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Waterhemp has evolved resistance to seven herbicide modes of action in the USA and five in Canada, which limits weed control options for producers. The objective of this research was to quantify the amount and duration of residual control of multiple-herbicide-resistant (MHR) waterhemp across various Group 15 herbicides applied preemergence in a non-crop area, including acetochlor, dimethenamid-p, flufenacet, pyroxasulfone, and S-metolachlor. Four field trials were conducted over a two-year period (2021, 2022) in southwestern Ontario, Canada. By four weeks after application (WAA) 91% of waterhemp had emerged in the nontreated control. Numerical control of waterhemp with all Group 15 herbicides, with the exception of pyroxasulfone, was greatest at 4 WAA then control declined. Flufenacet provided the lowest waterhemp control; dimethenamid-p and S-metolachlor provided intermediate control, and acetochlor and pyroxasulfone provided the highest control. Waterhemp control with pyroxasulfone peaked at 6 WAA with 99% and declined to 77% at 12 WAA. Flufenacet (low and high rate) is predicted to reduce waterhemp emergence by 50% for 42 to 44 days after application (DAA). Dimethenamid-p, S-metolachlor, and acetochlor (both formulations and three rates) are predicted to reduce waterhemp emergence by 80% for 36, 43, and 33 to 51 DAA, respectively; in contrast, pyroxasulfone is predicted to reduce waterhemp emergence by 80% for 82 DAA. This study concludes that of the Group 15 herbicides evaluated, flufenacet provides the lowest and shortest residual control of waterhemp and pyroxasulfone provides the highest and longest residual control of waterhemp.

The development of glufosinate-resistant soybean cultivars has created opportunities for use of glufosinate applied postemergence for weed control. Four field experiments were conducted in 2021 and 2022 to ascertain the effect of glufosinate rate and the addition of ammonium sulfate on annual weed control in glyphosate/glufosinate/2,4-D–resistant soybean. An increased glufosinate rate of 500 from 300 g ai ha−1 improved control of common ragweed, common lambsquarters, redroot pigweed, and foxtail species and resulted in decreased density and dry biomass of common lambsquarters and foxtail species. The addition of ammonium sulfate to glufosinate increased control of common lambsquarters, 2 and 8 wk after application (WAA), and of foxtail species, 2, 4, and 8 WAA, but did not improve control of common ragweed and redroot pigweed. Increasing the dose of glufosinate from 300 to 500 g ai ha−1 improves control of common ragweed, redroot pigweed, common lambsquarters, and foxtail species; however, the benefit of the addition of ammonium sulfate to glufosinate is weed species-specific.

Glyphosate-resistant (GR) biotypes of horseweed were first confirmed in southern Ontario in 2010 and have spread across southern Ontario. A total of four field experiments were conducted between 2021 and 2022 to determine GR horseweed control with one- and two-pass herbicide programs in glyphosate/glufosinate/2,4-D-resistant (GG2R) soybean. 2,4-D choline/glyphosate DMA, halauxifen-methyl, and saflufenacil applied preplant (PP) controlled GR horseweed by 59%, 72%, and 78% 8 wk after postemergence (POST) application (WAA-POST); there was no improvement of GR horseweed control when 2,4-D choline/glyphosate DMA was added to saflufenacil; in contrast, there was improved GR horseweed control when saflufenacil was added to 2,4-D choline/glyphosate DMA. Glufosinate and 2,4-D choline/glyphosate DMA applied POST controlled glyphosate-resistant horseweed by 71% and 86%, respectively, 8 WAA-POST. Two-pass herbicide programs of a PP followed by POST application provided greater GR horseweed control than a PP or POST herbicide applied alone. Glufosinate or 2,4-D choline/glyphosate DMA applied POST following 2,4-D choline/glyphosate DMA or halauxifen-methyl applied PP improved GR horseweed control by 29% to 38% and 24%, respectively at 8 WAA-POST. The application of 2,4-D choline/glyphosate DMA applied POST following saflufenacil applied PP improved control by 20% 8 WAA-POST; there was no improvement of GR horseweed control when glufosinate was applied POST following saflufenacil applied PP or when either POST herbicide was applied following saflufenacil + 2,4-D choline/glyphosate DMA applied PP. When used in a two-pass program, 2,4-D choline/glyphosate DMA POST provided 2% to 3% greater control of GR horseweed than glufosinate.

Potato is the third most important staple food crop globally following rice and wheat. In the United States, potato is grown on approximately 410,000 ha with a farm-gate value of US$1,032 million. In Canada, potato is grown on approximately 134,000 ha with a farm-gate value of US$235 million. The objective of this manuscript, compiled by the Weed Science Society of America Weed Loss Committee, was to estimate potato yield loss caused by weed interference. Potato yield data from weedy and weed-free plots (or plots with >95% weed control) was obtained from researchers working on weed management in potato in the United States and Canada or from published manuscripts from 2000 to 2018. Potato yield loss from weed interference was 12% to 61% when no weed management tactics were implemented. The average yield loss for all states/provinces (where data was obtained) due to weed interference was 44%. Weed interference would cause a farm-gate loss of approximately US$465 million and US$61 in the United States and Canada, respectively, if weeds are not controlled. These results indicate that weed management is critical for successful potato production, and that an ongoing need for research exists on weed management in this crop.

The development of an integrated weed management (IWM) strategy for control of multiple herbicide-resistant (MHR) waterhemp can provide field crop producers with a strategy to deplete the number of waterhemp seeds in the soil seedbank. Field experiments were established on two commercial farms in Ontario, Canada, with MHR waterhemp in 2017. The number of waterhemp seeds in the seedbank at the Cottam and Walpole Island sites prior to establishing the experiments was 413 and 40 million seeds ha−1, respectively. The goal of this 9-yr study is to document the depletion in the number of waterhemp seeds in the seedbank after Years 3, 6, and 9 (spring 2020, 2023, and 2026) and to identify management practices that can reduce the number of waterhemp seeds by 95% or more. Relative to the number of seeds in the soil seedbank when the experiment was initiated, at the Cottam site after 3 yr of this experiment, in the “control” treatment (continuous soybean seeded in rows spaced 75 apart, and sprayed with glyphosate) there was a numeric 31% increase in the number of waterhemp seeds in the seedbank; in contrast, in the three-crop rotation of corn/soybean/winter wheat (with or without a cover crop after winter wheat harvest), soybean seeded in rows spaced 37.5 cm apart, with herbicide applications using a total of eight different herbicide modes of action resulted in a 65% to 66% decrease in the number of waterhemp seeds in the soil seedbank. At the Walpole Island site after 3 yr of this experiment, the number of waterhemp seeds in the seedbank was not affected by the IWM programs evaluated. Results indicate that a diversified integrated waterhemp management program dramatically decreased the number of waterhemp seeds in the seedbank at one of two sites.

