Introduction
Farmers have traditionally mixed pesticides to increase the spectrum of control and to reduce the number of trips through the field, which may result in economic savings and delay the evolution of pesticide resistance (Green Reference Green1989; Putnam and Penner Reference Putnam, Penner and Gunthur1974; Wrubel and Gressel Reference Wrubel and Gressel1994). Mixtures of herbicides with different effective mechanisms of action (MOAs) may be applied in annual rotations and sequential applications to delay the evolution of resistance by minimizing the selection pressure imposed by the recurrent use of a single herbicide MOA (Beckie and Harker Reference Beckie and Harker2017; Gressel and Segel Reference Gressel and Segel1990; Norsworthy et al. Reference Norsworthy, Ward, Shaw, Llewellyn, Nichols, Webster, Bradley, Frisvold, Powles, Burgos, Witt and Barrett2012). Mixtures or sequential applications of herbicides with different MOAs can result in biochemically additive, antagonistic, or synergistic activity (Green Reference Green1989). Herbicide mixtures may result in lower control and may be exacerbated when applied to larger weeds (Colby Reference Colby1967; Green Reference Green1989; Putnam and Penner Reference Putnam, Penner and Gunthur1974). Sequential herbicide applications may result in decreased or increased control when compared to the herbicides when mixed (Putnam and Penner Reference Putnam, Penner and Gunthur1974).
Applying dicamba (Herbicide Group [HG] 4) and glufosinate (HG 10) alone or sequentially controls weed species resistant to other herbicides in North Carolina (Cahoon et al. Reference Cahoon, York, Jordan, Everman, Seagroves, Culpepper and Eure2015; Everman et al. Reference Everman, Burke, Allen, Collins and Wilcut2007; Oreja et al. Reference Oreja, Inman, Jordan and Leon2021; Schrage Reference Schrage2018). Dicamba and glufosinate mixtures could be useful for weed control, but this mixture is not currently labeled. However, dicamba and glufosinate have different MOAs that result in plant death; thus mixed and sequential applications could result in additive, antagonistic, or synergistic control (Belz and Duke Reference Belz and Duke2014; Gressel Reference Gressel2020). In addition, the herbicide mixtures and sequential applications could mitigate the evolution of target-site resistance (Beckie and Harker Reference Beckie and Harker2017; Gressel Reference Gressel2020; Gressel and Segel Reference Gressel and Segel1990). Dicamba is a selective, slow-acting systemic herbicide that binds to the transport inhibitor response gene, resulting in the uncontrolled transcription of auxin-responsive genes (Grossmann Reference Grossmann2009). Glufosinate is a nonselective, fast-acting contact herbicide that inhibits glutamine synthetase (EC 6.3.1.2) (Takano et al. Reference Takano, Beffa, Preston, Westra and Dayan2019). Because dicamba and glufosinate differentially affect plants, knowledge of potential interactions between these two herbicides when applied in combination is important for ensuring effective weed control (Comont et al. Reference Comont, Lowe, Hull, Crook, Hicks, Onkokesung, Beffa, Childs, Edwards, Freckleton and Neve2020; Green Reference Green1989; Gressel Reference Gressel2020).
