Control of Multiple-Herbicide-Resistant Green Pigweed (Amaranthus powellii) with Preemergence and Postemergence Herbicides in Ontario Soybean Production

Abstract

Green pigweed [Amaranthus powellii S. Wats.] is a competitive, annual, broadleaf weed that can significantly reduce soybean yield due to rapid early growth and biomass production. As a consequence of green pigweed’s high competitiveness with crops, the weed species is generally managed with the use of herbicides; this may, however, lead to the selection of resistance. A green pigweed biotype with resistance to MCPA, mecoprop, dichlorprop-p, aminocyclopyrachlor (synthetic auxins), and imazethapyr (acetolactate synthase-(ALS)-inhibitor) was recently confirmed in Ontario, Canada. Research was conducted to identify alternative effective preemergence (PRE) and postemergence (POST) herbicides for the control of multiple-herbicide-resistant (MHR) green pigweed in Ontario soybean production. Four field trials, two with PRE and two with POST herbicides were conducted near Dresden, Ontario in 2019 and 2020. Visible soybean injury, green pigweed control, density, aboveground biomass, and soybean yield were evaluated following application with 19 PRE herbicide treatments and 12 POST herbicide treatments in separate studies. At 8 wk after application (WAA), pyroxasulfone/flumioxazin applied PRE controlled green pigweed 95% and was the most effective soil-applied herbicide. In the POST study, glyphosate was the most effective herbicide, controlling green pigweed 94% at 8 WAA. Imazethapyr, applied PRE or POST did not control this green pigweed biotype as it is also resistant to ALS-inhibitors. Control with other ALS-inhibiting herbicides as well as with PPO-inhibiting herbicides was variable, and further research is needed to determine the reason for variable control with these herbicides.

1. Introduction

Green pigweed [Amaranthus powellii S. Wats.] is a monoecious, small-seeded, summer annual, competitive, C4, dicot weed that is a member of the Amaranthaceae family [1,2]. Originally from South America, green pigweed has been present in North America since the 1940s [1,3]. Germination of green pigweed begins in late spring when temperatures are 15 °C with germination continuing through the summer [1,4]. Green pigweed is primarily self-pollinating with high fecundity [5,6]; these features help this species to spread rapidly and establish genetically adapted biotypes.

Certain biological characteristics contribute to the competitive nature of green pigweed and to the difficulty in controlling this weed. Green pigweed is similar to other monoecious Amaranthus species such as redroot pigweed (Amaranthus retroflexus L.) and smooth pigweed (Amaranthus hybridus L.) in that it has prolific seed production with the ability to produce up to 100,000 seeds per plant [1]. Additionally, green pigweed exhibits rapid germination and early growth as well as prolonged dormancy, allowing for a persistent seedbank [7,8]. It has been previously documented that high densities of green pigweed and redroot pigweed have reduced soybean yields by up to 16% when weeds emerged with the crop [9]. Yield losses due to green pigweed have also been reported in sugar beets (Beta vulgaris L.), where 24 green pigweed plants per 30 m of crop row reduced sugar beet root yield by up to 25% and recoverable sucrose by up to 24% [10].

Green pigweed can be controlled with tillage or the use of effective soil-applied and postemergence (POST) dicot herbicides [4]. Due to their high efficacy, low cost, and ease of use, herbicides have replaced tillage as the most utilized method of weed control [11]. Repeated applications of the same herbicides have contributed to the evolution of herbicide resistance by selecting biotypes that are able to survive and reproduce following the application of a previously lethal herbicide dose [12,13,14]. Acetolactate synthase (ALS) inhibitor- and photosystem (PSII) inhibitor-resistant green pigweed biotypes have been previously confirmed in Ontario, Canada [15]. As of 2021, a multiple, herbicide-resistant (MHR) biotype of green pigweed with resistance to MCPA, dichlorprop-p, mecoprop, aminocyclopyrachlor, and imazethapyr has been confirmed from a field site near Dresden, Ontario, Canada [16]. The field site follows a six-crop rotation of corn, soybean, wheat, dry bean, peas, and sweet corn with minimal historical exposure to synthetic auxin herbicides with the use of ALS-inhibiting herbicides in rotation with other herbicide modes of action. Resistance to MCPA, dichlorprop-p, mecoprop, and aminocyclopyrachlor was confirmed through dose–response experiments, whereas resistance to imazethapyr was confirmed by sequencing the ALS gene to identify target site mutations [16,17]. Although MHR green pigweed is not currently widespread in Ontario, Canada, it could become a challenging problem if control strategies are not identified.

The presence of MHR green pigweed biotypes in Ontario, underscores the importance of identifying effective control solutions for farmers. The objective of this research is to determine the efficacy of preemergence (PRE) and POST herbicides for the control of green pigweed in soybean. The identification of efficacious herbicide programs with multiple herbicide sites of action will minimize soybean yield loss from green pigweed interference and will help to mitigate the spread of MHR green pigweed biotypes.

