Introduction
Benghal dayflower, a glyphosate-tolerant plant (Cerdeira et al. Reference Cerdeira, Gazziero, Duke and Matallo2011; Culpepper et al. Reference Culpepper, Flanders, York and Webster2004; Holkem et al. Reference Holkem, Silva, Bianchi, Corassa and Ulguim2022;), is among the most prevalent weed species in the world, affecting more than 25 crops in 29 countries (Holm et al. Reference Holm, Plucknett, Pancho and Herberger1977; Lucio et al. Reference Lucio, Kalsing, Adegas, Rossi, Correia, Gazziero and da Silva2019; Webster et al. Reference Webster, Burton, Culpepper, York and Prostko2005), and spreading rapidly through areas where glyphosate is used for weed management (Adegas et al. Reference Adegas, Correia, da Silva, Concenço, Gazziero and Dalazen2022; Silva et al. Reference Silva, Barbosa, Ferrari, Tronquini and Monquero2019; Webster et al. Reference Webster, Burton, Culpepper, Flanders, Grey and York2006). In Brazil, 93% of soybean [Glycine max (L.) Merr.] production area is planted with glyphosate-resistant (GR) cultivars (Lucio et al. Reference Lucio, Kalsing, Adegas, Rossi, Correia, Gazziero and da Silva2019). Glyphosate use in Brazil increased by 75% between 2009 and 2021 (Merotto et al. Reference Merotto, Gazziero, Oliveira, Scursoni, Garcia, Figueroa and Turra2022), which has led to the selection of glyphosate-tolerant weed species such as Benghal dayflower. Benghal dayflower now infests 41% of areas planted with soybean in Brazil, ranking it among top five most-troublesome weeds in cropping systems (Oliveira et al., Reference Oliveira, Lencina, Ulguim and Werle2021). The weed is widely dispersed, and particularly infests regions with year-round high temperatures (Lucio et al. Reference Lucio, Kalsing, Adegas, Rossi, Correia, Gazziero and da Silva2019). In western Paraná, in southwestern Brazil, phytosociological surveys conducted in fields planted with GR soybean identified Benghal dayflower as the second most invasive species (Dias et al. Reference Dias, Krenchinsky, Pereira, Moreno, Albrecht and Albrecht2018). Similarly, in Central-West Brazil, where glyphosate has been used for several years, Benghal dayflower was found to have the highest relative importance among weed species (Castro et al. Reference Castro, Lima, Tomquelski, Andrade and Martins2020). Furthermore, even at low densities, Benghal dayflower is more competitive than other common glyphosate-tolerant weeds and causes greater yield losses (Oliveira et al. Reference Oliveira, Pereira, Ferreira, Santos, Knezevic and Werle2018). Additionally, its control becomes challenging when plants exceed 10 cm in height (Culpepper et al. Reference Culpepper, Flanders, York and Webster2004; Osipe et al. Reference Osipe, Oliveira, Constantin, Takano and Biffe2017; Takano et al. Reference Takano, Oliveira, Constantin, Biffe, Franchini, Braz, Rios, Gheno and Gemelli2013).
Herbicide mixtures are commonly required to manage hard-to-control species. Glyphosate is often applied without a sequential herbicide and increasingly close to crop sowing (Merotto et al. 2022). Additionally, use of glyphosate in combination with other Group 4 herbicides (as categorized by the Herbicide Resistance Action Committee and Weed Science Society of America), has become more common. Glyphosate is often applied in emergencies to control bigger and older weeds that result from late, off-season burndown management. However, when herbicides with distinct modes of action are applied together, weed control with these mixtures can be termed antagonistic, additive, or synergistic based on whether their ability to control weeds is respectively less, equal, or greater than the sum of an individual herbicide (Colby Reference Colby1967; Fluttert et al. Reference Fluttert, Soltani, Galla, Hooker, Robinson and Sikkema2022). Farmers may use a combination of auxin herbicides combined with glyphosate in weed management because the mixture can hasten and improve weed control (Osipe et al. Reference Osipe, Oliveira, Constantin, Takano and Biffe2017; Takano et al. Reference Takano, Oliveira, Constantin, Biffe, Franchini, Braz, Rios, Gheno and Gemelli2013). Nevertheless, mixtures can reduce glyphosate uptake and translocation in some species, thus reducing their efficacy (Flint and Barrett Reference Flint and Barrett1989; Li et al. Reference Li, Han, Bai and Yu2020). It is unclear whether herbicides from the same group interact differently when mixed with a second common herbicide to control different weed species.
