Introduction

Most vegetable crops lack sufficient herbicide coverage to protect crops from weed competition without other inputs like cultivation and hand weeding (Fennimore and Doohan 2008). There are many reasons for this, including the diverse numbers of crops and crop varieties, small acreages and limited market potential, high crop values, and potential liability to the registrants for crop damage from herbicides (Fennimore and Cutulle 2019). Herbicides commonly used in vegetable crops, such as DCPA and pronamide, were developed before 1980, when costs were lower and the regulatory barriers were less demanding (Fennimore and Doohan 2008). DCPA is used in Allium vegetables, such as onion (Allium cepa L.), and many brassica vegetables, such as broccoli. DCPA was registered in 1958 and is labeled on many vegetable crops. However, regulatory concerns have been raised about a DCPA metabolite that is highly mobile in soil and has been found in groundwater (Istok et al. 1993; Lohstroh and Koshlukova 2017). Although DCPA remains available for use in brassica vegetables, there is no guarantee that this product used at rates as high as 11.2 kg ha^–1 will be available in the long term; therefore sustainable brassica vegetable production may require comparable preemergence alternatives to DCPA (Blecker et al. 2018; Daugovish et al. 2019).

The U.S. Environmental Protection Agency has granted food use tolerances for S-metolachlor and sulfentrazone for use on brassica head and stem Group 5-16 vegetables, which include broccoli, brussels sprouts, cabbage (Brassica oleracea L. var. capitata), bok choi, and cauliflower (Brassica oleracea L. var. botrytis), as well as brassica leafy greens Subgroup 4-16B, which includes 20 crops, such as kale (Anonymous 2022a, 2022c; USEPA 2017, 2018). S-metolachlor is a selective chloroacetamide herbicide that controls weeds by inhibiting the synthesis of long-chain fatty acids. S-metolachlor is widely used on corn (Zea mays L.), soybean [Glycine max (L.) Merr.], potato (Solanum tuberosum L.), sugar beet (Beta vulgaris L.), sunflower (Helianthus annuus L.), and tomato (Solanum lycopersicum L.). Soil half-life of S-metolachlor under California field conditions was estimated at 97 d (Shaner 2014). Sulfentrazone is an aryl triazinone herbicide that acts on the protoporphyrinogen oxidase enzyme that disrupts susceptible plant membranes and is primarily absorbed by roots. The soil half-life of sulfentrazone was determined to be in the range of 121 to 302 d (Shaner 2014). Sulfentrazone is labeled for use on brassica crops, such as cabbage (Anonymous 2022e).

S-metolachlor and sulfentrazone use in brassica head and stem Group 5-16 and brassica leafy greens Subgroup 4-16B has not been well characterized on seeded or transplanted crops under California conditions. Many of these crops, such as broccoli, are both seeded and transplanted (Le Strange et al. 2010). Therefore the objective of this work was to measure the selectivity of S-metolachlor on seeded root and tuber vegetables, such as radish and head and stem and leafy green vegetables, and the selectivity of S-metolachlor and sulfentrazone on transplanted brussels sprouts and kale.

Table 1.

Herbicide tolerance in Brassica vegetable crops: trial number, year, crop, variety, planting, and harvest dates.

Materials and Methods

Crop tolerance field studies were conducted in 2019 and repeated in 2020 at the Hartnell research farm at Salinas, CA (36.10°N, 121.36°W), on Antioch sandy loam soil, fine, smectitic, thermic Typic Natrixeralf (53% sand, 32% silt, and 15% clay) with a pH of 7.0 and 2.1% organic matter. S-metolachlor (DualMagnum^® 7.62E, Syngenta Crop Protection, Greensboro, NC, USA) was applied PRE at 0.37, 0.56, and 0.73 kg ha^–1, and the standard DCPA (DACTHAL^® 6 F, AMVAC, Los Angeles, CA, USA) was applied PRE at 8.41 kg ha^–1 on direct-seeded bok choi, broccoli rabe, collard, mizuna, mustard greens, and radish and on transplanted brussels sprouts and kale. The transplanted brussels sprouts and kale were also treated with sulfentrazone (Zeus^® 4F, FMC, Philadelphia, PA, USA) PRE at 0.08 and 0.11 kg ha^–1. All herbicides were applied with a CO₂ backpack sprayer. Application volumes in 2019 were 439 L ha^–1 for DCPA and 280 L ha^–1 for all other treatments, and in 2020, all treatments were applied at 374 L ha^–1. Plots were single 1-m-wide × 6.1-m-long beds. Treatments were replicated four times and arranged in a randomized complete block design. Trial numbers, years, crops, varieties, and planting and harvest dates are listed in Table 1.

