Glyphosate-Resistant Canada Fleabane Control with Three-Way Herbicide Tankmixes in Soybean ()
1. Introduction
Soybean [Glycine max (L.) Merr.] production is valuable to agriculture in Canada [1]. Growers in Canada annually produce nearly 5 million tonnes of soybean on 1.6 million hectares with a farm gate value of over one billion dollars [1] [2]. Nearly 70% of Canadian soybean production is grown in Ontario [1]. Effective weed management, particularly against recently evolved glyphosate-resistant (GR) weeds is rated as one of the greatest challenges faced by Ontario soybean producers.
Ontario farmers have identified GR Conyza canadensis (L.) Cronquist (common names include Canada fleabane, horseweed or marestail in North America) as the number one weed management issue in the province in a survey conducted in Ontario [3] [4]. C. canadensis is a prolific weed belonging to Asteraceae family that can produce as much as 1,000,000 seeds/plant which can subsequently be dispersed as far as 550 kilometers from the parent plant [5]. C. canadensis usually germinates in late summer and early fall, forming overwintering rosettes which bolt and produce numerous, elongated flowering branches in the following summer [6]. C. canadensis is very competitive with soybean crop and can decrease yield substantially if not adequately controlled. Bruce and Kells [7] reported 90% soybean yield reduction with 100 - 200 plant−2 densities of C. canadensis. Dauer et al. [8] found 97% reduction in yield due to C. canadensis interference in soybean. Byker et al. [9] also reported up to 93% reduction in soybean yield when GR C. canadensis was not controlled. It is critical for soybean growers to adequately control this troublesome weed in their fields to avoid crop yield loss and increase crop production efficiency.
Preplant (PP) herbicide combinations that have burndown and extended residual activity are the optimal choices for GR C. canadensis control as this prolific weed has a long emergence pattern and most of postemergence (POST) herbicides do not effectively control this problematic weed in GR soybean [10]. Saflufenacil, a pyrimidinedione herbicide that inhibits the PPO enzyme; 2,4-D ester, a phenoxy herbicide that up-regulates gene expression; and paraquat, a bipyridilium herbicide that causes cell membrane destruction in susceptible plants are three preplant herbicides with burndown activity, but all provide relatively short residual activity against GR C. canadensis in soybean [11].
Earlier studies have shown that saflufenacil can control GR C. canadensis greater than 90% in soybean [9]. However, some studies have shown inconsistent control of C. canadensis with saflufenacil in soybean in Ontario [3]. Additionally, 2,4-D ester and paraquat have been reported to provide inconsistent control of GR C. canadensis in soybean [3] [9]. Tankmix herbicide partners with residual activity such as sulfentrazone, flumioxazin and metribuzin have the potential to increase the uniformity of GR C. canadensis control when tankmixed with saflufenacil, 2,4-D or paraquat [3] [9] [12] [13]. Adding a third tankmix partner to these herbicides may increase efficacy and may provide the season-long residual control of GR C. canadensis in soybean. In addition, 3-way herbicide tankmixes can also be helpful by adding a new mode of action that is crucial for long-term sustainable weed management strategies in agricultural crops.
Few studies have collectively evaluated the 3-way tankmixes of glyphosate plus saflufenacil, 2,4-D ester or paraquat with sulfentrazone, flumioxazin or metribuzin for GR C. canadensis control in glyphosate-resistant soybean in Ontario. Therefore, the purpose of this study was to assess the effectiveness of glyphosate (900 g ae ha−1) plus saflufenacil (25 g ai ha−1), 2,4-D ester (500 g ai ha−1) or paraquat (1100 g ai ha−1) in 3-way tankmixes with sulfentrazone (140 g ai ha−1), flumioxazin (107 g ai ha−1) or metribuzin (400 g ai ha−1) for GR C. canadensis control in GR soybean.