Weeds represent one of the most important biotic threats to agricultural plant health, and the potential global impact of weeds on crop yields is similar to that of all other pests (animal pests and pathogens) combined. Canola is the most-grown crop in Canada based on seeded area and generates on average Can$29.9 billion in economic activity each year. The objective of this report, sponsored by the Weed Science Society of America Weed Loss Committee, was to provide an updated estimate of potential yield and monetary losses due to weed interference in spring canola grown in Canada and the United States. Quantitative yield data from field experiments were provided by researchers and weed science professionals in the northern Great Plains region; the major canola-producing area of North America. Overall, 89 yield loss estimates were compiled, covering the 18-yr period from 2003 to 2020. Average canola yield losses due to weed interference in Alberta, Saskatchewan, Manitoba, and North Dakota were 35%, 30%, 18%, and 28%, respectively. Potential yield losses weighted by canola harvested area averaged 30%, 28%, and 30% for Canada, the United States, and both countries combined, respectively. Therefore, unfettered weed interference in spring canola represents a potential monetary loss of Can$2.21 billion, $0.16 billion, and $2.37 billion for farmers in Canada, the United States, and both countries combined. The realization of such losses could manifest through continued selection for herbicide-resistant weeds, indicating the critical need for canola farmers to diversify resistance selection pressures by implementing proactive integrated weed management programs.

Limited information exists on the global economic impact of glyphosate-resistant (GR) weeds. The objective of this manuscript was to estimate the potential yield and economic loss from uncontrolled GR weeds in the major field crops grown in Ontario, Canada. The impact of GR weed interference on field crop yield was determined using an extensive database of field trials completed on commercial farms in southwestern Ontario between 2010 and 2021. Crop yield loss was estimated by expert opinion (weed scientists and Ontario government crop specialists) when research data were unavailable. This manuscript assumes that crop producers adjust their weed management programs to control GR weeds, which increases weed management costs but reduces crop yield loss from GR weed interference by 95%. GR volunteer corn, horseweed, waterhemp, giant ragweed, and common ragweed would cause an annual monetary loss of (in millions of Can$) $172, $104, $11, $3, and $0.3, respectively, for a total annual loss of $290 million if Ontario farmers did not adjust their weed management programs to control GR biotypes. The increased herbicide cost to control GR volunteer corn, horseweed, waterhemp, giant ragweed, and common ragweed in the major field crops in Ontario is estimated to be (in millions of Can$) $17, $9, $2, $0.1, and $0.02, respectively, for a total increase in herbicide expenditures of $28 million annually. Reduced GR weed interference with the adjusted weed management programs would reduce farm-gate monetary crop loss by 95% from $290 million to $15 million. This study estimates that GR weeds would reduce the farm-gate value of the major field crops produced in Ontario by Can$290 million annually if Ontario farmers did not adjust their weed management programs, but with increased herbicide costs of Can$28 million and reduced crop yield loss of 95% the actual annual monetary loss in Ontario is estimated to be Can$43 million annually.

Little information is available on the relative efficacy of Group 4 herbicides for glyphosate-resistant (GR) horseweed management in soybean. Five field research experiments were conducted in growers’ fields from 2020 to 2021 to determine GR horseweed control with Group 4 herbicides applied preplant (PP) alone and in a mixture. There was minimal soybean injury (≤4%) with herbicides evaluated. Dicamba, 2,4-D, or halauxifen-methyl applied PP controlled GR horseweed 92% to 96%, 73% to 76%, and 85% to 89%, respectively. The mixtures of dicamba + 2,4-D, dicamba + halauxifen-methyl and dicamba + 2,4-D + halauxifen-methyl provided 97% to 99% control of GR horseweed, similar to dicamba applied alone. The mixture of 2,4-D + halauxifen-methyl provided 93% to 94% control of GR horseweed. Dicamba + saflufenacil controlled GR horseweed at 98%. Dicamba alone, dicamba + 2,4-D ester, dicamba + halauxifen-methyl, and dicamba + 2,4-D ester + halauxifen-methyl decreased GR horseweed density 97%, 99%, 99%, and 98%, respectively, similar to a 98% density reduction with dicamba + saflufenacil. Other herbicide treatments had no effect on GR horseweed density. Dicamba, 2,4-D, and halauxifen-methyl applied PP decreased GR horseweed dry biomass by 99%, 76%, and 72%, respectively. The mixtures of dicamba + 2,4-D, dicamba + halauxifen-methyl, and dicamba + 2,4-D + halauxifen-methyl decreased GR horseweed dry biomass by 99% to 100%, similar to a 99% dry biomass reduction with dicamba + saflufenacil. The mixture of 2,4-D + halauxifen-methyl decreased GR horseweed dry biomass by 94%. Soybean yield was decreased by 61% when GR horseweed was left uncontrolled. Results show that Group 4 herbicides that include dicamba applied PP can be very effective in managing GR horseweed in soybean.

Many studies have documented the interaction between 4-hydroxyphenylpyruvate dioxygenase (HPPD)-inhibiting and photosystem II (PSII)-inhibiting herbicides. Most have focused on the interaction between mesotrione and atrazine, with only a few studies characterizing the nature of the interaction between tolpyralate and atrazine. Therefore, five field experiments were conducted in Ontario, Canada, over a 3-yr period (2019 to 2021) to characterize the interaction between three rates of tolpyralate (15, 30, and 45 g ai ha−1) and three rates of atrazine (140, 280, and 560 g ai ha−1) for the control of seven annual weed species in corn (Zea mays L.). Tolpyralate at 30 or 45 g ha−1 applied with atrazine at 280 or 560 g ha−1 controlled velvetleaf (Abutilon theophrasti Medik.), redroot pigweed (Amaranthus retroflexus L.), common ragweed (Ambrosia artemisiifolia L.), common lambsquarters (Chenopodium album L.), and wild mustard (Sinapis arvensis L.) >90% at 8 wk after application (WAA). Tolpyralate and atrazine were synergistic at each rate combination for the control of A. theophrasti at 8 WAA. In contrast, A. retroflexus and S. arvensis control at 8 WAA was additive with each rate combination. At 8 WAA, C. album control was generally additive, but one rate combination was synergistic. Ambrosia artemisiifolia control at 8 WAA was synergistic with five rate combinations and additive with the other four. Barnyardgrass [Echinochloa crus-galli (L.) P. Beauv.] control at 8 WAA was additive with seven of the rate combinations and synergistic with two. Setaria spp. control at 8 WAA was synergistic with one more rate combination compared with E. crus-galli, but the two weed species shared the same synergistic rate combinations. This study concludes that extrapolation or broad classifications of the interaction between tolpyralate and atrazine would be inappropriate, as the interaction can vary due to herbicide rate, weed species, and the response parameter analyzed.

