Introduction
Yellow toadflax is an invasive perennial broadleaf weed that negatively affects forage production and wildlife habitat. The weed can spread aggressively via rhizomes and seeds, and therefore warrants management (Mitich Reference Mitich1993; Sutton et al. Reference Sutton, Stohlgren and Beck2007; Ward et al. Reference Ward, Reid, Harrington, Sutton and Beck2008). Yellow toadflax plants germinate in the spring and can grow to height of 0.3 to 1 meter at maturity, and can overgrow desirable vegetation that has been grazed or hayed (Beck Reference Beck2014; Begay et al. Reference Begay, Alexander and Questad2011; Blatt et al. Reference Blatt, De Clerck-Floate and White2022; Noy-Meir Reference Noy-Meir1993). Tactics for managing yellow toadflax are few. Mechanical tactics can suppress yellow toadflax, but tillage is rarely implemented in pasture and rangeland. Mowing does not deplete root carbohydrates or seeds in the soil and is thus not a good management option (Lajuenesse Reference Lajeunesse, Sheley and Petroff1999; Morishita Reference Morishita, James, Evans, Ralphs and Child1991). Herbicide options are limited. Chlorsulfuron (a herbicide that inhibits acetolactate synthase, categorized as a Group 2 herbicide by the Herbicide-Resistance Action Committee [HRAC] and Weed Science Society of America [WSSA]) and picloram (a synthetic auxin herbicide; HRAC/WSSA Group 4) can be used to manage yellow toadflax in the short term but follow-up applications are frequently required (Almquist et al. Reference Almquist, Wirt, Adams and Lym2015; Johnson et al. Reference Johnson, Grovenburg, Perkins, Jenks, Inselman and Swanson2014). The appropriate time to apply herbicides to yellow toadflax occurs when flowering plants are likely at their maximum height (Almquist et al. Reference Almquist, Wirt, Adams and Lym2015; Beck Reference Beck2014).
Glyphosate (HRAC/WSSA Group 9) applied via broadcasting has been found to significantly reduce yellow toadflax stands in preharvest wheat fields in the following growing season (Baig et al. Reference Baig, Darwent, Harker and O’Donovan1999). While broadcast-applied glyphosate is effective, this application method is not commonly used in pasture and rangeland because desirable vegetation is killed (Lym and Kirby Reference Lym and Kirby1991; Sebastian et al. Reference Sebastian, Sebastian, Nissen and Beck2016). Applying herbicides with a wiper allows the herbicide to directly contact weeds that grow taller than the desirable vegetation. A wiper applicator uses an absorbent vessel (i.e., a cotton rope or sponge) that is saturated with a herbicide. The wiper applicator is affixed to a tractor (or similar vehicle) and set to a height that will contact the weeds that grow taller than the desirable vegetation (Harrington and Ghanizadeh Reference Harrington and Ghanizadeh2017). Glyphosate has been wiper-applied to pasture and rangeland to manage some perennial weeds with success (Dias et al. Reference Dias, Mncube, Sellers, Ferrell, Enloe, Vendramini and Moriel2024; Grekul et al. Reference Grekul, Cole and Bork2003; Krueger-Mangold et al. Reference Krueger-Mangold, Sheley and Roos2002).
Yellow toadflax can grow above desirable vegetation, thus presenting the opportunity to use a wiper to apply herbicides (Carder Reference Carder1963; Harrington and Ghanizadeh Reference Harrington and Ghanizadeh2017). Yellow toadflax treated with wiper-applied glyphosate has previously exhibited injury under greenhouse conditions, but reproductive and vegetative regrowth quantification has not been evaluated (Jones et al. Reference Jones, Alms and Vos2024). Because yellow toadflax reproduces both sexually and asexually, quantifying its vegetative regrowth and flowering after herbicide application is important for determining the persistence of treated plants. Previous research reported that yellow toadflax seedlings and ramets (plants produced from root pieces) exhibited differential growth, with shoot growth not differing between plant origin, but underground growth being greater in ramets than in seedlings (Nadeau et al. Reference Nadeau, King and Harker1992). That research also suggested that yellow toadflax plants grown from seed may be more susceptible to management tactics than plants that are vegetatively propagated (Nadeau et al. Reference Nadeau, King and Harker1992). No information exists on managing yellow toadflax with wiper-applied glyphosate followed by other broadcast-applied herbicides. Therefore, we sought to quantify yellow toadflax regrowth after wiper-applied glyphosate, followed by broadcast applications of chlorsulfuron or picloram on plants grown from seed and clonally propagated, to determine the effectiveness of herbicide applications across stands of mixed plant origins.
