Introduction
Horseweed [Conyza canadensis (L.) Cronquist; syn.: Erigeron canadensis L.] is a fall- or spring-germinating annual weed species that can germinate under a myriad environmental conditions (Buhler and Owen Reference Buhler and Owen1997; Main et al. Reference Main, Steckel, Hayes and Mueller2006; Nandula et al. Reference Nandula, Eubank, Poston, Koger and Reddy2006; Weaver Reference Weaver2001). The high fecundity of C. canadensis, along with the tendency of its seeds to germinate best when located on or near the soil surface, makes C. canadensis a particularly successful weed in no-tillage cropping systems (Brown and Whitwell Reference Brown and Whitwell1988; Nandula et al. Reference Nandula, Eubank, Poston, Koger and Reddy2006; Regehr and Bazzaz Reference Regehr and Bazzaz1979). An individual C. canadensis plant can produce 230,000 seeds, which then may be wind disseminated 500 km from the plant of origin by the aid of an attached pappus (Shields et al. Reference Shields, Dauer, VanGessel and Neumann2006; Weaver Reference Weaver2001). This favorable seed dispersal mechanism and high fecundity of C. canadensis allow for its quick and pervasive expansion in agroecosystems.
Conyza canadensis has been reported to be one of the most common and troublesome weeds in corn (Zea mays L.) in several U.S. states and in Ontario, Canada (Van Wychen Reference Van Wychen2020). A glyphosate-resistant (GR) biotype of C. canadensis was first reported in a population in Delaware, USA in 2000 (VanGessel Reference VanGessel2001). Ten years later, GR C. canadensis was found in a population from Essex County, Ontario, Canada (Byker et al. Reference Byker, Soltani, Robinson, Tardif, Lawton and Sikkema2013). GR biotypes of C. canadensis have since been identified in 30 counties across southern Ontario (Budd et al. Reference Budd, Soltani, Robinson, Hooker, Miller and Sikkema2018). GR C. canadensis is also widespread in the United States, where it has been reported in 25 states as of September 2021 (Heap Reference Heap2021).
Several herbicides applied preplant provide effective control (>90%) of GR biotypes of C. canadensis in corn (Brown et al. Reference Brown, Shropshire and Sikkema2016; Ford et al. Reference Ford, Soltani, Robinson, Nurse, McFadden and Sikkema2014). There are fewer postemergence herbicide options in corn that provide >90% control of GR C. canadensis. In Ontario, GR C. canadensis can be controlled >90% with a postemergence application of dicamba, dicamba/atrazine, bromoxynil + atrazine, or tolpyralate + atrazine (Langdon et al. Reference Langdon, Soltani, Raeder, Hooker, Robinson and Sikkema2020; Mahoney et al. Reference Mahoney, Shropshire and Sikkema2017; Metzger et al. Reference Metzger, Soltani, Raeder, Hooker, Robinson and Sikkema2019). Only four active ingredients (dicamba, atrazine, bromoxynil, and tolpyralate) are represented across these postemergence herbicide options for effective GR C. canadensis control. The incorporation of tillage or cover crops into a crop production system can suppress GR C. canadensis, but these two weed management strategies are not suitable in all corn-cropping systems for various agronomic and economic reasons (Chahal and Jhala Reference Chahal and Jhala2019; Cholette et al. Reference Cholette, Soltani, Hooker, Robinson and Sikkema2018). Suppression of GR C. canadensis can be achieved with previous crop residue left on the soil, but the suppression is often not commercially acceptable (Main et al. Reference Main, Steckel, Hayes and Mueller2006). The use of herbicides can complement various cultural and mechanical control options for effective C. canadensis control. When GR C. canadensis is present, effective control is imperative, as interference by this weed biotype has been reported to reduce grain corn yields up to 69% (Ford et al. Reference Ford, Soltani, Robinson, Nurse, McFadden and Sikkema2014). The competitiveness, difficulty of control, and geographic distribution of GR C. canadensis in corn emphasizes the need for efficacious postemergence herbicide options for its control in corn.