Auxin herbicides have been used for more than 70 yr, representing the third most-used herbicide class behind glyphosate and acetolactate synthase inhibitors (Todd et al. Reference Todd, Figueiredo, Morran, Soni, Preston, Kubeš, Napier and Gaines2020). Auxinic herbicides rank as the second most-used group of herbicides in Brazil, with their use expanding primarily for pre-plant burndown for summer crops under no-tillage systems and in pasture management. Between 2009 and 2017, the use of 2,4-D increased by 400%. Additionally, from 2018 to 2020, triclopyr usage surged by 253%, mostly driven by the emergence of weed resistance to 2,4-D. In 2019, auxin herbicides such as 2,4-D, triclopyr, fluroxypyr, and dicamba were estimated to be applied over an area of several million hectares (Merotto et al. 2022). The adoption of these herbicides is expected to increase with the adoption of auxin-resistant crops such as Enlist E3® and XtendFlex® soybeans. According to Westwood et al. (Reference Westwood, Charudattan, Duke, Fennimore, Marrone, Slaughter, Swanton and Zollinger2018), these new herbicides are short-term solutions to some of the current problems since resistance to 2,4-D and dicamba has already been reported (Heap Reference Heap2024). Therefore, it is crucial to use these herbicides properly to maintain the long-term viability of glyphosate, 2,4-D, and dicamba as weed management tools.
Little to no information has been reported on which auxin herbicides can or should be combined with glyphosate to control Benghal dayflower in advanced growth stages. This study addresses possible ways for managing Benghal dayflower plants that have surpassed the ideal growth stage for effective control, which often occurs in fallow fields, poorly managed areas, or where weeds can grow unchecked. These scenarios are common during off-season burndown times and in pastures and perennial crop fields where Benghal dayflower frequently establishes as a dominant and troublesome weed. Dicamba, in particular, is reported to have reduced efficacy against Benghal dayflower (Bottcher et al. Reference Bottcher, Albrecht, Albrecht, Kashivaqui, Cassol, de Souza, Wagner and Silva2022; Osipe et al. Reference Osipe, Oliveira, Constantin, Takano and Biffe2017), raising questions about whether this can be mitigated by combining glyphosate with increasing doses of dicamba. Given the potential for varying responses to different auxin herbicides and their interactions with glyphosate, this study aimed to evaluate the effectiveness of auxin herbicides, used singly and in combination with glyphosate, and to determine whether higher dicamba doses can overcome its reduced efficacy against Benghal dayflower plants in advanced growth stages.
Material and Methods
Experimental Site and Design
Two field trials were conducted during the 2021 and 2022 growing seasons in no-till agricultural research fields in Palotina, West Paraná State, Brazil: 1) the interaction of glyphosate with auxin herbicides (Trial 1); and 2) a dicamba dose-response study (Trial 2). Experimental areas were naturally infested with glyphosate-tolerant (GT) Benghal dayflower plants that were approximately 1 yr old, with an average height of 50 cm and 100% soil coverage. In southern Brazil, Benghal dayflower behaves as a perennial species, with the ability to grow and reproduce year-round, which allows it to persist and dominate in areas that are left unmanaged. The infestation originated from the previous soybean crop seasons, when the fields were left fallow after harvesting in January 2020 for Year 1 and February 2021 for Year 2. The infestation continued to grow until herbicide application dates (March 24, 2021, for Year 1, and February 4, 2022, for Year 2). Site information, location coordinates, soil properties, weather conditions at the time of treatment, and herbicide application dates are listed in Table 1. Weather data during the experimental period are shown in Figure 1.
