Introduction
Soybean plays a large role in U.S. agriculture. Growers in Mississippi growers planted 931,000 ha of soybean in 2024, which accounted for a greater number of hectares than any other row crop in the state (USDA-NASS 2025). Weeds reduce crop yield by competing for light, soil nutrients, space, and moisture (Buehring Reference Buehring2008). Chemical management of weeds generally requires the use of herbicides to reduce the competition between crop and weed plants. However, managing weeds has come with an ecological penalty because they have evolved resistance to 21 of the 31 herbicide sites of action (SOAs) (Heap Reference Heap2025). At present, 273 weed species worldwide have evolved resistance to one or more herbicide SOA.
Within the midsouthern United States, multiple weed species have developed resistance to the more widely used herbicide SOAs (Heap Reference Heap2025). As a result of herbicide resistance, Palmer amaranth (Amaranthus palmeri S. Wats.) and barnyardgrass are among the most common and problematic weed species in Mississippi soybean production (Van Wychen Reference Van Wychen2022). In 2013, Palmer amaranth was ranked as the most common and troublesome weed of soybean in the midsouthern United States (Webster Reference Webster2013). In more recent years, barnyardgrass has increased in rankings and become one of the more troublesome weeds of soybean (Van Wychen Reference Van Wychen2022).
Glyphosate inhibits the 5-enolpyruvylshikimate-3-phosphate (EPSPS) enzyme and was commercialized in 1974 as a broad-spectrum nonselective herbicide (Amrhein et al. Reference Amrhein, Deus, Gehrke and Steinrucken1980). Less than 6 yr after soybean with engineered resistance to glyphosate (GR) was commercialized, approximately 80% of soybean hectares in the United States were planted with GR cultivars (Green Reference Green2012). By 2009, 91% of soybean in the United States was GR soybean (Reddy and Norsworthy Reference Reddy, Norsworthy and Nandula2010). The widespread adoption of GR crops was credited with greater weed control, excellent crop safety, simplicity of application, relatively low cost associated with weed control, soil conservation, and crop yield benefits (Feng et al. Reference Feng, Cajacob, Martino-Catt, Cerny, Elmore, Heck, Haung, Kruger, Malven, Miklos, Padgette and Nandula2010; Zhou et al. Reference Zhou, Roberts, Larson, Lambert, English, Mishra, Falconer, Hogan, Johnson and Reeves2016). Continued use of glyphosate resulted in resistance by Palmer amaranth as early as 2005 in Georgia (Culpepper et al. Reference Culpepper, Grey, Vencill and Kichler2006). To date, 34 states have confirmed glyphosate-resistant Palmer amaranth (Heap Reference Heap2025).
Dicamba-resistant soybean was developed and commercialized by Monsanto in 2016 (Hart Reference Hart2015). Before then, research on dicamba-resistant crops included an evaluation of Palmer amaranth control with dicamba alone and as a component of other herbicide mixtures (Cahoon et al. Reference Cahoon, York, Jordan, Everman, Seagroves, Culpepper and Eure2015; Houston et al. Reference Houston, Norsworthy, Barber and Brabham2019; Underwood et al. Reference Underwood, Soltani, Hooker, Robinson, Vink, Swanton and Sikkema2017). A second area of research focused on crop tolerance to dicamba applied at different rates and times (Jones et al. Reference Jones, Norsworthy and Barber2019; Montgomery et al. Reference Montgomery, McClure, Hayes, Walker and Steckel2018). However, evaluations of grass control with dicamba were not a main focus of any studies. While early research suggested that dicamba formulations could effectively manage some of the major broadleaf weed species in soybean production, growers were cautioned to not use the herbicide due to its lack of grass control.