Tolpyralate is an herbicide that is usually mixed with atrazine for broad-spectrum weed control in corn. Previous research has provided information on the effective dose (ED) of tolpyralate applied alone and in a 1:33.3 mixture with atrazine; however, tolpyralate is commercially applied at a dose of 30 to 40 g ai ha−1 with a minimum of 560 g ai ha−1 of atrazine. Therefore, five field trials were conducted over 3 yr (2019 to 2021) to determine the ED of atrazine to complement 30 g ai ha−1 of tolpyralate to achieve 80%, 90%, and 95% control of seven weed species 2, 4, and 8 wk after application (WAA). Tolpyralate was applied alone and in a mixture with atrazine doses ranging from 50 to 2,000 g ai ha−1. At 8 WAA, the ED of atrazine for 95% control of velvetleaf, common ragweed, common lambsquarters, and wild mustard was below the minimum label dose of atrazine on the commercial tolpyralate label, ranging from 430 to 520 g ai ha−1, which supports the use of the minimum label dose of atrazine. In contrast, redroot pigweed required 1,231 g ai ha−1 of atrazine to complement tolpyralate for 95% control 8 WAA. At 8 WAA, barnyardgrass and a mixture of green foxtail and giant foxtail (Setaria spp.) were not controlled by 80%, 90%, or 95% with tolpyralate applied alone or co-applied with any dose of atrazine evaluated in this study. The results of this study conclude that tolpyralate + atrazine is highly efficacious on several weed species at atrazine doses of 40 to 130 g ai ha−1 below the label dose of 560 g ai ha−1, but the use of the higher dose of tolpyralate or another herbicide may be required to improve control of redroot pigweed and grass weed species.

Common bean and azuki bean are poor competitors with weeds and demonstrate sensitivity to herbicides used for weed control in soybean. S-metolachlor, flufenacet, and acetochlor are categorized as Group 15 herbicides and provide control of multiple annual grass and select small-seeded broadleaf weeds. By way of field trials near Exeter and Ridgetown, Ontario, in 2019, 2020, and 2021, four dry bean market classes (azuki, kidney, small red, and white bean) were evaluated for their tolerance to 1× established label rates and 2× rates of S-metolachlor (1,600 and 3,200 g ai ha−1), flufenacet (750 and 1,500 g ai ha−1) and acetochlor (1,700 and 3,400 g ai ha−1) applied preplant incorporated (PPI). Injury was evaluated by assessing visible injury symptoms, density, shoot biomass, height, seed moisture content, and seed yield. Azuki bean was more sensitive to the Group 15 herbicides than other dry bean market classes; the Group 15 herbicides caused a 12% reduction in azuki bean growth at 2 wk after emergence; growth reduction was ≤2% in the other bean classes. Flufenacet (2× rate) was the most injurious treatment, causing a 27% reduction in azuki bean yield. This study concludes that kidney, small red, and white bean have a sufficient margin of crop safety to flufenacet, acetochlor, and S-metolachlor applied PPI. Azuki bean was sensitive to flufenacet; additional research is needed to investigate azuki bean tolerance to acetochlor and S-metolachlor applied PPI.

The complementary modes of action of 4-hydroxyphenylpyruvate dioxygenase (HPPD) and photosystem II (PSII) inhibitors have been credited for the synergistic weed control improvement of several species. Recent research discovered that reactive oxygen species (ROS) generation and subsequent lipid peroxidation is the cause of cell death by the glutamine synthetase inhibitor glufosinate. Therefore, a basis for synergy exists between glufosinate and HPPD inhibitors, but the interaction has not been well reported. Four field experiments were conducted in Ontario, Canada, in 2020 and 2021 to determine the interaction between HPPD-inhibiting (mesotrione and tolpyralate) and ROS-generating (atrazine, bromoxynil, bentazon, and glufosinate) herbicides on control of annual weed species in corn (Zea mays L.). The ROS generators were synergistic with the HPPD inhibitors and provided ≥95% control of velvetleaf (Abutilon theophrasti Medik.), except for tolpyralate + glufosinate, which was additive at 8 wk after application (WAA) and provided 87% control. Tank mixes of HPPD inhibitors plus ROS generators were synergistic for the control of common ragweed (Ambrosia artemisiifolia L.), except for tolpyralate + glufosinate, which was antagonistic at 8 WAA. Tolpyralate + glufosinate was antagonistic for the control of barnyardgrass [Echinochloa crus-galli (L.) P. Beauv.] and Setaria spp. at 8 WAA. Common lambsquarters (Chenopodium album L.) control at 8 WAA was synergistic and ≥95% with mesotrione plus atrazine, bromoxynil, or glufosinate and with tolpyralate plus bromoxynil or bentazon. Herbicide tank mixes were generally additive for the control of wild mustard (Sinapis arvensis L.) at 8 WAA, except for the synergistic tank mixes of tolpyralate plus atrazine or bromoxynil; however, each tank mix provided 97% to 100% control of S. arvensis. Results from this study demonstrate that co-application of ROS generators with mesotrione or tolpyralate controlled all broadleaf weed species >90% at 8 WAA, with the exceptions of A. artemisiifolia and C. album control with tolpyralate + glufosinate. Mesotrione plus PSII inhibitors controlled E. crus-galli and Setaria spp. 48 to 68 percentage points less than tolpyralate plus the respective PSII inhibitor at 8 WAA; however, mesotrione + glufosinate and tolpyralate + glufosinate controlled the grass weed species similarly.