Previous research demonstrated mixtures of herbicides, including dicamba or glufosinate, resulted in lower control due to biochemical antagonism on several weed species (Besançon et al. Reference Besançon, Penner and Everman2018; Burke et al. Reference Burke, Askew, Corbett and Wilcut2005; Ou et al. Reference Ou, Thompson, Stahlman, Bloedow and Jugulam2018). Previous research has also demonstrated that sequential herbicide applications and the order in which herbicides were applied can circumvent antagonism (Burke et al. Reference Burke, Askew, Corbett and Wilcut2005; Culpepper et al. Reference Culpepper, York, Jordan, Corbin and Sheldon1999; Koger et al. Reference Koger, Burke, Miller, Kendig, Reddy and Wilcut2007). Dicamba and glufosinate have label restrictions that require sequential applications to occur between 7 to 14 days after the initial herbicide application (Anonymous 2017; Anonymous 2018). Investigations assessing the potential interaction of sequential applications of dicamba and glufosinate should include treatment timings within this labeled timeframe. Studies have investigated dicamba and glufosinate applied alone, mixed, and sequentially on Palmer amaranth; however the studies investigating potential herbicides interactions were applied to plants larger than the labeled-size to mimic delayed application induced by environmental conditions (Browne et al. Reference Browne, Li, Price, Langemeier, Sanz-Saez de Jaureguy, McElroy, Feng and Price2020; Meyer and Norsworthy Reference Meyer and Norsworthy2019; Priess et al. Reference Priess, Popp, Norsworthy, Mauromoustakos, Roberts and Butts2022a; Vann et al. Reference Vann, York, Cahoon, Buck, Askew and Seagroves2017a, Reference Vann, York, Cahoon, Buck, Askew and Seagroves2017b).
Weeds other than Palmer amaranth are often present in sufficient amounts to justify treatment; thus it is of interest to determine the level of control from dicamba and glufosinate applications on common weed species. Dicamba does not provide grass control and may cause increased population densities of grass weeds if recurrently applied (Canode and Robocker Reference Canode and Robocker1966; Hodgskiss et al. Reference Hodgskiss, Legleiter, Young and Johnson2020; Oreja et al. Reference Oreja, Inman, Jordan and Leon2021). Although glufosinate is effective on large crabgrass (Corbett et al. Reference Corbett, Askew, Thomas and Wilcut2004; Everman et al. Reference Everman, Burke, Allen, Collins and Wilcut2007; Tharp et al. Reference Tharp, Schabenberger and Kells1999), it provides variable control on other grass species, such as goosegrass (Burke et al. Reference Burke, Askew, Corbett and Wilcut2005). There is also a goosegrass population from Malaysia that has evolved resistance to glufosinate (Seng et al. Reference Seng, Lun, San and Sahid2010; Zhang et al. Reference Zhang, Yu, Han, Yu, Nyporko, Tian, Beckie and Powles2022). Decreased control of goosegrass when treated with glufosinate could be exacerbated when mixed or applied sequentially with dicamba (Flint and Barrett Reference Flint and Barrett1989; Hart and Wax Reference Hart and Wax1996; Meyer et al. Reference Meyer, Falco, Norsworthy and Beffa2020). Currently there is no literature providing information about the control of goosegrass and large crabgrass with dicamba and glufosinate applied in combination or sequentially.
A similar situation occurs with common ragweed, ivyleaf morningglory, and sicklepod, which are also pervasive and hard-to-control weeds in the southeastern United States (Jones et al. Reference Jones, Cahoon, Leon and Everman2022; Van Wychen Reference Van Wychen2017; Webster and Nichols Reference Webster and Nichols2012). Previous research has shown that dicamba and glufosinate are effective on these species; however, mixtures and sequential applications of these herbicides have not been evaluated (Everman et al. Reference Everman, Burke, Allen, Collins and Wilcut2007; Kalina et al. Reference Kalina, Corkern, Shilling, Basinger and Gret2021; Leon et al. Reference Leon, Ferrell and Sellers2016; Schrage Reference Schrage2018). The objectives of this study were to determine (1) if the order of dicamba and glufosinate in a sequential application influence control on certain weed species and sizes and (2) if dicamba + glufosinate mixtures demonstrate an antagonistic interaction on certain weed species and sizes.
