Radish, a member of the Brassicaceae family is an important cool-season crop grown in rotation with sugarcane (Saccharum spp. hybrids), leafy vegetables, and rice (Oryza sativa L.) in the EAA of south Florida. The EAA is dominated by organic soils (Histosols) that were formed under flooded conditions, which precluded decomposition of organic matter, allowing those materials to form organic soils with up to 85% or more organic matter (Snyder, 1994; Wright and Hanlon, 2009). About 3200 ha of radish valued at $3 million are planted annually in the EAA (Chris Miller, personal communication).
Radish has a relatively short growth period and is primarily planted in the EAA from October to April. ‘Red silk’ is the predominant variety of radish cultivated in the EAA because of its high adaptability to organic soils (Miller et al., 2014). This variety is an open pollinated, globe, red, medium top size with excellent flesh quality, and matures in 26 to 30 d (Miller et al., 2014). Weed competition can limit radish production despite its relative short growth period. For instance, Santos et al. (1998) reported complete radish crop failure as a result of season-long competition from 100 to 200 purple nutsedge (Cyperus rotundus L.) plants/m2 over a 30-d growing season. In the organic soils of the EAA, negative effects of weed interference on radish are due to lack of registered effective preemergence or postemergence herbicides for broad-spectrum weed control. Many soil-applied preemergence herbicides are prone to adsorption and metabolism by soil microorganisms in these organic soils because of their high cation exchange capacity, large soil microbial populations, and relatively high soil moisture and temperature (Schueneman and Sanchez, 1994; Shea, 1989). Binding and degradation of soil-applied herbicides on these organic soils due to adsorption and metabolism combine to reduce their efficacy, resulting in poor weed control (Schueneman and Sanchez, 1994). Also, mechanical weed control by cultivation is not an option for radish grown on these organic soils because of concerns of disturbance of the crop’s rooting system.
Radish is a Group 1B vegetable along with carrot (Daucus carota L.) and garden beet (Beta vulgaris L.) (Legal Information Institute, 2010). Recently, S-metolachlor was registered for preemergence weed control in root and tuber vegetable crops Group 1B under Special Local Needs 24 (c) registration through the Third Party Registrations, Inc., a subsidiary of the Florida Fruit and Vegetable Association (Florida Fruit & Vegetable Association, 2015). S-metolachlor is a preemergence chloroacetanilide herbicide for broadleaf weed and grass control thought to inhibit the very long-chain fatty acid synthesis (Böger et al., 2000; Shaner, 2014). This herbicide more readily adsorbs to organic matter and is primarily absorbed by emerging shoots of susceptible grasses just above the seed and in broadleaf weeds through the root and shoot (Shaner, 2014). S-metolachlor is an 88:12 mixture of the S- and R-isomers, respectively; however, it is the S-isomer that provides 95% of the herbicidal activity (Moser et al., 1982; Muller et al., 2001; O’Connell et al., 1998). Also, S-metolachlor is more active at the site of action in susceptible plants and allows for lower use rates than the racemic (50:50) mixture of S- and R-isomers of metolachlor (Shaner, 2014). The lower use rate of S-metolachlor compared with the racemic mixture of metolachlor on an active ingredient basis has provided growers with the opportunity to reduce herbicide load applied to the environment while still maintaining biological performance (O’Connell et al., 1998; Shaner, 2014). For example, S-metolachlor provided the same efficacy on major grass weeds and tolerance to different corn (Zea mays L.) varieties at 65% use rate of racemic metolachlor (O’Connell et al., 1998). Thus, radish growers in the EAA have an opportunity to use S-metolachlor at lower rates compared with the racemic metolachlor in the environmentally sensitive EAA for weed management. But, the residual activity of soil-applied S-metolachlor should be long enough on organic soils to prevent deleterious effects of weed interference for an extended period until radish achieves a competitive advantage. Currently, there is limited information on the efficacy and level of crop safety of S-metolachlor on radish grown on organic soils of the EAA. Therefore, this study was conducted to determine the efficacy of soil-applied preemergence S-metolachlor on weed control and radish tolerance on organic soils of the EAA using a dose–response bioassay.
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