Photosystem II (PSII) and 4-hydroxyphenylpyruvate dioxygenase (HPPD) inhibitors are commonly applied in tank mixes. Herbicides that inhibit the HPPD enzyme block the production of homogentisic acid, which prevents the synthesis of plastoquinone and tocopherols in susceptible plants (Schulz et al. Reference Schulz, Ort, Beyer and Kleinig1993; Secor Reference Secor1994; Tsegaye et al. Reference Tsegaye, Shintani and DellaPenna2002). A lack of plastoquinone and tocopherols limits the ability of a susceptible plant to quench reactive oxygen species, which leads to the destruction of plant cells (Kruk et al. Reference Kruk, Holländer-Czytko, Oettmeier and Trebst2005; Trebst et al. Reference Trebst, Depka and Holländer-Czytko2002). Mesotrione, topramezone, and tolpyralate are commonly used postemergence HPPD inhibitors in Ontario with different weed control spectrums (Kohrt and Sprague Reference Kohrt and Sprague2017; Metzger et al. Reference Metzger, Soltani, Raeder, Hooker, Robinson and Sikkema2018). PSII inhibitors such as atrazine, bentazon, and bromoxynil are complementary to HPPD inhibitors because of their interrelated modes of action (Abendroth et al. Reference Abendroth, Martin and Roeth2006; Armel et al. Reference Armel, Hall, Wilson and Cullen2005; Creech et al. Reference Creech, Monaco and Evans2004; Kim et al. Reference Kim, Jung, Hwang and Cho1999). PSII inhibitors disrupt electron flow in the photosynthetic electron transport chain by competing with plastoquinone for the QB binding site on the D1 protein (Hess Reference Hess2000). PSII inhibitors cause cell death by triggering a massive influx of reactive oxygen species, which induces lipid peroxidation (Hess Reference Hess2000). Herbicides that inhibit HPPD therefore can increase the productivity of a PSII inhibitor to bind to the D1 protein by limiting the biosynthesis of plastoquinone when the herbicides are applied in conjunction (Armel et al. Reference Armel, Hall, Wilson and Cullen2005). Also, because HPPD inhibitors induce a loss in quenching of reactive oxygen species and PSII inhibitors cause an influx of reactive oxygen species, greater herbicidal activity may occur with joint applications of the two herbicides (Armel et al. Reference Armel, Hall, Wilson and Cullen2005; Creech et al. Reference Creech, Monaco and Evans2004). Therefore, PSII inhibitors, typically atrazine, and HPPD inhibitors are commonly tank mixed to improve the efficacy and broaden the weed control spectrum of a single herbicide application (Armel et al. Reference Armel, Hall, Wilson and Cullen2005, Reference Armel, Wilson, Richardson, Whaley and Hines2008, Reference Armel, Richardson, Wilson and Hines2009; Johnson et al. Reference Johnson, Young and Matthews2002; Kohrt and Sprague Reference Kohrt and Sprague2017; Metzger et al. Reference Metzger, Soltani, Raeder, Hooker, Robinson and Sikkema2018, Reference Metzger, Soltani, Raeder, Hooker, Robinson and Sikkema2019; Whaley et al. Reference Whaley, Armel, Wilson and Hines2006).
When herbicides from distinct modes of action are co-applied, weed control with these tank-mix combinations can be termed “antagonistic,” “additive,” or “synergistic.” Colby’s equation is used to calculate the expected weed control of a herbicide mix based on the level of weed control from the component herbicides applied individually (Colby Reference Colby1967). If the observed weed control from the tank mix is less, equal, or greater than expected, the interaction between the two herbicides is either antagonistic, additive, or synergistic, respectively (Colby Reference Colby1967).
In many cases, the joint activity of HPPD and PSII inhibitors has been reported as synergistic or additive for the control of several weed species; however, the reported interaction between HPPD and PSII inhibitors can vary among HPPD inhibitors, PSII inhibitors, herbicide rates evaluated, weed species, and weed biotype resistance profiles (Abendroth et al. Reference Abendroth, Martin and Roeth2006; Hugie et al. Reference Hugie, Bollero, Tranel and Riechers2008; Kohrt and Sprague Reference Kohrt and Sprague2017; Woodyard et al. Reference Woodyard, Bollero and Riechers2009a). The synergy between postemergence-applied HPPD and PSII inhibitors has been documented for the control of waterhemp [Amaranthus tuberculatus (Moq.) Sauer], Palmer amaranth (Amaranthus palmeri S. Watson), redroot pigweed (Amaranthus retroflexus L.), wild radish (Raphanus raphanistrum L.), velvetleaf (Abutilon theophrasti Medik.), giant ragweed (Ambrosia trifida L.), common cocklebur (Xanthium strumarium L.), red morningglory (Ipomoea coccinea L.), common lambsquarters (Chenopodium album L.), and giant foxtail (Setaria faberi Herrm.) (Armel et al. Reference Armel, Rardon, McComrick and Ferry2007; Hugie et al. Reference Hugie, Bollero, Tranel and Riechers2008; Kohrt and Sprague Reference Kohrt and Sprague2017; Walsh et al. Reference Walsh, Stratford, Stone and Powles2012; Willemse et al. Reference Willemse, Soltani, Hooker, Jhala, Robinson and Sikkema2021; Woodyard et al. Reference Woodyard, Bollero and Riechers2009a, Reference Woodyard, Hugie and Riechers2009b). Additive interactions between HPPD and PSII inhibitors have also been reported for the control of Amaranthus spp., A. theophrasti, A. trifida, C. album, and R. raphanistrum (Hugie et al. Reference Hugie, Bollero, Tranel and Riechers2008; Kohrt and Sprague Reference Kohrt and Sprague2017; Walsh et al. Reference Walsh, Stratford, Stone and Powles2012; Willemse et al. Reference Willemse, Soltani, Hooker, Jhala, Robinson and Sikkema2021; Woodyard et al. Reference Woodyard, Bollero and Riechers2009a, Reference Woodyard, Hugie and Riechers2009b).