Materials and Methods
Separate field experiments were conducted in 2019 and 2021 at Rocky Mount (35.89°N, 77.68°W) and Kinston (35.29°N, 77.65°W), NC, for a total of 4 site-years. The soil at the Rocky Mount location is an Aycock very fine sandy loam (fine-silty, siliceous, subactive, thermic Typic Paleudult), whereas the Kinston location encompasses a mosaic of Lumbee sandy loam (sandy-skeletal, siliceous, subactive, thermic Typic Endoaquult) and Portsmouth loam (sandy-skeletal, mixed, semiactive, thermic Typic Umbraquult) soils. Natural populations of large crabgrass and Palmer amaranth occurred at both locations both years (4 site-years). Ivyleaf morningglory populations were evaluated at both locations in 2021 (2 site-years). Goosegrass populations were evaluated at Kinston in 2019 and at both locations in 2021 (3 site-years). The populations of common ragweed and sicklepod were evaluated each year at Rocky Mount and Kinston, respectively, resulting in 2 site-years of data for each weed. The Palmer amaranth populations had evolved resistance to acetolactate synthase–inhibiting herbicides and glyphosate, whereas the other weed species at both experiment sites were herbicide susceptible. The field sites were tilled prior to experiment initiation to control established weeds, but preemergence herbicides were not applied to ensure maximum weed seedling emergence. Both experiment locations remained fallow for the duration of the study.
Treatments were arranged as a two-way factorial in a randomized complete block design with four replications. Individual plots were 3.6 m wide by 9.0 m long. Treatment factors were herbicide and weed size (Table 1). Herbicide treatments were applied to plots with a CO2-pressurized backpack sprayer calibrated to deliver 140 L ha−1 at 165 kPa and 46 cm above the target weed height. The sequential herbicide applications were made 1 wk after the initial herbicide application. A nontreated control was included in the experiment. Dicamba-only treatments were applied at the maximum labeled rate for use in dicamba-tolerant cotton and soybean (560 g ae ha−1) with TeeJet® TTI11002-VP spray nozzles (TeeJet® Technologies, Spraying Systems, Wheaton, IL, USA) (Anonymous 2018). Glufosinate-only treatments were applied at a rate commonly used in North Carolina (590 g ai ha−1) (W. J. E. Everman, personal communication, 2018) with 10 g L−1 of ammonium sulfate and with TeeJet® XR11002-VS spray nozzles. Ammonium sulfate was included with the glufosinate treatments to prevent the herbicide molecules from binding to cations in the water carrier and was excluded from the treatments including dicamba to prevent herbicide volatility per label requirements (Anonymous 2018; Mueller and Steckel Reference Mueller and Steckel2019; Pratt et al. Reference Pratt, Kells and Penner2003). The dicamba + glufosinate treatments were applied with TeeJet® TTI11002-VP spray nozzles. The different nozzles were used to ensure the highest control based on the herbicide activity and to mimic a farmer application per label requirements (Anonymous 2017; Anonymous 2018; Sikkema et al. Reference Sikkema, Brown, Shropshire, Spieser and Soltani2008). Herbicide treatments were applied at 2 ± hours of solar noon and at temperatures ranging between 30 C and 35 C with relative humidity greater than 30% to avoid environment-induced control reductions (Coetzer et al. Reference Coetzer, Al-Khatib and Loughin2001; Johnston et al. Reference Johnston, Eure, Grey, Culpepper and Vencill2018; Sellers et al. Reference Sellers, Smeda and Johnson2003).
Table 1. Dicamba (560 g ae ha−1) and glufosinate (590 g ai ha−1) treatments tested in the fallow field experiment conducted at Rocky Mount and Kinston, NC, in 2019 and 2021. a, b
a Each herbicide treatment was tested on two sizes of common ragweed (5 and 15 cm), goosegrass (10 and 20 cm), ivyleaf morningglory (5 and 15 cm), large crabgrass (10 and 20 cm), Palmer amaranth (5 and 15 cm), and sicklepod (5 and 15 cm).
b Abbreviations: D, dicamba; DG, dicamba + glufosinate; fb, followed by; G, glufosinate.
Weed control evaluations were made using estimates based on a scale ranging from 0% to 100%, where 0% equals no control (i.e., no injury symptoms on any tissue) and 100% equals complete control (i.e., total necrosis). Height reduction was determined by measuring from the soil surface to the apical growing point for three plants (representative of overall plot average height) of each species growing in the central region of each plot. Percentage height reduction was calculated by dividing the heights of the plants in the treated plots by the heights of the plants in the nontreated plots. Control and height reduction evaluations were made 28 d after initial treatment (DAIT) for each application timing. Clethodim was applied at the maximum labeled rate (560 g ai ha−1) with TeeJet® XR11002-VS spray nozzles to dicamba-only treatments 21 DAIT to control grass weeds that could confound the control for broadleaf weeds. Thus no response variable data were recorded for grass species in these plots.