Table 1. Site information for field trials conducted in Western Paraná, Brazil, in 2021 and 2022.
a Trial 1: assessment of interaction of glyphosate with auxin herbicides; Trial 2: dicamba dose-response study.
Figure 1. Rainfall indices, and maximum and minimum temperatures during (A) 2021 and (B) 2022 experimental periods, demonstrating the time of spraying and evaluations at 2, 4, and 8 wk after application (WAA) in Western Paraná, Brazil. Source: weather station in Palotina, Paraná, Brazil (24.1790ºS, 53.8379ºW).
Field trials were set up as a randomized complete block with four replicates. Plots were 5 m long and 3 m wide. The auxin herbicides tested in Trial 1 included 2,4-D at 966 g ae ha−1, triclopyr at 720 g ae ha−1, fluroxypyr at 400 g ae ha−1, and dicamba at 720 g ae ha−1, applied singly or with glyphosate at 1,550 g ae ha−1, as detailed in Table 2. In addition, Trial 2 (the dicamba dose-response study) aimed to evaluate dicamba rates of 0, 288, 432, 576, 720, 864, and 1,008 g ae ha−1. Herbicide treatments were applied with a CO2-pressurized backpack sprayer equipped with six 110015 AIXR nozzles (Teejet Technologies South America) spaced 50 cm apart, adjusted to deliver 150 L ha−1 at 245 kPa, at a speed of 3.6 km h−1, producing a spray width of 3.0 m.
Table 2. Herbicides and adjuvants used in field trials between 1 and 8 wk after herbicide application. a–c
a Soybean methylated oil (Mees™; BASF S.A.) was included at a concentration of 0.5% (5 mL L-1) in treatments with dicamba-methyl.
b Soybean methylated oil (Rumba®; Adama S.A.) was included at a concentration of 0.5% (5 mL L-1) in treatments with fluroxypyr-methyl.
c Ethoxylated alkyl ester oil (Lanzar®; Arysta Lifescience) was included at a concentration of 0.5% (5 mL L-1) in treatments with triclopyr-butotyl.
Weed control was evaluated 2, 4, and 8 wk after herbicide application (WAA). For these, scores were assigned by visible control analysis to each experiment using a scale of 0% (no effect) to 100% (death of the weed), based on the comparison of the treated plot with a nontreated control plot (Velini et al. Reference Velini, Osipe and Gazziero1995). At 8 WAA, the Benghal dayflower aboveground biomass was determined by clipping all plants within a random 0.25-m2 quadrant at each plot. Aboveground dry mass was determined by oven-drying at 75 C to constant moisture.
Statistical Analysis
Statistical analysis was conducted using R software (v. 3.6.1; R Core Team 2024). Data were pooled between years after confirming homogeneity of variances using Levene’s test (P > 0.05). The nonsignificant test result indicates that variances were equal between years, suggesting no significant year effect. Therefore, data from different years were combined for analysis. In Trial 1, visible control levels in percentages were transformed to proportions (values between 0 and 1) prior to analysis and the results were then back-transformed and presented on the original percentage scale (0%–100%). A generalized linear mixed model (GLMM) using Template Model Builder (TMB) was then fitted with the glmmTMB package. The emmeans and multcomp packages were used to obtain the back-transformed visible control values from the logit scale, with tests performed on the log odds ratio scale at a significance level of α = 0.05. Herbicides were compared using Fisher’s protected LSD test (P < 0.05). Data were pooled across years based on Levene‘s test, and the DHARMa package was used to create readily interpretable scaled (quantile) residuals for our fitted (generalized) linear mixed models to confirm the statistical assumptions were met. The same procedure was adopted to analyze the dry mass levels of Benghal dayflower at 8 WAA according to the herbicide treatments. The statistical model included herbicide as a fixed effect evaluated weekly, while year and block (with blocks nested within years) were considered as random effects. The coefficient of variation (CV) was calculated for each least square mean to determine the consistency of the control by auxin herbicide and week (Shechtman Reference Shechtman, Doi and Williams2013), then the control consistency by treatments was compared using Fisher’s protected LSD test (P < 0.05).