Most of the published literature on the interaction between HPPD and PSII inhibitors involves the use of atrazine, with few studies incorporating other PSII inhibitors such as bentazon and bromoxynil. Given that the use of atrazine has become more restricted due to atrazine detected in groundwater and surface water, identifying the interaction and level of control of GR C. canadensis with HPPD inhibitors when applied with atrazine-alternative PSII inhibitors is valuable (Graymore et al. Reference Graymore, Stagnitti and Allinson2001). Additionally, much of the peer-reviewed literature focuses on the interaction of HPPD and PSII inhibitors on Amaranthus spp., while the interaction has not been intensively studied for GR C. canadensis control in corn. Therefore, the two objectives of this study were to evaluate the level of GR C. canadensis control with HPPD and PSII inhibitors applied alone and in tank mixes and to classify the interaction of HPPD and PSII inhibitors for the control of GR C. canadensis in corn.
Materials and Methods
Four no-tillage field trials were conducted in 2019 and 2020 in commercial corn fields in southwestern Ontario, Canada (Table 1). In 2019, two trials were conducted near the community of Zone Centre on two separate fields located at 42.62°N, 81.94°W and 42.62°N, 81.95°W, with the trials being separated both geographically and temporally. In 2020, one field trial was near Highgate (42.55°N, 81.84°W) and another one was near Thamesville (42.53°N, 81.91°W). Populations of C. canadensis from these sites were previously confirmed as GR by greenhouse screening (unpublished data). All sites contained natural infestations of confirmed GR C. canadensis. GR corn hybrids, DKC45-65RIB® and DKC42-60RIB® (Bayer CropScience Canada, 160 Quarry Boulevard SE, Calgary, AB T2C 3G3, Canada) were planted at a seeding rate of approximately 80,000 seeds ha−1 in 2019 and 2020, respectively. Corn was planted approximately 4-cm deep in 75-cm row spacing. Plot dimensions were 8 m in length and 2.25 m wide. Trials were set up as randomized complete block designs with four replicated blocks in each trial. The trials were fertilized according to provincial recommendations for corn production in Ontario (OMAFRA 2017). Soil characteristics, corn planting and harvest dates, herbicide application dates, and GR C. canadensis size and density at application are listed in Table 1.
Table 1. Year, location, soil characteristics, corn planting dates, corn harvest dates, treatment application dates, corn development stages at treatment application, and glyphosate-resistant Conyza canadensis size and density at treatment application for four field trials in southwestern Ontario, Canada, in 2019 and 2020.
a Soil characteristics were obtained from soil cores taken to a depth of 15 cm and subsequent analysis at A&L Canada Laboratories Inc. (2136 Jetstream Road, London, ON N5V 3P5, Canada). OM, organic matter; PP, preplant.
b Size determined as height of bolting plants or diameter of rosettes at time of treatment application. Mean of measurements taken in two randomly placed 0.25-m2 quadrats in each nontreated control plot.
c Mean density of two stand counts in each nontreated control plot.
d Treatments applied before corn emergence, as weed size dictated application date.
Glyphosate-susceptible C. canadensis and other competing weed species were removed before treatment application with an application of glyphosate (Roundup WeatherMax®, 540 g ae L−1, Bayer CropScience Canada) at a rate of 900 g ae ha−1 to ensure a monoculture weed population of GR C. canadensis. No pesticide other than the herbicide treatments was applied for the rest of the study. The study was conducted as a two-factor factorial. Factor 1 included four levels of HPPD-inhibiting herbicides: nontreated control, mesotrione, tolpyralate, and topramezone. Factor 2 comprised the PSII-inhibiting herbicide factor: nontreated control, atrazine, bromoxynil, and bentazon. Detailed treatment information is presented in Table 2. All herbicide treatments were sprayed with a CO2-powered backpack plot sprayer, outfitted with four ULD120-02 spray nozzles (Pentair, 375 5th Avenue NW, New Brighton, MN 55112, USA) at 50-cm spacing calibrated to deliver 200 L ha−1 spray volume at 240 kPa pressure. Herbicide treatments were sprayed when the GR C. canadensis in the nontreated control plots reached an average of approximately 10 cm in height or rosette diameter. The herbicide treatments were intended to be sprayed postemergence to the corn; however, in 2020, the herbicide treatments were sprayed before corn emergence in the interest of targeting 10-cm GR C. canadensis, because the control of GR C. canadensis was the research focus.
Table 2. Herbicide active ingredient, rate, trade name, and manufacturer for the study of the interaction between 4-hydroxyphenylpyruvate dioxygenase and photosystem II inhibitors on the control of glyphosate-resistant C. canadensis in southwestern Ontario, Canada, in 2019 and 2020.
a The adjuvant used depended on the herbicide: mesotrione included Agral® 90 (Syngenta Canada Inc., 140 Research Lane, Guelph, ON N1G 4Z3, Canada) at 0.2% v/v; tolpyralate included methylated seed oil (MSO Concentrate®) (Loveland Products Inc., 3005 Rocky Mountain Avenue, Loveland, CO 80538, USA) at 0.5% v/v and urea ammonium nitrate (UAN 28-0-0) at 2.5% v/v; topramezone included Merge® (BASF Canada Inc., 100 Milverton Drive, Mississauga L5R 4H1, ON, Canada) at 0.5% v/v.