Limited information exists on the critical time of weed removal (CTWR) with the currently used soybean cultivars in Ontario. A study consisting of eight field experiments was conducted from 2017 to 2019 in Ontario, Canada, to determine the impact of delayed postemergence (POST) herbicide application on soybean yield based on average weed height at application, days after crop emergence (DAE), accumulated crop heat units (CHU) from the date of planting, and soybean growth stage. The regression model estimated the weed size at herbicide application that led to 1%, 2.5%, and 5% yield loss in soybean was 9, 14, and 20 cm under low weed density (averaging 73 to 134 plants m−2) and 3, 4, and 6 cm under high weed density (143 to 153 plants m−2) conditions, respectively. The estimated DAE at herbicide application time that led to 2.5%, 5%, 10%, and 25% yield loss in soybean was 24, 30, 37, and 53 DAE under low weed density and 8, 10, 14, and 23 DAE under high weed density, respectively. The predicted crop stage at herbicide application that resulted in 2.5%, 5%, 10%, and 25% yield loss in soybean was V4, V5, R2, and R5 under low weed density and VE, VC, V1, and V4 under high weed density, respectively. This study concludes that soybean yield loss is influenced by the weed density (low vs/high) and the time of the first POST herbicide application. When the first POST herbicide application was delayed until soybean was at the V2 stage the monetary loss was Can$20.46 and Can$221.20 ha−1 in low and high weed-density environments, respectively.

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 wk 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 at 45, 89, 134, and 268 g ai ha–1 controlled MHR waterhemp up to 72%, 89%, 92%, and 95%, respectively. Flumioxazin applied preemergence at 35, 70, 106, and 211 g ai ha–1 controlled MHR waterhemp up to 78%, 90%, 93%, and 96%, respectively. Pyroxasulfone/flumioxazin applied preemergence 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 trend similar trend to 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 a synergistic increase in soybean injury with the co-application of pyroxasulfone plus flumioxazin.

Tolpyralate is a 4-hydroxyphenylpyruvate dioxygenase–inhibiting herbicide that is applied postemergence for control of annual broadleaf and grass weeds in corn. Current Canadian label recommendations for tolpyralate specify the addition of a methylated seed oil (MSO) adjuvant (MSO Concentrate®) for improved weed control. The efficacy of tolpyralate applied with other proprietary adjuvants has not been widely reported in the peer-reviewed literature. Therefore, four field trials were conducted in corn over 2020 and 2021 in Ontario, Canada, to evaluate MSO Concentrate®, Agral® 90 (nonionic surfactant), Assist® Oil Concentrate (blended surfactant), Carrier® (blended surfactant), LI 700® (nonionic surfactant), and Merge® (blended surfactant) as adjuvants with tolpyralate for the control of annual broadleaf and grass weeds. At 8 wk after application (WAA), tolpyralate applied with MSO Concentrate®, Agral® 90, Assist® Oil Concentrate, Carrier®, or Merge® controlled velvetleaf, wild mustard, barnyardgrass, and foxtail species similarly. These adjuvants also enhanced the efficacy of tolpyralate similarly for the control of common ragweed at 8 WAA with the exception that Agral® 90 was inferior to Merge®. At 8 WAA, tolpyralate controlled common lambsquarters the greatest when applied with MSO Concentrate®, Agral® 90, Carrier®, or Merge®; these adjuvants with the exception of Agral® 90 were superior to Assist® Oil Concentrate. At 8 WAA, tolpyralate applied with LI 700® controlled common ragweed, barnyardgrass, and foxtail species less than when tolpyralate was applied with the other adjuvants tested; control of these weed species with tolpyralate was not improved with LI 700® when compared to tolpyralate applied without an adjuvant. Overall, tolpyralate applied with either MSO Concentrate®, Carrier®, or Merge® controlled all annual broadleaf and grass weed species similarly or greater than tolpyralate applied without an adjuvant or tolpyralate with Agral® 90, Assist® Oil Concentrate, or LI 700® at 8 WAA.

Recent research reported synergism between glufosinate plus very low rates of protoporphyrinogen oxidase (PPO)–inhibiting herbicides on select broadleaf weeds. Two field studies, each consisting of four trials, were conducted in 2020 and 2021 in commercial fields with glyphosate-resistant (GR) horseweed in Ontario, Canada. Study 1 evaluated GR horseweed control with glufosinate plus five PPO inhibitors at 5% of the label rate; study 2 evaluated what dose of saflufenacil is needed when co-applied with glufosinate to improve GR horseweed control. In study 1, glufosinate plus very low rates of PPO-inhibiting herbicides provided low GR horseweed control. At site 1, despite the synergistic increase in GR horseweed control with saflufenacil (1.25 g ai ha–1) plus glufosinate (300 g ai ha–1), the level of control did not exceed 42% at 2 and 4 wk after application (WAA); the interaction was additive at 8 WAA. The co-application of glufosinate (300 g ai ha–1) with pyraflufen-ethyl (0.34 g ai ha–1), pyraflufen-ethyl/2,4-D (26.4 g ai ha–1), flumioxazin (5.35 g ai ha–1), fomesafen (12 g ai ha–1), or sulfentrazone (7 g ai ha–1) resulted in an additive interaction for GR horseweed control at 2, 4, and 8 WAA. However, glufosinate plus pyraflufen-ethyl or sulfentrazone was antagonistic at 8 WAA. In study 2, similar doses of saflufenacil were required for 50%, 80%, and 95% GR horseweed control whether glufosinate was included in the mixture or not. Interactions between glufosinate (300 g ai ha–1) plus saflufenacil at 1.56, 3.13, 6.25, and 12.5 g ai ha–1 were antagonistic at 2, 4, and 8 WAA at sites 1, 2, and 3; all other interactions were additive. The results of this research indicate there was little to no benefit of adding very low rates of PPO-inhibiting herbicides to glufosinate to improve GR horseweed control under field conditions.