Shergill et al. (Reference Shergill, Bish, Biggs and Bradley2018) suggested that weed shifts resulting from the use of dicamba in dicamba-resistant soybean were likely to occur. Vanlieshout and Loux (Reference Vanlieshout and Loux2020) suggested that glyphosate rates would likely need to be increased when it was applied with residual herbicides, especially when barnyardgrass was a target. Quackgrass (Elymus repens L. Gould) was controlled by 78% 28 d after an application of glyphosate + alachlor + atrazine; however, control was reduced to 61% when dicamba was added to this combination (Selleck and Baird Reference Selleck and Baird1981). O’Sullivan and O’Donovan (Reference O’Sullivan and O’Donovan1980) also identified that glyphosate toxicity to barley (Hordeum vulgare L.), oat (Avena fatua L.), and wheat (Triticum aestivum L.) was reduced when dicamba or 2,4-D were added to it. Similar reductions in barnyardgrass control with glyphosate + dicamba were reported by growers soon after the commercialization of dicamba-resistant soybean. Therefore, research was conducted to 1) determine whether common residual herbicides applied postemergence in combination with glyphosate-based herbicides influenced the control of barnyardgrass in dicamba-resistant soybean, and 2) evaluate barnyardgrass control with sequential applications of glyphosate or glyphosate + dicamba.
Materials and Methods
Field studies were conducted in 2020 and 2021 at the Delta Research and Extension Center in Stoneville, Mississippi, to evaluate how barnyardgrass was affected by residual herbicide mixtures that contained glyphosate and how it was affected by sequential applications of herbicides. The soil texture was Sharkey clay (Very-fine, smectitic, thermic Chromic Epiaquerts) with a pH ranging from 7.5 to 8.2 and approximately 2.4% organic matter. Global positioning system coordinates and herbicide application dates are presented in Table 1. Soybean cultivar Asgrow AG46X6 (Bayer Crop Science, Creve Coeur, MO) was planted at 320,000 seeds ha−1 to a depth of 2.5 cm. Plots consisted of four rows spaced 102 cm apart and 9 m long. Plots were separated by a fallow, weed-free alley that was approximately 3 m long. Barnyardgrass in each study was obtained from a naturally occurring population with no known herbicide resistance.
Table 1. Coordinates and dates of initial herbicide application and harvest.
a The two studies were carried out at the Mississippi State University Delta Research and Extension Center in Stoneville, Mississippi, from 2020 to 2021.
Visible estimates of barnyardgrass control and soybean injury were recorded 14, 21, and 28 d after treatment (DAT) in the Residual Herbicide Study and 14 and 28 d after final treatment (DAFT) in the Sequential Application Study on a 0% to 100% scale, with 0% representing no control and 100% equaling complete control. Soybean yield was collected and recorded at maturity using a small-plot combine equipped with an onboard weigh system (Kincaid 8XP; Kincaid Equipment Manufacturing, Haven, KS), and moisture was adjusted to 13%. A percentage of nontreated was calculated by dividing the yield from each treated plot by the average from nontreated plots and multiplying by 100. All data were subjected to ANOVA using the GLIMMIX procedure in SAS software (v.9.4; SAS Institute, Cary, NC) with site-year and replication (nested within site-year) as random effects (Blouin et al. Reference Blouin, Webster and Bond2011). Estimates of the least square means at the 5% significance level were used to separate means.
Residual Herbicide Study
A field study to evaluate barnyardgrass control using residual herbicide mixtures and glyphosate was designed as a randomized complete block with a two-factor arrangement of treatments and four replications. Factor A was postemergence herbicide and included glyphosate (1,120 g ha−1, Roundup PowerMax 2; Bayer Crop Science, Research Triangle, NC) and glyphosate + dicamba (560 g ha−1, Engenia; BASF, Research Triangle, NC). Factor B was residual herbicide and included no residual herbicide, pyroxasulfone (93 g ai ha−1, Zidua SC; BASF), pyroxasulfone + fluthiacet (91 + 3 g ai ha−1, Anthem Max; FMC Agricultural Solutions, Philadelphia, PA), acetochlor (1,267 g ai ha−1, Warrant; Bayer Crop Science), dimethenamid-P (527 g ai ha−1, Outlook; BASF), and S-metolachlor (1,064 g ai ha−1, Dual Magnum; Syngenta Crop Protection, Greensboro, NC). A nontreated control was included for comparison. Herbicides were applied when barnyardgrass reached 15 to 20 cm in height using an air-pressurized, tractor-mounted spray boom equipped with drift-reducing nozzles (TTI11002; TeeJet Technologies, Glendale Heights, IL), a water output of 140 L ha−1, and a ground speed of 4.8 km h−1.