Visible corn injury was evaluated at 1, 2, and 4 wk after application (WAA) on a 0% to 100% scale; 0% represented no visible corn injury, and 100% was complete corn death. Depending on the herbicide treatment, leaf bleaching, chlorosis, and necrosis were the symptoms observed on corn leaf tissue exposed at herbicide application. Visible GR C. canadensis control at 2, 4, and 8 WAA was assessed on a scale of 0% to 100% as a visual assessment of aboveground GR C. canadensis biomass reduction compared with aboveground GR C. canadensis biomass in the nontreated control within the corresponding replication. Depending on the herbicide treatment, the herbicides induced bleaching, chlorosis, and necrosis on GR C. canadensis. The density and aboveground biomass of GR C. canadensis was collected at 8 WAA by counting the GR C. canadensis plants and clipping them at the soil surface within two 0.25-m2 quadrats placed indiscriminately in each plot. The clipped GR C. canadensis plants from each plot were placed in separate paper bags and kiln-dried at 60 C until the biomass reached constant moisture. The GR C. canadensis dry biomass contents of each bag were measured with an analytical scale. Grain corn yield and harvest moisture were recorded at corn harvest maturity by combining two corn rows in each plot using a small plot research combine. Grain corn yields were corrected to 15.5% moisture before statistical analysis of yield data.
Statistical Analysis
All response parameters were subject to mixed model variance analysis within PROC GLIMMIX in SAS v. 9.4 (SAS Institute, 100 SAS Campus Drive, Cary, NC 27513, USA). The variance was subdivided into the fixed effects of HPPD inhibitor (Factor 1), PSII inhibitor (Factor 2), and the interaction between the two factors. The random effects included the environment (a collective term for the trial year and location combinations), replicated block within environment, and the interaction of environment with Factors 1 and 2. The significance of random effects was ascertained with a log-likelihood ratio test, and fixed effects with an F-test, with α = 0.05 set for all tests. All data were pooled across environments, except for corn injury data, because herbicide treatments were applied postemergence in 2019 and before corn emergence in 2020. Studentized residual plots and the Shapiro-Wilk test for normality were analyzed to ensure that the residuals were random, independent of treatment and design effects, normally distributed, homogeneous, and had a mean of zero. A gaussian distribution was used to analyze corn injury and yield data. GR C. canadensis control data at all assessment timings were arcsine square-root transformed to meet the assumptions of variance analysis. Control data were back-transformed from the analysis scale for the presentation of results. A lognormal distribution was used to analyze GR C. canadensis density and dry biomass data. For the presentation of GR C. canadensis density and dry biomass, the least-square means were back-transformed using the omega method of back-transformation (M Edwards, Ontario Agricultural College Statistics Consultant, University of Guelph, personal communication). The main effects (HPPD inhibitor or PSII inhibitor) least-square means were only assessed when the interaction between HPPD and PSII inhibitors was not significant. When the interaction between HPPD and PSII inhibitors was significant, the simple effects for each factor were presented. The Tukey-Kramer multiple-range test was used to separate main and simple effect least-square means at a type I error of α = 0.05.
Colby’s equation (Equation 1) was used to compute the expected visible GR C. canadensis control and corn injury for each herbicide tank mix within each block by utilizing the observed values for HPPD inhibitor alone (X) and PSII inhibitor alone (Y).
$${\rm{Expected}} = (X + Y) - [(X*Y)/100]$$
Expected GR C. canadensis density and dry biomass were also calculated for each herbicide tank mix within each replication in each trial by using the adjusted Colby’s equation, which incorporates the value from the nontreated control (Z) within the replication (Equation 2).
$${\rm{Expected}} = [(X*Y)/Z]$$
The observed and expected values for GR C. canadensis control, density, dry biomass, and corn injury were compared using a two-tailed t-test. Statistically similar observed and expected values were reported as additive interactions. Antagonistic or synergistic interactions occurred when the observed and expected values were statistically different. Data analysis was conducted with a significance level set to α = 0.05; significance levels of α = 0.01 are also presented in Tables 4 and 6.
Results and Discussion
GR Conyza canadensis Control
There was an interaction between HPPD- and PSII-inhibiting herbicides for GR C. canadensis control at 2, 4, and 8 WAA; therefore, the effect of HPPD inhibitor was analyzed by each PSII inhibitor and the effect of PSII inhibitor was analyzed by each HPPD inhibitor (Table 3).