Furthermore, to investigate the behavior of auxin herbicides to the addition of glyphosate, the Colby equation (Colby Reference Colby1967) was used to compare the expected values with the corresponding observed values using a two-tailed t-test (α = 0.05). Accordingly, the expected control mean was calculated using the observed control means for A (Group 4 herbicide) and B (glyphosate) in the Colby equation.
$${\rm{Expected}} = \;\left( {{\rm{A\; + \;B}}} \right)\; - \;\left( {{{{\rm{A\;}} \times {\rm{B}}}}\over{{100}}} \right)$$
This means that if the observed value of the control is greater or less than the expected value, the interaction is considered synergistic or antagonistic, respectively, and if the expected responses are the same, the combination is additive.
Based on results from Trial 2, the dicamba dose-response study, we estimated the effective dose required for a 50% visible control (ED50) and a 50% reduction in dry mass of Benghal dayflower plants at an advanced growth stage, assessed at 2, 4, and 8 WAA. The selection of a suitable fitting model was based on the Akaike information criterion (AIC) and/or restricted maximum likelihood (REML) at convergence and verification of the fit of the dose-response model through formal significance tests such as ANOVA. According to the AIC and ANOVA results, a three-parameter log-logistic dose-response model (Equation 2), with a lower limit fixed at 0 and an upper limit at 100 from the drc package (Ritz et al. Reference Ritz, Baty, Streibig and Gerhard2015) was used to describe the relationship between dicamba doses (g ae ha−1) and the visible control levels of Benghal dayflower as shown in the following equation:
$$Y = {{{100}}\over{{1 + {\rm{exp}}\left( {b\left( {\log \left( x \right) - e} \right)} \right)}}}$$
where Y is the visible control, x is the dicamba dose;
$e$
is the ED50, and b is the slope of the curve around the inflection point.
For dry mass reduction levels at 8 WAA in response to dicamba doses, a four-parameter log-logistic dose-response model (Equation 3), was used to describe the relationship between dicamba doses (g ae ha−1) and the dry mass reduction of Benghal dayflower, as described by Equation 3:
$$Y = c + {{{d - {\rm{\;}}c}}\over{{1 + {\rm{exp}}\left( {b\left( {\log \left( x \right) - {\rm{log}}(e} \right)} \right))}}}$$
where Y is the dry mass reduction, c is the lower limit, d is the upper limit, x is the dicamba dose,
$e$
is the ED50, and b is the slope of the curve around the inflection point. The ED50 was calculated using the ED function of the drc package.
Results and Discussion
Interaction of Glyphosate with Auxin Herbicides (Trial 1)
Significant interactions (P < 0.05) were observed in visible control (Figure 2) levels and dry biomass (Figure 3) between auxin herbicides, applied with or without glyphosate, over 2, 4, and 8 WAA. However, no complete control of Benghal dayflower plants was achieved at an advanced growth stage by any of the treatments.
Figure 2. Visible control of auxin herbicides mixed with glyphosate or not mixed with glyphosate against glyphosate-tolerant Benghal dayflower plants at an advanced growth stage at 2, 4, and 8 wk after application (WAA) in Western Paraná, Brazil, in 2021 and 2022, evaluated by a generalized linear mixed model. Data were pooled between years according to Levene’s test for homogeneity of variance, P = 0.894. Means followed by the same letter within weeks are not significantly different according to Fisher’s protected LSD test (P < 0.05). The red dashed line represents the mean visible control of herbicide treatments excluding the nontreated control.
