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Article

The Addition of Saflufenacil to Glyphosate plus Dicamba Improves Glyphosate-Resistant Canada Fleabane (Erigeron canadensis L.) Control in Soybean

by
Meghan Dilliott
,
Nader Soltani
*,
David C. Hooker
,
Darren E. Robinson
and
Peter H. Sikkema
Department of Plant Agriculture, University of Guelph, 120 Main St. East, Ridgetown, ON N0P 2C0, Canada
*
Author to whom correspondence should be addressed.
Agronomy 2022, 12(3), 654; https://doi.org/10.3390/agronomy12030654
Submission received: 25 January 2022 / Revised: 23 February 2022 / Accepted: 1 March 2022 / Published: 8 March 2022
Review Reports Versions Notes

Abstract

:
Glyphosate + dicamba has provided variable glyphosate-resistant Canada fleabane (GRCF) control in glyphosate/dicamba-resistant (GDR) soybean. Previous research has indicated improved GRCF control when a third herbicide was added to glyphosate + dicamba, though research is limited. The objective of this research was to ascertain if the level and consistency of GRCF control can be improved when adding tiafenacil, metribuzin, bromoxynil, pyraflufen-ethyl/2,4-D, 2,4-D ester, halauxifen-methyl or saflufenacil to glyphosate + dicamba applied preplant (PP) in GDR soybean. Four field trials were conducted in 2020 and 2021 in commercial fields in southwestern Ontario, Canada. Glyphosate + dicamba controlled GRCF 57, 93 and 94% at 2, 4 and 8 WAA, respectively. Adding bromoxynil to glyphosate + dicamba improved GRCF control from 57 to 77% at 2 WAA; adding saflufenacil to glyphosate + dicamba improved GRCF control from 57 to 92, 93 to 99, and 94 to 99% at 2, 4 and 8 WAA, respectively. All three-way tank-mixtures improved the consistency of GRCF control, except for glyphosate + dicamba + 2,4-D ester at 2 WAA, glyphosate + dicamba + 2,4-D ester, tiafenacil or metribuzin at 4 WAA, and glyphosate + dicamba + tiafenacil or bromoxynil at 8 WAA. This study concludes that the level and consistency of GRCF control was improved when saflufenacil was added to a PP application of glyphosate + dicamba in soybean.