Table 3. Least-square means and significance of main effects and interaction for glyphosate-resistant Conyza canadensis control (2, 4, and 8 wk after application), density, and dry biomass in corn after the application of HPPD, PSII, and HPPD + PSII inhibitors across four field trials in southwestern Ontario, Canada, in 2019 and 2020. a
a Abbreviations: HPPD, 4-hydroxyphenylpyruvate dioxygenase; PSII, photosystem II; WAA, weeks after application.
b Means within the same main effect and column followed by the same lowercase letter are not statistically different according to the Tukey-Kramer multiple-range test (P < 0.05).
c The adjuvant used depended on the herbicide: mesotrione included Agral® 90 (Syngenta Canada Inc., 140 Research Lane, Guelph, ON N1G 4Z3, Canada) at 0.2% v/v; tolpyralate included methylated seed oil (MSO Concentrate®) (Loveland Products Inc., 3005 Rocky Mountain Avenue, Loveland, CO 80538, USA) at 0.5% v/v and urea ammonium nitrate (UAN 28-0-0) at 2.5% v/v; topramezone included Merge® (BASF Canada Inc., 100 Milverton Drive, Mississauga, ON L5R 4H1, Canada) at 0.5% v/v.
At 2 WAA, the HPPD inhibitors mesotrione, tolpyralate, and topramezone controlled GR C. canadensis 79%, 79%, and 70%, respectively (Table 4). In a previous study, tolpyralate controlled GR C. canadensis 85% at 2 WAA (Metzger et al. Reference Metzger, Soltani, Raeder, Hooker, Robinson and Sikkema2019). The PSII inhibitors bromoxynil and bentazon controlled GR C. canadensis 39 and 43 percentage points greater than atrazine, respectively, at 2 WAA (Table 4). The application of atrazine, bromoxynil, or bentazon with mesotrione improved GR C. canadensis control to 98%, 100%, and 99%, respectively (Table 4). The co-application of bromoxynil or bentazon with tolpyralate increased the control of GR C. canadensis to 99% and 98%, respectively (Table 4). Similarly, bromoxynil or bentazon addition to topramezone improved GR C. canadensis control by 22 percentage points (Table 4). The addition of atrazine to tolpyralate or topramezone did not increase GR C. canadensis control at 2 WAA (Table 4). In contrast, the addition of atrazine to tolpyralate increased GR C. canadensis control at 2 WAA in a study by Metzger et al. (Reference Metzger, Soltani, Raeder, Hooker, Robinson and Sikkema2019). The addition of an HPPD inhibitor to atrazine, bromoxynil, or bentazon improved GR C. canadensis except for the addition of topramezone to bentazon (Table 4). The tank mix of mesotrione + atrazine controlled GR C. canadensis 14 percentage points greater than topramezone + atrazine (Table 4). Additionally, the tank mix of mesotrione + bromoxynil controlled GR C. canadensis 8 percentage points greater than topramezone + bromoxynil at 2 WAA (Table 4).
Table 4. Glyphosate-resistant Conyza canadensis control (2, 4, and 8 wk after application), density, and dry biomass in corn after the application of HPPD, PSII, and HPPD + PSII inhibitors from four field trials in southwestern Ontario, Canada, in 2019 and 2020. a
a Abbreviations: HPPD, 4-hydroxyphenylpyruvate dioxygenase; PSII, photosystem II; WAA, weeks after application.
b The adjuvant used depended on the herbicide: mesotrione included Agral® 90 (Syngenta Canada Inc., 140 Research Lane, Guelph, ON N1G 4Z3, Canada) at 0.2% v/v; tolpyralate included methylated seed oil (MSO Concentrate®) (Loveland Products Inc., 3005 Rocky Mountain Avenue, Loveland, CO 80538, USA) at 0.5% v/v and urea ammonium nitrate (UAN 28-0-0) at 2.5% v/v; topramezone included Merge® (BASF Canada Inc., 100 Milverton Drive, Mississauga, ON L5R 4H1, Canada) at 0.5% v/v.
c Means followed by the same lowercase letter within the same column and response parameter or means followed by the same uppercase letter within a row are not statistically different according to the Tukey-Kramer multiple-range test (P < 0.05).
d Values in parentheses are expected values calculated from Colby’s equation. Asterisks (*)indicate significant differences of *P < 0.05 and **P < 0.01, respectively, between observed and expected values based on a two-tailed t-test.
The addition of any PSII inhibitor to the HPPD inhibitors improved GR C. canadensis control by 13 to 22 percentage points at 4 WAA (Table 4). Metzger et al. (Reference Metzger, Soltani, Raeder, Hooker, Robinson and Sikkema2019) also observed that adding atrazine to tolpyralate increased GR C. canadensis control 13 percentage points at 4 WAA. GR C. canadensis control with atrazine, bromoxynil, and bentazon was improved 59 to 69, 20 to 27, and 15 to 22 percentage points, respectively, with the addition of an HPPD inhibitor (Table 4). Similarly, Armel et al. (Reference Armel, Richardson, Wilson and Hines2009) reported a 63 percentage point increase in glyphosate-susceptible C. canadensis control when mesotrione was added to atrazine at 3 WAA in no-tillage corn. Mesotrione tank mixed with atrazine, bromoxynil, or bentazon controlled GR C. canadensis 7 to 10 percentage points greater than topramezone tank mixed with atrazine, bromoxynil, or bentazon (Table 4). Mesotrione or tolpyralate tank mixed with atrazine, bromoxynil, or bentazon controlled GR C. canadensis 99% to 100% and 96% to 98%, respectively (Table 4). In two previous studies conducted in Ontario, tolpyralate + atrazine controlled GR C. canadensis 99% and 96% at 4 WAA (Langdon et al. Reference Langdon, Soltani, Raeder, Hooker, Robinson and Sikkema2020; Metzger et al. Reference Metzger, Soltani, Raeder, Hooker, Robinson and Sikkema2019).