Sequential Application Study
The field study to evaluate sequential herbicide applications for barnyardgrass control was designed as a randomized complete block with four replications and treatments arranged as two factorials that included initial herbicide treatment and sequential herbicide treatment. Initial herbicide treatment included glyphosate (1,120 g ha−1), glyphosate (1,120 g ha−1) + dicamba (560 g ha−1), and glyphosate (1,120 g ha−1) + dicamba (560 g ha−1) + S-metolachlor (1,064 g ha−1). Sequential treatments consisted of glyphosate (1,120 g ha−1) and glyphosate (1,120 g ha−1) + dicamba (560 g ha−1) applied 3 and 7 d after the initial herbicide treatment (DA-I). A nontreated control was included for comparison. Initial herbicide treatments were applied when barnyardgrass reached 15 to 20 cm tall as previously described in the Residual Herbicide Study.
Results and Discussion
Residual Herbicide Study
An interaction of postemergence and residual herbicide treatments was detected for barnyardgrass control 14, 21, and 28 DAT (Table 2). When evaluated at 14 DAT, barnyardgrass control was greater when a mixture of acetochlor and glyphosate had been applied compared with glyphosate applied alone. Barnyardgrass control was reduced by 19 and 5 percentage points when acetochlor and S-metolachlor, respectively, were added to glyphosate + dicamba. Bollman and Sprague (Reference Bollman and Sprague2007) observed greater control of giant foxtail (Setaria faberi Herrm.) with dimethenamid-P compared to S-metolachlor applied with glyphosate. At 14 DAT, with the exception of treatments that contained dimethenamid-P, mixtures of glyphosate + dicamba + residual herbicides, less barnyardgrass control was observed than with the same treatments that did not contain dicamba. This is similar to the findings reported by Bowman (Reference Bowman2022) that less barnyardgrass control was observed with glyphosate + dicamba compared with glyphosate alone.
a Abbreviation: DAT, days after treatment.
b Data were pooled over two experiments. Means followed by the same letter within a column are not different at α = 0.05.
c Herbicide application rates were as follows: glyphosate, 1,120 g ae ha−1; dicamba, 560 g ae ha−1; pyroxasulfone, 93 g ai ha−1; pyroxasulfone + fluthiacet, 91 + 3 g ai ha−1; acetochlor, 1,267 g ai ha−1; dimethenamid-P, 527 g ai ha−1; S-metolachlor, 1,064 g ai ha−1.
Adding acetochlor to glyphosate + dicamba reduced barnyardgrass control by 33 and 38 percentage points at 21 and 28 DAT, respectively, compared with glyphosate + dicamba alone (Table 2). Additionally, barnyardgrass control at 21 DAT was decreased when S-metolachlor or pyroxasulfone were mixed with glyphosate + dicamba compared with the same residual herbicides applied with only glyphosate. No main effects or interaction of postemergence herbicide or residual herbicide treatments were detected for soybean yield (P = 0.6638 to 0.99; data not presented).
Sequential Application Study
At 7 d following the initial herbicide treatment, barnyardgrass was controlled by 92% with glyphosate alone (Table 3). When glyphosate was mixed with dicamba or S-metolachlor, control was reduced by 5 and 6 percentage points, respectively. These results are similar to those reported by Bowman (Reference Bowman2022), that adding dicamba to glyphosate resulted in a 5 percentage point reduction in barnyardgrass control in a non-crop weed control evaluation. Bowman (Reference Bowman2022) also indicated that reducing carrier volumes from 140 to 47 L ha−1 negatively influenced barnyardgrass control; however, using drift-reducing nozzles did not affect barnyardgrass control compared with using regular flat-fan nozzles to apply herbicides.
