Introduction

Dry bean and azuki bean are important food crops. In 2020, the United States was the fourth largest producer of dry bean; the highest dry bean-producing states were North Dakota, Michigan, Minnesota, Nebraska, and Idaho (FAO 2021; Lucier and Davis 2020). Although production occurs on smaller hectarages in Canada, farm cash receipts from Canadian dry bean production totaled more than Can$315 million in 2020 (Government of Alberta 2021; Manitoba Agriculture 2021; OMAFRA 2021d). Ontario accounts for 38% of Canadian dry bean production with approximately 63,500 hectares seeded to dry bean in 2020 (OMAFRA FCT 2020). White (navy) bean accounts for 50% of Ontario dry bean production, the remaining hectares are seeded primarily with black, kidney, cranberry, and azuki bean market classes (OMAFRA FCT 2020; OMAFRA 2021a, 2021b). Dry bean demonstrates sensitivity to weeds, in part, resulting from its small stature (Ghamari and Ahmadvand 2012; Sikkema et al. 2007). A meta-analysis by Soltani et al. (2017a) concluded that 56% yield loss in Ontario dry bean would result when weeds are not controlled.

Although dry bean is sensitive to weed interference, far fewer herbicide options are available for weed management in Ontario dry beans relative to soybean. EPTC, dimethenamid-P, pendimethalin, S-metolachlor, and trifluralin are registered for soil application in Ontario and are used primarily for annual grass control; clethodim, fluazifop-p-butyl, quizalofop-p-ethyl, and sethoxydim are registered postemergence (POST) herbicides for grass weed control. For broadleaf weed control, halosulfuron and imazethapyr are the only two herbicides registered for soil application, and POST broadleaf herbicides in Ontario are limited to halosulfuron, fomesafen, and bentazon (OMAFRA 2021c). Dry beans are sensitive to many herbicides, which prevents the use of some herbicides that are used for soybean from being registered with dry bean crops (Cowan and Sikkema 2018; Shaner 2014).

Flufenacet is an oxyacetamide herbicide that belongs to the Group 15 herbicides (WSSA; Soltani et al. 2005). It is a very long-chain fatty acid elongases (VLCFAE) inhibitor. Absorption takes place through the roots and shoots of emerging weeds, providing control of many annual grass and select small-seeded broadleaf weeds (Gajbhiye and Gupta 2001; Johnson et al. 2012; Soltani et al. 2005). Flufenacet is registered for the control of green foxtail [Setaria viridis (L.) P. Beauv.], yellow foxtail [Setaria pumila (Poir.) Roem. & Schult.], giant foxtail (Setaria faberi Herrm.), witchgrass (Panicum capillare L.), barnyardgrass [Echinochloa crus-galli (L.) P. Beauv.], fall panicum (Panicum dichotomiflorum Michx.), proso millet (Panicum miliaceum L.), smooth crabgrass [Digitaria ischaemum (Schreb.) Schreb. ex Muhl.], large crabgrass [Digitaria sanguinalis (L.) Scop.], and yellow nutsedge (Cyperus esculentus L.). In addition, flufenacet is registered for the suppression of common lambsquarters (Chenopodium album L.), redroot pigweed (Amaranthus retroflexus L.), and Powell amaranth (Amaranthus powellii S. Wats.) (A Kaastra, Bayer Crop Science, personal communication, February 25, 2022).

Acetochlor is a WSSA Group 15 chloroacetanilide herbicide that is also a VLCFAE inhibitor (Shaner 2014). It is commonly used in U.S. corn, soybean, and cotton (Gossypium hirsutum L.) production but has not been registered for use in Canada (Cahoon et al. 2015; Murschell and Farmer 2019). Acetochlor provides effective control of broadleaf signalgrass [Urochloa platyphlla (Munro ex C. Wright) R. D. Webster], barnyardgrass, redroot pigweed, hairy nightshade (Solanum physalifolium Rusby), common lambsquarters, and Palmer amaranth (Amaranthus palmeri S. Watson; Cahoon et al. 2015; Jursik et al. 2013; Mueller and Steckel 2011). It exists as two different stand-alone formulations that include an emulsifiable concentrate (EC) and a micro-encapsulated (ME) product. The ME product, as used in this study, is a slow-release acetochlor via the use of a polymer coating that increases crop safety in certain crops (Cahoon et al. 2015; Fogleman et al. 2018). Although injury with ME acetochlor exceeds 20% in select nonregistered, sensitive crops such as pumpkin (Curcurbita pepo) and rice (Oryza sativa), less injury occurs than when the EC formulation is used (Ferebee et al. 2019; Fogleman et al. 2018).