1. Introduction

Canada fleabane (Erigeron canadensis L.) is a problematic weed from the Asteraceae family [1,2]. It emerges primarily in the spring from April to June, and in the fall from September to October [3,4]. Biotypes can germinate at low temperatures of 8 °C [5] and have been reported to germinate 11 months of the year excluding January [3,4]. Fall-emerged Canada fleabane overwinters as a rosette, which gives it a competitive edge over spring-seeded annual crops and later-emerging weeds [4,6]. Canada fleabane is highly prolific, with individual plants producing thousands of seeds [7,8]; fecundity is correlated with plant height [1,9,10]. The seeds have a feather-like structure attached—known as the pappus—which allows them to be disseminated by the wind [1]. Most seeds fall within meters of their source [11], but one study collected a seed up to 500 km from its source [12]. The Canada fleabane seed is adapted to no-till production systems, since the seeds are non-dormant and can germinate in the top 0.3 cm of the soil [4,13].
Glyphosate is a non-selective herbicide with systemic activity [14]. When applied to a susceptible plant, glyphosate primarily translocates in the symplast to actively growing meristems [14], but it has ambimobile properties [15,16]. The mode of action of glyphosate involves the inhibition of the enzyme 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) in the shikimate pathway, which can cause a buildup of shikimate in the plant and a lack of carbon for other essential pathways; this is thought to be a main factor of glyphosates’ phytotoxicity [16]. In 1996, glyphosate-resistant (GR) soybean and canola were first introduced [17]. This allowed for in-crop glyphosate applications during the growing season without harming the crop [17,18]. By 2012, approximately 93, 85 and 82% of the soybean, corn, and cotton produced in the USA were GR hybrids/cultivars. In 2000, Monsanto’s glyphosate patent expired, resulting in a dramatic reduction in cost to growers since other manufacturers could produce it [19]. The rapid adoption of GR crops and the concomitant over-reliance on and overuse of glyphosate resulted in more simplified weed-control programs, which contributed to the evolution of GR weeds [20,21].
The first GR weed was found in 1996 in a monocot species, rigid ryegrass (Lolium rigidum Gaudin) [22]. In 2000, the first dicot weed to evolve glyphosate resistance was Canada fleabane in Delaware, USA [23]. Ten years later, the first GR Canada fleabane (GRCF) biotype was reported in Canada in Essex county, Ontario [24]. GRCF is currently found in 30 Ontario counties [25].
Dicamba is a synthetic auxin inhibitor classified as a WSSA Group 4 herbicide [26,27] or HRAC Group O [28]. Dicamba can be absorbed by the roots, stem, or foliage of the plant and is symplastically translocated to the actively growing meristems [29,30]. Dicamba is available in several salt formulations including potassium (K), sodium (Na), isopropylamine (IPA), diglycolamine (DGA), dimethylamine (DMA), and bis-aminopropyl methylamine (BAPMA) salts, which affect volatility. Glyphosate/dicamba-resistant (GDR) soybean and cotton cultivars allow in-crop applications of dicamba for weed management throughout the growing season [31,32]. These cultivars are stacked with glyphosate- and dicamba-resistant transgenes, which provides growers with another option for the control of GR weed biotypes [33].
Dicamba and two-way tank-mixtures of dicamba + glyphosate are commonly used to control GRCF in GDR soybean. Byker et al. [24] reported 67 to 100% GRCF control with glyphosate (900 g ae ha−1) + dicamba (300 g ae ha−1) applied postemergence (POST) at 8 WAA. In the same study, when double the rate of dicamba (600 g ae ha−1) was tank-mixed with glyphosate (900 g ae ha−1), GRCF control improved to 90 to 100% applied POST 8 WAA. Eubank et al. [34] observed that glyphosate (860 g ae ha−1) + dicamba (280 g ae ha−1) provided 78 and 90% GRCF control at 2 and 4 WAA, respectively. Bolte et al. [35] observed that glyphosate (840 g ae ha−1) + dicamba (420 g ae ha−1) applied POST to 10 to 20 cm GRCF provided 46 to 93% control, in a two-year study. Zimmer et al. [36] reported that glyphosate (1120 g ae ha−1) + dicamba (280 g ae ha−1) controlled GRCF 89%. There has been variable GRCF control with two-way tank-mixtures of glyphosate + dicamba.
Improved GRCF control has been reported when a third herbicide is tank-mixed with glyphosate + dicamba. Budd et al. [25] observed that glyphosate (900 g ae ha−1) + dicamba (300 g ae ha−1) + saflufenacil (25 g ai ha−1) applied PP controlled GRCF 95% at 8 WAA. Zimmer et al. [1] observed 95% GRCF control with glyphosate (560 g ae ha−1) + dicamba (280 g ae ha−1) + halauxifen-methyl (5 g ai ha−1) or 2,4-D (560 g ae ha−1) at 5 WAA. When a third herbicide is added to the glyphosate + dicamba, the level of GRCF control appears to improve; adding a third herbicide is also useful for decreasing the selection intensity for the evolution of herbicide resistance.
GRCF is a challenge for Ontario soybean growers since postemergence (POST) applications are not effective in non-genetically modified (GMO) and GR soybean [24], and preplant (PP) herbicide applications have provided variable control. The use of GDR soybean provides an alternative technology that growers could utilize to manage GRCF. Research with two-way tank-mixtures of glyphosate + dicamba in GDR soybean has provided variable control of GRCF. Based on preliminary research, three-way tank-mixtures with dicamba have provided improved GRCF control, though further investigation is needed since research is limited. Therefore, the objective of this study is to ascertain if the addition of tiafenacil, metribuzin, bromoxynil, pyraflufen-ethyl/2,4-D (preformulated mixture), 2,4-D ester, halauxifen-methyl or saflufenacil to glyphosate + dicamba, applied PP, will improve the level and consistency of GRCF control in soybean.