Tolpyralate is commonly mixed with atrazine for improved control of common annual weed species in corn production systems in the United States and Canada. Weed control efficacy with this mixture is enhanced with the addition of methylated seed oil (MSO) Concentrate®; however, there is little information on the efficacy of tolpyralate + atrazine with other proprietary adjuvants. Therefore, four trials were conducted at field research sites in Ontario, Canada, to evaluate the efficacy of tolpyralate + atrazine when applied with six different commercially available adjuvants on four annual broadleaf and two annual grass weed species in corn. The adjuvants evaluated were MSO Concentrate®, Agral® 90, Assist® Oil Concentrate, Carrier®, LI 700®, and Merge®. A treatment of tolpyralate + atrazine applied with no adjuvant was also included in the study. For the control of velvetleaf and wild mustard, the adjuvants evaluated with tolpyralate + atrazine did not improve control. At 8 wk after application (WAA), the use of Agral® 90, Assist® Oil Concentrate, Carrier®, MSO Concentrate®, or Merge® with tolpyralate + atrazine provided similar or greater control of common ragweed than tolpyralate + atrazine applied with LI 700®. At 8 WAA, the adjuvants performed similarly with tolpyralate + atrazine for the control of common lambsquarters; however, LI 700® was the only adjuvant that did not improve control compared to tolpyralate + atrazine applied without an adjuvant. At 8 WAA, MSO Concentrate®, Carrier®, and Merge® improved control of barnyardgrass and foxtail species with tolpyralate + atrazine to a similar or greater level than Assist® Oil Concentrate, Agral® 90, and LI 700®. Overall, MSO Concentrate®, Carrier®, or Merge® should be added to tolpyralate + atrazine for control of the myriad of weed species interfering with corn production.

The objectives of this study were to determine if the level and consistency of glyphosate-resistant (GR) horseweed control prior to soybean planting can be improved by (i) adding halauxifen-methyl, 2,4-D ester, saflufenacil, metribuzin, or dicamba to glufosinate, (ii) increasing the rate of glufosinate from 500 to 1,000 g ai ha–1, and (iii) adding 28% urea ammonium nitrate (UAN) as the carrier solution. During a 2-yr period (2020–2021), four field trials were conducted on commercial farms located in southwestern Ontario, Canada, with confirmed GR horseweed. Glufosinate controlled GR horseweed 65%, 66%, and 63% at 2, 4, and 8 wk after application (WAA), respectively, and reduced density and biomass 46% and 33% at 8 WAA, respectively. There was no improvement in GR horseweed control from the addition of halauxifen-methyl, 2,4-D ester or saflufenacil to glufosinate and no decrease in density and biomass, with the exception that the addition of saflufenacil to glufosinate reduced density 30% compared to glufosinate alone. The addition of metribuzin to glufosinate improved GR horseweed control by 22%, 22%, and 28% at 2, 4, and 8 WAA, respectively, and further reduced density and biomass 50% and 47%, respectively, at 8 WAA, respectively. The addition of dicamba to glufosinate improved GR horseweed control by 19%, 26%, and 30% at 2, 4, and 8 WAA, respectively, and further reduced density and biomass 54% and 60%, respectively, at 8 WAA. There was no improvement in GR horseweed control by increasing the rate of glufosinate from 500 to 1,000 g ai ha–1 or when using 28% UAN as the carrier solution. The addition of all herbicides to glufosinate, increasing the rate of glufosinate, or using 28% UAN as the carrier solution improved the consistency of GR horseweed control.

Nine field experiments were conducted from 2017 to 2019 in Ontario to determine the impact of early weed interference on corn yield based on corn growth stage, days after emergence (DAE), accumulated crop heat units (CHU), and weed size. The predicted weed size at herbicide application that resulted in a 1%, 2.5%, 5%, 10%, 25%, and 50% yield loss in corn was estimated to be 1, 4, 11, 53, non-estimable (N est.*), and N est.* cm under low weed density and 3, 5, 7, 11, 27, and N est.* cm under high weed density, respectively. The predicted DAE at herbicide application time that resulted in a 1%, 2.5%, 5%, 10%, 25%, and 50% yield loss in corn was predicted to be 14, 20, 27, 44, N est.*, and N est.* DAE under low weed density and 5, 7, 11, 17, 25, and 59 DAE under high weed density, respectively. The predicted CHU from planting at herbicide application time that led to a 1%, 2.5%, 5%, 10%, 25%, and 50% yield loss in corn was 468, 636, 821, 1,271, N est.*, and N est.* CHU from planting under low weed density and 207, 283, 385, 551, 972, and 1,748 CHU from planting under high weed density, respectively. The predicted crop stage at herbicide application that led to a 1%, 2.5%, 5%, 10%, 25%, and 50% yield loss in corn was V5, V6, V7, V11, N est.*, and N est.* under low weed density and V1, V2, V3, V4, V8, and V14 under high weed density, respectively. Results indicate that weeds must be controlled before they reach 7 cm in height, prior to 11 d after crop emergence, prior to 385 accumulated CHU from emergence, or prior to the V3 stage under high weed density to avoid greater than 5% yield loss.

Glyphosate-resistant (GR) horseweed interference in soybean can reduce soybean yield up to 93%. Glyphosate plus dicamba, 2,4-D ester, halauxifen-methyl or pyraflufen-ethyl/2,4-D applied preplant (PP) provide variable GR horseweed control in soybean. The objective of this study was to determine if the addition of saflufenacil or metribuzin to glyphosate plus dicamba, 2,4-D ester, halauxifen-methyl, or pyraflufen-ethyl/2,4-D will improve the level and consistency of GR horseweed control. Four trials were conducted over the 2020 and 2021 field seasons in fields with GR horseweed populations. Glyphosate plus dicamba, 2,4-D ester, halauxifen-methyl, or pyraflufen-ethyl/2,4-D controlled GR horseweed 96%, 77%, 71%, and 52%, respectively, at 8 wk after application (WAA). When saflufenacil or metribuzin was added to glyphosate plus dicamba or 2,4-D ester, GR horseweed control was not improved at 8 WAA. When saflufenacil or metribuzin was added to glyphosate plus halauxifen-methyl, GR horseweed control improved by 27% and 25%, respectively, at 8 WAA. When saflufenacil or metribuzin was added to glyphosate plus pyraflufen-ethyl/2,4-D, GR horseweed control was improved by 47% and 37%, respectively, at 8 WAA. The consistency of GR horseweed control was improved when saflufenacil or metribuzin was added to glyphosate plus dicamba, 2,4-D ester, halauxifen-methyl, or pyraflufen-ethyl/2,4-D compared to each herbicide applied alone. Synergism was observed when metribuzin was added to glyphosate plus halauxifen-methyl and when saflufenacil or metribuzin was added to glyphosate plus pyraflufen-ethyl/2,4-D at 8 WAA. Though GR horseweed control was improved with the addition of saflufenacil or metribuzin to glyphosate plus halauxifen-methyl or pyraflufen-ethyl/2,4-D, all treatments including saflufenacil resulted in the highest level and most consistent control.

