Cole crop [broccoli, cabbage (Brassica oleracea var. oleracea L.), and cauliflower (Brassica oleracea var. botrytis L.)] and leafy green [collards and kale (Brassica oleracea var. sabellica L.)] production in the United States comprised 61,263 ha harvested during 2017, with nearly 22% of that occurring in the state of Georgia (Coolong, 2017; U.S. Department of Agriculture, 2019). In the Southeast, these crops have wide planting windows ranging from August to October and January to April (Coolong et al., 2016; Coolong, 2017). The vast range of planting dates provide unique challenges for weed control, with the potential for both problematic summer and winter annual weeds to be present before and at planting.
Among the most problematic weeds for these crops in Georgia are wild radish (Raphanus raphanistrum L.), cutleaf evening primrose (Oenothera laciniata Hill), Amaranthus spp., and Ipomoea spp. (Webster, 2014). These weeds cannot be controlled with herbicides when cole crops or leafy greens are transplanted or when seedlings have emerged. Therefore, it is essential to control weeds before planting in conjunction with residual herbicide use to minimize in-season weed emergence (Bitterlich et al., 1996; Smart et al., 2001). Controlling emerged weeds before seeding or transplanting can be extremely challenging because few herbicides are registered for use by growers. Preplant burndown herbicides with broad-spectrum postemergence activity are limited to glyphosate and paraquat (Kemble et al., 2019). With widespread glyphosate resistance in Palmer amaranth (Amaranthus palmeri S. Wats), paraquat is the only effective option if applied to small weeds (Chaudhari et al., 2017; Culpepper et al., 2006). Glyphosate and paraquat provide variable and often inadequate control of susceptible annual morningglory, wild radish, and cutleaf evening primrose (Culpepper et al., 2005; Hydrick and Shaw, 1995; Leon et al., 2016). Additionally, other vegetable-producing states, including California and Florida, have noted resistance to paraquat in horseweed (Conyza canadensis L.) (Moretti and Hanson, 2016) and American black nightshade (Solanum americanum Mill.) (Chase et al., 1998). Therefore, further control options are necessary to start the season weed-free and allow for the incorporation of multiple mechanisms of action for resistance management purposes.
Tillage in bareground production systems is effective for controlling these weeds and incorporating residual herbicides (Brainard et al., 2013; Cutulle et al., 2019). However, in Georgia, cole crops and leafy greens are often grown in plastic mulch systems. These systems eliminate the potential for tillage as a weed management tool and can increase weed densities and the species present. Although an effective fumigant system is used when laying plastic mulch, it is generally only effective for the first crop produced, often tomato (Solanum lycopersicum L.) or pepper (Capsicum annuum L.), which are considered high-value crops (Csinos et al., 1997; Culpepper et al., 2017; Eure and Culpepper, 2017; Webster et al., 2001). Cole crops and leafy greens are subsequently grown in the plastic mulch system and are often planted 6 to 24 months after the initial fumigation application and plastic mulch installation (Cutulle et al., 2019). By the time cole crops or leafy greens are planted, weeds emerge and establish not only between the plastic mulched beds (i.e., row middles) but also in holes of the plastic mulch from previous crops or areas of natural degradation of the plastic.
Mixtures of 2,4-D with glyphosate or paraquat could benefit growers if applied before planting cole crops and leafy greens because of its ability to control Palmer amaranth, annual morningglory, wild radish, and cutleaf evening primrose (Chaudhari et al., 2017; Culpepper et al., 2005; Leon et al., 2016). Although new 2,4-D formulations have been improved in regard to off-target movement (Corteva, 2018; Sosnoskie et al., 2015), their interactions with plastic mulch are unknown in terms of sorption and dissipation. Herbicide interaction with plastic mulch is variable and specific to a given herbicide. For example, herbicides have been shown to wash off the plastic mulch with an initial irrigation or rainfall event, partially wash off of plastic mulch over time, or bind to the plastic mulch without release (Culpepper et al., 2009; Grey et al., 2009, 2018; Randell et al., 2020). Glyphosate and paraquat are examples of herbicides that can be effectively washed off plastic mulch, whereas other herbicides, such as carfentrazone, bind without release (Culpepper et al., 2009; Grey et al., 2009). Flumioxazin applied over plastic mulch is an example of an herbicide that partially washes off the herbicide over time (Grey et al., 2009) and has caused significant injury and yield reductions for squash (Cucumis melo L.) and tomato (Culpepper et al., 2009). Similarly, halosulfuron-methyl applied over plastic mulch, even after 17 to 19 cm of rainfall, has been shown to damage squash, broccoli, and cabbage (Grey et al., 2018; Randell et al., 2020).
Another factor (in addition to rainfall or irrigation) to consider when understanding the relationship of a given herbicide and plastic mulch is the time interval between application and planting. When halosulfuron was applied over plastic mulch before crop planting, injury generally increased as applications were made closer to planting for squash, broccoli, and cabbage (Randell et al., 2020). In the case of broccoli and cabbage, halosulfuron removal from plastic mulch was more dependent on time than rainfall, with 52% to 58% injury for applications 7 d before planting (DBP) and 36% to 37% injury for applications 14 DBP when similar amounts of rainfall were received for both application timings (Randell et al., 2020). Therefore, some herbicides will require not only rainfall or irrigation for removal from plastic mulch but also time.
Understanding the interaction of 2,4-D with plastic mulch is critical to determining the potential for using this herbicide over plastic mulch for cole crop and leafy green production in these systems. If 2,4-D dissipates from the plastic mulch with rainfall, irrigation, or degradation, then it would offer a significant improvement in preplant herbicide options. Therefore, analytical and bioassay experiments were conducted to quantify variable rates of 2,4-D removal from plastic mulch over time and with irrigation before planting broccoli and collards.
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