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.
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 1997; Main et al. 2006; Nandula et al. 2006; Weaver 2001). 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 1988; Nandula et al. 2006; Regehr and Bazzaz 1979). 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. 2006; Weaver 2001). 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 2020). A glyphosate-resistant (GR) biotype of C. canadensis was first reported in a population in Delaware, USA in 2000 (VanGessel 2001). Ten years later, GR C. canadensis was found in a population from Essex County, Ontario, Canada (Byker et al. 2013). GR biotypes of C. canadensis have since been identified in 30 counties across southern Ontario (Budd et al. 2018). GR C. canadensis is also widespread in the United States, where it has been reported in 25 states as of September 2021 (Heap 2021).
Several herbicides applied preplant provide effective control (>90%) of GR biotypes of C. canadensis in corn (Brown et al. 2016; Ford et al. 2014). 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. 2020; Mahoney et al. 2017; Metzger et al. 2019). 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 2019; Cholette et al. 2018). 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. 2006). 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. 2014). 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. 1993; Secor 1994; Tsegaye et al. 2002). 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. 2005; Trebst et al. 2002). Mesotrione, topramezone, and tolpyralate are commonly used postemergence HPPD inhibitors in Ontario with different weed control spectrums (Kohrt and Sprague 2017; Metzger et al. 2018). PSII inhibitors such as atrazine, bentazon, and bromoxynil are complementary to HPPD inhibitors because of their interrelated modes of action (Abendroth et al. 2006; Armel et al. 2005; Creech et al. 2004; Kim et al. 1999). 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 2000). PSII inhibitors cause cell death by triggering a massive influx of reactive oxygen species, which induces lipid peroxidation (Hess 2000). 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. 2005). 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. 2005; Creech et al. 2004). 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. 2005, 2008, 2009; Johnson et al. 2002; Kohrt and Sprague 2017; Metzger et al. 2018, 2019; Whaley et al. 2006).
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 1967). 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 1967).
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. 2006; Hugie et al. 2008; Kohrt and Sprague 2017; Woodyard et al. 2009a). 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. 2007; Hugie et al. 2008; Kohrt and Sprague 2017; Walsh et al. 2012; Willemse et al. 2021; Woodyard et al. 2009a, 2009b). 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. 2008; Kohrt and Sprague 2017; Walsh et al. 2012; Willemse et al. 2021; Woodyard et al. 2009a, 2009b).
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. 2001). 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.
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.
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.
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).
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).
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).
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. 2019). 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. (2019). 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).
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. (2019) 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. (2009) 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. 2020; Metzger et al. 2019).
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
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. (2017) 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. 2020; Metzger et al. 2019). 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. 2017). The differences in control between this study and the study by Mahoney et al. (2017) 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. (2016) 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. 2007; Hugie et al. 2008; Kohrt and Sprague 2017; Walsh et al. 2012; Woodyard et al. 2009a, 2009b). 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. 2006; Hugie et al. 2008; Woodyard et al. 2009a). A study on A. tuberculatus control documented synergy between mesotrione plus bromoxynil or bentazon, but additive interactions with mesotrione + atrazine (Willemse et al. 2021). 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. (2021) reported synergy with tolpyralate + bromoxynil, but not with tolpyralate plus atrazine or bentazon for the control of A. tuberculatus. Kohrt and Sprague (2017) 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 2017; Willemse et al. 2021).
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
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.
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
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
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).
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).
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. (2019) 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.
