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Twenty field experiments were conducted from 2021-2022 at nine locations in Ontario, Canada to optimize weed control in E3 soybean. Two studies compared one- and two-pass weed control programs for control of glyphosate-resistant (GR) Canada fleabane and multiple herbicide-resistant (MHR) waterhemp. Two-pass weed control programs provided greater control of GR Canada fleabane and MHR waterhemp. Another study investigated herbicide interactions for volunteer corn control in E3 soybean. The co-application of 2,4-D choline or dicamba with quizalofop-p-ethyl or clethodim resulted in antagonistic herbicide interactions and reduced control. Glufosinate co-applied with quizalofop-p-ethyl or clethodim resulted in a synergistic interaction. The effect of glufosinate rate, the addition of AMS, and weed size at application was established for five annual weed species. The effect of glufosinate rate, the addition of AMS, and weed size at application is weed species specific. Generally, control is improved with increased rate and reduced weed size at application. BASF Canada Inc.; Ontario Agri-Food Innovation Alliance

Canada Fleabane (Conyza canadensis) is an economically important, resistant weed species. The objective of the study was to characterize paraquat and diquat resistance in geographically isolated, evolutionarily unique biotypes of C. canadensis, and evaluate inheritance using segregating F2 families. Resistant (R) biotypes were crossed to a common susceptible (S), and amongst themselves. Discriminating and parental eliminating doses of paraquat and diquat were identified through dose responses. Results indicated F2 families deviated from expected 3:1 ratio for resistance to paraquat, indicating that resistance in C. canadensis was not attributable to single gene inheritance. Evaluated digenic models suggested an 11:5 ratio may best describe paraquat resistance. These results are some of the first to evaluate impacts of herbicide resistance on expression of transgressive segregation. F2 progeny from the paraquat RxS cross demonstrated transgressive segregation at discriminating and parental eliminating doses. Transgressive segregation was not observed in progeny from a paraquat RxR cross.

Thirty field experiments were conducted from 2019-2021 at six locations in Ontario, Canada to optimize the efficacy of tolpyralate for weed control in corn. Two studies investigated the interaction between 4-hydroxyphenylpyruvate dioxygenase (HPPD)-inhibitors (tolpyralate, mesotrione, and topramezone) and reactive oxygen species (ROS)-generators (atrazine, bromoxynil, bentazon, and glufosinate) on annual weed species and one study focused on the interaction on glyphosate-resistant Canada fleabane. Co-application of HPPD-inhibitors and ROS-generators were generally synergistic or additive for weed control, but the response depended on the herbicide, rate of herbicide, weed species, and response parameter. In contrast to mesotrione, tolpyralate was antagonistic with glufosinate for common ragweed, Setaria spp., and barnyardgrass control at 8 weeks after application. The effective dose of atrazine to complement tolpyralate was established for seven annual weed species. Three separate studies identified that MSO Concentrate®, Merge®, or Carrier® adjuvants should be added to tolpyralate, tolpyralate + atrazine, or tolpyralate + bromoxynil. ISK Biosciences Corporation; Grain Farmers of Ontario

Six studies consisting of twenty four field experiments were conducted in 2020 and 2021 in Ontario, Canada to investigate how to improve the level and consistency of glyphosate-resistant (GR) Canada fleabane control with herbicides applied preplant (PP) to soybean. Study One concluded the adding saflufenacil or metribuzin to glyphosate plus halauxifen-methyl or pyraflufen-ethyl/2,4-D resulted in a synergistic increase in GR Canada fleabane control and improved the consistency of control. Study Two and Three determined adding dicamba or saflufenacil to glyphosate plus bromoxynil and adding saflufenacil to glyphosate plus dicamba improved the level and consistency of GR Canada fleabane control. Study Four determined adding metribuzin to glyphosate plus glufosinate provided the greatest level of GR Canada fleabane control and improved the consistency of control. Study Five and Six determined there was no benefit of adding low rates of protoporphyrinogen oxidase (PPO)-inhibiting herbicides to glufosinate to improve GR Canada fleabane control. OMAFRA Alliance program, Grain Farmers of Ontario, BASF Canada Inc., Bayer CropScience Inc., Nufarm Canada, ISK Biosciences, Syngenta Canada Inc., FMC Canada, and Dow AgroSciences Canada Inc.

An analog of glyphosate, glycine, has been shown to be a competitive inhibitor of glyphosate sequestration which could allow separation of target site and non-target site resistance mechanisms. Therefore, the objective was to determine if the addition of glycine to glyphosate will reduce the level of resistance in glyphosate resistant Canada fleabane. Glycine and glyphosate were applied in a hydroponic system with the resistance levels being measured by shikimate accumulation and growth variables. The addition of glycine caused an increase in the resistance level for the glyphosate resistant populations of Canada fleabane. Also, shikimate accumulation was inhibited by the glycine treatment in the glyphosate susceptible plants at low dose (0.1 mM) of glyphosate and at the higher dose (0.2 mM) for the resistant plants. This antagonistic effect observed from the combination treatment was unexpected and may be due to glycine interfering with photosynthesis. University of Guelph and the Ontario Ministry of Agriculture, Food and Rural Affairs through the Highly Qualified Personnel Scholarship Program.