Materials and Methods
Plant Establishment
Yellow toadflax seeds were collected from rangeland in Edmunds County, South Dakota, in 2009, and stored in a freezer at −17 C until the experiments began. Seeds were sown into pots containing potting media (Miracle-Gro; Scotts Company, Marysville, OH) at the beginning of October 2024. Seeded pots were maintained at a constant 25 C with a 12-h photoperiod supplemented with LED light (12 lm m−2). Plants were topically watered to saturation weekly. Individual plants were transplanted into 20-cm-diam (668 cm3) pots containing the potting media described above, once they reached approximately 7 cm in height, approximately 1 mo after planting. The transplants were moved from a growing room to the greenhouse with a diurnal temperature of 29/18 C and a 16-h photoperiod supplemented with metal halide lights (600–1,000 mol m−2 s−1 photosynthetic photon flux density).
Clones were produced from established plants to determine whether susceptibility differed between clonal propagations and plants grown from seed, approximately 2 mo after planting on 20-cm-tall plants. Clones were produced by cutting axillary stems from the plants, submerging them in water for approximately 30 s to prevent xylem cavitation, dipping in a rooting hormone (indole-3-butyric acid; 0.1% concentration; Garden Safe Take-Root, Spectrum Brands, Middleton, WI), and transplanting them into separate 20-cm-diam pots. Plants were watered from above to saturation every other day for the entirety of the study. Plants began to flower in late February 2025.
Treatments
Treatments were designed as a two-way factorial in a completely randomized setup with three replications, and the experiment was conducted twice, a week apart. The total duration of the experiment lasted until 4 mo after treatment (MAT), which included the initial 3 MAT with an additional 1 MAT for regrowth. The factors included plant origin (seed or clone) and herbicide treatment (Table 1). A nontreated control was included for comparison. The frame of the wiper applicator was constructed with 1.9-cm polyvinyl chloride pipes with two 1.6-cm-diam cotton ropes (the wiper surface was approximately 2.5 cm wide and 18 cm long) affixed to the end of the frame (Figure 1). Wiper applications of glyphosate (Roundup Powermax 3, 575 g ae L−1; Bayer CropScience, St. Louis, MO) included treatments of 0% (no herbicide) and 50% concentration (Bayer 2020). The 50% solution of glyphosate was obtained by diluting the herbicide concentrate with distilled water. The wiper frames were disassembled before treatment, and the cotton rope of the wiper was submerged in a 300-mL solution (150 mL of distilled water and 150 mL of glyphosate concentrate) of the respective herbicide concentrations until it was saturated. Yellow toadflax plants were treated when they were approximately 40 cm tall, which coincided with the reproductive stage, when each plant exhibited flowers or had visible flower buds. The wiper applicator was positioned approximately halfway up the plant (20 cm) to simulate an application of herbicide above the desirable vegetation growth height. The glyphosate label states that the wiper should be positioned 5 cm above desirable vegetation (Bayer 2020). The upper portion of the plant was treated until it was wet.
Table 1. Visual estimates of yellow toadflax injury with wiper-applied glyphosate, chlorsulfuron, or picloram at 1, 2, and 3 mo after treatment a–c .
a Abbreviations: fb, followed by; MAT, months after treatment.
b Data are pooled over plant origin and two experimental runs for 1 mo after treatment. Data are pooled over experimental runs and analyzed by plant origin for each herbicide treatment for 2 and 3 mo after treatment.
c Means that share the same letter within columns are not statistically different based on Fisher’s LSD (P < 0.05).
d Glyphosate was wiper-applied at 50% concentration. Chlorsulfuron (26 g ai ha−1) or picloram (560 g ae ha−1) was broadcast-applied.
Figure 1. Schematic for the wiper. Glyphosate was delivered via the cotton rope that wipes against the yellow toadflax plant, suspended between two polyvinyl chloride posts.
Chlorsulfuron (Telar XP, 75% ai; Environmental Science U.S., Cary, NC) or picloram (Tordon, 240 g ae L−1; Corteva Agriscience, Indianapolis, IN) were applied immediately after the plants had been wiped with glyphosate, which mimics a typical field application. Herbicides were applied using a spray chamber (EDA, Folsom, CA) with an output of 187 L ha−1 and 193 kPa pressure using a single TeeJet XR5001 nozzle (TeeJet Technologies, Glendale Heights, IL) positioned 50 cm above the plants. Chlorsulfuron (26 g ai ha−1) was applied with crop coil concentrate (Maximizer, 10 mL L− 1; Loveland Products, Greeley, CO), and picloram was applied at 560 g ae ha−1.