2. Materials and Methods

2.1. Experimental Methods

Four trials were conducted on commercial farms in southwestern Ontario near Ridgetown (42°27′41.1″ N 81°51′18.8″ W) and Moraviantown (42°33′04.2″ N 81°50′19.4″ W) in 2020, and near Kintyre (42°33′54.9″ N 81°46′22.6″ W) and Bothwell (42°37′04.3″ N 81°54′45.7″ W) in 2021. Table 1 includes location, treatment application dates, soybean seeding, and emergence dates. The soil texture at Ridgetown was sandy loam with 1.9% organic matter (OM) and a pH of 7.1, and the soil texture at Moraviantown was loamy sand with 2.5% OM and a pH of 6.6. The soil texture at Kintyre was sandy loam with 4.4% OM and a pH of 6.9 and the soil texture at Bothwell was loamy sand with 3.3% OM and a pH of 6.8. The level of resistance of each Canada fleabane population is reported in Table 2.
The experiment was setup as a randomized complete block design (RCBD). The study had 10 treatments that were applied PP. Table 3 includes the herbicides used in this study. The treatments included a weedy control treated with glyphosate (900 g ae ha−1) and a weed-free control treated with glyphosate (900 g ae ha−1) + saflufenacil (25 g ai ha−1) + metribuzin (400 g ai ha−1) + the surfactant Merge (1 L ha−1). The remaining treatments were glyphosate (900 g ae ha−1) + dicamba (600 g ae ha−1); glyphosate (900 g ae ha−1) + dicamba (600 g ae ha−1) + tiafenacil (25 g ai ha−1) + the surfactant MSO (0.5% v/v); glyphosate (900 g ae ha−1) + dicamba (600 g ae ha−1) + metribuzin (400 g ai ha−1); glyphosate (900 g ae ha−1) + dicamba (600 g ae ha−1) + bromoxynil (280 g ai ha−1); glyphosate (900 g ae ha−1) + dicamba (600 g ae ha−1) + pyraflufen-ethyl/2,4-D (532 g ai ha−1); glyphosate (900 g ae ha−1) + dicamba (600 g ae ha−1) + 2,4-D ester (528 g ae ha−1); glyphosate (900 g ae ha−1) + dicamba (600 g ae ha−1) + halauxifen-methyl (5 g ai ha−1) + the surfactant methylated seed oil (MSO) (1% v/v); and glyphosate (900 g ae ha−1) + dicamba (600 g ae ha−1) + saflufenacil (25 g ai ha−1) + the surfactant Merge (1 L ha−1). A CO2-pressurized backpack sprayer was used to make the PP applications when the average height of GRCF was approximately 10 cm. The sprayer was calibrated to dispense 200 L ha−1 at 240 kPa. The sprayer boom measured 1.5 m and had four ULD 120-02 nozzles (Hypro, New Brighton, MN, USA) producing a 2 m spray width. The plots were 2.25 m wide (3 soybean rows distanced 76 cm apart) by 8 m long. The entire experimental area was treated with a POST application of glyphosate (450 g ae ha−1) to remove glyphosate-susceptible Canada fleabane and other weed species.
Visible control ratings were collected at 2, 4, and 8 weeks after application (WAA) based on the estimated percent biomass reduction of GRCF, relative to the weedy control. A 0 to 100% rating scale was used; no GRCF control was indicated by a rating of 0%, while complete GRCF control was indicated by a rating of 100% [37]. At 8 WAA, two 0.25 m2 quadrats were set randomly near the front and back of each plot. The number of GRCF in each quadrat were counted to indicate density for each plot at 8 WAA. All GRCF within each quadrat were cut at the base of each stem, placed in sample bags separated by plot, dried in a kiln at 60°C for a minimum of 48 h, and weighed for biomass. Visible control ratings, density, and biomass were recorded in a notebook and input into ARM Software.
The seeding of GDR soybean (DKB12-16) was at a 3.75 cm depth at about 400,000 seeds ha−1. Soybean injury assessments were conducted using a 0 to 100 scale, 2 and 4 weeks after soybean emergence (WAE); a score of 0 represented no soybean injury, and 100 represented complete soybean death. At maturity, two soybean rows within each plot were harvested and the yield and moisture were recorded. Soybean grain yields were adjusted to 13.5% moisture content before statistical analysis. The herbicides were selected for this study since they are common herbicides used to control GRCF. The herbicides rates were applied at the recommended field rates indicated on the label.