2. Materials and Methods

Four field trials, two with PRE and two with POST herbicides, were conducted at a single field site near Dresden, Ontario (42.582811, −82.113953) in 2019 and 2020. The trials were arranged as a randomized complete block design with four blocks. Information on soil characteristics, soybean planting, emergence, and harvest dates, and herbicide application dates are presented in

Table 1. Soil, soybean agronomic, and herbicide application information for four trials conducted near Dresden, ON, Canada in 2019 and 2020.

Year	Soil Characteristics a (%)						Soybean Information			Date of Herbicide Application
	Texture	Sand	Silt	Clay	OM b	pH	Planting	Emergence	Harvest	PRE c	POST d
2019	Sandy Loam	62	23	15	2.9	7	8 June	15 June	1 October	11 June	3 July
2020	Sandy Loam	55	27	19	3.4	7.1	27 May	1 June	30 October	28 May	19 June

^a Soil characteristics were obtained from samples taken to a 15 cm depth below the soil surface and analyzed by A&L Canada Laboratories Inc. (2136 Jetstream Rd., London, ON, Canada, N5V 3P5). ^b OM, organic matter. ^c PRE, preemergence. ^d POST, postemergence.

. The soil was conventionally tilled, and soybean was planted between mid-May to early June at a rate of approximately 400,000 seeds ha⁻¹ to a depth of 3.75 cm. Glyphosate-resistant soybean cultivars DKB 12–16 and DKB 10–20 (Bayer CropScience Inc., 160 Quarry Park Boulevard, Calgary, AB, Canada, T2C 3G3) were planted in 2019 and 2020, respectively. The plots were 2.25 m wide (3 soybean rows spaced 75 cm apart) and 8 m long. Weed-free control plots were maintained with S-metolachlor/metribuzin (Boundary^® LQD, 1943 g a.i. ha⁻¹, Syngenta Canada Inc., 140 Research Lane, Guelph, ON, Canada, N1G 4Z3) applied PRE followed by glyphosate (Roundup WeatherMax^®, 900 g a.e. ha⁻¹, Bayer CropScience Inc., 160 Quarry Park Boulevard, Calgary, AB, Canada, T2C 3G3) applied POST as required. Quizalofop-p-ethyl (AMVAC Assure^® II, 36 g a.i. ha⁻¹, plus Sure-Mix, 0.5% v/v, Belchim Crop Protection Canada, 104 Copper Dr., Unit 3, Guelph, ON, Canada, N1C 0A4) was applied POST for annual grass control over the entire trial area. At the field site, other broadleaf weeds were present; however, the competitive nature of this green pigweed biotype based on its density and biomass, allowed green pigweed to dominate in the trial area eliminating any confounding factors due to the presence of other broadleaf weeds. Average soybean height and growth stage and green pigweed height, leaf number, and density at the POST herbicide application are presented in

Table 2. Average soybean height and development stage and green pigweed (A. powellii) height, leaf number, and density at POST herbicide application timing for two trials conducted near Dresden, ON, Canada in 2019 and 2020.

Year	Soybean		Green Pigweed b
	Height (cm)	Development Stage a	Height (cm)	Leaf Number	Density (Plants m−2)
2019	21	V2	10	13	134
2020	32	V2	5	6	179

^a Based on soybean growth staging by McWilliams et al. 1999. ^b Average height, leaf number, and density were recorded from two 0.25 m² quadrats in the non-treated control plots.

. Herbicide active ingredients, trade names, and manufacturers for the PRE study are presented in

Table 3. Herbicide active ingredients, trade names, and manufacturers for PRE herbicide treatments.

Herbicide Active Ingredients	Trade Name	Manufacturer
Clomazone	Command 360 ME	FMC of Canada Limited, 6755 Mississauga Road, Suite 204, Mississauga, ON, Canada, L5N 7Y2
Sulfentrazone	Authority® 480
Pyroxasulfone/sulfentrazone	Focus®
Saflufenacil/dimethenamid-p	Integrity®	BASF Canada Inc., 100 Milverton Drive, Mississauga, ON, Canada, L5R 4H1
Saflufenacil/imazethapyr	Optill®
Imazethapyr	Pursuit®
Cloransulam-methyl	FirstRate™	Corteva Agriscience Canada Company, 215—2nd Street SW, Suite 2450, Calgary, AB, Canada, T2P 1M4
Chlorimuron-ethyl	Classic™
Pyroxasulfone/flumioxazin	Fierce®	Valent Canada, Inc., 201–230 Hanlon Creek Blvd., Guelph, ON, Canada, N1C 0A1
Flumioxazin	Valtera™
Metribuzin	Sencor®	Bayer CropScience Inc., 160 Quarry Park Boulevard, Calgary, AB, Canada, T2C 3G3
Linuron	Lorox® L	Tessenderlo Kerley Inc., 2910 N. 44th Street, Suite 100, Phoenix, AZ, USA, 85018
S-metolachlor/metribuzin	Boundary® LQD	Syngenta Canada Inc., 140 Research Lane, Guelph, ON, Canada, N1G 4Z3

and for the POST study are presented in

Table 4. Herbicide active ingredients, trade names, and manufacturers for POST herbicide treatments.