2. Materials and Methods
Eight experiments (2 in 2016, 3 in 2017, 3 in 2018) were conducted in farmers’ fields in Ontario. Experiments were arranged as a random complete block design (RBCD) with 4 replications. Plot measurements were 2.25 m wide (3 rows of soybean) by 8 m long. Glyphosate-resistant soybean (“DKB 12-57”) was seeded to a depth of approximately 4 cm, in rows spaced 0.75 m apart, at the rate of 400,000 seeds per ha−1. Herbicide treatments included glyphosate (900 g ae ha−1) plus saflufenacil (25 g ai ha−1), 2,4-D ester (500 g ai ha−1) or paraquat (1100 g ai ha−1) applied alone and in combination with sulfentrazone (140 g ai ha−1), flumioxazin (107 g ai ha−1) or metribuzin (400 g ai ha−1) as listed in Table 1. All saflufenacil treatments included a surfactant (Merge® at 1 L·ha−1).
Herbicides were applied PP (up to 6 days before seeding) with a CO2-pressurized backpack sprayer when C. canadensis was 10 cm in width/height. The sprayer was adjusted to deliver 200 L·ha−1 of water solution at 240 kPa using a hand boom that was 1.5 m long and had four ultra-drift ULD-12-002 nozzles (Hypro® Ultra-Lo Drift 120-02 nozzle, 375 5th Ave. NW, New Brighton, MN 55112) spaced 0.5 m apart.
Soybean injury was assessed visually at two and four weeks after crop emerged (WAE) using a scale of 0% to 100% (0 represented no visible injury and 100 represented completely dead plants). Percent GR C. canadensis control was evaluated four and eight weeks after herbicide application (WAA) using a scale of 0 to 100 (0 represented no GR C. canadensis control and 100 represented completely dead GR C. canadensis plants). Density and shoot aboveground dry weight/biomass (counted and oven dried at 60˚C in a paper bag for two weeks) of GR C. canadensis was assessed eight WAA within two 0.25 m−2 quadrats which were randomly placed within each plot. At harvest time, soybean plants in the two centre rows of each plot were harvested using a small plot research combine. Soybean seed yields were then adjusted to 13% seed moisture content.
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Table 1. Percent glyphosate-resistant Conyza canadensis (Canada fleabane) control 4 and 8 WAA, fleabane density and dry biomass 8 WAA, soybean seed moisture content at harvest and seed yield for herbicide combinations applied preplant in soybean at 8 sites in southwestern Ontario from 2016 to 2018. Means followed by a different letter within a column are significantly different according to a Tukey-Kramer multiple range test at P < 0.05. a
a Abbreviations: WAA, weeks after herbicide application. b All herbicide treatments included glyphosate (900 g ae ha−1). c Soybean seed moisture content and seed yield collected in 2017 and 2018 at 6 of 8 sites. d All saflufenacil treatments included non-ionic surfactant (Merge® at 1 L·ha−1).
For the statistical analysis, the GLIMMIX method was used to analyze data in SAS (2016; SAS Institute Inc, Cary, NC.). In the analysis, the preplant herbicide treatment was the fixed effect and year-location combinations, replicate within the year-location and the year-location by treatment interaction were the random effects. The best distribution and associated link function for each parameter was chosen by comparing fit statistics, residual plots and the Shapiro-Wilk statistic among the potential distributions. LSMEANS were calculated by using the inverse link function, and pairwise comparisons were subjected to Tukey’s adjustment before determining treatment differences at P < 0.05. The Gaussian distribution and identity link were used for GR C. canadensis control (%) 4 and 8 WAE, and yield. Additionally, the lognormal distribution and identity link were used to analyze C. canadensis density and biomass, while the gamma distribution and log link were used to analyze percent soybean seed moisture at harvest. The two control treatments were kept out from the analysis due to zero variance. Comparisons were still possible between the other treatments and the value zero using the LSMEANS output and differences were identified. A correction for log bias was applied to treatment means that were analyzed using the lognormal distribution.
3. Results and Discussion
3.1. GR Conyza Canadensis Control
Glyphosate plus saflufenacil applied PP provided as much as 90% control of GR C. canadensis 4 and 8 WAA (Table 1). The addition of sulfentrazone, flumioxazin or metribuzin to the tankmix provided as much as 93%, 96% and 97% GR C. canadensis control, respectively. The control with the 3-way tankmixes was similar to glyphosate plus saflufenacil. Glyphosate plus 2,4-D ester applied PP controlled C. canadensis only 53% - 59% 4 and 8 WAA. The addition of sulfentrazone, flumioxazin or metribuzin to the tankmix provided as much as 60%, 59% and 91% control of GR C. canadensis, respectively. Metribuzin addition to glyphosate plus 2,4-D improved GR C. canadensis control. Glyphosate plus paraquat applied PP provided as much as 85% GR C. canadensis control 4 and 8 WAA (Table 1). The addition of sulfentrazone, flumioxazin or metribuzin to the tankmix provided as much as 88%, 89% and 98% GR C. canadensis control, respectively. Metribuzin addition to glyphosate plus paraquat improved GR C. canadensis control.