At 8 WAA, the addition of an HPPD inhibitor to atrazine, bromoxynil, or bentazon increased GR C. canadensis control 60 to 69, 22 to 29, and 16 to 21 percentage points, respectively (Table 4). Atrazine applied alone controlled GR C. canadensis nearly 30% at all visible control intervals (Table 4). Similarly, Mahoney et al. (Reference Mahoney, Shropshire and Sikkema2017) observed 37% control of GR C. canadensis at 8 WAA with atrazine applied alone. Atrazine tank mixed with mesotrione, tolpyralate, or topramezone controlled GR C. canadensis 100%, 96%, and 91%, respectively (Table 4). In previous studies, tolpyralate + atrazine controlled GR C. canadensis 97% and 98% at 8 WAA (Langdon et al. Reference Langdon, Soltani, Raeder, Hooker, Robinson and Sikkema2020; Metzger et al. Reference Metzger, Soltani, Raeder, Hooker, Robinson and Sikkema2019). When the PSII inhibitor tank-mix partner was atrazine or bromoxynil, mesotrione control of GR C. canadensis was 9 and 7 percentage points greater, respectively, than topramezone (Table 4). In a previous study on GR C. canadensis control, mesotrione + atrazine and topramezone + atrazine controlled GR C. canadensis 76% and 67%, respectively, at 8 WAA (Mahoney et al. Reference Mahoney, Shropshire and Sikkema2017). The differences in control between this study and the study by Mahoney et al. (Reference Mahoney, Shropshire and Sikkema2017) could be because those authors sprayed 15-cm GR C. canadensis, while 10-cm GR C. canadensis was treated in the present study. GR C. canadensis control with mesotrione + bromoxynil and tolpyralate + bromoxynil was 100% and 98%, respectively (Table 4). The tank mixes of mesotrione, tolpyralate, or topramezone with bentazon controlled GR C. canadensis 100%, 98%, and 95%, respectively, with no statistical differences detected between these tank-mix combinations (Table 4).
According to the two-tailed t-test comparisons of observed and expected control, the tank mixes of mesotrione or tolpyralate with atrazine, bromoxynil, or bentazon were synergistic for the control of GR C. canadensis at 2, 4, and 8 WAA (Table 4). Ditschun et al. (Reference Ditschun, Soltani, Robinson, Tardif, Kaastra and Sikkema2016) also documented synergy between HPPD and PSII inhibitors for GR C. canadensis control, but that study was conducted with isoxaflutole and metribuzin in no-crop field trials. As per Colby’s equation, the tank mixes of mesotrione plus atrazine, bromoxynil, or bentazon controlled GR C. canadensis 11, 6, and 4 percentage points greater than expected at 8 WAA, respectively (Table 4). The synergy between mesotrione and atrazine has also been recorded for the control of A. palmeri, A. tuberculatus, A. retroflexus, A. trifida, A. theophrasti, C. album, I. coccinea, R. raphanistrum, X. strumarium, and S. faberi (Armel et al. Reference Armel, Rardon, McComrick and Ferry2007; Hugie et al. Reference Hugie, Bollero, Tranel and Riechers2008; Kohrt and Sprague Reference Kohrt and Sprague2017; Walsh et al. Reference Walsh, Stratford, Stone and Powles2012; Woodyard et al. Reference Woodyard, Bollero and Riechers2009a, Reference Woodyard, Hugie and Riechers2009b). In addition, synergy has also been detected between mesotrione and bromoxynil for the control of A. palmeri, A. tuberculatus, A. retroflexus, A. theophrasti, A. trifida, and C. album in previous studies (Abendroth et al. Reference Abendroth, Martin and Roeth2006; Hugie et al. Reference Hugie, Bollero, Tranel and Riechers2008; Woodyard et al. Reference Woodyard, Bollero and Riechers2009a). A study on A. tuberculatus control documented synergy between mesotrione plus bromoxynil or bentazon, but additive interactions with mesotrione + atrazine (Willemse et al. Reference Willemse, Soltani, Hooker, Jhala, Robinson and Sikkema2021). Tolpyralate plus atrazine, bromoxynil, or bentazon controlled GR C. canadensis 6, 4, and 2 percentage points greater than expected at 8 WAA, respectively (Table 4). Willemse et al. (Reference Willemse, Soltani, Hooker, Jhala, Robinson and Sikkema2021) reported synergy with tolpyralate + bromoxynil, but not with tolpyralate plus atrazine or bentazon for the control of A. tuberculatus. Kohrt and Sprague (Reference Kohrt and Sprague2017) also did not find synergy between tolpyralate and atrazine for the control of A. palmeri. In contrast to mesotrione and tolpyralate tank mixes, the interaction of topramezone with atrazine, bentazon, or bromoxynil was additive for the control of GR C. canadensis at all assessment timings (Table 4). Similarly, synergy was generally not documented between topramezone and PSII inhibitors in previous studies focusing on A. palmeri and A. tuberculatus control (Kohrt and Sprague Reference Kohrt and Sprague2017; Willemse et al. Reference Willemse, Soltani, Hooker, Jhala, Robinson and Sikkema2021).