Table 3. Control of barnyardgrass 7 d after initial herbicide treatment in the Sequential Application Study.a
a Data were pooled over four sequential herbicide treatments and two experiments. Sequential treatments had not been applied at this evaluation. Means followed by the same letter are not different at α = 0.05.
By 14 DAFT, the interaction between initial and sequential herbicide treatments was significant for barnyardgrass control following different application timings (Table 4). Delaying an application of glyphosate + dicamba to 7 d following an initial application of glyphosate + dicamba + S-metolachlor resulted in less barnyardgrass control than with all other herbicide mixtures and timings. No main effects or interaction of initial and sequential herbicide treatments were detected for barnyardgrass control 28 DAFT (P = 0.0731, data not presented) or soybean yield (P = 0.3305, data not presented). However, a main effect of sequential treatment was significant (Table 5). Applying glyphosate + dicamba 3 d following an initial application resulted in the lowest soybean yield compared with all other herbicide combinations. Lawrence et al. (Reference Lawrence, Hydrick, Bond, Golden, Allen and Sanders2020) reported similar results, noting that various mixtures of postemergence soybean herbicides and foliar fertilizers had no effect on soybean yield.
Table 4. Barnyardgrass control 14 d after herbicide treatment in the Sequential Application Study.a–c
a Abbreviation: DA-I, days after the initial herbicide treatment.
b Data were pooled over two experiments. Means followed by the same letter are not different at α = 0.05.
c Glyphosate was applied at 1,120 g ae ha−1; dicamba was applied at 560 g ae ha−1; S-metolachlor was applied at 1,064 g ai ha−1.
a Abbreviation: DA-I, days after the initial herbicide treatment.
b Data were pooled over three initial herbicide treatments and two experiments. Means followed by the same letter are not different at α = 0.05.
c Average soybean yield in nontreated plots was 3,420 kg ha−1.
Practical Implications
Results from these studies indicate that glyphosate alone provided greater barnyardgrass control than glyphosate + dicamba. Additionally, at 28 DAT, pyroxasulfone, pyroxasulfone + fluthiacet, dimethenamid-P, and S-metolachlor did not affect postemergence control of barnyardgrass after glyphosate + dicamba treatments. Furthermore, sequential herbicide treatments of glyphosate or glyphosate + dicamba led to no difference in barnyardgrass control 28 d after the sequential treatment. Sequential applications of glyphosate or glyphosate + dicamba 3 or 7 d after initial treatments with glyphosate, glyphosate + dicamba, or glyphosate + dicamba + S-metolachlor provided >87% barnyardgrass control for up to 28 DAFT (data not presented).
Proper herbicide stewardship and weed control should be emphasized in soybean production because glyphosate-resistant barnyardgrass has already been documented in the midsouthern United States (Heap Reference Heap2025). These results indicate that not all residual herbicides reduce barnyardgrass control when mixed with glyphosate or glyphosate + dicamba, and that sequential herbicide treatments can be used for barnyardgrass control. These data also suggest that growers have options to combat reduced barnyardgrass control associated with glyphosate + dicamba applications by applying the herbicides sequentially (Nalewaja and Matysiak Reference Nalewaja and Matysiak1991).
Acknowledgments
We thank personnel at the Mississippi State University Delta Research and Extension Center for their assistance.
Funding
This publication is a contribution of the Mississippi Agricultural and Forestry Experiment Station. This study is based on a larger study that is supported by Hatch project 153300, which is funded by the U.S. Department of Agriculture–National Institute of Food and Agriculture. In addition, we extend gratitude to the Mississippi Soybean Promotion Board for providing partial funding this research.
Competing interests
The authors declare they have no competing interests.
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