Byker et al. [9] found 88% - 100% GR C. canadensis control with glyphosate tankmixed with saflufenacil, 49% - 80% GR C. canadensis control with glyphosate tankmixed with 2,4-D ester, and 59% - 95% GR C. canadensis control with glyphosate tankmixed with paraquat 4 WAA in soybean. Similarly, Loux et al. [14] showed that a tankmix of glyphosate with saflufenacil, 2,4-D ester or metribuzin provided season-long control of GR C. canadensis in glyphosate-resistant soybean. Additionally, Budd et al. [3] reported as much as 99% GR C. canadensis control with glyphosate tankmixed with saflufenacil applied PP alone or in tankmix combinations with 2,4-D, metribuzin or paraquat in soybean. In contrast, other studies have found only 57% control of GR C. canadensis with glyphosate tankmixed with saflufenacil in glyphosate-resistant soybean [15]. Other studies have also found only 73% control of GR C. canadensis with glyphosate plus metribuzin in soybean [16]. Glyphosate plus saflufenacil and glyphosate plus paraquat were shown to control GR C. canadensis as much as 96% - 97% and 81% - 95% in cotton, respectively [17]. Keeling et al. [18] reported 92% - 100% control of C. canadensis in cotton with glyphosate tankmixed with 2,4-D ester applied PP.
In this study, adding sulfentrazone and flumioxazin to the tankmixes of glyphosate plus saflufenacil, 2,4-D ester, or paraquat did not significantly increased the control of GR C. canadensis. Glyphosate tankmixed with flumioxazin has been shown to minimally (28% - 46%) control GR C. canadensis in glyphosate-resistant soybean [9]. Norsworthy [13] and Owen et al. [12] reported only 50% - 63% GR C. canadensis control with flumioxazin tankmixes in cotton.
3.2. GR Conyza Canadensis Density
Glyphosate plus saflufenacil decreased density of GR C. canadensis 95% 8 WAA (Table 1). The addition of sulfentrazone, flumioxazin or metribuzin to the tankmix decreased GR C. canadensis density 98%, 98% and 99% compared to the weedy control, respectively. Density of GR C. canadensis was not reduced significantly with the addition of sulfentrazone, flumioxazin or metribuzin to glyphosate plus saflufenacil. Glyphosate plus 2,4-D ester decreased density of GR C. canadensis only 62% 8 WAA. The addition of sulfentrazone, flumioxazin or metribuzin to the tankmix reduced GR C. canadensis density 65%, 63% and 96%, respectively. Adding metribuzin to glyphosate plus 2,4-D reduced GR C. canadensis density. Glyphosate plus paraquat decreased GR C. canadensis density 90% 8 WAA. Adding sulfentrazone, flumioxazin or metribuzin to the tankmix decreased GR C. canadensis density 94%, 94% and 99% compared to the weedy control, respectively. Adding metribuzin to glyphosate plus paraquat decreased GR C. canadensis density.
In another study, Budd et al. [3] reported 97%, 98% and 97% decrease in GR C. canadensis density when a 3-way tankmixes of glyphosate plus saflufenacil plus 2,4-D ester, metribuzin or paraquat were applied PP in GR soybean, respectively. However, other studies have reported only 66% decrease in GR C. canadensis density when glyphosate plus metribuzin were applied PP in soybean [19]. Glyphosate plus saflufenacil and glyphosate plus paraquat were shown to reduce C. canadensis density only 60% and 54% in cotton, respectively [17].