GR Conyza canadensis Density and Dry Biomass
No interaction effect was detected between the two herbicide factors for GR C. canadensis density and aboveground dry biomass at 8 WAA, so the main effects are analyzed (Table 3). Mesotrione, tolpyralate, and topramezone reduced the density of GR C. canadensis by 97%, 91%, and 78%, respectively, when averaged across the PSII inhibitors (Table 3). The density reduction was greater with mesotrione than topramezone; the density reduction with tolpyralate was intermediate and similar to both (Table 3). Atrazine, bromoxynil, and bentazon reduced the density of GR C. canadensis by 75%, 89%, and 92%, respectively, when averaged across the HPPD inhibitors (Table 3). Bentazon reduced the density of GR C. canadensis more than atrazine; the density reduction with bromoxynil was intermediate and similar to both (Table 3). The co-application of mesotrione with the PSII inhibitors and the tank mix of topramezone + atrazine synergistically decreased GR C. canadensis density (Table 4).
Averaged across the PSII inhibitors, all the HPPD inhibitors reduced the dry biomass of GR C. canadensis 91% to 99% (Table 3). Topramezone reduced GR C. canadensis dry biomass less than mesotrione and tolpyralate (Table 3). The PSII inhibitors atrazine, bromoxynil, and bentazon reduced GR C. canadensis dry biomass 72%, 91%, and 93%, respectively (Table 3). Atrazine reduced GR C. canadensis dry biomass less than bromoxynil and bentazon (Table 3). The interaction between mesotrione and the three PSII inhibitors was synergistic for the reduction of the aboveground dry biomass of GR C. canadensis (Table 4). In contrast, the interactions between tolpyralate or topramezone and atrazine, bromoxynil, or bentazon were all additive for the reduction of dry biomass of GR C. canadensis (Table 4).
Corn Injury and Grain Yield
No visible corn injury was observed at both trials in 2020 at 1, 2, and 4 WAA. Corn was not emerged at the time of application in 2020, because GR C. canadensis reached an average height or rosette diameter of 10 cm before corn emergence. Therefore, corn injury was only analyzed for the trials in 2019. Corn injury at 42.62°N, 81.95°W was 0% for all treatments at 2 WAA, so these data were removed from analysis, and only injury data from 42.62°N, 81.94°W are presented 2 WAA. No corn injury was observed at 4 WAA at both trials in 2019.
There was an interaction effect between the two herbicide factors for corn injury at 2 WAA and corn yield at maturity; therefore, the effect of HPPD inhibitor was analyzed by each PSII inhibitor and the effect of PSII inhibitor was analyzed by each HPPD inhibitor (Table 5).
Table 5. Least-square means and significance of main effects and interaction for corn injury (1 and 2 wk after application) and corn grain yield after the application of HPPD, PSII, and HPPD + PSII inhibitors from four field trials in southwestern Ontario, Canada, in 2019 and 2020. a
a Abbreviations: HPPD, 4-hydroxyphenylpyruvate dioxygenase; PSII, photosystem II; WAA, weeks after application.
b Corn injury at 1 WAA in 2020 is omitted, because no corn injury was observed.
c Corn injury at 2 WAA in 2020 and at 42.62°N, 81.95°W in 2019 is omitted, because no corn injury was observed.
d The adjuvant used depended on the herbicide: mesotrione included Agral® 90 (Syngenta Canada Inc., 140 Research Lane, Guelph, ON N1G 4Z3, Canada) at 0.2% v/v; tolpyralate included methylated seed oil (MSO Concentrate®) (Loveland Products Inc., 3005 Rocky Mountain Avenue, Loveland, CO 80538, USA) at 0.5% v/v and urea ammonium nitrate (UAN 28-0-0) at 2.5% v/v; topramezone included Merge® (BASF Canada Inc., 100 Milverton Drive, Mississauga, ON L5R 4H1, Canada) at 0.5% v/v.
