Search

Photosystem II (PS II)-inhibitor herbicide resistance in Ontario waterhemp [Amaranthus tuberculatus (Moq.) Sauer] populations 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–inhibitor 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 preemergence and postemergence 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 preemergence at the calculated doses of 133, 350, and 1,070 g ai ha−1 controlled MS A. tuberculatus 50%, 80%, and 95%, respectively, whereas the calculated doses of 7,868 and 17,533 g ai ha−1 controlled MR A. tuberculatus 50% and 80%, respectively, at 12 wk after application (WAA). Metribuzin applied postemergence at the calculated doses of 245 and 1,480 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 (17,920 g ai ha−1) included in this study. Metribuzin at 560 and 1,120 g ha−1 and pyroxasulfone/flumioxazin (240 g ai ha−1) applied preemergence controlled MS A. tuberculatus 88%, 95%, and 98%, respectively, at 12 WAA, whereas the same treatments only controlled MR A. tuberculatus 0%, 4%, and 93%, respectively, at 12 WAA. Metribuzin at 560 and 1,120 g ha−1 and fomesafen (240 g ai ha−1) applied postemergence controlled MS A. tuberculatus 65%, 70%, and 78%, and MR A. tuberculatus 0%, 1%, and 49%, respectively, at 12 WAA. Based on these results, PS II–inhibitor resistant A. tuberculatus with NTSR (enhanced metabolism) is controlled with metribuzin applied preemergence and postemergence; in contrast, PS II–inhibitor resistant A. tuberculatus with TSR (glycine-264-serine 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 in soybean. Pyraflufen-ethyl/2,4-D is a premixed herbicide formulation sold under the tradename Blackhawk®. Five field experiments were conducted over a 2-yr 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 preplant to soybean. Soybean visible injury for all treatments was 2,108 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% control of GR horseweed to 19, 46, and 201 g ha−1, respectively. Pyraflufen-ethyl/2,4-D + metribuzin controlled GR horseweed by 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.

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 wk 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 (1,200/600 + 25 g ai ha−1) that provided 98% and 100% control, respectively, at 8 WAA. This study presents the potential utility of tiafenacil + metribuzin as a GR horseweed management strategy in soybean.

Glyphosate-resistant (GR) horseweed was first confirmed in Ontario in 2010. GR horseweed interference can reduce soybean yield by 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 for control of GR horseweed in soybean in Ontario. Five field experiments were conducted over a 2-yr period (2019–2020) to determine the predicted dose of bromoxynil with or without metribuzin that would control GR horseweed 50%, 80%, and 95%. No soybean injury was observed. The predicted doses of bromoxynil to achieve 50% and 80% control of GR horseweed were 98 and 277 g ai ha−1, respectively, at 8 wk after application (WAA). When mixed with metribuzin (400 g ai ha−1), the predicted doses of bromoxynil for 50%, 80%, and 95% control of GR horseweed 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%, a finding that was similar to the industry standards of saflufenacil + metribuzin (99% control) and glyphosate/dicamba + saflufenacil (100% control) at 8 WAA. This study concludes that bromoxynil + metribuzin applied before planting provides excellent control of GR horseweed in soybean.