3.3. GR Conyza Canadensis Biomass
Glyphosate plus saflufenacil applied PP decreased GR C. canadensis biomass 98% 8 WAA (Table 1). The addition of sulfentrazone, flumioxazin or metribuzin to the tankmix decreased GR C. canadensis biomass 99% 8 WAA (Table 1). GR C. canadensis biomass was similar with glyphosate plus saflufenacil and in the 3-way tankmixes. Glyphosate plus 2,4-D ester applied PP caused an 80% decrease in GR C. canadensis biomass in soybean 8 WAA (Table 1). The addition of sulfentrazone, flumioxazin or metribuzin to the tankmix caused an 81%, 74% and 98% reduction in GR C. canadensis biomass in soybean 8 WAA, respectively (Table 1). Adding metribuzin to glyphosate plus 2,4-D decreased GR C. canadensis biomass. Glyphosate plus paraquat applied PP provided a 91% reduction in GR C. canadensis biomass in soybean 8 WAA (Table 1). The addition of sulfentrazone, flumioxazin or metribuzin to the tankmix provided a 96%, 95% and 99% reduction in GR C. canadensis biomass in soybean 8 WAA, respectively. Adding metribuzin to glyphosate plus paraquat decreased GR C. canadensis biomass.
In another study, Budd et al. [3] reported 93%, 92% and 86% decrease in biomass of GR C. canadensis with a 3-way tankmixes of glyphosate plus saflufenacil plus 2,4-D ester, metribuzin or paraquat in GR soybean, respectively. Additionally, Byker et al. [9] reported as much as 99% and 95% decrease of GR C. canadensis biomass with glyphosate plus saflufenacil and glyphosate plus 2,4-D ester applied PP in glyphosate-resistant soybean, respectively. However, other studies have reported only 45% reduction in biomass of GR C. canadensis with glyphosate plus saflufenacil in soybean [15]. Other 2-way or 3-way herbicide tankmixes such as glyphosate + flumioxazin, glyphosate + flumioxazin + chorimuron-ethyl and glyphosate + flumioxazin + pyroxasulfone were shown to reduce GR C. canadensis biomass 65%, 88% and 47% in soybean, respectively [9].
3.4. Soybean Crop Responses
There was no injury in soybean at 2 and 4 WAE at all site-years with treatments evaluated, therefore data were not analyzed (data not presented).
Seed moisture of soybean was 3.7% higher in weedy control plots compared to the plots with no weeds indicating that the presence of GR C. canadensis delayed soybean maturity (Table 1). Seed moisture of soybean was not different between the weed-free control plots and herbicide treated plots (Table 1).
GR C. canadensis interference reduced soybean yield 66% (Table 1). This finding is consistent with another study that has shown 73% reduction in yield due to GR C. canadensis interference in GR soybean [3]. However, other studies have shown as much as 82% - 97% reduction in yield from GR C. canadensis interference in GR soybean [8] [9] [19].
Reduced GR C. canadensis interference with the preplant herbicides evaluated provided soybean yield similar to the weed-free control. Budd et al. [3] also reported no soybean seed yield loss with a 3-way tankmixes of glyphosate plus saflufenacil plus 2,4-D ester, metribuzin or paraquat.
4. Conclusion
Glyphosate plus saflufenacil applied PP has the potential to adequately control GR C. canadensis in soybean. Adding sulfentrazone, flumioxazin or metribuzin to glyphosate plus saflufenacil did not significantly enhance C. canadensis control. Glyphosate plus paraquat applied PP has the potential to adequately control GR C. canadensis in soybean. Adding metribuzin to glyphosate plus paraquat enhanced C. canadensis control and reduced density and biomass of C. canadensis, however, there was no benefit of adding sulfentrazone and flumioxazin to this tankmix for the control of C. canadensis. Glyphosate plus 2,4-D ester applied PP provided poor control of GR C. canadensis. The addition of sulfentrazone or flumioxazin to glyphosate plus 2,4-D did not enhance the control of C. canadensis and there was no decrease in density and biomass of GR C. canadensis. In contrast, adding metribuzin to glyphosate plus 2,4-D increased C. canadensis control and decreased density and biomass of GR C. canadensis. This study concludes that metribuzin tankmixed with glyphosate plus saflufenacil, glyphosate plus 2,4-D ester, or glyphosate plus paraquat can be used for consistent season-long control of GR C. canadensis in GR soybean. The use of a 3-way tankmix can be helpful in decreasing the natural selection of herbicide-resistant C. canadensis.
Acknowledgements
Funding for this project was provided in part by Grain Farmers of Canada (GFO) and the Growing Forward II program of the Agricultural Adaptation Council.