Dicamba + glufosinate mixtures were further evaluated to determine whether control was additive, antagonistic, or synergistic 28 DAIT. Colby’s method (Colby Reference Colby1967) has been frequently used to evaluate herbicide interactions (de Sanctis and Jhala Reference de Sanctis and Jhala2021; Meyer and Norsworthy Reference Meyer and Norsworthy2019). Colby’s method calculates an expected control value for an herbicide mixture based on the control of the individual herbicides and compared to the control of the tested mixture. If the observed control of the herbicide mixture deviates from the expected control, then the herbicide mixture can be considered antagonistic or synergistic, depending on the nature of the deviation. Dicamba + glufosinate treatments were analyzed using the equation for Colby’s method:
$$E = \left( {X + Y} \right) - \left( {{{xy} \over {100}}} \right)$$
where E is expected control (%) of two herbicides applied in a mixture, X is control (%) of X herbicide when applied alone, and Y is control (%) of Y herbicide when applied alone. The expected control was compared with the observed control using a two-sided t-test (α = 0.05). If the control was greater than the expected, the mixture was considered synergistic, whereas if the control was lower than the expected, the mixture was considered antagonistic (Colby Reference Colby1967). If the observed and expected controls were equal, the mixture was considered additive (Colby Reference Colby1967). The height reduction of the weeds when treated with dicamba, glufosinate, and dicamba + glufosinate was subjected to Colby’s equation as well. Because dicamba does not control goosegrass or large crabgrass, statistical deviations from the single and mixed applications of dicamba and glufosinate can provide evidence of antagonism or synergism (Flint and Barrett Reference Flint and Barrett1989; Meyer and Norsworthy Reference Meyer and Norsworthy2019). The control of sequential herbicide treatments was compared to the control of the dicamba + glufosinate mixture to determine if incurred antagonism could be overcome with a particular sequential herbicide treatment (Burke et al. Reference Burke, Askew, Corbett and Wilcut2005).
Control and height reduction 28 DAIT data were subjected to analysis of variance (ANOVA) using the Glimmix procedure in SAS 9.4 (Statistical Analysis Software Institute, Cary, NC, USA) (α = 0.05). Herbicide, weed size, and their interactions were considered fixed effects, whereas block, year, location, and their interactions were considered random effects. Year and location were considered random to allow inferences to be made across broader conditions and locations (Blouin et al. Reference Blouin, Webster and Bond2011; Moore and Dixon Reference Moore and Dixon2015). Treatment means were separated using Tukey’s honestly significant difference test (P ≤ 0.05). Control and height reduction data were also subjected to the Corr procedure in SAS 9.4 to determine Pearson’s correlation coefficients between the evaluations. The nontreated control was excluded from control and height reduction analyses for all weed species. All dicamba-only treatments were excluded from the statistical analyses for goosegrass and large crabgrass, as no control was incurred and clethodim was applied.
Results
Common Ragweed
Common ragweed control was affected by herbicide (P < 0.0001) but not size (P = 0.69). The interaction was significant (P = 0.0005); thus common ragweed control data were analyzed by herbicide and size. The single herbicide treatments controlled the 5-cm and 15-cm common ragweed similarly, respectively (Table 2). All sequential herbicide treatments completely controlled the 5-cm and 15-cm common ragweed, respectively (Table 2). No initial treatment fb glufosinate controlled the 5-cm common ragweed approximately 11% less than all sequential herbicide treatments (Table 2). No initial treatment fb dicamba controlled 15-cm common ragweed approximately 24% less than all sequential herbicide treatments (Table 2).