Herbicide Name	Trade Name	Manufacturer
Cloransulam-methyl ab Chlorimuron-ethyl ab	FirstRate™ Classic™	Corteva Agriscience Canada Company, 215—2nd Street SW, Suite 2450, Calgary, AB, Canada, T2P 1M4
Imazethapyr ab Bentazon	Pursuit® Basagran® Forté	BASF Canada Inc., 100 Milverton Drive, Mississauga, ON, Canada, L5R 4H1
Acifluorfen	Ultra Blazer®	UPL NA Inc., 630 Freedom Business Center, Suite 402, King of Prussia, PA, USA, 19406
Fomesafen c Glyphosate/fomesafen c	Reflex® FlexstarTM GT	Syngenta Canada Inc., 140 Research Lane, Guelph, ON, Canada, N1G 4Z3
Thifensulfuron-methyl ab	Pinnacle® SG Toss-N-Go®	FMC of Canada Limited, 6755 Mississauga Road, Suite 204, Mississauga, ON, Canada, L5N 7Y2
Glyphosate	Roundup WeatherMax®	Bayer CropScience Inc., 160 Quarry Park Boulevard, Calgary, AB, Canada, T2C 3G3

^a Thifensulfuron-methyl, chlorimuron-ethyl, cloransulam-methyl and imazethapyr applied POST were mixed with 0.1% v/v, 0.2% v/v, 0.25% v/v, and 0.25% v/v of the adjuvant Agral^®90 (Syngenta Canada Inc., 140 Research Lane, Guelph, ON, Canada, N1G 4Z3), imazethapyr mixed with bentazon was applied with urea ammonium nitrate (UAN-28-0-0) alone at 2.0 L ha⁻¹. ^b Chlorimuron-ethyl, imazethapyr, cloransulam-methyl and thifensulfuron-methyl applied POST were mixed with 2.0 L ha⁻¹, 2.0 L ha⁻¹, 2.5 L ha⁻¹ and 8.0 L ha⁻¹ of urea ammonium nitrate (UAN-28-0-0). ^c Fomesafen and glyphosate/fomesafen applied POST were mixed with 0.5% v/v and 0.25% v/v of the adjuvant Turbocharge (Syngenta Canada Inc., 140 Research Lane, Guelph, ON, Canada, N1G 4Z3).

.

The herbicide treatments were applied using a CO₂-pressurized backpack sprayer equipped with a hand-held boom containing four ULD-120-02 (Pentair Canada Inc., 490 Pinebush Rd., Cambridge, ON, Canada, N1T 0A5) nozzles spaced 50 cm apart at an operating pressure of 207 kPa that was calibrated to deliver a spray volume of 200 L ha⁻¹. Application dates for the PRE- and POST-applied herbicide trials are presented in Table 1. The PRE herbicides were applied within 3 days of soybean planting. The POST herbicides were applied when green pigweed plants reached 5 to 10 cm in height (Table 1 and Table 2). The weather at both application timings was as follows; in 2019, the average temperature was 26 °C, relative humidity was between 50% (PRE) and 84% (POST), and wind speed was between 1.3 and 2.2 km h⁻¹, not exceeding 9 km h⁻¹. In 2020, average temperature at the application timings was 26 °C, relative humidity was between 82% (PRE) and 65% (POST), and wind speed was between 2.1 and 5.9 km h⁻¹, not exceeding 13 km h⁻¹. In both years the PRE trials received activating rainfall, with the 2020 trial receiving 25 mm of rainfall within 11 days of application (C. Kramer, University of Guelph, personal communication, 2022). The amount of rainfall in 2019 was not recorded (C. Kramer, University of Guelph, personal communication, 2022).