2.2. Statistical Analysis

The analysis was conducted in SAS 9.4 (Statistical Analysis Systems Institute, Cary, NC, USA) with a GLIMMIX procedure. The fixed effect was herbicide treatment. The random effects were location, treatment by location, and block within location. Normality assumptions were confirmed using the Shapiro–Wilk test and residual plots. To meet normality assumptions, GRCF control at 2, 4 and 8 WAA was arcsine transformed prior to the analysis. A lognormal distribution was used for density and biomass. Transformed data were converted back to the original scale to present the results. Soybean yield was evaluated with a normal distribution. The Tukey–Kramer multiple range test (p = 0.05) was used to separate treatment means. Significance was noted by letter codes that differed within each column in Table 4. The coefficient of variation (CV) was calculated and used as an indicator for the consistency of GRCF control.

3. Results and Discussion

3.1. Soybean Injury

At all locations, soybean injury did not exceed 10% over the two-year period (2020, 2021). The data are not presented.

3.2. Control of Glyphosate-Resistant Canada Fleabane

At 2, 4 and 8 WAA, glyphosate + dicamba controlled GRCF 57, 93 and 94% respectively, and reduced density and biomass by 95 and 98%, respectively, relative to the weedy control (Table 4). Byker et al. [1] observed that glyphosate (900 g ae ha−1) + dicamba (600 g ae ha−1) provided a minimum of 63, 93 and 98% GRCF control at 2, 4 and 8 WAA, respectively. Eubank et al. [34] reported 78 and 90% GRCF control at 2 and 4 WAA, respectively, with glyphosate (860 g ae ha−1) + dicamba (280 g ae ha−1). In contrast, Bolte et al. [35] observed that glyphosate (840 g ae ha−1) + dicamba (420 to 840 g ae ha−1) provided 61 to 91% control in 2013 and in 2014; the same tank-mixture provided 84 to 97% GRCF control at 4 WAA. Eubank et al. [34] reported a 94% decrease in GRCF density with glyphosate (860 g ae ha−1) + dicamba (280 g ae ha−1). Byker et al. [38] observed a 97% biomass reduction with glyphosate (900 g ae ha−1) + dicamba (600 g ae ha−1), which is comparable to the present study but contrasts McCauley et al. (2019), who observed a 72% reduction in biomass with dicamba (10 g ae ha−1).
Adding tiafenacil to glyphosate + dicamba did not improve GRCF control at 2, 4, or 8 WAA, and there was no reduction in density or biomass relative to glyphosate + dicamba (Table 4). Recent research by Westerveld et al. [39] reported similar results with no improvement in GRCF control when adding tiafenacil (25 g ai ha−1) to glyphosate (900 g ae ha−1) + metribuzin (400 g ai ha−1) at 2, 4 and 8 WAA. In their study, there was also no decrease in GRCF density or biomass relative to glyphosate (900 g ae ha−1) + metribuzin (400 g ai ha−1).
Adding metribuzin to glyphosate + dicamba did not improve GRCF control at 2, 4, or 8 WAA, and there was no reduction in density or biomass relative to glyphosate + dicamba (Table 4). Similar to the present study, Soltani et al. [40] reported no improvement in GRCF control at 4 and 8 WAA, and no decrease in density or biomass when adding metribuzin (400 g ai ha−1) to glyphosate/dicamba (1800 g ae ha−1) compared to glyphosate/dicamba (1800 g ae ha−1).
Adding bromoxynil to glyphosate + dicamba improved GRCF control from 57 to 77% at 2 WAA, but there was no improvement in control at 4 and 8 WAA, and no decrease in density and biomass compared to glyphosate + dicamba (Table 4). There is limited research investigating GRCF control using glyphosate + dicamba + bromoxynil; however, previous research has demonstrated the benefit of adding bromoxynil to other herbicides for GRCF control. Westerveld et al. [41] reported 97% GRCF control with bromoxynil (280 g ai ha−1) + glyphosate (900 g ae ha−1) + metribuzin (400 g ai ha−1) at 2 WAA. Similar to the current study, GRCF density or biomass was not reduced when bromoxynil (280 ai ha−1) was tank-mixed with glyphosate (900 g ae ha−1) + metribuzin (400 g ai ha−1) [41].