Glyphosate-resistant (GR) horseweed [Conyza canadensis (L.) Cronquist; syn.: Erigeron canadensis L.] interference can substantially reduce corn (Zea mays L.) yield. The complementary activity of 4-hydroxyphenylpyruvate dioxygenase (HPPD) and photosystem II (PSII) inhibitors has been investigated for the control of several weed species, and in many cases has been synergistic; however, there is little information on the interaction of HPPD- and PSII-inhibiting herbicides for postemergence control of GR C. canadensis in corn. Four field trials were studied over 2 yr (2019, 2020) in Ontario, Canada, in commercial corn fields with natural infestations of GR C. canadensis to evaluate the level of GR C. canadensis control with three HPPD-inhibiting herbicides (mesotrione, tolpyralate, and topramezone) and three PSII-inhibiting herbicides (atrazine, bromoxynil, and bentazon) applied individually and in tank-mix combinations, and to document the interaction of the three HPPD inhibitors tank mixed with the three PSII inhibitors. Mesotrione, tolpyralate, and topramezone controlled GR C. canadensis 83%, 84%, and 72%, respectively, at 8 wk after application (WAA). Bromoxynil and bentazon controlled GR C. canadensis 71% and 79%, respectively, while atrazine provided only 31% control at 8 WAA. The joint application of atrazine, bromoxynil, or bentazon with mesotrione increased GR C. canadensis control from 83% to 100% at 8 WAA. Tolpyralate tank mixed with atrazine, bromoxynil, or bentazon controlled GR C. canadensis 96%, 98%, and 98%, respectively, which was comparable to the mesotrione tank mixes at 8 WAA. Topramezone plus atrazine, bromoxynil, or bentazon controlled GR C. canadensis 91%, 93%, and 95%, respectively, at 8 WAA. Interactions between HPPD and PSII inhibitors were synergistic for all combinations of mesotrione or tolpyralate with atrazine, bromoxynil, or bentazon. The interaction between topramezone and PSII inhibitors was additive. All nine tank mixes controlled GR C. canadensis >90%. This study concludes that bromoxynil or bentazon, instead of atrazine, can be co-applied with mesotrione, tolpyralate, or topramezone without compromising GR C. canadensis control in corn.

Glyphosate’s efficacy is influenced by the amount absorbed and translocated throughout the plant to inhibit 5-enolpyruvyl shikimate-3-phosphate synthase (EPSPS). Glyphosate resistance can be due to target-site (TS) or non–target site (NTS) resistance mechanisms. TS resistance includes an altered target site and gene overexpression, while NTS resistance includes reduced absorption, reduced translocation, enhanced metabolism, and exclusion/sequestration. The goal of this research was to elucidate the mechanism(s) of glyphosate resistance in common ragweed (Ambrosia artemisiifolia L.) from Ontario, Canada. The resistance factor for this glyphosate-resistant (GR) A. artemisiifolia biotype is 5.1. No amino acid substitutions were found at positions 102 or 106 of the EPSPS enzyme in this A. artemisiifolia biotype. Based on [14C]glyphosate studies, there was no difference in glyphosate absorption or translocation between glyphosate-susceptible (GS) and GR A. artemisiifolia biotypes. Radio-labeled glyphosate metabolites were similar for GS and GR A. artemisiifolia 96 h after application. Glyphosate resistance in this A. artemisiifolia biotype is not due to an altered target site due to amino acid substitutions at positions 102 and 106 in the EPSPS and is not due to the NTS mechanisms of reduced absorption, reduced translocation, or enhanced metabolism.

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 wk 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 wk 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 visible 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. The dose of pyroxasulfone predicted for 80% reduction in MHR A. tuberculatus density was 117 g ai ha−1, and the doses of pyroxasulfone predicted 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. The predicted doses of pyroxasulfone required to obtain 50%, 80%, and 95% yield relative to the weed-free plots were 6, 24, and 112 g ai ha−1, respectively. Flumioxazin and pyroxasulfone applied preemergence at the appropriate doses provided early-season MHR A. tuberculatus control in soybean.

Yield losses due to weeds are a major threat to wheat production and economic well-being of farmers in the United States and Canada. The objective of this Weed Science Society of America (WSSA) Weed Loss Committee report is to provide estimates of wheat yield and economic losses due to weeds. Weed scientists provided both weedy (best management practices but no weed control practices) and weed-free (best management practices providing >90% weed control) average yield from replicated research trials in both winter and spring wheat from 2007 to 2017. Winter wheat yield loss estimates ranged from 2.9% to 34.4%, with a weighted average (by production) of 25.6% for the United States, 2.9% for Canada, and 23.4% combined. Based on these yield loss estimates and total production, the potential winter wheat loss due to weeds is 10.5, 0.09, and 10.5 billion kg with a potential loss in value of US$2.19, US$0.19, and US$2.19 billion for the United States, Canada, and combined, respectively. Spring wheat yield loss estimates ranged from 7.9% to 47.0%, with a weighted average (by production) of 33.2% for the United States, 8.0% for Canada, and 19.5% combined. Based on this yield loss estimate and total production, the potential spring wheat loss is 4.8, 1.6, and 6.6 billion kg with a potential loss in value of US$1.14, US$0.37, and US$1.39 billion for the United States, Canada, and combined, respectively. Yield loss in this analysis is greater than some previous estimates, likely indicating an increasing threat from weeds. Climate is affecting yield loss in winter wheat in the Pacific Northwest, with percent yield loss being highest in wheat-fallow systems that receive less than 30 cm of annual precipitation. Continued investment in weed science research for wheat is critical for continued yield protection.

Tiafenacil is a new nonselective, protoporphyrinogen IX oxidase–inhibiting pyrimidinedione herbicide that is under consideration for registration to control grass and broadleaf weeds in corn, soybean, wheat, cotton, and other crops prior to crop emergence. The sensitivity of dry beans to tiafenacil is not known. Four field experiments were completed at Exeter and Ridgetown, ON, Canada, during the 2019 and 2020 growing seasons, to determine the sensitivity of azuki, kidney, small red, and white beans to tiafenacil applied preemergence (PRE) at 12.5, 25, 50, and 100 g ai ha−1. Tiafenacil applied at 100 g ai ha−1 caused 5% or less injury to azuki, kidney, small red, and white beans: 0% to 3% injury to azuki bean; 1% to 5% injury to kidney bean; and 1% to 4% injury to both small red bean and white bean. Tiafenacil applied PRE at 12.5, 25, 50, and 100 g ai ha−1 caused up to 1%, 4%, 4%, and 5% visible dry bean injury, respectively, but had no negative effect on other measured growth parameters including seed yield. Crop injury was generally greatest when tiafenacil was appled at the 100 g ai ha−1 rate in dry beans. Generally, kidney, small red, and white bean were more sensitive to tiafenacil than azuki bean. Dry bean injury was persistent and increased with time with the greatest injury observed 8 wk after emergence. Tiafenacil applied PRE can be a useful addition to the current strategies to control grass and broadleaf weeds, especially glyphosate-resistant horseweed and amaranth species prior to bean emergence.