Visible soybean injury and yield and green pigweed control, density, and biomass data were collected. Soybean injury and green pigweed control were evaluated by the same individual to ensure consistent data collection. Visible soybean injury was assessed at 1, 2, and 4 wk after crop emergence (WAE) for the PRE herbicide study and 1, 2, and 4 wk after herbicide application (WAA) for the POST herbicide study. Soybean injury was assessed as a percentage with 0% being no visible soybean injury and 100% being complete soybean death. Visible green pigweed control was evaluated on a 0 to 100% scale as a visual estimation of the biomass reduction in each plot compared to the non-treated control in each replicate. Visible green pigweed control was assessed at 2, 4, and 8 WAA for the PRE herbicide study and 1, 2, 4, and 8 WAA for the POST study with 0 representing no decrease in green pigweed biomass and 100 being complete control. Green pigweed density and aboveground biomass were assessed at 8 WAA for both the PRE and POST studies. Green pigweed density and aboveground biomass were obtained using a 0.25 m² quadrat. For green pigweed density, the number of plants within the quadrat was counted and recorded for two separate quadrats in each plot. The aboveground green pigweed biomass was obtained by cutting the green pigweed plants at the soil surface and placing them into paper bags. The aboveground biomass samples were then placed in a kiln for approximately 2 weeks at 60 °C, and the dry biomass was weighed and recorded. At soybean harvest maturity, the center two rows of each plot were harvested with a small-plot combine; the weight and moisture content were recorded. Soybean yield was adjusted to 13% moisture.

The statistical analysis was conducted in SAS 9.4 (SAS Institute Inc., 100 SAS Campus Dr., Cary, NC, USA, 27513) using PROC GLIMMIX. The fixed effects were determined to be the environment (year) and treatment. The block was the only random effect. To meet the objective of identifying the most efficacious PRE- and POST-applied herbicides for the control of green pigweed, the data across the two site-years were pooled. The normality assumptions were met by plotting the residuals against predicted, treatment, year, and replicate. To generate the Shapiro-Wilk statistic to test for normality, PROC UNIVARIATE was used. Data for all variables except for density data in the PRE study were fitted to a normal distribution. To ensure the assumptions of normality were met, a lognormal distribution was used to analyze green pigweed density data for the POST study. The data were back-transformed to a normal distribution for presentation. Tukey’s HSD was used to separate treatments at a significance level of p = 0.05.

3. Results and Discussion

3.1. Preemergence Herbicide Study

Many of the herbicides applied PRE provided very high control of the multiple-herbicide-resistant (MHR) green pigweed biotype (

Table 5. Visible green pigweed (A. powellii) control, density, biomass, and soybean yield as impacted by PRE herbicide treatments from two field trials conducted in 2019 and 2020 near Dresden, Ontario.

Treatment a	Rate (g a.i. ha−1)	Green Pigweed Control (%)			Density c (No. m−2)	Biomass c (g m−2)	Soybean Yield d (t ha−1)
		2 WAA b	4 WAA	8 WAA
Non-treated control	-	0	0	0	64 a	208 ab	3.4 h
Weed-free control	-	100	100	100	0	0	4.8 a
Imazethapyr	100	41 d	4 f	0 g	62 ab	245 a	3.5 gh
Sulfentrazone	140	81 c	16 ef	7 fg	39 a–c	216 a	3.7 e–h
Saflufenacil/imazethapyr	25 + 75	95 a–c	36 de	17 e–g	28 b–d	209 ab	3.6 f–h
Saflufenacil/dimethenamid-p	25 + 222	97 ab	46 d	24 ef	19 c–e	194 a–c	3.8 d–h
Cloransulam-methyl	35	97 ab	56 cd	29 e	40 a–c	152 a–d	4.0 c–h
Clomazone	846	87 bc	42 d	29 e	22 c–e	170 a–c	4.1 b–g
Chlorimuron-ethyl + imazethapyr	9 + 75	95 a–c	72 bc	56 d	20 c–e	104 b–e	4.3 a–e
Linuron	2160	99 a	77 ab	68 cd	11 c–e	96 c–e	4.3 a–d
Imazethapyr + metribuzin	100 + 542	99 a	78 ab	73 b–d	14 c–e	47 c–e	4.3 a–d
Flumioxazin	108	99 a	84 ab	78 a–c	6 de	40 e	4.5 a–d
S-metolachlor/metribuzin	1620 + 323	99 a	86 ab	81 a–c	6 de	42 e	4.5 a–c
Chlorimuron-ethyl + metribuzin	9 + 412	98 a	87 ab	85 a–c	6 de	26 e	4.5 a–c
Pyroxasulfone/sulfentrazone	150 + 150	99 a	90 ab	89 ab	2 de	23 e	4.3 a–d
Flumioxazin + metribuzin	72 + 413	98 a	92 ab	90 ab	3 de	11 e	4.7 ab
Metribuzin	1120	99 a	94 a	91 ab	5 de	14 e	4.2 b–f
Flumioxazin + metribuzin + imazethapyr	75 + 413 + 77	99 a	95 a	91 ab	2 de	10 e	4.7 ab
Pyroxasulfone/flumioxazin	134 + 106	99 a	96 a	95 a	1 e	19 e	4.8 a

Note: Means followed by the same letter within each variable column are not statistically different based on Tukey’s HSD (p < 0.05). ^a Treatments containing multiple active ingredients are separated into either pre-formulated mixtures using “/” or separate products as part of a mixture using “+”. ^b Wk after application. ^c Green pigweed density and biomass was collected at 8 WAA. ^d Soybean yield was collected at crop maturity with crop yield adjusted to 13% moisture.