S-metolachlor is a registered chloroacetanilide, WSSA Group 15 herbicide used for weed control in dry bean in Canada (Soltani et al. 2018). It primarily controls annual grasses with activity on select small-seeded annual broadleaf weeds (Osborne et al. 1995; Sikkema et al. 2009). Although visible dry bean injury has been documented with S-metolachlor; dry bean shoot biomass, height, density, seed moisture, and yield are rarely affected (Sikkema et al. 2009; Soltani et al. 2018).

Studies investigating the tolerance of dry bean to flufenacet and acetochlor applied preplant incorporated (PPI) have not been completed to our knowledge. Flufenacet and acetochlor have activity primarily on annual grasses, with select activity on small-seeded broadleaf weeds. Canadian dry bean growers would benefit from additional herbicide options for weed control, which may lead to improved weed control, lower yield losses from weeds, and increased net returns for producers, assuming that there is sufficient dry bean tolerance to these products, and registrations are permissible.

The research objective of this study was to evaluate azuki, kidney, small red, and white bean tolerance to PPI applications of three Group 15 herbicides (flufenacet, acetochlor, and S-metolachlor) at the 1× and 2× rates.

Materials and Methods

From 2019 to 2021, six field trials were conducted (two trials per year) in Ridgetown, ON, at the University of Guelph Ridgetown Campus, and near Exeter, ON, at the Huron Research Station. Soil characteristics for each site-year are summarized in Table 1. The cumulative rainfall received and the average daily temperature for 7 and 14 d following herbicide application at each site-year is listed in Table 2.

Table 1.

Soil characteristics for the six field trials.^a

Table 2.

Cumulative precipitation and average daily temperature during the six field trials conducted.^a

The experimental design used for the study was a split-plot with four replications. Herbicide treatment was the main plot factor and was arranged in a randomized complete block design. A nontreated control, flufenacet (750 and 1,500 g ai ha^–1), acetochlor (1,700 and 3,400 g ai ha^–1), and S-metolachlor (1,600 and 3,200 g ai ha^–1) was included in each replicate. The rates used for S-metolachlor represent the manufacturer's recommended 1× label rate and 2× rate, while the rate structure for flufenacet and acetochlor were determined based on previously identified rate ranges. The 2× rates were included to imitate an application overlap in the field. Main plots differed slightly in size; plots measured 6 m wide by 8 m long in Ridgetown, ON, with plots being slightly longer in Exeter, ON, at 6 m wide by 10 m long. Subplot factor was dry bean market class, which included azuki, white, kidney, and small red beans represented by the cultivars ‘Erimo’, ‘T9905’, ‘Dynasty’, and ‘Viper’, respectively. Due to equipment limitations, dry bean was seeded in a continuous fashion and was not randomized within each main plot. Two rows of each market class were sown 4 cm deep in rows spaced 75 cm apart. Azuki, kidney, small red, and white bean were seeded at 230,000, 188,000, 207,000, and 254,000 seeds ha^–1, respectively, in Exeter; and 232,900, 175,500, 232,900, and 232,900 seeds ha^–1, respectively, in Ridgetown.

Site preparation began in the fall when sites were moldboard plowed. One pass of spring cultivation was conducted prior to herbicide application with an S-tine cultivator with rolling basket harrows. The entire experimental area was maintained weed-free with a cover spray of pendimethalin (1,000 g ai ha^–1) + imazethapyr (37.5 g ai ha^–1) applied preemergence (PRE). Fomesafen (240 g ai ha^–1) was applied POST when needed in addition to hand hoeing as necessary. Herbicides were applied using a CO₂-pressurized backpack sprayer that delivered 200 L ha^–1 at 240 kPa with ultra-low drift 120-02 nozzles (ULD120-02; Hypro, Pentair Ltd., London, UK). Within 1 h of application, the herbicides were incorporated with an S-tine cultivator with rolling basket harrows. Two passes in opposite directions were used to incorporate the herbicides. All dry bean market classes emerged at similar times.

Visible dry bean injury assessments were completed at 1, 2, 4, and 8 wk after emergence (WAE) on a percent scale of 0 to 100 where 0% represented no injury and 100% indicated complete plant death. All injury symptoms that were present at a specific site-year were evaluated. Density and biomass assessments were completed at 3 WAE by counting and clipping bean plants in 1 m of row at the soil line; samples were placed in paper bags, kiln-dried at 60 C for 2 wk, and the biomass was recorded. Shoot dry weight per plant was determined by taking the weight of the biomass in 1 m of row and dividing it by the number of plants. Bean height measurements were taken at 6 WAE by averaging the height of 10 arbitrarily selected plants from both rows in each plot. Dry beans were straight-cut and threshed with a small-plot combine at harvest maturity; seed moisture content and seed yield weight were documented. Seed moisture content was adjusted to standard moistures of 13% for azuki bean and 18% for Phaseolus vulgaris classes prior to statistical analysis.