Table 2. Broadleaf weed control with dicamba and glufosinate 28 d after the initial herbicide application in fallow field experiments conducted at Kinston and Rocky Mount, NC, in 2019 and 2021. a, b
a Abbreviations: ‘AMAPA,’ Palmer amaranth; ‘AMBEL,’ common ragweed; ‘CASOB,’ sicklepod; D, dicamba; DG, dicamba + glufosinate; fb, followed by; G, glufosinate; N, no initial treatment.
b Means followed by the same letter are not statistically different based on Tukey’s honestly significant difference (P < 0.05).
Common ragweed height reduction was affected by treatment (P < 0.0001) but not size (P = 0.97), and a significant interaction was not detected (P = 0.06); thus common ragweed height reduction data were averaged over size. Common ragweed height reduction was the same when treated with dicamba, glufosinate, and dicamba + glufosinate (Table 3). No initial treatment fb glufosinate and no initial treatment fb dicamba + glufosinate reduced common ragweed height similarly. All sequential herbicide treatments controlled common ragweed, resulting in no vegetative growth (Table 3). No initial treatment fb dicamba and no initial treatment fb glufosinate reduced common ragweed height approximately 25% less than dicamba and sequential herbicide treatments (Table 3). A high correlation between common ragweed control and height reduction was detected (R = 0.82; P < 0.0001).
Table 3. Broadleaf weed height reduction with dicamba and glufosinate 28 d after the initial herbicide application in fallow field experiments conducted at Kinston and Rocky Mount, NC, in 2019 and 2021. a, b
a Abbreviations: ‘AMAPA,’ Palmer amaranth; ‘AMBEL,’ common ragweed; ‘CASOB,’ sicklepod; D, dicamba; DG, dicamba + glufosinate; fb, followed by; G, glufosinate; N, no initial treatment.
b Means that share the same letter are not statistically different based on Tukey’s honestly significant difference (P < 0.05).
Dicamba + glufosinate and no initial treatment fb dicamba + glufosinate additively controlled 5-cm common ragweed (Table 4). Dicamba + glufosinate and no initial treatment fb dicamba + glufosinate also additively controlled 15-cm common ragweed. Furthermore, dicamba + glufosinate and no initial treatment fb dicamba + glufosinate additively reduced common ragweed height (Table 4).
Table 4. Broadleaf weed control and height reduction (expected and observed) with dicamba and glufosinate 28 d after initial herbicide application in fallow field experiments conducted at Kinston and Rocky Mount, NC, in 2019 and 2021. a
a Abbreviations: ‘AMAPA,’ Palmer amaranth; ‘AMBEL,’ common ragweed; ‘CASOB,’ sicklepod; DG, dicamba + glufosinate; fb, followed by; N, no initial treatment; NC, not calculated.
Goosegrass
Goosegrass control was affected by herbicide (P < 0.0001) but not size (P = 0.53). The interaction was not significant (P = 0.12); thus goosegrass control data were averaged over size. Goosegrass control was 20% and 17% greater with glufosinate and no initial treatment fb glufosinate compared to dicamba + glufosinate and no initial treatment fb dicamba + glufosinate, respectively (Table 5). Sequential herbicide treatments controlled goosegrass similarly, with the exception that dicamba fb dicamba + glufosinate control was 16% less than glufosinate fb dicamba + glufosinate (Table 5). All other sequential treatments provided similar goosegrass control compared to glufosinate and no initial treatment fb glufosinate (Table 5). Glufosinate fb dicamba + glufosinate and glufosinate fb glufosinate provided 18% to 31% more control compared to dicamba + glufosinate and no initial treatment fb dicamba + glufosinate (Table 5).
Table 5. Grass weed control with dicamba and glufosinate 28 d after the initial herbicide application in fallow field experiments conducted at Kinston and Rocky Mount, NC, in 2019 and 2021. a, b
a Abbreviations: D, dicamba; DG, dicamba + glufosinate; ‘DIGSA,’ large crabgrass; ‘ELEIN,’ goosegrass; fb, followed by; G, glufosinate; N, no initial treatment.
b Means that share the same letter are not statistically different based on Tukey’s honestly significant difference (P < 0.05).