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.

Four field experiments were completed in commercial corn fields during 2019 and 2020 to determine glyphosate-resistant (GR) horseweed control in corn with tiafenacil alone or in combination with bromoxynil, dicamba, or tolpyralate applied preplant (PP). Corn planted 1 to 10 d after herbicide application was not injured with any of the herbicides tested. GR horseweed interference reduced corn grain yield 32% when left uncontrolled. Herbicides reduced GR horseweed interference and resulted in corn grain yield that was similar to the weed-free control. Glyphosate (900 g ae ha−1) + tiafenacil at 12.5, 25, and 37.5 g ha−1 controlled GR horseweed 63%, 68%, and 72% at 4 wk after treatment (WAT) and decreased GR horseweed density 64%, 43%, and 83% and dry biomass 69%, 55%, and 83%, respectively. Glyphosate + tiafenacil at 12.5, 25, and 37.5 g ha−1 plus bromoxynil (280 g ai ha−1) controlled GR horseweed 81%, 88%, and 87% at 4 WAT and reduced GR horseweed density 82%, 94%, and 93% and dry biomass 93%, 93%, and 98%, respectively. Glyphosate + tiafenacil at 12.5, 25, and 37.5 g ha−1 plus dicamba (300 g ai ha−1) controlled GR horseweed 86%, 88%, and 88% at 4 WAT and decreased GR horseweed density 76%, 89%, and 86% and dry biomass 94%, 98%, and 98%, respectively. Glyphosate + tiafenacil at 12.5, 25, and 37.5 g ha−1 plus tolpyralate (30 g ai ha−1) controlled GR horseweed 90%, 90%, and 91% at 4 WAT and decreased GR horseweed density 93%, 91%, and 95% and dry biomass 98%, 97%, and 97%, respectively. The industry standards in Ontario, glyphosate + dicamba/atrazine and glyphosate + saflufenacil/dimethenamid-p controlled GR horseweed 95% and 100% at 4, 8, and 12 WAT and caused 99% and 100% density or biomass reduction, respectively.

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.

The complementary activity of 4-hydroxphenylpyruvate dioxygenase (HPPD) inhibitors and atrazine is well documented, but the use of atrazine is restricted in some geographic areas, including the province of Quebec in Canada, necessitating the evaluation of atrazine alternatives and their interactions with HPPD inhibitors. The objectives of this study were to determine whether mixing HPPD inhibitors with atrazine alternative photosystem II (PS II) inhibitors, such as metribuzin and linuron applied PRE or bromoxynil and bentazon applied POST, results in similar control of multiple herbicide–resistant (MHR) waterhemp [Amaranthus tuberculatus (Moq.) Sauer] in corn (Zea mays L.). Ten field trials, five with herbicides applied PRE and five with herbicides applied POST, were conducted in Ontario, Canada, in fields infested with MHR A. tuberculatus. Isoxaflutole, applied PRE, controlled MHR A. tuberculatus 58% to 76%; control increased 17% to 34% with the addition of atrazine, metribuzin, or linuron at three of five sites across 2, 4, 8, and 12 wk after application (WAA). The interaction between isoxaflutole and PS II inhibitors, applied PRE, was additive for MHR A. tuberculatus control and biomass and density reduction. Mesotrione, tolpyralate, and topramezone, applied POST, controlled MHR A. tuberculatus 54% to 59%, 61%, and 44% to 45%, respectively, at two of five sites across 4, 8, and 12 WAA. The addition of atrazine, bromoxynil, or bentazon to mesotrione improved MHR A. tuberculatus control 29%, 34%, and 22%; to tolpyralate, improved control 2%, 20%, and 10%; and to topramezone, improved control 3%, 14%, and 8%, respectively. Interactions between HPPD and PS II inhibitors were mostly additive; however, synergistic responses were observed with mesotrione + bromoxynil or bentazon, and tolpyralate + bromoxynil. Mixing atrazine alternatives metribuzin or linuron with isoxaflutole, applied PRE, and bromoxynil or bentazon with mesotrione or tolpyralate, applied POST, resulted in similar or better control of MHR A. tuberculatus in corn.

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.

The biologically-effective-dose of tolpyralate, a new 4-hydroxyphenyl-pyruvate dioxygenase (HPPD)-inhibitor, applied alone or tank-mixed with atrazine, for the control of multiple-herbicide-resistant (MHR) waterhemp [Amaranthus tuberculatus (Moq.) J. D. Sauer] has not been studied in corn. Seven field experiments were conducted during a three-year period (2018, 2019, 2020) in Ontario, Canada with MHR waterhemp to determine: 1) the dose-response of MHR waterhemp to tolpyralate and tolpyralate plus atrazine, and 2) the relative efficacy of tolpyralate and tolpyralate plus atrazine to post-emergence corn herbicides, dicamba/atrazine (500/1000 g·ha−1) and mesotrione + atrazine (100 + 280 g·ha−1). Tolpyralate + atrazine (120 + 4000 g·ha−1) caused 13% corn injury at one site two weeks after application (WAA), which was observed as transient foliar chlorosis and bleaching of new leaves. At 12 WAA, the predicted dose of tolpyralate for 50% control of MHR waterhemp at Cottam and on Walpole Island was 8 and 2 g·ha−1, respectively; the predicted dose of tolpyralate + atrazine for 50% control of MHR waterhemp at Cottam and on Walpole Island was 5 + 160 and 1 + 21 g·ha−1, respectively. The difference in predicted dose at the two sites is likely due to differences in MHR density and resistance profile. Applied at the registered rate, tolpyralate (30 g·ha−1) and tolpyralate + atrazine (30 + 1000 g·ha−1) controlled MHR waterhemp similar to dicamba/atrazine and mesotrione + atrazine across sites. This study demonstrates that tolpyralate + atrazine, applied POST, provides season-long control of MHR waterhemp in corn.