Adding pyraflufen-ethyl/2,4-D to glyphosate + dicamba did not improve GRCF control at 2, 4, or 8 WAA, and there was no reduction in density or biomass relative to glyphosate + dicamba (Table 4). Previous research reported similar results when pyraflufen-ethyl/2,4-D was tank-mixed with other herbicides for GRCF control. Westerveld et al. [42] reported no benefit in GRCF control when adding low rates of pyraflufen-ethyl/2,4-D (≤132 g ai ha−1) to glyphosate (900 g ae ha−1) + metribuzin (400 g ai ha−1); however, GRCF control was improved by 20% when greater rates of pyraflufen-ethyl/2,4-D (≥527 g ai ha−1) were added to glyphosate (900 g ae ha−1) + metribuzin (400 g ai ha−1) at 2, 4 and 8 WAA. In contrast to the present study, Westerveld et al. [42] observed 87 and 95% decreases in GRCF density and biomass, respectively, when pyraflufen-ethyl/2,4-D (527 g ai ha−1) was added to glyphosate (900 g ae ha−1) + metribuzin (400 g ai ha−1), compared to glyphosate (900 g ae ha−1) + metribuzin (400 g ai ha−1).
Adding 2,4-D ester to glyphosate + dicamba did not improve GRCF control at 2, 4, or 8 WAA, and there was no reduction in density or biomass relative to glyphosate + dicamba (Table 4). Similarly, Hedges et al. [43] reported that GRCF control was not improved by adding 2,4-D ester (500 g ae ha−1) to glyphosate/dicamba (1800 g ae ha−1) at 2, 4 and 8 WAA. Zimmer et al. [36] observed no decrease in GRCF when 2,4-D (560 g ae ha−1) was added to glyphosate (560 g ae ha−1) + dicamba (280 g ae ha−1), compared to glyphosate (560 g ae ha−1) + dicamba (280 g ae ha−1). In contrast, Hedges et al. [43] observed 99 and 99.7% reductions in GRCF density and biomass, respectively, when 2,4-D ester (500 g ae ha−1) was added to glyphosate/dicamba (1800 g ae ha−1).
Adding halauxifen-methyl to glyphosate + dicamba did not improve GRCF control at 2, 4 or 8 WAA, and there was no reduction in density or biomass compared to glyphosate/dicamba (Table 4). Zimmer et al. [36] observed 95% control of GRCF with glyphosate (560 g ae ha−1) + dicamba (280 g ae ha−1) + halauxifen-methyl (5 g ai ha−1) at 5 WAA; however, there was no benefit when halauxifen-methyl (5 g ai ha−1) was added to glyphosate (560 g ae ha−1) + dicamba (280 g ae ha−1) compared to glyphosate (560 g ae ha−1) + dicamba (280 g ae ha−1). Similarly, Zimmer et al. [36] observed no reduction in GRCF density when halauxifen-methyl (5 g ai ha−1) was added glyphosate (560 g ae ha−1) + dicamba (280 g ae ha−1) relative to glyphosate (560 g ae ha−1) + dicamba (280 g ae ha−1).
Adding saflufenacil to glyphosate + dicamba improved GRCF control from 57 to 92, 93 to 99, and 94 to 99% at 2, 4 and 8 WAA, respectively; however, neither density nor biomass were improved relative to glyphosate + dicamba (Table 4). The addition of saflufenacil to glyphosate + dicamba provided the highest level of control and had the quickest activity on GRCF at 2 WAA. Similar to this study, Budd et al. [25] observed that glyphosate (900 g ae ha−1) + dicamba (600 g ae ha−1) + saflufenacil (25 g ai ha−1), applied PP in soybean, controlled 99.8 and 97.5% of GRCF at 4 and 8 WAA, respectively. Soltani et al. [40] observed that GRCF control increased from 79 to 99% with the addition of saflufenacil (25 g ai ha−1) to glyphosate/dicamba (1800 g ae ha−1), applied PP in soybean at 4 WAA; however, control was not improved at 8 WAA. Hedges et al. [43] observed that GRCF control improved when saflufenacil (25 g ai ha−1) was added to glyphosate/dicamba (1800 g ae ha−1), applied PP in soybean at 2 and 4 WAA. Preceding research reported that GRCF density and biomass was reduced by 98% with glyphosate (900 g ae ha−1) + dicamba (600 g ae ha−1) + saflufenacil (25 g ai ha−1) [25]. Similarly, Hedges et al. [43] and Soltani et al. [40] observed no reduction in GRCF density or biomass from the addition of saflufenacil (25 g ai ha−1) to glyphosate/dicamba (1800 g ae ha−1), compared to glyphosate/dicamba (1800 g ae ha−1).
In the present study, it was difficult to detect differences when adding a third herbicide to glyphosate + dicamba since glyphosate + dicamba provided a high level of GRCF control at 4 and 8 WAA. The addition of saflufenacil to glyphosate + dicamba provided the highest level of control at 2, 4 and 8 WAA, and had the quickest activity on GRCF relative to glyphosate + dicamba at 2 WAA. The addition of an effective third herbicide, such as saflufenacil, to glyphosate + dicamba is also beneficial for decreasing selection intensity for the evolution of herbicide resistance [44].