). At 2 WAA, 14 of the 17 herbicide treatments provided ≥95% control of the green pigweed biotype. However, by 8 WAA, the efficacy of many of these treatments had declined considerably resulting in only 8 of the 17 treatments providing control that was not different from the most efficacious treatment (78–95%). The decrease in control with all the herbicides tested over the course of the study highlights that some herbicides provided longer residual control of MHR green pigweed over time than others.

Although control at 2 and 4 WAA will be discussed, emphasis will be placed on control with the treatments at 8 WAA. The highest and most consistent control was provided by the pyroxasulfone/flumioxazin treatment with 96% control at 4 WAA and 95% control at 8 WAA (Table 5). Similarly, flumioxazin, S-metolachlor/metribuzin, chlorimuron-ethyl + metribuzin, pyroxasulfone/sulfentrazone, flumioxazin + metribuzin, metribuzin, flumioxazin + metribuzin + imazethapyr, provided 99% control at 2 WAA, but this declined to 84 to 96% control at 4 WAA and 78 to 95% control at 8 WAA. Chlorimuron-ethyl + imazethapyr, linuron, imazethapyr + metribuzin provided very high control of green pigweed at 2 WAA (95 to 99%), but by 8 WAA control declined to between 56 and 73%. Sulfentrazone, saflufenacil/imazethapyr, saflufenacil/dimethenamid-p, cloransulam-methyl, and clomazone controlled green pigweed by 81% to 97% at 2 WAA, but control rapidly declined to 16 to 56% at 4 WAA and <30% at 8 WAA (Table 5). Imazethapyr was the least effective PRE herbicide with very poor control at all time points.

Imazethapyr, sulfentrazone, and cloransulam-methyl did not reduce green pigweed density relative to the non-treated control at 8 WAA (Table 5). Saflufenacil/dimethenamid-p, clomazone, chlorimuron-ethyl + imazethapyr, linuron, imazethapyr + metribuzin, flumioxazin, S-metolachlor/metribuzin, chlorimuron-ethyl + metribuzin, pyroxasulfone/sulfentrazone, flumioxazin + metribuzin, metribuzin, flumioxazin + metribuzin + imazethapyr, and pyroxasulfone/flumioxazin reduced green pigweed density similarly by 66 to 98% at 8 WAA. Saflufenacil/imazethapyr reduced green pigweed density 56%, which was similar to all herbicide treatments evaluated with the exception that it was less than the pyroxasulfone/flumioxazin treatment (Table 5).

Imazethapyr, sulfentrazone, saflufenacil/imazethapyr, saflufenacil/dimethenamid-p, cloransulam-methyl, and clomazone did not reduce green pigweed biomass relative to the non-treated control (Table 5). Chlorimuron-ethyl + imazethapyr, linuron, imazethapyr + metribuzin, flumioxazin, S-metolachlor/metribuzin, chlorimuron-ethyl + metribuzin, pyroxasulfone/sulfentrazone, flumioxazin + metribuzin, metribuzin, flumioxazin + metribuzin + imazethapyr, and pyroxasulfone/flumioxazin reduced green pigweed biomass similarly by 50 to 95% at 8 WAA.

The interference from uncontrolled green pigweed reduced soybean yield by 29% from 4.8 to 3.4 t ha⁻¹ in this study (Table 5). Poor control of green pigweed with imazethapyr, sulfentrazone, saflufenacil/imazethapyr, saflufenacil/dimethenamid-p, and cloransulam-methyl resulted in a soybean yield that was similar to that in the non-treated control plots. Reduced green pigweed interference with chlorimuron-ethyl + imazethapyr, linuron, imazethapyr + metribuzin, flumioxazin, S-metolachlor/metribuzin, chlorimuron-ethyl + metribuzin, pyroxasulfone/sulfentrazone, flumioxazin + metribuzin, metribuzin, flumioxazin + metribuzin + imazethapyr, and pyroxasulfone/flumioxazin resulted in soybean yields that were similar to the weed-free control. Interference from green pigweed escapes in the clomazone treatment reduced soybean yield by 15%, which was similar to all herbicide treatments except for pyroxasulfone/flumioxazin (Table 5). The finding that the pyroxasulfone/flumioxazin treatment reduced weed interference such that soybean yield was similar to that of the weed-free control was also observed by Mahoney et al. [18] who had applied pyroxasulfone/flumioxazin at the same rate. The results indicate that yield was correlated to the efficacy of the treatments at controlling green pigweed and that pyroxasulfone/flumioxazin is consistently effective for MHR green pigweed control across all variables evaluated and in protecting soybean yield. As there was no visible phytotoxic effect greater than 5% from the herbicides on the soybean (data not shown), soybean yields were impacted mostly by the level of green pigweed control and reflect the efficacy of the treatment.