Thirty-six experiments were conducted over two years (2018, 2019) to evaluate tolpyralate, a benzoylpyrazole 4-hydroxyphenyl-pyruvate dioxygenase inhibitor, for weed management in corn in Ontario. The importance of adjuvants (MSO and UAN) with tolpyralate + atrazine and the influence of Roundup Weathermax® was determined for control of nine annual weed species, glyphosate-resistant (GR) Canada fleabane and GR waterhemp. Optimal control was achieved with tolpyralate + atrazine + MSO and when tolpyralate + atrazine was co-applied with Roundup Weathermax®. Tolpyralate and tolpyralate + atrazine has residual activity on GR Canada fleabane, comparable to current industry standards. Three weed species exhibited a time-of-day effect to tolpyralate + atrazine, although, consistent control was achieved regardless of application time. Corn injury was greatest from tolpyralate + atrazine when applied at the 2X rate alone and/or co-applied with glyphosate at 1 WAA; there was no further increase in corn injury when co-applied with glyphosate.

The new herbicide halauxifen-methyl was evaluated for the control of glyphosate-resistant (GR) Canada fleabane and giant ragweed in winter wheat and soybean, and the tolerance of corn and soybean to halauxifen-methyl. Halauxifen-methyl applied alone or in a tankmix controlled GR Canada fleabane in winter wheat, while a tankmix partner was required to provide control in soybean. Halauxifen-methyl applied alone did not control GR giant ragweed in winter wheat or soybean, however, when applied in a tankmix, control was achieved. Halauxifen-methyl applied preplant and pre-emergence at 5 g ai ha-1 and 10 g ai ha-1 was safe in corn however yield was reduced when applied early postemergence at the 10 g ai ha-1 rate. Soybean was extremely sensitive to halauxifen-methyl applied early postemergence at both rates, however minimal injury was observed when either rate was applied preplant and pre-emergence. Halauxifen-methyl could provide an effective solution for the control of GR weeds in corn, soybean and winter wheat.

Twenty-seven field experiments were conducted from 2015-2018 at nine locations to evaluate tolpyralate, a new pyrazolone 4-hydroxyphenyl-pyruvate dioxygenase inhibitor, for weed management in corn. The biologically-effective dose of tolpyralate and tolpyralate + atrazine was established in eight common annual weed species, and separately in multiple herbicide-resistant (MR) Canada fleabane and waterhemp biotypes. Co-application of tolpyralate with atrazine improved weed control; tolpyralate + atrazine at label rates provided similar or improved control of eight common weed species, MR Canada fleabane and MR waterhemp compared to industry standards. Four weed species were controlled equally regardless of POST application timing and two species were controlled with tolpyralate + atrazine applied pre-emergence; annual grass control declined with delayed applications. Corn was at greatest risk of injury with tolpyralate + atrazine applied at V1 or V3, and at 2X rates. Injury was influenced by environmental variables, but did not translate to grain yield loss. ISK Biosciences Corporation; Grain Farmers of Ontario; Growing Forward 2 Program

Field studies were conducted in Ontario, Canada on the control of glyphosate-resistant (GR) Canada fleabane and waterhemp in 2016 and 2017. Glyphosate/dicamba was applied to Canada fleabane at 3 postemergent (POST) application timings (5, 15 and 25 cm) and 4 herbicide rates (0, 900, 1350 and 1800 g ae ha-1). An increase in plant height at the application timing, led to a decrease in control of Canada fleabane; an increase in herbicide rate led to an increase in efficacy. There was an interaction between application timing and rate for density and biomass only. This same study was conducted with GR waterhemp, the same trend followed except there was no interaction between application timing and herbicide rate. Control of GR Canada fleabane with several preplant herbicide tankmixes was evaluated; there was no increase in control with the addition of a second-effective site-of-action. Similarly, GR waterhemp control did not increase with the addition of a second-effective mode-of-action. Acceptable control was not obtained with glyphosate/dicamba applied pre-emergent (PRE), therefore, another mode-of-action is needed for effective control of GR waterhemp. A two-pass program of an effective PRE herbicide followed by glyphosate/dicamba provided season-long control of GR waterhemp and was more efficacious than a PRE or POST program alone.

In 2010, glyphosate-resistant (GR) biotypes of Canada fleabane were first confirmed in Essex County, Ontario. From surveys conducted in 2010 to 2015, inclusive, GR Canada fleabane has now been confirmed in 30 counties and multiple-resistant biotypes (glyphosate and cloransulam-methyl) in 22 counties. Saflufenacil was recommended for GR Canada fleabane control in soybean, however variable control had been reported. The biologically effective rate (BER) of saflufenacil is 25 g a.i. ha-1 for 90% control eight weeks after application (WAA). At 8 WAA, the BER of metribuzin when tankmixed with glyphosate (900 g a.i. ha-1) plus saflufenacil (25 g a.i. ha-1) was 61, 261, and 572 g a.i. ha-1 for 90, 95 and 98% control, respectively. Tankmix partners with glyphosate (900 g a.i. ha-1) plus saflufenacil (25 g a.i. ha-1) were investigated, and the best three-way tankmixes were with either 2,4-D or metribuzin for full season control of multiple-resistant Canada fleabane. The optimal time of day (TOD) for controlling GR Canada fleabane with glyphosate (900 g a.i. ha-1) plus saflufenacil (25 g a.i. ha-1) was 09:00-21:00 h. GR Canada fleabane height and density were found to have a minimal effect on control with glyphosate (900 g a.i. ha-1) plus saflufenacil (25 g a.i. ha-1). BASF Canada, Grain Farmers of Ontario, Agricultural Adaptation Council.