3.3. Consistency of Glyphosate-Resistant Canada Fleabane Control

In Table 5, the CV is listed as an indicator of the consistency of GRCF control for each treatment.
At 2 WAA, the consistency of GRCF control was improved (>1 decrease in the CV relative to the CV of glyphosate + dicamba) when tiafenacil, metribuzin, bromoxynil, pyraflufen-ethyl/2,4-D or saflufenacil was added to glyphosate + dicamba, relative to glyphosate + dicamba. The consistency of control was not improved when 2,4-D ester or halauxifen-methyl were tank-mixed with glyphosate + dicamba. Adding saflufenacil to glyphosate + dicamba resulted in the most consistent GRCF control at 2 WAA.
At 4 WAA, the consistency of GRCF control was improved by adding saflufenacil to glyphosate + dicamba compared to glyphosate + dicamba; there was no improvement in the consistency of control when tiafenacil, metribuzin, bromoxynil, pyraflufen-ethyl/2,4-D, 2,4-D ester, or halauxifen-methyl were added to glyphosate + dicamba compared to glyphosate + dicamba.
At 8 WAA, the consistency of GRCF control improved by adding halauxifen-methyl or saflufenacil to glyphosate + dicamba compared to glyphosate + dicamba; the addition of tiafenacil, metribuzin, bromoxynil, pyraflufen-ethyl/2,4-D, or 2,4-D ester to glyphosate + dicamba did not improve the consistency of GRCF control compared to glyphosate + dicamba.
Previous research has indicated improved consistency of GRCF control as a result of adding a third herbicide into the tank. Soltani et al. [44] and Budd et al. [25] observed consistent control of GRCF when adding metribuzin (400 g ai ha−1) to glyphosate (900 g ae ha−1) + saflufenacil (25 g ai ha−1). Budd et al. [25] observed consistent GRCF control when adding 2,4-D (500 g ae ha−1) to glyphosate (900 g ae ha−1) + saflufenacil (25 g ai ha−1).

3.4. Soybean Yield

GRCF interference in the weedy control reduced soybean yield by 1.15 t ha−1 compared to the weed-free control (Table 4). Reduced GRCF interference with applications of dicamba, dicamba + tiafenacil, metribuzin, bromoxynil, pyraflufen-ethyl/2,4-D, 2,4-D ester, or halauxifen-methyl resulted in a soybean yield increase of 1.20 to 1.36 t ha−1 compared to the weedy control. Hedges et al. [43] observed a 1.40 to 1.70 t ha−1 yield increase with glyphosate/dicamba (1800 g ae ha−1)-based tank-mixtures. Eubank et al. [33] reported a yield increase of 2.51 t ha−1 with glyphosate (860 g ae ha−1) + dicamba (280 g ae ha−1).