Visual injury evaluations occurred 1, 2, and 3 MAT using a scale of 0% to 100%; where 0% equals no injury observed and 100% equals plant death. At 3 MAT, aboveground yellow toadflax biomass was harvested by excising the plant stem at the potting media surface, drying the samples at 50 C for 48 h, and weighing them. After removing the aboveground biomass, pots were left unwatered for 1 wk, and irrigation was resumed thereafter every other day for 1 mo. Regrowth biomass was collected and quantified as described above. Visual evaluations were conducted before regrowth biomass sampling to determine whether plants were flowering; the evaluation used a binomial scale, with 0 indicating no flowering and 1 indicating flowering. After regrowth sampling, pots remained in the greenhouse without water for approximately 1 wk, and roots were then extracted and cleaned from the dried potting media. Roots were then dried and weighed as described above. After the injury and flowering evaluations, dry biomass reduction for the 3 MAT biomass, 4 MAT biomass regrowth, and roots at 4 MAT were calculated by dividing the dry biomass of the treated plants by the dry biomass of the nontreated plants.
Statistical Analysis
Visual injury estimates, flowering evaluations, and biomass (aboveground and root) data from experiments were subjected to ANOVA using the Glimmix procedure with SAS software (v.9.4; Statistical Analysis System, Cary, NC) at a significance level of α = 0.05. Plant origin, herbicide treatment and their interactions were considered fixed effects, while replication and experimental runs were considered random effects to allow inferences to be made across broader conditions (Blouin et al. Reference Blouin, Webster and Bond2011; Moore and Dixon Reference Moore and Dixon2015; Nadeau et al. Reference Nadeau, King and Harker1992). Treatment means from the experiments were separated using Fisher’s LSD (P ≤ 0.05). The visual injury estimates were analyzed at three separate timepoints: 1, 2, and 3 MAT. The visual injury estimates and biomass reductions from the nontreated plants (i.e., 0% or 100%) were removed from the analysis to avoid violation of constant variance assumptions required for ANOVA. Biomass data of treated plants were normalized relative to the nontreated plants, which were assigned 100%.
Wiper-applied glyphosate followed by (fb) a broadcast application of chlorsulfuron or picloram was evaluated to determine whether the effects were additive, antagonistic, or synergistic for injury estimates, treated-biomass reduction, biomass regrowth, and root biomass using Colby’s method (Colby Reference Colby1967). The Colby method is used to calculate an expected response value for a herbicide treatment based on the control of the individual herbicides, and the expected response value is compared with the response of the tested herbicide treatment. Wiper-applied glyphosate fb applications of chlorsulfuron or picloram were analyzed using the Colby equation:
$$E = \left( {X + Y} \right) - \left( {{xy}\over{100}} \right)$$
where E is the expected percent of response of wiper-applied glyphosate fb chlorsulfuron or picloram, X is the percent response of wiper-applied glyphosate alone, and Y is the percent response from chlorsulfuron or picloram alone. The expected response was compared with the observed response using a two-sided t-test (α = 0.05). A significantly greater observed response indicated synergism, a significantly lower response indicated antagonism, and no significant difference indicated an additive effect (Colby Reference Colby1967). While the Colby method is used for herbicide mixtures and not sequential applications, this method was selected because the wiper and broadcast applications occurred with minimal time interval between the applications.
Results and Discussion
Injury Estimates
Plant origin did not affect yellow toadflax injury when evaluated at 1 MAT (P = 0.2) but it did at 2 MAT (P = 0.03) and 3 MAT (P = 0.004). Clonal propagations incurred injury (75%), 10% greater than plants grown from seed, when averaged across herbicide treatments (65% to 68%) at both 2 MAT and 3 MAT (data not shown). Herbicide treatment affected yellow toadflax injury (P < 0.0001) at 1, 2, and 3 MAT. The interaction was not significant at 1 MAT (P = 0.59); therefore, injury data were pooled across plant origin and analyzed by herbicide treatment. The interaction was significant at 2 MAT (P = 0.18) and 3 MAT (P = 0.08), but data were analyzed by plant origin for each herbicide treatment.
Chlorsulfuron or picloram applied alone caused the least injury at 1 and 2 MAT, respectively (Table 1). Picloram-treated clonal yellow toadflax plants incurred greater injury than chlorsulfuron-treated plants at 3 MAT (Table 1), although at 3 MAT, seed-grown yellow toadflax plants incurred similar injury with chlorsulfuron and glyphosate fb picloram (Table 1). Wiper-applied glyphosate treatments (alone or fb a broadcast-applied herbicide) consistently caused the greatest injury across all evaluation time points (Table 1). Injury to yellow toadflax at 3 MAT from glyphosate fb chlorsulfuron or picloram was additive when averaged across plant origin (Table 2).