3.2. Postemergence Herbicide Study

While crop injury was observed for most herbicides tested at the 1 and 2 WAA evaluations, injury was generally low (< 5%) with most of the treatments except for thifensulfuron-methyl, which caused 38 and 36% injury at 1 and 2 WAA, respectively (

Table 6. Visible green pigweed (A. powellii) control, density, biomass, and soybean yield as impacted by POST herbicide treatments from two field trials conducted in 2019 and 2020 near Dresden, Ontario.

Treatment a	Rate (g a.i. ha−1)	Soybean Injury (%)			Green Pigweed Control (%)				Density c (No. m−2)	Biomass c (g m−2)	Soybean Yield d (t ha−1)
		1 WAA b	2 WAA	4 WAA	1 WAA	2 WAA	4 WAA	8 WAA
Non-treated control	-	0 b	0 b	0 a	0	0	0	0	93 a–c	404 a	2.9 ab
Weed-free control	-	0 b	0 b	0 a	100	100	100	100	0	0	4.7 a
Bentazon	1080	3 b	2 b	0 a	3 g	0 f	0 e	0 d	86 a–c	333 ab	3.2 ab
Imazethapyr	100	3 b	1 b	0 a	6 g	4 f	2 e	0 d	116 a	389 a	2.8 ab
Imazethapyr + bentazon	75 + 840	4 b	2 b	0 a	3 g	0 f	0 e	0 d	101 ab	355 a	2.4 b
Cloransulam-methyl	18	8 b	4 ab	1.3 a	15 f	11 e	7 e	4 d	115 a	338 ab	3.0 ab
Chlorimuron-ethyl	9	27 ab	24 ab	8.6 a	47 e	46 d	26 d	19 c	75 a–d	248 a–c	3.3 ab
Acifluorfen	600	13 ab	13 ab	2.8 a	70 cd	64 c	37 c	27 c	29 c–e	160 c–e	3.2 ab
Fomesafen	240	11 ab	6 ab	0 a	64 d	62 c	39 c	31 c	47 b–e	184 b–d	3.8 ab
Thifensulfuron-methyl	6	38 a	36 a	15 a	71 c	79 b	76 b	74 b	12 de	50 de	3.4 ab
Glyphosate/fomesafen	962 + 238	10 ab	7 ab	0 a	93 b	94 a	85 ab	85 ab	16 de	16 e	3.9 ab
Glyphosate	900	0 b	0 b	0 a	98 a	84 ab	94 a	94 a	2 e	0.4 e	4.6 a

Note: Means followed by the same letter within each variable column are not statistically different based on Tukey’s HSD (p < 0.05). ^a Treatments containing multiple active ingredients are separated into either pre-formulated mixtures using “/” or separate products as part of a mixture using “+”. ^b Wk after application. ^c Green pigweed density and biomass was collected at 8 WAA. ^d Soybean yield was collected at crop maturity with crop yield adjusted to 13% moisture.

). As the soybean cultivars selected were glyphosate resistant, the glyphosate treatment caused no visible soybean injury. Bentazon, imazethapyr, imazethapyr + bentazon, cloransulam-methyl, chlorimuron-ethyl, acifluorfen, fomesafen, and glyphosate/fomesafen caused similar soybean injury ranging from 3 to 27% at 1 WAA and 1 to 24% at 2 WAA, respectively. Thifensulfuron-methyl caused 38, 36, and 15% soybean injury at 1, 2, and 4 WAA, respectively; it caused the highest level of soybean injury.

Although 10 POST treatments were tested, only two treatments provided over 80% control at 8 WAA (Table 6). Bentazon, imazethapyr, imazethapyr + bentazon, and cloransulam-methyl provided very little green pigweed control (<15%) from 1 WAA onward. For example, bentazon provided only 3% control at 1 WAA, and this dropped to 0% from 2 to 8 WAA; a similar trend of no appreciable decrease in control was observed with imazethapyr and imazethapyr + bentazon. Glyphosate provided 99% control of green pigweed at 1 WAA, and while control decreased over time, control was still 94% at 8 WAA. Similarly, control with glyphosate/fomesafen was just marginally less than with glyphosate alone across the time points. Control with thifensulfuron-methyl was marginal, ranging between 71 and 79% from 1 to 8 WAA. Control of green pigweed with acifluorfen was 70% at 1 WAA, but by 8 WAA, it was only 27%. A similar trend of rapid decrease in control over time occurred with cloransulam-methyl, chlorimuron-ethyl, and fomesafen.