4. Conclusions

Though glyphosate + dicamba provided a high level of GRCF control at 4 and 8 WAA, adding saflufenacil to glyphosate + dicamba improved control from 57 to 92, 93 to 99, and 94 to 99% at 2, 4 and 8 WAA, respectively. There was an improvement in the consistency of GRCF control when adding saflufenacil to glyphosate + dicamba; this treatment also had the quickest activity on GRCF at 2 WAA. Adding saflufenacil to glyphosate + dicamba did not reduce density or biomass relative to glyphosate + dicamba. The addition of an effective third tank-mix partner such as saflufenacil is also useful for decreasing the selection intensity for the evolution of herbicide resistance. There was no consistent benefit of adding tiafenacil, metribuzin, bromoxynil, pyraflufen-ethyl/2,4-D, 2,4-D ester, or halauxifen-methyl to glyphosate + dicamba at 2, 4, or 8 WAA, and no decrease in GRCF density or biomass. In conclusion, adding saflufenacil to glyphosate + dicamba improved the level and consistency of GRCF control applied PP in soybean.

Author Contributions

M.D. conceived of, designed, and performed experiments, analyzed data, and wrote the article; N.S., D.C.H., D.E.R. and P.H.S. assisted in the design and implementation of the study from its conception to publication. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Conflicts of Interest

The authors declare no conflict of interest.