After planting, the trials were sprinkler irrigated for 2 h, during which time 1.7 cm of water was applied to set transplants or germinate seed. The plots were cultivated and hand weeded as needed to minimize weed competition, fertilized with 330 kg ha^–1 21-0-0-24 (S), and sprinkler irrigated twice weekly until emergence, then once per week until harvest.

Data collected were crop injury estimates at 2 w after treatment based on a scale ranging from 0 (no injury) to 10 (plant death), which was converted to percentages for presentation in tables. The crop injury assessments included stunting and foliar injury in an overall injury score. Weed densities were measured 18 to 28 d after planting on the tops of the raised beds using a 48.3 × 53.3 cm (0.257 m²) quadrat, with the longer side laid across the width of the bed top and the sample area covering all plant lines. After weed density counts, all trials were cultivated and hand weeded. Crops were harvested at commercial maturity typical for the Salinas Valley. Bok choi, kale, and brussels sprouts were harvested from 2.13 m of bed, mizuna from 1.52 m of bed, and broccoli rabe from 3.05 m of bed both years. Collard was harvested from 3.05 m of bed in 2019 and 1.52 m of bed in 2020. Radish and mustard greens were harvested from 2.13 m of bed in 2019 and 1.52 m of bed in 2020. Data were subjected to analysis of variance, and mean separation was performed using Fisher's protected LSD. Agriculture Research Management (ARM) 7, version 7.0.5 (Gyllings Data Management Inc., Brookings, SD, USA) was used for data analysis.

Table 2.

Crop injury estimates at 15 d after treatment and fresh weights (at harvest) on direct-seeded bok choi.

Table 3.

Crop injury estimates at 15 d after treatment and fresh weights (at harvest) for direct-seeded collards.

Results and Discussion

Seeded Crops

DCPA and S-metolachlor caused little or no visible injury to bok choi, collard, radish, or mustard greens (Tables 2, 3, 4, and 5). S-metolachlor resulted in slight injury to broccoli rabe in 2019, possibly due to unusually cool and wet weather during May 15 to 26, 2019 (9 to 20 d after planting), when temperatures were 7 C below normal and 5 cm of rain fell (Table 6; UCIPM 2022). S-metolachlor, on the other hand, caused much greater initial injury in 2020 during normal warm and dry weather typical of the area. Injury to sweet potato [Ipomoea batatas (L.) Lam.] from S-metolachlor was less under cooler conditions of 25 C than at 35 C (Abukari et al. 2015). The year-to-year variation in broccoli rabe injury may suggest reduced sensitivity to S-metolachlor in cool weather and increased sensitivity in warm weather, but verification of this will require more research. Also, broccoli rabe visible injury declined to low levels by 42 d after treatment (data not shown) and resulted in similar fresh weights at harvest both years (Table 6). DCPA caused slight to moderate injury to broccoli rabe—greater than the nontreated both years—but did not reduce yields (Table 6). DCPA and S-metolachlor caused slight injury to mizuna in 2019. However, both DCPA and S-metolachlor treatments resulted in 6% to 15% injury to mizuna in 2020, significantly greater than the nontreated, with the exception of the 0.37 kg ai ha^–1 S-metolachlor treatment (Table 7). Mizuna in 2020 was grown during late September to early November, during much warmer conditions, when 750 GDD base 10 C occurred, compared to 572 GDD base 10 C in 2019 (UCIPM 2022). Increased mizuna injury may have been due to warmer conditions in 2020 than in 2019. The lower mizuna fresh weights in 2020 than in 2019 were across all treatments, which suggests that conditions for crop development were more ideal in 2019 than in 2020. The DCPA and S-metolachlor did not reduce harvestable yields in any of the seeded crops, including broccoli rabe and mizuna, relative to the nontreated (Tables 2, 3, 4, 5, 6, and 7).