Goosegrass height reduction was affected by herbicide (P < 0.0001) but not size (P = 0.12), and the interaction was not significant (P = 0.35); thus goosegrass height reduction data were averaged over size. Goosegrass height reduction was similar across all single herbicide treatments, with the exception that glufosinate reduced height by 23% more than no initial treatment fb dicamba + glufosinate (Table 6). Sequential herbicide treatments reduced goosegrass height similarly (Table 6). The sequential treatments provided similar goosegrass height compared to glufosinate and no initial treatment fb glufosinate (Table 6). Most treatments that included glufosinate (alone or mixed with dicamba) reduced goosegrass height more than dicamba + glufosinate and no initial treatment fb dicamba + glufosinate (Table 6). A high correlation between goosegrass control and height reduction was detected (R = 0.84; P < 0.0001).
Table 6. Grass weed height reduction with dicamba and glufosinate treatments 28 d after the initial herbicide application in fallow field experiments conducted at Kinston and Rocky Mount, NC, in 2019 and 2021. a, b
a Abbreviations: D, dicamba; DG, dicamba + glufosinate; ‘DIGSA,’ large crabgrass; ‘ELEIN,’ goosegrass; fb, followed by; G, glufosinate; N, no initial treatment.
b Means that share the same letter are not statistically different based on Tukey’s honestly significant difference (P < 0.05).
Differential control and height reduction of goosegrass when treated with glufosinate and dicamba + glufosinate suggests that the mixture may be antagonistic (Table 5). Reduced control was realized between no initial treatment fb glufosinate and no initial treatment fb dicamba + glufosinate, further suggesting that the mixture may be antagonistic (Table 6). Conversely, goosegrass height reductions were no different with the no initial treatment fb herbicide treatments. These results suggest that applying dicamba + glufosinate to goosegrass may be antagonistic.
Ivyleaf Morningglory
All herbicides controlled ivyleaf morningglory at both locations in 2021 (data not shown). Because there was no variation of control or height reduction, ANOVA could not be conducted. This result was expected, as ivyleaf morningglory is highly susceptible to both dicamba and glufosinate applied individually (Everman et al. Reference Everman, Burke, Allen, Collins and Wilcut2007; Kalina et al. Reference Kalina, Corkern, Shilling, Basinger and Gret2021; Merchant et al. Reference Merchant, Sosnoskie, Culpepper, Steckel, York, Braxton and Ford2013). Control and height reduction data were not subjected to Colby’s equation because of the complete control achieved.
Large Crabgrass
Large crabgrass control was affected by herbicide (P < 0.0001) and size (P = 0.009). The interaction was not significant (P = 0.28); thus large crabgrass control data were averaged over size. All single herbicide treatments provided similar control of large crabgrass (Table 5). Differential control was realized across the sequential herbicide treatments. Largely, treatments containing glufosinate in both applications provided 24% to 27% greater control than treatments containing a single glufosinate (alone or mixed with dicamba) application (initial or sequential) (Table 5). Glufosinate provided similar control to sequential herbicide treatments (Table 5).
Large crabgrass height reduction was affected by herbicide (P < 0.001) but not size (P = 0.23), and the interaction was not significant (P = 0.42). Thus large crabgrass height reduction data were averaged over size. Large crabgrass height reduction was similar across all single herbicide treatments, with the exception that glufosinate reduced height by 15% more than no initial treatment fb dicamba + glufosinate (Table 6). Similar to the control evaluations, treatments containing glufosinate (alone or mixed with dicamba) in both applications provided 27% to 32% greater height reduction than treatments containing a single glufosinate (alone or mixed with dicamba) application (initial or sequential) (Table 6). A high correlation between large crabgrass control and height reduction was detected (R = 0.72; P < 0.0001).
Control and height reduction of large crabgrass when treated with glufosinate and dicamba + glufosinate were similar (Tables 5 and 6). Similar control and height reduction were also observed with no initial treatment fb dicamba + glufosinate and no initial treatment fb glufosinate (Tables 5 and 6). These results suggest that dicamba + glufosinate additively controls and reduces the height of large crabgrass. Differential control of the sequential herbicide treatments suggests that including glufosinate (alone or mixed with dicamba) in both applications provides greater large crabgrass control over a single application of glufosinate (alone or mixed with dicamba) (Tables 5 and 6).