The relative green pigweed control with the POST herbicides evaluated followed a similar trend at 1, 2, 4, and 8 WAA, so green pigweed control at 8 WAA will be discussed. Bentazon, imazethapyr, imazethapyr + bentazon, and cloransulam-methyl controlled green pigweed 0 to 4% at 8 WAA (Table 6). Chlorimuron-ethyl, acifluorfen, and fomesafen controlled green pigweed similarly at 19 to 31% at 8 WAA. Thifensulfuron-methyl and glyphosate/fomesafen controlled green pigweed 74 and 85%, respectively, at 8 WAA. Glyphosate was the most efficacious herbicide; it controlled green pigweed 94% at 8 WAA. Glyphosate was identified as the most efficacious POST herbicide in this study. Glyphosate/fomesafen provided 93 and 94% control at 1 WAA and 2 WAA, respectively; however, the control dropped below 90% at 4 WAA and 8 WAA (Table 6). Culpepper et al. [19], found that glyphosate alone at 840 g a.i. ha⁻¹ controlled smooth pigweed 96 and 100%, respectively, across two trial years at 8 WAA. When fomesafen was co-applied with glyphosate, control of smooth pigweed was 95 and 100% across two trial years; however, 95% control in one trial year was statistically different from the 96% control obtained by glyphosate alone [19]. In the present study, glyphosate/fomesafen reduced green pigweed density and aboveground biomass by only 83 and 96% compared to glyphosate alone, which reduced green pigweed density and aboveground biomass by 98 and 100% relative to the non-treated control.

Bentazon, imazethapyr, imazethapyr + bentazon, cloransulam-methyl, chlorimuron-ethyl, acifluorfen, and fomesafen did not reduce green pigweed density relative to the non-treated control at 8 WAA (Table 6). Glyphosate/fomesafen, thifensulfuron-methyl, and glyphosate reduced green pigweed density by 83, 87, and 98%, respectively; there was no statistical difference among these three herbicide treatments. Bentazon, imazethapyr, imazethapyr + bentazon, cloransulam-methyl, and chlorimuron-ethyl reduced green pigweed biomass by <40%; there was no reduction in biomass relative to the non-treated control at 8 WAA (Table 6). While fomesafen and acifluorfen reduced green pigweed biomass by 54% and 60% relative to the untreated control, thifensulfuron-methyl, glyphosate/fomesafen, and glyphosate reduced green pigweed density by 88% or more.

Imazethapyr, bentazon, and imazethapyr + bentazon provided the lowest green pigweed control (0%) at 8 WAA in this study and are not viable options for the control of synthetic auxin and ALS-resistant green pigweed biotypes. Bauer et al. [20] reported that bentazon applied alone at 840 g a.i. ha⁻¹ controlled redroot pigweed 33 to 38% at 14 days after treatment. Although the bentazon rate applied was lower than that used in this study, it is evidence that bentazon applied alone does not provide adequate control of Amaranthus (spp.). Due to poor control with bentazon at 8 WAA, there was a reduction in green pigweed density and biomass of 8 and 18%, respectively. When bentazon was co-applied with imazethapyr, there was no reduction in green pigweed density and a 12% reduction in aboveground biomass compared to the non-treated control. Bauer et al. [20] found that when bentazon (840 g a.i. ha⁻¹) was co-applied with imazethapyr (53 g a.i. ha⁻¹), redroot pigweed was controlled by 95%.

Green pigweed interference reduced soybean yield by 38% compared to the weed-free control (Table 6). Reduced green pigweed interference with all the POST herbicide treatments resulted in a soybean yield that was similar to that in the weed-free control. The exception was the bentazon + imazethapyr treatment where green pigweed interference reduced soybean yield by 49% compared to the weed-free control. Glyphosate was the most efficacious treatment across all weed control assessment variables, and this resulted in the highest soybean yield.

Imazethapyr applied either PRE or POST controlled green pigweed 0% at 8 WAA and did not reduce green pigweed density and biomass. The other ALS inhibitors, cloransulam-methyl applied PRE and POST and chlorimuron-ethyl applied POST, provided poor control of green pigweed ranging from 4 to 29% at 8 WAA, the only exception being thifensulfuron applied POST which provided 74% control of green pigweed at 8 WAA. Sulfonylurea herbicides such as chlorimuron-ethyl and thifensulfuron-methyl are expected to provide control levels of Amaranthus (spp.) of at least 90% in biotypes where resistance to ALS-inhibitor herbicides is not present [21]. The ALS gene was sequenced using 6F-6R primers with conditions as described by McNaughton et al. [22] to identify target site mutations. This biotype has the Serine₆₅₃Threonine substitution on the ALS gene, conferring resistance to imazethapyr [17]. This mutation is well known to confer resistance to imidazolinones such as imazethapyr and to a lesser extent some of the other classes of ALS-inhibitors such as the sulfonylureas [23,24]. The result from the present study suggests that there may be differences in control within the ALS-inhibitor chemical families in this biotype.