Table 2. Expected and observed visual injury estimates of yellow toadflax, and reductions in treated biomass, biomass regrowth, and root biomass with wiper-applied glyphosate followed by applications of chlorsulfuron or picloram a,b .
a Abbreviation: fb, followed by.
b Data were calculated using the Colby method (Colby Reference Colby1967).
x Glyphosate was wiper-applied at 50% concentration. Chlorsulfuron (26 g ai ha−1) or picloram (560 g ae ha−1) was broadcast-applied.
Treated Biomass
Plant origin influenced the treated biomass (P = 0.001). Herbicide treatment had no effect (P = 0.08), nor was the interaction between the two main effects significant (P = 0.61). Therefore, treated biomass values were pooled across treatments and analyzed by plant origin. The clonal propagation biomass was reduced more (57%) than it was in plants from seed (30%), averaged across herbicide treatments (data not shown). While no differences in treated biomass reductions were detected between herbicide treatments, glyphosate fb chlorsulfuron or picloram additively reduced treated biomass when averaged across plant origin (Figure 2; Tables 2 and 3).
Figure 2. Visual representation of yellow toadflax vegetative regrowth 4 mo after treatment. A) 1, nontreated; 2, wiper-applied glyphosate; 3, chlorsulfuron; 4, wiper-applied glyphosate followed by chlorsulfuron. B) 1, nontreated; 2, wiper-applied glyphosate; 3, picloram; 4, wiper-applied glyphosate followed by picloram.
Table 3. Reductions in treated, regrowth, and root biomass yellow toadflax in response to chlorsulfuron, picloram, and wiper-applied glyphosate 3 mo after treatment a–c .
a Abbreviation: fb, followed by.
b Data are pooled over plant origin and two experimental runs.
c Means that share the same letter within columns are not statistically different based on Fisher’s LSD (P < 0.05).
d Glyphosate was wiper-applied at 50% concentration. Chlorsulfuron (26 g ai ha−1) or picloram (560 g ae ha−1) was broadcast applied.
Flowering
Herbicide treatment influenced the flowering on vegetative regrowth (P < 0.001), while the plant origin (P = 0.12) and interaction between the two main effects (P = 0.80) did not affect flowering data. Therefore, flowering data were pooled across plant origin and analyzed by treatment. Chlorsulfuron treatment resulted in greater flowering (50%) on vegetative regrowth than the other herbicide treatments (8% to 17%) (Table 4). Nontreated plants exhibited the greatest flowering on vegetative regrowth (83%) as expected (Table 4).
Table 4. Yellow toadflax regrowth flowering in response to chlorsulfuron, picloram, and wiper-applied glyphosate 4 mo after treatment a–c .
a Abbreviation: fb, followed by.
b Data are pooled over plant origin and two experimental runs.
c Means that share the same letter within columns are not statistically different based on Fisher’s LSD (P < 0.05).
d Glyphosate was wiper-applied at 50% concentration. Chlorsulfuron (26 g ai ha−1) or picloram (560 g ae ha−1) was broadcast applied.
Biomass Regrowth
Treatment influenced biomass (P = 0.006), while plant origin (P = 0.14) and the interaction between the two main effects (P = 0.70) had no effect; therefore, regrowth biomass data were pooled across plant origin and analyzed by treatment. Chlorsulfuron caused the least biomass reduction and the biomass reduction was less than that of picloram (Table 3). The greatest reduction in biomass regrowth occurred with wiper-applied glyphosate fb chlorsulfuron but neither was that much different from wiper-applied glyphosate alone or wiper-applied glyphosate fb picloram (Table 3). Biomass regrowth was similar between picloram and wiper-applied glyphosate and wiper-applied glyphosate fb picloram (Table 3). Glyphosate fb chlorsulfuron or picloram additively reduced biomass regrowth (Table 2).
Root Growth
The main effects of herbicide treatment (P = 0.15), plant origin (P = 0.55), and their interaction (P = 0.86) did not affect root biomass, and root biomass reductions ranged from 10% to 83% when averaged across treatments (Table 3). Although no differences were detected, Colby’s method indicates that glyphosate fb chlorsulfuron, or picloram additively reduced root biomass (Figure 3; Table 2).
Figure 3. Visual representation of yellow toadflax root biomass 4 m after treatment. A) 1, nontreated; 2, wiper-applied glyphosate; 3, chlorsulfuron; 4, wiper-applied glyphosate followed by chlorsulfuron. B) 1, nontreated; 2, wiper-applied glyphosate; 3, picloram; 4, wiper-applied glyphosate followed by picloram.