Farmers in southern Ontario face a significant threat from glyphosate-resistant weeds such as Canada fleabane and giant ragweed. Left uncontrolled, glyphosate-resistant weed interference in soybean can result in substantial yield loss. The three objectives of this study were: 1) to determine the biologically effective rate of isoxaflutole, metribuzin, and glyphosate on glyphosate-resistant Canada fleabane and giant ragweed biotypes, 2) to assess the response of isoxaflutole and glyphosate applied as a tank-mix on glyphosate-resistant and glyphosate-susceptible biotypes of both species, and 3) to ascertain the response of isoxaflutole, metribuzin and glyphosate tank-mix on the glyphosate-resistant biotypes of both species. In growth room experiments, antagonism is observed at low rates of glyphosate plus isoxaflutole in glyphosate-susceptible giant ragweed and at field rates of the same tank-mix in glyphosate-resistant Canada fleabane. The addition of metribuzin to the tank-mix resulted in an additive or synergistic response across all rates in the glyphosate-resistant biotypes of both species tested. Field experiments confirmed the additive and synergistic response of the three-way tank-mix in both species tested. An estimated 631 (126+504) and 420 (84+336) g a.i. ha-1 of isoxaflutole plus metribuzin in a 1:4 ratio are required for control of glyphosate-resistant giant ragweed and Canada fleabane respectively. Bayer CropScience

Conyza canadensis (Conyza canadensis (L.) Cronq. var. canadensis) is a surface-germinating ruderal facultative winter annual. The ruderal nature is a key adaptive characteristic that implicates emergence timing as an important recruitment factor. Experiments were established at three sites in southern Ontario, Canada from 2009 to 2012 and at the Thermogradient Plate at the University of Saskatchewan in 2011. Experiments in Ontario aimed to determine the possible effect of emergence timing of Conyza canadensis on survival, fecundity, and flowering timing in populations emerged seed sourced from Spain, United Kingdom, Ontario, and Iran as well as determine the effects of warming spell timing on survival, fecundity, aboveground biomass, and flowering timing for a population sourced from Ontario. Experiments at the Thermogradient Plate at the University of Saskatchewan aimed to determine a base germination temperature range and the amount of GGDs to 50% germination in seed sourced from populations in Spain, United Kingdom, Ontario, and Iran. Results indicated that seed shed timing parsed out into early/late spring or early/late fall helped observe differences between experimental factors. Overall, early fall and early spring emerging plants had significantly greater survival and fecundity, as well as shorter times to flowering when compared to late fall and late spring emerging cohorts in all populations. Results also indicated that warming spells closer to the spring season had a significantly larger negative impact on survival, fecundity, aboveground biomass, and flowering timing compared to warming spells occurring earlier in the winter season. Results conducted on the thermogradient plate indicated significant differences in base germination temperature range and the amount of GGDs to 50% germination in seed sourced from populations in Spain, United Kingdom, Ontario, and Iran, suggesting a genetic and therefore evolutionary difference between the international populations. Overall the findings emerging from this research suggest emergence timing plays a critical role in the population dynamics of Conyza canadensis and that this species has the potential to rapidly adapt to new climatic areas. These findings are especially important in a changing climate and may be exploited for better management approaches as herbicide resistant populations increase in frequency.

Glyphosate resistant Canada fleabane (GRCF) exists in Ontario due to the repeated use of glyphosate on Roundup Ready crops. GR weeds must now be managed using an integrated approach including herbicides like, 2,4-D choline/glyphosate DMA, a premixed herbicide solution. The objective of this research was to determine the ideal application timing of 2,4-D choline/glyphosate DMA herbicide for the control of GRCF. Single applications of 2,4-D choline/glyphosate DMA (1720 g ae ha-1) provided 71-93% control, while sequential applications provided 98-100% control of GRCF 8 WAA. S-metolachlor (1600 g ai ha-1) + flumetsulam (50 g ai ha-1) + clopyralid (135 g ae ha-1) applied preemergence provided the best consistent control of GRCF (95-99%) of the preplant corn residual herbicides evaluated. 2,4-D choline/glyphosate DMA applied post-emergence provided 97-100% control of GRCF following any preplant corn residual herbicide. The size of the GRCF (10, 20 and 30 cm tall) at the time of the 2,4-D choline/glyphosate DMA application did not affect the efficacy of the herbicide. The 2,4-D choline/glyphosate DMA formulation and a tank mix of 2,4-D amine and glyphosate DMA provide equivalent control of GRCF. Dow AgroSciences LLC