Figure 3. Dry mass reduction according to auxin herbicides mixed or not mixed with glyphosate against glyphosate-tolerant Benghal dayflower plants at an advanced growth stage at 8 wk after application (WAA) in Western Paraná, Brazil, in 2021 and 2022, evaluated by a generalized linear mixed model. Data were pooled between years according to Levene’s test for homogeneity of variance. P = 0.987. Means followed by the same letter within weeks are not significantly different according to Fisher’s protected LSD test (α = 0.05). The red dashed line represents the mean dry mass of herbicide treatments excluding the nontreated control.
We assessed the visible control consistency by calculating the coefficient of variation (CV) (Shechtman Reference Shechtman, Doi and Williams2013). Each mean least squares CV was applied to reflect the consistency in Benghal dayflower control. Following the approach reported in previous studies, lower CV values were interpreted as indicators of more consistent control outcomes (Dilliott et al. Reference Dilliott, Soltani, Robinson, Hooker and Sikkema2021, Reference Dilliott, Soltani, Hooker, Robinson and Sikkema2022a, Dilliott et al., Reference Dilliott, Soltani, Hooker, Robinson and Sikkema2022b). The control consistency improved when glyphosate at 1,550 g ae ha−1 was mixed with triclopyr or fluroxypyr at 2 and 4 WAA, but not 8 WAA (Table 3). Conversely, when glyphosate was mixed with 2,4-D, control was not affected at 2 and 4 WAA. However, at 8 WAA, the mixture led to more control compared to 2,4-D applied alone. While adding glyphosate to dicamba improved control consistency at 2 WAA, it had no effect at 4 WAA and decreased at 8 WAA.
Table 3. Control consistency of glyphosate-tolerant Benghal dayflower at advanced growth stages at 2, 4, and 8 wk after herbicide application with or without glyphosate. a–d
a Abbreviations: EPSPS, 5-enolpyruvylshikimate-3-phosphate synthase; GT, glyphosate-tolerant; WAA, weeks after application.
b Soybean methylated oil (Mees™; BASF S.A.) (0.5% v/v) was included in treatments with dicamba-methyl.
c Soybean methylated oil (Rumba®; Adama S.A.) (0.5% v/v) was included in treatments with fluroxypyr-methyl.
d Ethoxylated alkyl ester oil (Lanzar®: Arysta Lifescience) (0.5% v/v) was included in treatments with triclopyr-butotyl.
e Data were pooled between years. Consistency of variation values followed by the same letter within weeks are not significantly different according to Fisher’s protected LSD test (P < 0.05).
At 2 WAA, visible control levels ranged from 55% to 74% (Figure 2). Adding glyphosate to all auxin herbicides resulted in ≥7% control. Even when Fisher’s protected LSD test (P < 0.05) detected statistical differences between auxin herbicides applied alone or in combination with glyphosate, these differences were not sufficient to characterize a synergistic effect since a Colby’s t-test (P < 0.05) did not reveal a synergistic interaction with any of the auxin herbicides evaluated here (Table 4). The herbicide interactions varied according to the auxin herbicide and application timing. Adding glyphosate to dicamba, 2,4-D, and fluroxypyr initially showed (at 2 WAA) additive effects, which means that adding glyphosate did not increase or decrease the efficacy of those herbicides. However, all herbicides tested transitioned to antagonism between 4 and 8 WAA. In addition, adding glyphosate to triclopyr resulted in a consistently antagonistic effect during all weeks evaluated. These findings highlight the dynamic of herbicide interactions on Benghal dayflower plants at advanced growth stages, which can be related to the species’ tolerance to glyphosate.
Table 4. Auxin herbicide interactions when mixed with glyphosate according to the Colby test, t-test (α = 0.05), and percent control of glyphosate-tolerant Benghal dayflower from 1 to 8 wk after herbicide application.a–d
Abbreviations: Exp, expected; GT, glyphosate-tolerant; Obs, observed; WAA, weeks after application.
a Soybean methylated oil (Mees™; BASF S.A.) (0.5% v/v) was included in treatments with dicamba-methyl.
b Soybean methylated oil (Rumba®; Adama S.A.) (0.5% v/v) was included in treatments with fluroxypyr-methyl.
c Ethoxylated alkyl ester oil (Lanzar®; Arysta Lifescience) (0.5% v/v) was included in treatments with triclopyr-butotyl.
d Observed visible control values (Obs) were used to calculate the expected control values (Exp) between the Colby formula 1) where X represents glyphosate and Y represents auxin herbicide. The relationship between Exp and Obs is presented in the Colby column, where Colby values >1 indicate synergism and Colby values <1 indicate antagonism when supported by α-values (<0.05) from the t-test. α-valuesns indicates an additive effect, whereas asterisks (*) indicate α < 0.05.