Data analysis was performed using SAS software v. 9.4 (SAS Institute Inc., Cary, NC) and the GLIMMIX procedure. Fixed effects included herbicide treatment, dry bean market class, and the interaction between herbicide treatment and dry bean market class, while environment (site-year combination), block nested within the environment, the interaction of dry bean market class, herbicide treatment, and environment, and the interaction of herbicide treatment by block nested within environment were the random effects. The F-test and Z-test were used to assess the significance of the fixed and random effects, respectively. Studentized residual plots were analyzed, the Shapiro-Wilk test statistic was verified, and a check for overdispersion was conducted to satisfy the assumptions of homogeneity and normality using the UNIVARIATE procedure. Injury assessments were transformed to normalize data using the arcsine square root transformation. Dry bean density, shoot biomass, height, seed moisture, and seed yield were analyzed as a percentage of the nontreated control to allow market classes to be compared. Dry bean density, shoot biomass, height, and seed yield were transformed using the square root function, while seed moisture content used a lognormal distribution. All transformed data were back-transformed to the original scale for the presentation of results. Comparisons between herbicide treatments were made using a Tukey-Kramer grouping test with a significance of P < 0.05.

Results and Discussion

Visible dry bean leaf deformation, growth reduction, stand reduction, chlorosis and necrosis, delayed emergence, and bleaching were evaluated at 1, 2, 4, and 8 WAE, though for clarity of discussion only, the prevalent symptoms of leaf deformation, growth reduction, stand reduction, and chlorosis and necrosis at 2 WAE are presented. Injury symptoms were evaluated only when present; not all symptoms were present at each site-year, thus explaining why not all injury symptoms were evaluated at all six site-years. Main effects are presented when there was no interaction between dry bean market class and herbicide treatment. Where a significant interaction was detected, simple effects are presented. No injury was observed from the PRE cover spray of pendimethalin (1,000 g ai ha^–1) + imazethapyr (37.5 g ai ha^–1). Some injury from the POST application of fomesafen did occur in Exeter in 2021, however, the timing of application allowed for recovery. Additionally, the remaining injury in the nontreated control was considered when injury assessments were performed.

Stand Reduction

Visible stand reduction was evaluated at five site-years. The simple effects are presented (Table 4) as an interaction was detected (Table 3). The Group 15 herbicides did not cause stand reductions in kidney, small red, or white bean at 2 WAE. Flufenacet (1× rate), acetochlor (1× and 2× rate), and S-metolachlor (1× rate) reduced azuki bean stand 1% to 3% at 2 WAE, whereas flufenacet (2× rate) and S-metolachlor (2× rate) caused a stand reduction of 18% and 5%, respectively. At the 1× rate, flufenacet, acetochlor, and S-metolachlor reduced azuki bean stand similarly. At the 2× rate, S-metolachlor caused a greater stand reduction than acetochlor; flufenacet (2× rate) reduced azuki bean stand by 18% at 2 WAE, which was greater than all other herbicide treatments evaluated. At 2 WAE, flufenacet (1× rate) caused a greater stand reduction in azuki bean than small red bean; stand reduction in kidney and white bean was intermediary and similar to that of the other dry bean market classes. There was a greater stand reduction in azuki bean than kidney, small red, and white bean with flufenacet (2× rate) and S-metolachlor (2× rate) at 2 WAE.

Table 3.

Mean values of main effects and their interaction.^a,c,d

Table 4.

Percent visible stand reduction.^a,b

Plant Height

Dry bean height was evaluated at all six site-years. The simple effects are presented (Table 6); there was a significant interaction (Table 5). Flufenacet, acetochlor, and S-metolachlor (1× and 2× rates) did not cause a decrease in kidney, small red, or white bean height. Acetochlor and S-metolachlor at the 1× rate did not reduce azuki bean height; flufenacet at the 1× rate reduced azuki bean height by 10%. Acetochlor, S-metolachlor, and flufenacet at the 2× rate reduced plant height of azuki bean by 12%, 16%, and 23%, respectively. Similarly, Soltani et al. (2018) concluded that relative to P. vulgaris market classes, azuki bean height was much lower when treated with Group 15 herbicides such as pethoxamid, S-metolachlor, dimethenamid-P, and pyroxasulfone at 1× and 2× rates.

Table 5.

Mean values of main effects of herbicides on dry beans as a percentage of the nontreated control and their interaction.^a,b

Table 6.

Height and yield of dry beans as a percentage of the nontreated control.^a