Palmer Amaranth
Palmer amaranth control was affected by herbicide and size (P < 0.0001), and a significant interaction was detected (P = 0.02); thus Palmer amaranth control data were analyzed by herbicide and size. Dicamba, glufosinate, and dicamba + glufosinate controlled the 5-cm Palmer amaranth similarly (Table 2). No initial treatment fb dicamba and no initial treatment fb glufosinate provided 19% to 30% less control of 5-cm Palmer amaranth compared to dicamba and glufosinate, respectively (Table 2). However, no initial treatment fb dicamba + glufosinate was as effective as all other treatments. All sequential herbicide treatments completely controlled the 5-cm Palmer amaranth (Table 2). The single herbicide treatments controlled 15-cm Palmer amaranth similarly, but control was lower compared to when these treatments were applied to 5-cm Palmer amaranth (Table 2). The sequential herbicide treatments provided approximately 30% higher control on 15-cm Palmer amaranth when compared to no initial treatment fb dicamba and no initial treatment fb glufosinate (Table 2). No initial treatment fb dicamba + glufosinate was efficacious on 15-cm Palmer amaranth, where only the dicamba fb dicamba + glufosinate and glufosinate fb dicamba + glufosinate provided 20% higher control (Table 2).
Palmer amaranth height reduction was affected by herbicide (P < 0.0001) but not size (P = 0.74), and a significant interaction (P = 0.70) was not detected; thus Palmer amaranth height reduction data were averaged over size. Palmer amaranth height was reduced similarly with all single herbicide treatments (Table 3). All sequential herbicide treatments reduced Palmer amaranth height similarly (Table 3). Height reduction with dicamba and dicamba + glufosinate was no different than the sequential herbicide treatments (Table 3). Palmer amaranth height reduction from glufosinate was similar to dicamba fb dicamba, dicamba + glufosinate fb dicamba, and glufosinate fb glufosinate; all other sequential herbicide treatments reduced Palmer amaranth height to a greater degree (Table 3). The no initial treatment fb herbicide treatments reduced Palmer amaranth height 22% to 33% less than all the sequential herbicide treatments (Table 3). A significant correlation between Palmer amaranth control and height reduction was detected (R = 0.46; P < 0.0001).
Dicamba + glufosinate additively controlled 5-cm Palmer amaranth (Table 4). Additive control of 15-cm Palmer amaranth was also achieved with dicamba + glufosinate and no initial treatment fb dicamba + glufosinate. Furthermore, dicamba + glufosinate and no initial treatment fb dicamba + glufosinate additively reduced Palmer amaranth height (Table 4). Although dicamba + glufosinate did not have an antagonistic or synergistic interaction in the presented study, other research has demonstrated that this mixture can incur control antagonism or synergism on Palmer amaranth of similar sizes (Merchant et al. Reference Merchant, Sosnoskie, Culpepper, Steckel, York, Braxton and Ford2013; Priess et al. Reference Priess, Popp, Norsworthy, Mauromoustakos, Roberts and Butts2022a). The dissimilar results between the studies suggest that control with dicamba + glufosinate is influenced by the environment (e.g., humidity, light intensity, temperature) when applied on larger Palmer amaranth (Hammerton Reference Hammerton1967; Richter et al. Reference Richter, Zwerger and Bottcher2002).