A similar effect was observed by Martins et al. (Reference Martins, Carneiro Santana, Sasso, Santana, de Souza and Bagatta2012) who applied auxin herbicides with or without glyphosate to Benghal dayflower that were 25 to 35 cm tall, in which the addition of glyphosate to 2,4-D did not enhance its efficacy compared to using 2,4-D alone. Auxin herbicide mixes with glyphosate were also tested by Freitas et al. (Reference Freitas, Freitas, Furtado, Teixeira and Silva2018) against Benghal dayflower that was 25 cm tall, and found an antagonistic effect with the mixture of glyphosate + 2.4-D at 4 WAA, achieving 60% control with 2.4-D alone and 76.6% control when glyphosate was added, while an 86% control was expected for the mixture based on the Colby test.
The greatest visible control level during the experimental period, reaching approximately 89%, was achieved at 4 WAA with triclopyr applied either alone or in combination with glyphosate, fluroxypyr alone, and 2,4-D in combination with glyphosate (Figure 2). These values were the highest observed across all herbicide treatments. Curiously, 2,4-D combined with glyphosate (which provided 89% control) exhibited greater control (78%) than 2,4-D applied alone; however, the Colby test indicated an additive effect (α = 0.416ns) (Table 4). In contrast, fluroxypyr performed better when it was applied alone, with 89% control, than combined with glyphosate, which provided 84% control, revealing an antagonistic effect (α = 0.001) according to the Colby test (Table 4). Dicamba, whether applied alone or in combination with glyphosate, provided 65% and 71% visual control, respectively, at 4 WAA (Figure 2), which raises questions about the suitability of using dicamba for Benghal dayflower control in a scenario in which plants have surpassed the ideal growth stage for achieving effective control. Furthermore, all herbicides showed decreases in their control levels after 4 WAA (Figure 2), suggesting that a sequential herbicide application could be effective since plants are more vulnerable.
Our results demonstrate the challenges in controlling year-old Benghal dayflower plants. Osipe et al. (Reference Osipe, Oliveira, Constantin, Takano and Biffe2017) studied younger Benghal dayflower plants, at the 4- to 6-leaf and 10- to 12-leaf stages, and reported 60% control with a C80 >1,008 g ae ha−1 when dicamba was applied alone (C80 represents the herbicide dose required to achieve 80% control) at the 10- to 12-leaf stage, whereas 100% control was achieved (C80 > 349.4 g ae ha−1 when 2,4-D was applied. The greater effectiveness of 2,4-D on larger plants may be attributed to the higher sensitivity of Benghal dayflower mitochondria to this herbicide than others (Duke Reference Duke1985). Additionally, Takano et al. (Reference Takano, Oliveira, Constantin, Biffe, Franchini, Braz, Rios, Gheno and Gemelli2013) observed that applying 670 g ae ha−1 of 2,4-D resulted in 28% control of Benghal dayflower at stages when the plant had 10 leaves compared with 74% control of plants with just 4 to 6 leaves and 98% of plants at the 2- to 4-leaf stage.
Benghal dayflower at advanced growth stages exhibited varying responses to auxin herbicides. The best visible control (79%) at 8 WAA (Figure 2) was achieved with triclopyr applied alone followed by 77% control with 2,4-D combined with glyphosate, and 74% with triclopyr combined with glyphosate. No differences were detected between fluroxypyr applied alone or combined with glyphosate (71% and 69%, respectively) and 2,4-D applied alone (69%). Dicamba provided the lowest level of control, even when combined with glyphosate (56% and 51%, respectively).
