Weed Management in Azuki Bean with Postemergence Herbicides

A limited number of postemergence (POST) herbicides are available for weed management in azuki bean production in Ontario. A total of three field tri-als were conducted during 2017 to 2019 at the Huron Research Station, University of Guelph, Exeter, Ontario, Canada to evaluate the efficacy of various postemergence (POST) herbicides for weed control in azuki bean. Fomesafen, acifluorfen and acifluorfen + Assist ® caused up to 6% crop injury and weed interference reduced azuki bean yield up to 42%. Bentazon, bentazon Forte, bentazon/acifluorfen, bentazon + fomesafen + Assist ® , bentazon + fomesafen + Turbocharge ® and bentazon Forte + fomesafen caused up to 16% injury and weed interference reduced azuki bean yield up to 53%. Fomesafen, acifluorfen, acifluorfen + Assist ® , bentazon, bentazon Forte, bentazon/acifluorfen, bentazon + fomesafen + Assist ® , bentazon + fomesafen + Turbocharge ® and bentazon Forte + fomesafen provided 36% to 88% control of redroot pigweed (Amaranthus retroflexus L.), common ragweed (Ambrosia artemesiifolia L.), and common lambsquarters (Cheno-podium album L.). Fomesafen, acifluorfen + Assist ® and acifluorfen provided 59% to 83% control of flower-of-an-hour (Hibiscus trionum L.), but bentazon, bentazon Forte, bentazon/acifluorfen, bentazon + fomesafen + Assist ® , bentazon + fomesafen + Turbocharge ® and bentazon Forte + fomesafen provided 78% to 99% control of H. trionum. All POST herbicides evaluated provide 99% to 100% control of wild mustard (Sinapis arvensis L.) in azuki bean. Based on these results, none of the POST herbicide evaluated that included bentazon can be used safely for the control of annual broadleaved weeds in azuki bean under Ontario environmental conditions.


Introduction
Dry bean production is important to the economy and agriculture in Canada. Dry bean growers in Canada produced 249,000, 322,000, 341,000 and 317,000 tonnes of dry bean in 2016, 2017, 2018 and 2019, respectively [1]. Ontario farmers produce most of the dry beans grown in Canada. In 2019, dry bean growers in Ontario planted 53,000 hectares of dry bean including white, black, cranberry, kidney, azuki and other market classes with a total farm gate value of approximately $100,000,000 [2]. Azuki bean [Vigna angularis (Willd.) Ohwi & Ohashi] is a specialty market class of dry bean grown mostly for the export market to Asia where it is used in confectionery products [3]. Azuki bean thrives well under Ontario environmental conditions and has become popular among dry bean growers in Ontario. There was an increase of 46% in azuki production in 2019 compared to 2018 [2]. One of the largest production obstacles in azuki bean production is yield loss from weed interference [4]. The Weed Science Society of America (WSSA) reported that dry bean yield was reduced 71% compared to 50% in corn and 52% in soybean due to weed competition [5] [6] [7]. Azuki bean growers need new herbicide options to control problematic weeds in their production.
Fomesafen, acifluorfen and bentazon are often used in combination with an adjuvant to improve weed control efficacy and provide a greater degree of weed control consistency under varying environments [8]. Assist ® is a mineral oil/surfactant adjuvant that consists of 83% paraffin base mineral oil plus 17% surfactant blend that is often added to fomesafen or acifluorfen [8]. Turbocharge ® is a surfactant/solvent adjuvant that consists of a 39.5% surfactant blend plus 50% solvent (mineral oil) and is often added to fomesafen [8]. Bentazon Forte does not require additional adjuvants [8].
Currently, fomesafen is the only POST herbicide registered for the control of broadleaved weeds in azuki bean production in Ontario [8]. Fomesafen does not adequately control common weeds in Ontario such as C. album, giant ragweed (Ambrosia trifida L.), P. persicaria and A. theophrasti [8]. The co-application of fomesafen with bentazon and bentazon with acifluorfen using different adjuvants available can be new herbicide options for weed management in azuki bean production.
There is little knowledge on the crop safety and effectiveness of fomesafen, acifluorfen and bentazon (with or without adjuvants), applied POST alone and in combination for weed control in azuki bean under Ontario environmental conditions. The co-application of these herbicides has the potential to produce efficacious full-season control of troublesome weeds in azuki bean in Ontario.