Table 4.

Crop injury estimates at 15 d after treatment and fresh weights (at harvest) on direct-seeded radish.^a

Table 5.

Crop injury estimates at 15 d after treatment and fresh weights (at harvest) on direct-seeded mustard greens.

Table 6.

Crop injury estimates at 15 d after treatment and fresh weights (at harvest) on direct-seeded broccoli rabe.^a

Table 7.

Crop injury estimates at 15 d after treatment and fresh weights (at harvest) on direct-seeded mizuna.^a

Table 8.

Crop injury estimates at 14 d after treatment and fresh weights (at harvest) on transplanted brussels sprouts.

Table 9.

Crop injury estimates at 15 d after treatment and fresh weights (at harvest) on transplanted kale.

Weed Control

The predominant weeds in the trial site were burning nettle (Urtica urens L.), common purslane (Portulaca oleracea L.), and shepherd's-purse [Capsella bursa-pastoris (L.) Medik.] (Tables 10 and 11). S-metolachlor at 0.56 and 0.73 kg ai ha^–1 generally controlled common purslane and shepherd's-purse as well or better than DCPA. The S-metolachlor 0.37 kg ai ha^–1 controlled shepherd's-purse as well or better than DCPA but was inconsistent on common purslane (Table 10). In the transplanted trials, sulfentrazone at 0.08 and 0.11 kg ai ha^–1 controlled common purslane and burning nettle as well or better than DCPA. S-metolachlor at 0.56 and 0.73 kg ai ha^–1 generally controlled common purslane and burning nettle in the transplanted trials as well or better than DCPA. S-metolachlor at 0.37 kg ai ha^–1 did not adequately control common purslane and burning nettle (Table 11).

Table 10.

Common purslane and shepherd's-purse densities in direct-seeded vegetable trials.^a

Table 11.

Common purslane and burning nettle densities in transplanted vegetable trials.^a

S-metolachlor is already an important vegetable herbicide and has potential for expanded use. In Florida, sulfentrazone and S-metolachlor were evaluated on tomato and sulfentrazone on strawberry [Fragaria ×ananassa (Weston) Duchesne ex Rozier] (Sandhu et al. 2022). Sulfentrazone was safe on both tomato and strawberry, and S-metolachlor was safe on tomato. In the Pacific Northwest, S-metolachlor is registered on radish grown for seed (Peachey 2021). S-metolachlor was applied to 12 vegetable and flower crops in California during 2018, with the largest uses in carrot (Daucus carota L.), flowers, pepper (Capsicum annuum L.), spinach (Spinacia oleracea L.), tomato, and potato (CDPR 2021). S-metolachlor has been tested in combination with sulfentrazone for use in pepper in Canada (Robinson et al. 2008). The S-metolachlor label for Canada lists 14 vegetables (Anonymous 2022d). The U.S. label for S-metolachlor (DualMagnum^®) has plant-back restrictions of 60 d for a number of vegetable crop groups, including Group 1B root vegetables, Group 3.07 green onion, Group 4-16 brassica leafy greens, and Group 9 cucurbits (Anonymous 2022b).

Results of this work indicate that expansion of labeled crops for S-metolachlor should include direct-seeded bok choi, collard, mizuna, radish, and mustard greens as well as transplanted brussels sprouts and kale. We also recommend that sulfentrazone be labeled for use on transplanted brussels sprouts and kale.