Herbicides that achieved higher levels of visible control (Figure 2) also led to greater reductions in dry mass (r = −0.856, P < 0.05). Triclopyr and 2,4-D either with glyphosate (105 g, 106 g) or without glyphosate (103 g, 109 g) achieved the most significant mass reductions (Figure 3) compared with nontreated control plants (246 g). Fluroxypyr, when applied with glyphosate (116 g) or without it (115 g), had a lesser impact on biomass reduction. Dicamba-based treatments resulted in the lowest reduction in dry mass (173 g) even when combined with glyphosate (169 g).
Dicamba Dose-Response (Trial 2)
While significant interactions between dicamba doses and WAA were observed for visible control and dry mass (Tables 5 and 6), we found that dicamba was unable to completely control this weed species at advanced growth stages, even at high doses. The maximum visible control observed at 4 WAA was 71% with dicamba at 1,008 g ae ha−1, representing only a 5% increase over the highest labeled dose (66% at 720 g ae ha−1). This marginal improvement suggests that increasing dicamba rates beyond 720 g ae ha−1 provided limited additional control (Figures 4 and 5). The estimated ED50 values at 2 and 8 WAA were similar (507.94 ± 25 and 519.79 ± 19 g ae ha−1, respectively), indicating a slow initial control followed by weed recovery, leading to a maximum control of only about 60% at 8 WAA. In contrast, at 4 WAA, a significantly lower ED50 of 214.84 g ae ha−1 of dicamba was required to achieve 50% control of Benghal dayflower. The predicted dose to reduce dry mass levels at 8 WAA by at least 50% was 606 (±33.2) g ae ha−1 of dicamba (Figure 5). Moreover, the ED50 values from visible control levels at 2 and 8 WAA, as well as from dry mass reduction, fall within the recommended label dose range (432 to 720 g ae ha−1) for controlling Benghal dayflower at the appropriate growth stage by up to 80% (up to the 6-leaf stage or 10 cm), which highlights the challenge of managing Benghal dayflower at advanced growth stages as described in this study.
Table 5. Parameters for the nonlinear three-parameter log-logistic regression used to describe visible control levels of glyphosate-tolerant Benghal dayflower at at 2, 4, and 8 wk after herbicide application. a
a Abbreviation: parameter b, slope of the curve around the inflection point (see Equation 2); ED50, the effective dose that causes 50% mortality of Benghal dayflower plants; SE, standard error; WAA, weeks after application.
b Estimated ED50 values followed by different letters indicate significant differences according to Tukey’s HSD test (P < 0.05).
c Asterisks (***) indicate P < 0.001 by model’s ANOVA. Data were pooled between years based on Levene’s test for homogeneity of variance, P = 0.974; verification of the fit of the dose-response model through formal significance tests (ANOVA), P-value (0.125).
Table 6. Parameters for the nonlinear four-parameter log-logistic regression used to estimate dry mass content of glyphosate-tolerant Benghal dayflower at 8 wk after herbicide application.
a Abbreviations: b, slope of the curve around the inflection point; c, lower limit; d, upper limit (see Equation 3); ED50, herbicide dose required for 50% Benghal dayflower control; SE, standard error.
b Asterisks indicate P-values: ***, P < 0.001; *, P < 0.05 by model’s ANOVA. Data were pooled between years according to Levene’s test for homogeneity of variance P = 0.387; verification of the fit of the dose-response model through formal significance tests (ANOVA), P = 0.731.
Figure 4. Estimated dicamba dose response for glyphosate-tolerant Benghal dayflower plants at an advanced growth stage at (A) 2 wk after application (WAA), (B) 4 WAA, and (C) 8 WAA evaluated as described by Equation [2] and estimated effective dose for a 50% visible control of the weed.
Figure 5. Estimated dicamba dose response for glyphosate-tolerant Benghal dayflower plants at an advanced growth stage at 8 wk after application evaluated as described by Equation [3] and estimated effective dose for a 50% dry mass reduction.