There was no corn injury observed at 2 WAA when mesotrione, tolpyralate, and topramezone were applied alone (Table 6). Bromoxynil and bentazon caused 1% corn injury at 2 WAA, while atrazine did not injure corn (Table 6). Adding mesotrione to atrazine did not increase corn injury, but the addition of tolpyralate or topramezone to atrazine injured corn 1% (Table 6). Corn injury with atrazine plus tolpyralate or topramezone was synergistic (Table 6). In contrast, the addition of any HPPD inhibitor to bromoxynil did not accentuate corn injury (Table 6). The addition of mesotrione or tolpyralate to bentazon increased corn injury to 2%; however, the addition of topramezone to bentazon did not increase corn injury (Table 6). The addition of atrazine, bromoxynil, or bentazon increased the level of corn injury at 2 WAA similarly when applied with tolpyralate or topramezone (Table 6). In contrast, bentazon was the most injurious tank mix partner with mesotrione (Table 6).
Table 6. Corn injury (1 and 2 wk after application) and corn grain yield after the application of HPPD, PSII, and HPPD + PSII inhibitors from four field trials in southwestern Ontario, Canada, in 2019 and 2020. a
a Abbreviations: HPPD, 4-hydroxyphenylpyruvate dioxygenase; PSII, photosystem II; WAA, weeks after application.
b The adjuvant used depended on the herbicide: mesotrione included Agral® 90 (Syngenta Canada Inc., 140 Research Lane, Guelph, ON N1G 4Z3, Canada) at 0.2% v/v; tolpyralate included methylated seed oil (MSO Concentrate®) (Loveland Products Inc., 3005 Rocky Mountain Avenue, Loveland, CO 80538, USA) at 0.5% v/v and urea ammonium nitrate (UAN 28-0-0) at 2.5% v/v; topramezone included Merge® (BASF Canada Inc., 100 Milverton Drive, Mississauga, ON L5R 4H1, Canada) at 0.5% v/v.
c Means followed by the same lowercase letter within the same column and response parameter or means followed by the same uppercase letter within a row are not statistically different according to the Tukey-Kramer multiple-range test (P < 0.05).
d Values in parentheses are expected values calculated from Colby’s equation. Asterisks (*) indicate significant differences of *P < 0.05 and **P < 0.01, respectively, between observed and expected values based on a two-tailed t-test.
e Corn injury at 1 WAA in 2020 is omitted, because no corn injury was observed.
f Corn injury 2 WAA in 2020 and at 42.62°N, 81.95°W in 2019 is omitted, because no corn injury was observed.
In this study, GR C. canadensis interference decreased corn yield up to 53% (highest-yielding treatment compared with the nontreated control) (Table 6). Reduced GR C. canadensis interference with mesotrione, tolpyralate, or topramezone treatments increased corn yield 106%, 98%, and 96%, respectively (Table 6). Reduced GR C. canadensis interference with atrazine, bromoxynil, or bentazon increased corn yield 59%, 78%, and 84%, respectively (Table 6). The addition of mesotrione, tolpyralate or topramezone to atrazine increased corn yield 28% to 33% (Table 6). The improved GR C. canadensis control with the co-application of HPPD inhibitors and atrazine may explain the increased corn yield with these tank mixes compared with atrazine alone. In contrast, the addition of an HPPD inhibitor to bromoxynil or bentazon did not improve corn yield (Table 6). The use of a PSII inhibitor with mesotrione, tolpyralate, or topramezone did not increase the yield of corn relative to the HPPD inhibitors applied alone (Table 6). Similarly, Metzger et al. (Reference Metzger, Soltani, Raeder, Hooker, Robinson and Sikkema2019) found that reduced GR C. canadensis interference with tolpyralate and tolpyralate + atrazine resulted in higher corn yield than the nontreated control, but the two herbicide treatments did not differ in respect to corn yield.
In summary, nine tank mixes of HPPD inhibitors + PSII inhibitors controlled GR C. canadensis >90% at 8 WAA. Among these tank mixes, mesotrione or tolpyralate plus atrazine, bromoxynil, or bentazon controlled GR C. canadensis 96% to 100%, while topramezone tank mixes controlled GR C. canadensis 91% to 95% at 8 WAA. These results increase the known available herbicide options for excellent (>90%) GR C. canadensis control in corn. Bromoxynil or bentazon can be used in place of atrazine when co-applied with mesotrione, tolpyralate, or topramezone with no compromise in GR C. canadensis control. This is particularly advantageous in geographic regions where the use of atrazine is prohibited or restricted. The tank mixes of mesotrione or tolpyralate plus atrazine, bromoxynil, or bentazon were all synergistic for the control of GR C. canadensis at 2, 4, and 8 WAA; however, topramezone tank mixed with atrazine, bromoxynil, or bentazon was additive for the control of GR C. canadensis at all assessment timings. These findings expand the understanding of the interaction between HPPD and PSII inhibitors over several active ingredients within each herbicide mode of action. To our knowledge, this is the first report of the interaction of mesotrione, tolpyralate, or topramezone with atrazine, bromoxynil, or bentazon for GR C. canadensis control. The judicious use of effective herbicide tank mixes with cultural and mechanical weed control tactics may help to maintain the efficacious long-term use of the herbicide tank mixes for GR C. canadensis control in corn.
Acknowledgments
This research was funded in part by ISK Biosciences Inc. and the Grain Farmers of Ontario. No other conflicts of interest have been declared.
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