The results of this experiment provide support that wiper-applied glyphosate fb chlorsulfuron, or an application of picloram improves yellow toadflax management, whether plants originate from seed or clones. These results contradict the hypothesis posed by Nadeau et al. (Reference Nadeau, King and Harker1992) that yellow toadflax grown from seed may be more susceptible to herbicides than plants that originate from ramets, and they demonstrate that clonal propagations were more susceptible to herbicide treatments. Despite differential susceptibility across plant origins, the lack of interactions between plant origin and herbicide treatments suggests that the herbicide treatments we tested are equally effective, indicating that all herbicide treatments were additive (Table 2). Previous research reported similar injury estimates with wiper-applied glyphosate but it did not investigate vegetative regrowth (Jones et al. Reference Jones, Alms and Vos2024). While yellow toadflax incurred similar injury in this study, the vegetative regrowth had reduced flowering and biomass, suggesting that yellow toadflax is likely to have less reproductive potential when subjected to these treatments compared with broadcast applications of chlorsulfuron or picloram alone.
Decreased biomass and population density have been documented in other perennial weed species when treated with wiper-applied glyphosate compared with broadcast applications of glyphosate and other herbicides (Fryman Reference Fryman2009; Krueger-Mangold et al. Reference Krueger-Mangold, Sheley and Roos2002). Decreased regrowth was observed with leafy spurge (Euphorbia esula L.) when glyphosate was wiper-applied after a broadcast application of 2,4-D compared with 2,4-D broadcast-applied alone (Jones et al. Reference Jones, Alms and Vos2025). Previous research has also demonstrated that applying multiple herbicides improves yellow toadflax management compared with single herbicides (Almquist et al. Reference Almquist, Wirt, Adams and Lym2015; Johnson et al. Reference Johnson, Grovenburg, Perkins, Jenks, Inselman and Swanson2014). Therefore, wiper-applied glyphosate fb an application of chlorsulfuron or picloram for managing yellow toadflax is recommended. The herbicide to include in the broadcast application depends on the other weed species present. In addition, the broadcast application will be effective in managing most weeds (including yellow toadflax) growing within the desirable vegetation.
Herbicide mixtures (i.e., 2,4-D + picloram) should be evaluated for their effects following wiper-applied glyphosate to determine whether the effectiveness of wiper application is further increased (Agbakoba and Goodin Reference Agbakoba and Goodin1970; Sebastian et al. Reference Sebastian, Nissen, Sebastian, Meinman and Beck2017). Various glyphosate concentrations (i.e., 33% to 100%) and additional herbicide rates (i.e., picloram applied at 560 vs. 1,120 g ae ha−1) should be tested in combinations of sequential applications to determine the most effective concentration and rate combination. Future field research should also be conducted to assess how environmental conditions will affect wiper-applied glyphosate efficacy and its effects on desirable vegetation. The effectiveness of wiper-applied glyphosate fb a broadcast application of another herbicide had not been investigated on yellow toadflax before, but the utility of this treatment is evident. Our research used a herbicide (glyphosate) that is not commonly applied to pastures and rangelands, thus the herbicide treatments, including wiper-applied glyphosate, could disrupt selection pressure imposed on yellow toadflax by recurrent applications of chlorsulfuron and/or picloram (Almquist et al. Reference Almquist, Wirt, Adams and Lym2015; Ward et al. Reference Ward, Reid, Harrington, Sutton and Beck2008). However, we caution against the recurrent use of these herbicide combinations because this may result in increased selection pressure (Darmency Reference Darmency2018).
Practical Implications
Yellow toadflax is a perennial weed that is difficult to manage despite extensive efforts. Wiper-applied glyphosate fb an application of chlorsulfuron or picloram resulted in the greatest injury and reductions in flowering and biomass regrowth compared with single herbicide treatments. Since few effective broadcast herbicide treatments are available for controlling yellow toadflax, wiper-applied glyphosate fb a broadcast-applied herbicide can improve our ability to manage it. Based on the results of this research, glyphosate fb chlorsulfuron or picloram were equally effective; thus, choosing which broadcast herbicide to use should be based on other weed species present. While these herbicide treatments improve yellow toadflax management, overreliance should be avoided to reduce selection pressure.
Acknowledgments
We thank Micheal D. K. Owen for reviewing the manuscript prior to submission.
Funding statement
Funding for this project was provided by the South Dakota Weed and Pest Control Commission.
Competing interests
The authors declare they have no competing interests.
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