Similarly, Osipe et al. (Reference Osipe, Oliveira, Constantin, Takano and Biffe2017) analyzed five doses of dicamba between 202 and 1,008 g ae ha−1 to gauge their efficacy against two stages of Benghal dayflower. They achieved no maximum control with the tested doses, concluding that for larger plants (with 10 to 12 leaves), dicamba was not effective regardless of the acid equivalent dose tested. Bottcher et al. (Reference Bottcher, Albrecht, Albrecht, Kashivaqui, Cassol, de Souza, Wagner and Silva2022) also reported low control levels of Benghal dayflower when dicamba at doses between 144 and 576 g ae ha−1 were applied. Bottcher et al. (Reference Bottcher, Albrecht, Albrecht, Kashivaqui, Cassol, de Souza, Wagner and Silva2022) initially observed control levels of 45% and 51% at 1 and 2 WAA respectively, and losses of performance after 4 WAA until the 6 WAA, with maximum control of 43% at the highest dose, with no feasibility of increasing the dose to obtain greater control. Although auxin herbicides induce similar plant responses, they are members of different chemical families such as arylpicolinates (triclopyr and fluroxypyr), phenoxycarboxylates (2,4-D), and benzoates (dicamba). These herbicides can act through different mechanisms at distinct sites of action. Key proteins involved in auxin transport, such as the transport inhibitor response (TIR1) protein, exhibit varying levels of tolerance to different auxin chemical families. Additionally, mutations in receptor proteins such as AFB5, which are homologous to TIR1, confer specific resistance to dicamba but not to 2,4-D (Gleason et al. Reference Gleason, Foley and Singh2011). These findings may help explain why our dicamba dose-response study suggests that the limitations of dicamba in controlling Benghal dayflower cannot be mitigated through a dose increase.
Practical Implications
Auxinic herbicides rank as the second most-used herbicide group in Brazil, with their use expanding primarily in pre-plant burndown applications for summer crops under no-tillage systems and in pasture management, and these herbicides often are combined with glyphosate. Benghal dayflower infests 41% of fields in Brazil, ranking among the country’s top five most-troublesome weeds, with control becoming difficult when the plant is 10 cm tall. Benghal dayflower is highly dispersed in warm regions, and is the second most-important species in Western Paraná. This study highlights challenging scenarios in which Benghal dayflower plants have surpassed the ideal growth stage for effective control, as may be found during off-season burndown applications, and in pastures and perennial crop fields. We found that Benghal dayflower exhibited variable responses to auxin herbicides, that it was influenced by both the chemical family and the herbicide molecule, reinforcing the idea that not all auxinic herbicides exhibit the same control efficacy. By demonstrating the challenges in controlling Benghal dayflower plants at an advanced growth stage, our results underscore that early-stage control remains the most effective strategy. Farmers must control weeds when they are still small to achieve effective control and resistance management. Producers must also carefully assess which auxin herbicides to use and when glyphosate mixtures are necessary in scenarios where weeds have surpassed the ideal growth stage. In this scenario, triclopyr (applied alone) and 2,4-D (combined with glyphosate), and fluroxypyr (applied alone or combined with glyphosate) are the best options for producers to achieve the best visible control levels and dry mass reductions. Adding glyphosate at 1,550 g ae ha−1 did not always enhance control efficacy, nor did it result in synergistic interactions with any of the auxin herbicides evaluated. Dicamba-based treatments (alone or combined with glyphosate) provided the least control and dry mass reduction. Additionally, dicamba doses up to 1,008 g ae ha−1 did not provide complete control, suggesting that the limitations of dicamba against Benghal dayflower cannot be mitigated through a dose increase.
Acknowledgments
We are grateful for support provided by the Federal University of Paraná (UFPR), C. Vale Agroindustrial Cooperative.
Funding statement
Author W.F. Larini received financial support from CAPES (Brazilian Coordination for the Improvement of Higher Education Personnel).
Competing interests
The authors declare they have no competing interests.
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