Experimental Methods
Field experiments were established in late May to early June of 2017, 2018 and 2019 at the Huron Research Station, University of Guelph, Exeter, Ontario, Canada with a five-crop rotation of winter wheat-corn-soybean-oats-azuki bean.
Each experiment was arranged in a randomized complete block design with 4 replicates. Treatments evaluated are listed in Table 1. The experimental plots were 3.0 m wide and 10.0 m long. Azuki bean "Erimo" was seeded 4 cm deep at the rate of approximately 200,000 seeds ha −1 in rows that were 75 cm apart in late May to early June.
Herbicide treatments were applied postemergence 3 -4 weeks after seeding when azuki beans were at the 1 -2 trifoliate leaf stage. Herbicides were applied with a CO 2 -pressurized backpack sprayer calibrated to deliver 200 L·ha −1 at 240 kPa. The spray boom was 1.5 m long equipped with 4 ultra-low drift (ULD 120-02, Pentair-Hypro, New Brighton, Minnesota) nozzles spaced 0.5 m apart, producing a spray width of 2.0 m. Azuki bean visible injury was evaluated 2 and 4 weeks after herbicide application (WAT) and weed control were assessed 4 and 8 WAT on a scale of 0 (no injury/control) to 100% (total plant necrosis/weed control). Weed density and aboveground dry weight (biomass) were measured 8 WAT from two 0.25 m 2 quadrats placed between the centre two rows from each plot. Azuki bean seed yield (adjusted to 13% moisture) was determined by harvesting the middle two rows of each plot at maturity.

Statistical Analyses
The experimental design was a randomized complete block with 4 replications. Data were analyzed using the GLIMMIX procedure in SAS [10]. The fixed effect was herbicide treatment and random effects were year-location combinations (environment), replicate within environment and the environment by treatment interaction. The Shapiro-Wilk statistic, fit statistics, residual plots and the potential distributions were used to identify the best distribution and associated link function for each parameter. Least square means (LSMEANS) were calculated on the data scale by using the inverse link function, and pairwise comparisons were subjected to Tukey's adjustment before determining treatment differences at P < 0.05. The normal distribution and identity link were used for adzuki bean injury 4 WAT, percent visible weed control at 4 and 8 WAT for redroot pigweed, ragweed and common lambsquarters and percent adzuki bean moisture and yield at harvest. The arcsine square root distribution and identity link were used for percent visible adzuki bean injury 2 WAT as well as percent visible weed control at 4 and 8 WAT for flower-of-an-hour and wild mustard. Weed density and dry biomass were analyzed using the lognormal distribution and identity link. The weedy control was assigned a value of 0 for injury and weed control, and the weed-free control was assigned a value of 0 for injury, weed density and biomass, or 100 for weed control and was excluded from the analysis due to zero variance. Comparisons were still possible between the other treatments and the value zero using the LSMEANS output and differences were identified. Arcsine square root and lognormal distributions were back-transformed for the presentation of results.
There was generally no difference between bentazon and bentazon Forte for the control of weeds evaluated except for C. album which was controlled more effectively with bentazon Forte (78% to 87%) compared to bentazon (55% to 66%). Bentazon or bentazon Forte applied POST at 1080 g ai ha −1 did not reduce density or dry weight of A. retroflexus and A. artemesiifolia, but decreased density and dry weight of H. trionum up to 98% and S. arvensis 100% (Tables  2-6). Bentazon did not reduce density and dry weight of C. album, but bentazon Forte decreased density and dry weight of C. album 67% and 66%, respectively. In other studies, bentazon Forte applied POST at 840 g ai ha −1 controlled A. retroflexus 85% to 86%, A. artemesiifolia 63% to 66%, C. album 88% to 91% and S. arvensis 96% to 97% [14].
Bentazon/acifluorfen (with Assist ® ) applied POST at 840 g ai ha −1 did not reduce density or dry weight of A. retroflexus, A. artemesiifolia and C. album, but decreased density and dry weight of H. trionum as much as 84% and S. arvensis 100% (Tables 2-6).