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Table
Table 1. Location, treatment spray date, and soybean seeding plus emergence dates for field trials in Ontario, Canada in 2020 and 2021.
Table 1. Location, treatment spray date, and soybean seeding plus emergence dates for field trials in Ontario, Canada in 2020 and 2021.
SiteYear LocationAgronomic Information
Treatment Spray DateSeeding DateEmergence Date
S12020Ridgetown26 May5 June11 June
S22020Moraviantown12 June18 June23 June
S32021Kintyre18 May19 May25 May
S42021Bothwell24 May12 June18 June
Table
Table 2. Approximate size and density of glyphosate-resistant Canada fleabane (GRCF) at the time of treatment application and the level of resistance of the populations at field sites in Ontario, Canada in 2020 and 2021.
Table 2. Approximate size and density of glyphosate-resistant Canada fleabane (GRCF) at the time of treatment application and the level of resistance of the populations at field sites in Ontario, Canada in 2020 and 2021.
SiteYear LocationGRCFResistance (%)
Size
(cm)		Density (m−2)GlyphosateCloransulam-Methyl
S12020Ridgetown838610099
S22020Moraviantown811192100
S32021Kintyre92939885
S42021Bothwell954--
Table
Table 3. Herbicides (active ingredient, mode of action, trade name, and manufacturer) used in the present study conducted in Ontario, Canada.
Table 3. Herbicides (active ingredient, mode of action, trade name, and manufacturer) used in the present study conducted in Ontario, Canada.
Active IngredientMode of ActionTrade NameManufacturer
DicambaSynthetic auxinXtendimaxBayer CropScience Inc., 160 Quarry Park Blvd S. E.,
Calgary, AB, Canada
MetribuzinPhotosystem II (PS II) inhibitorSencor 480SC
2,4-D ester Synthetic auxinEster 700Nufarm Canada., 5101, 333—96th Ave N.E., Calgary, AB, Canada
Pyraflufen-ethyl/2,4-DPPO inhibitor/
Synthetic auxin		Blackhawk
Halauxifen-methylSynthetic auxinElevoreDow AgroSciences Canada Inc., 2400, 215—2nd Street S. W., Calgary, AB, Canada
TiafenacilProtoporphyrinogen
oxidase (PPO) inhibitor		InsightISK Biosciences., 7470 Auburn Rd, Painesville, OH 44077, United States.
BromoxynilPS II inhibitorPardnerBayer CropScience Inc., 160 Quarry Park Blvd S. E.,
Calgary, AB, Canada
SaflufenacilPPO inhibitorEragon LQ BASF Canada Inc., 100
Milverton Drive,
Mississauga, ON, Canada
Table
Table 4. Means for glyphosate-resistant Canada fleabane (GRCF) control, density, biomass, and soybean yield with dicamba-based tank-mixes from four field trials in Ontario, Canada.
Table 4. Means for glyphosate-resistant Canada fleabane (GRCF) control, density, biomass, and soybean yield with dicamba-based tank-mixes from four field trials in Ontario, Canada.
TreatmentRate
(g ai ha−1)		GRCF Control (%)Density a
(Plants m−2)		Biomass a
(g m−2)		Soybean Yield
(t ha−1)
2 WAA4 WAA8 WAA
Weedy control-000324 d246 d1.42 b
Weed-free control-1001001000 a0 a2.57 a
Glyphosate (G) + dicamba (D)900 + 60057 c93 b94 b,c16 b,c4 a,b,c2.73 a
G + D + tiafenacil900 + 600 + 2564 b,c87 b90 c23 c7 a,b,c2.65 a
G + D + metribuzin900 + 600 + 40065 b,c88 b92 c25 b,c49 c2.66 a
G + D + bromoxynil900 + 600 + 28077 a,b94 a,b93 b,c19 b,c7 a,b,c2.62 a
G + D + pyraflufen-ethyl/2,4-D900 + 600 + 53266 b,c95 a,b96 a,b,c13 b,c2 a,b,c2.70 a
G + D + 2,4-D ester900 + 600 + 52854 c92 b97 a,b,c6 b,c5 a,b,c2.62 a
G + D +
halauxifen-methyl 		900 + 600 + 557 b,c93 b98 a,b7 a,b1 a,b2.78 a
G + D + saflufenacil600 + 2592 a99 a99 a4 a,b1 a,b2.04 a,b
Means with a different letter in a column (a–d) are statistically significant with Tukey Kramer’s LSD (p = 0.05). Dicamba and 2,4-D ester rate units are listed as g ae ha−1. a At 8 WAA, density and biomass were collected.
Table
Table 5. The consistency of glyphosate-resistant Canada fleabane (GRCF) control with dicamba-based tank-mixes from four field trials in Ontario, Canada.
Table 5. The consistency of glyphosate-resistant Canada fleabane (GRCF) control with dicamba-based tank-mixes from four field trials in Ontario, Canada.
Consistency of GRCF Control
TreatmentRate
(g ai ha−1)		2 WAA4 WAA8 WAA
Glyphosate (G) + dicamba (D)900 + 600661722
G + D + tiafenacil900 + 600 + 25611923
G + D + metribuzin900 + 600 + 400601922
G + D + bromoxynil900 + 600 + 280521721
G + D + pyraflufen-ethyl/2,4-D900 + 600 + 532601721
G + D + 2,4-D ester900 + 600 + 528681820
G + D + halauxifen-methyl900 + 600 + 5661720
G + D + saflufenacil900 + 600 + 25441619
Dicamba and 2,4-D ester rate units are listed as g ae ha−1.
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Dilliott, M.; Soltani, N.; Hooker, D.C.; Robinson, D.E.; Sikkema, P.H. The Addition of Saflufenacil to Glyphosate plus Dicamba Improves Glyphosate-Resistant Canada Fleabane (Erigeron canadensis L.) Control in Soybean. Agronomy 2022, 12, 654. https://doi.org/10.3390/agronomy12030654

AMA Style

Dilliott M, Soltani N, Hooker DC, Robinson DE, Sikkema PH. The Addition of Saflufenacil to Glyphosate plus Dicamba Improves Glyphosate-Resistant Canada Fleabane (Erigeron canadensis L.) Control in Soybean. Agronomy. 2022; 12(3):654. https://doi.org/10.3390/agronomy12030654

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Dilliott, Meghan, Nader Soltani, David C. Hooker, Darren E. Robinson, and Peter H. Sikkema. 2022. "The Addition of Saflufenacil to Glyphosate plus Dicamba Improves Glyphosate-Resistant Canada Fleabane (Erigeron canadensis L.) Control in Soybean" Agronomy 12, no. 3: 654. https://doi.org/10.3390/agronomy12030654

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