Limited information is available on the sensitivity of dry beans to tribenuron and carfentrazone applied preplant (PP). Four field trials were conducted at Exeter and Ridgetown, ON, Canada in 2019 and 2020 to determine the tolerance of azuki, kidney, small red and white beans to glyphosate (1800 g·ae·ha-1) + tribenuron (15 g·ai·ha-1), glyphosate (1800 g·ae·ha-1) + carfentrazone (35 g·ai·ha-1) and glyphosate (1800 g·ae·ha-1) + tribenuron (15 g·ai·ha-1) + carfentrazone (35 g·ai·ha-1) applied PP 1 - 2 days prior to seeding dry beans. Glyphosate + tribenuron, glyphosate + carfentrazone, and glyphosate + tribenuron + carfentrazone, applied PP, caused 5%, 5% and 9% bean injury at 1 WAE; 7%, 6% and 10% bean injury at 2 WAE and 6%, 5% and 8% bean injury at 4 WAE, respectively. At 8 WAE, there was 0, 1% and 4% injury in azuki bean; 1%, 2% and 2% injury in kidney bean; 3%, 2% and 3% injury in small red bean; and 6%, 3% and 2% injury in white bean with glyphosate + tribenuron, glyphosate + carfentrazone, and glyphosate + tribenuron + carfentrazone applied PP, respectively. The injury was significantly greater with glyphosate + tribenuron in small red and white beans compared to the azuki and kidney beans. There was no difference between injury levels among market classes of dry been with glyphosate + carfentrazone or glyphosate + tribenuron + carfentrazone applied PP. There was no effect of glyphosate + tribenuron, glyphosate + carfentrazone and glyphosate + tribenuron + carfentrazone on dry bean plant stand, maturity and seed yield. However, dry bean biomass was reduced as much as 15% with glyphosate + tribenuron and 13% with glyphosate + tribenuron + carfentrazone compared to the untreated control. Dry bean height was reduced 4% with glyphosate + tribenuron + carfentrazone but was not affected with other treatments. Based on these results, there is potential for using glyphosate plus tribenuron or carfentrazone for preplant weed management in dry bean production.

Multiple-herbicide-resistant (MHR) waterhemp has been confirmed and is difficult to control for growers in Ontario, Canada and in the Midwestern United States. The objective of this study was to evaluate early post-emergence (EPOST) herbicides for control of MHR waterhemp in field corn. Five field trials were conducted over a two-year period (2019, 2020) at sites on Walpole Island, ON and near Cottam, ON, Canada. Thirteen herbicide tank-mixtures containing multiple modes-of-action (MOA) were applied EPOST to 5 cm MHR waterhemp in field corn. Control of MHR waterhemp varied by site due to variable plant density, plant biomass, and number of herbicide-resistant individuals across research sites and years. Control of MHR waterhemp ranged from 90% to 100% with glyphosate + S-metolachlor/mesotrione/ bicyclopyrone/atrazine, glyphosate/2,4-D choline + rimsulfuron + mesotrione + atrazine, glyphosate + S-metolachlor/atrazine/mesotrione, glyphosate + mesotrione + atrazine, glyphosate/S-metolachlor/mesotrione + atrazine, glyphosate + S-metolachlor/mesotrione/bicyclopyrone, glyphosate/2,4-D choline + rimsulfuron + mesotrione, and glyphosate + pyroxasulfone + dicamba/atrazine at 4, 8, and 12 WAA. Control of MHR waterhemp ranged from 70% to 100% with glyphosate + topramezone/dimethenamid-P + dicamba/atrazine, glyphosate + isoxaflutole + atrazine, and glyphosate + tolpyralate + atrazine at 4, 8, and 12 WAA. Control of MHR waterhemp was similar for all herbicide programs, except glyphosate + dicamba/atrazine and glyphosate + S-metolachlor/atrazine which resulted in the lowest control at three of five sites that ranged from 63% to 89% and 61% to 76%, respectively. Crop injury was ≤10% for herbicide programs tested, except 28% to 31% corn injury with glyphosate/2,4-D choline + rimsulfuron + mesotrione + atrazine; however, without effect on corn grain yield. Corn yield was comparable with all herbicide programs evaluated in this study. It is concluded that there are herbicide programs that provide control of emerged and full-season residual control of MHR waterhemp in field corn.

Six field trials were conducted over a four-year (2017-2020) period near Exeter and Ridgetown, Ontario to determine the efficacy of halosulfuron tankmixes applied postemergence to control broadleaf weeds in white bean. Halosulfuron caused up to 4% injury in white bean at 2 and 4 weeks after treatment (WAT). Bentazon, acifluorfen, fomesafen, bentazon/acifluorfen, and bentazon + fomesafen caused 2% - 16% injury at 2 WAT and up to 3% injury at 4 WAT in white bean. The addition of halosulfuron to the aforementioned herbicides did not accentuate white bean injury. Reduced weed interference with the herbicides evaluated increased white been yield 50% - 90% compared to the weedy control; there was no difference in seed yield among herbicide treatments evaluated. At 4 WAT, halosulfuron at 25, 37.5 and 50 g ai ha-1 controlled velvetleaf 86%, 93% and 97%; redroot pigweed 83%, 85% and 89%; common ragweed 90%, 93% and 94%; common lambsquarters 27%, 28% and 36%; flower-of-an-hour 66%, 76% and 69%; and wild mustard 100%, 100% and 100%, respectively. Bentazon, acifluorfen, fomesafen, bentazon/acifluorfen, and bentazon + fomesafen controlled velvetleaf 73%, 14%, 52%, 42% and 68%; redroot pigweed 40%, 91%, 85%, 75% and 80%; common ragweed 36%, 81%, 92%, 68% and 84%; common lambsquarters 87%, 39%, 48%, 60% and 76%; flower-of-an-hour 90%, 66%, 63%, 73% and 83%; and wild mustard 97%, 97%, 100%, 99% and 100%, respectively. Halosulfuron tankmixed with bentazon, acifluorfen, fomesafen, bentazon/acifluorfen or bentazon + fomesafen controlled velvetleaf 90%, 51%, 68%, 75% and 90%; redroot pigweed 80%, 99%, 95%, 92% and 91%; common ragweed up to 94%, 97%, 93%, 94% and 95%; common lambsquarters 74%, 62%, 43%, 62% and 66%; flower-of-an-hour 92%, 78%, 74%, 82% and 87%; and wild mustard 100%, 100%, 100%, 100% and 100%, respectively. Weed density and dry biomass followed the same trend. This study concludes that the optimal halosulfuron tankmix is broadleaf weed species specific for weed management in dry bean production.