedione chemical class that is proposed for use as a preplant (PP) burndown in soybean. Glyphosate-resistant (GR) horseweed is a troublesome weed often found in no-till systems that can dramatically reduce soybean yield; control in soybean has been variable. Five field experiments were conducted over 2019 and 2020 in commercial soybean fields with GR horseweed to determine the biologically-effective-dose (BED) of tiafenacil and tiafenacil + metribuzin, and to compare their efficacy to currently accepted industry standard herbicide treatments in identity-preserved (IP, non-GMO), GR, and glyphosate/dicamba-resistant (GDR) soybean systems. There was no soybean injury with treatments evaluated. The calculated doses of tiafenacil for 50, 80, and 95% control of GR horseweed control were 21, 147 and >200 g ai ha−1, respectively, at 8 weeks after application (WAA). Lower doses were calculated with the addition of metribuzin (400 g ai ha−1) to tiafenacil for 50 and 80% control, with no dose of tiafenacil + metribuzin providing 95% control. Tiafenacil + metribuzin at 25 + 400 and 50 + 400 g ai ha−1 controlled GR horseweed 88 and 93% , respectively which was similar to the industry standards of saflufenacil + metribuzin (25 + 400 g ai ha−1) and glyphosate/dicamba + saflufenacil (1200/600 + 25 g ai ha−1) that provided 98 to 100% control, respectively at 8 WAA. This study presents the potential utility of tiafenacil + metribuzin as a GR horseweed management strategy in soybean.