There was variable control of green pigweed with the PPO-inhibiting herbicides. Sulfentrazone PRE, saflufenacil/imazethapyr PRE, acifluorfen POST, fomesafen POST, and flumioxazin PRE controlled MHR green pigweed 7, 17, 27, 31, and 78%, respectively, at 8 WAA (Table 5). Reduction in green pigweed density and biomass was variable with PPO-inhibiting herbicides. Sulfentrazone PRE, acifluorfen POST, and fomesafen POST did not reduce green pigweed density relative to the non-treated control; in contrast, saflufenacil/imazethapyr PRE and flumioxazin PRE reduced green pigweed density 70 and 91%, respectively. Sulfentrazone PRE and saflufenacil/imazethapyr PRE did not reduce green pigweed biomass relative to the non-treated control; in contrast, fomesafen POST, acifluorfen POST, and flumioxazin PRE reduced green pigweed biomass 54, 60, and 71%, respectively. Sulfentrazone PRE, and saflufenacil/imazethapyr PRE resulted in a soybean yield that was similar to the non-treated control; in contrast, reduced green pigweed interference with flumioxazin PRE resulted in a soybean yield increase of 32%, which was similar to the weed-free control. Acifluorfen POST and fomesafen POST resulted in a soybean yield that was statistically similar to the weed-free control. Variable green pigweed control with the PPO-inhibiting herbicides in this study is not consistent with findings from other studies. A study evaluating the efficacy of flumioxazin and pyroxasulfone applied alone and in combination found that pyroxasulfone/flumioxazin applied PRE at 240 g a.i. ha⁻¹ controlled Amaranthus spp. 100% at 4 WAA [18], which is similar to the 96% control obtained in this study. In the same study, flumioxazin co-applied with imazethapyr or metribuzin controlled Amaranthus spp. 100% at 4 WAA [18]. In another study, sulfentrazone applied alone at 140 g a.i. ha⁻¹ controlled green pigweed 96% at 8 WAA [25] compared to 7% in the present study. In this study pyroxasulfone/sulfentrazone and pyroxasulfone/flumioxazin controlled green pigweed 89 and 95%, respectively, at 8 WAA. This is similar to the average green pigweed control across Ontario with pyroxasulfone/sulfentrazone and pyroxasulfone/flumioxazin of 97 and 99%, respectively, at 6 to 8 WAA (M. Cowbrough, Ontario Ministry of Agriculture, Food and Rural Affairs, personal communication, 2022). Average control of green pigweed with saflufenacil/dimethenamid-p across Ontario field sites at 6 to 8 WAA was found to be 100% at the highest application rate in corn (M. Cowbrough, Ontario Ministry of Agriculture, Food and Rural Affairs, personal communication, 2022). Sweat et al. [26] found that the control of redroot pigweed at 21 DAT with fomesafen and acifluorfen at 280 g a.i. ha⁻¹ was 74 to 76% and 75 to 81%, respectively, across trial years. In this study, control of green pigweed between the 2 and 4 WAA evaluation timepoints with acifluorfen and fomesafen was 64 to 37% and 62 to 39% (Table 6). These numbers suggest poor control when considering that the acifluorfen and fomesafen rates used in this study were higher than those used by Sweat et al. [26]. Green pigweed control with PPO-inhibiting herbicides is lower in this study than in previous research conducted in Ontario. Further research is needed to determine the reason for reduced control of this biotype of green pigweed with some PPO-inhibiting herbicides. It is possible that this green pigweed biotype may be resistant to some PPO-inhibiting herbicides; however, research has not been conducted to confirm herbicide resistance.

4. Conclusions

Although resistance to synthetic auxin and ALS-inhibiting herbicides has been confirmed in this biotype, the results from these studies suggest there are effective PRE and POST herbicide options for the control of this MHR-green pigweed biotype in Ontario soybean production. Pyroxasulfone/flumioxazin PRE and glyphosate POST are the most efficacious herbicides for the control of green pigweed in soybean. The results from these studies suggest that the best strategy to control MHR-green pigweed is to begin with an effective PRE herbicide and then, if there are weed escapes, apply an effective POST herbicide with a different mode of action. If possible, both PRE and POST herbicide options should be included in a program to control MHR-green pigweed. The use of a diversified integrated weed management program with multiple effective modes of action can provide excellent control of green pigweed and mitigate the spread of herbicide-resistant biotypes in the province.