Sicklepod
Sicklepod control was affected by herbicide (P < 0.0001) but not size (P = 0.06). The interaction between the main effects was not significant (P = 0.65); thus sicklepod control data were averaged over size. Most single and sequential herbicide treatments controlled sicklepod similarly (Table 3). No initial treatment fb dicamba controlled sicklepod 21% to 24% less than most of the single and all sequential herbicide treatments (Table 3). Sicklepod height reduction was affected by herbicide (P = 0.0008) and size (P = 0.004). The interaction was not significant (P = 0.37); thus sicklepod height reduction data were averaged over size (Table 4). Most single and sequential herbicide treatments reduced sicklepod height similarly (Table 5). The no initial treatment fb dicamba treatment reduced sicklepod height 20% less than many of the sequential herbicide treatments (Table 5). A high correlation between sicklepod control and height reduction was detected (R = 0.83; P < 0.0001). Dicamba + glufosinate and no initial treatment fb dicamba + glufosinate resulted in additive control of sicklepod (Table 4). Furthermore, dicamba + glufosinate and no initial treatment fb dicamba + glufosinate additively reduced sicklepod height (Table 4).
Discussion
The results of this experiment provide evidence that differential weed control is achieved when glufosinate is applied before dicamba for grass species compared to the broadleaf species. Grass weeds were controlled better when two applications of glufosinate (alone or mixed with dicamba) were applied in sequential herbicide treatments. Additionally, the results of the experiments provide evidence that dicamba + glufosinate mixtures result in additive control, with the exception that goosegrass control was antagonized. While dicamba + glufosinate additively controlled the other tested weed species, this mixture is not labeled and should be not applied or recommended. If dense stands of goosegrass and large crabgrass are present in the field, glufosinate should be applied initially to enhance control. Because glufosinate was not applied at the maximum labeled rate (875 g ai ha−1) in this experiment, the glufosinate rate could be increased to improve grass control. Future research should investigate the interaction between dicamba and glufosinate with varying rates of each for optimal grass and broadleaf weed control. However, across all tested broadleaf species, control never increased compared to the initial glufosinate application. If glufosinate is applied to the weed species at the tested sizes, a sequential application may not be necessary. Although a scheduled sequential application with these herbicides seems unnecessary on the tested weed species and sizes, the sequential treatments could be of great value by reducing selection pressure on a single herbicide group. Though weed size did not predominantly affect control or height reduction, the no initial treatment fb herbicide treatments provided less control for many of the tested weed species. This result further demonstrates that herbicides should be applied in a timely fashion and not delayed.
Dicamba and glufosinate are among the few effective postemergence herbicide options to control herbicide-resistant weeds in cotton and soybean production systems of the southeast United States (Heap Reference Heap2022). Palmer amaranth has evolved resistance to both dicamba and glufosinate in the United States (Carvalho-Moore et al. Reference Carvalho-Moore, Norsworthy, González-Torralva, Hwang, Patel, Barber, Butts and McElroy2022; Heap Reference Heap2022; Priess et al. Reference Priess, Norsworthy, Godara, Mauromoustakos, Butts, Roberts and Barber2022b). Thus these two unique herbicides should be stewarded to increase the longevity of usefulness instead of continuing the herbicide selection cycle (Comont et al. Reference Comont, Lowe, Hull, Crook, Hicks, Onkokesung, Beffa, Childs, Edwards, Freckleton and Neve2020; Wrubel and Gressel Reference Wrubel and Gressel1994; Young Reference Young2006). It is also important to note that glufosinate has no residual activity and that dicamba has minimal residual activity (Altom and Stritzke Reference Altom and Stritzke1973; Anonymous 2017; Edwards Reference Edwards2013). Thus additional management inputs should be included to control species that exhibit prolonged emergence (Hartzler Reference Hartzler2017; Mohler and Callaway Reference Mohler and Callaway1995; Myers et al. Reference Myers, Curran, VanGessel, Calvin, Mortensen, Majek, Karsten and Roth2004; Reinhardt Piskackova et al. Reference Reinhardt Piskackova, Reberg-Horton, Richardson, Jennings and Leon2020).
Acknowledgments
Project funding was provided by the North Carolina Soybean Producers Association. The authors extend thanks to John Sanders, Diego Contreras, Marco Fajardo, and the crew at the Caswell Research Station and Upper Coastal Plain Research Station for the technical support and to Dr. Micheal D. K. Owen for critically reviewing the manuscript. The authors declare no conflicts of interest.
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