Watermelon yield losses due to weed competition in the southeastern United States (Georgia, Florida, Alabama, and South Carolina) are estimated to be 15% annually (Chandler et al., 1984). Weed control in vegetable crops can be difficult because of the slow growth of the crops and the limited number of registered herbicides (Gilreath and Santos, 2004). Pressure from weed competition increases costs and reduces profit margin because of control costs, difficulty in harvesting, and reduction in crop quality and yield (Brandenberger et al., 2005).
Weed control is especially difficult in watermelon because of their vining growth habit as cultivation is not feasible after the vines begin to spread into the row middles. The combination of wide row spacing and slow growth of watermelon complicates weed control measures (Terry et al., 1997). Yellow nutsedge (Cyperus esculentus) can reduce watermelon yield by 10% at 2 plants/m2 and 66% at 37 plants/m2 (Buker et al., 2003). Transplanted watermelons competing with crabgrass (Digitaria sp.) have a critical weed-free period of 0–6 weeks (Monks and Schultheis, 1998). Direct-seeded watermelon competing with smooth pigweed (Amaranthus hybridus) has a critical weed-free period of 0–3 weeks (Terry et al., 1997). A blend of chemical and cultural controls is often needed to move beyond the critical weed-free period in watermelon (Monday et al., 2015).
Multiple PRE herbicides have been evaluated for use in watermelon and compared based on plant injury and yield (Brandenberger et al., 2005). Halosulfuron applied at any rate injured watermelon; however, in most cases plants recovered by 5–7 WAT. This also occurred when halosulfuron was tank-mixed with other PRE herbicides. Watermelon yield was greatest and weed control was best with a tank mix of clomazone, ethalfluralin, and halosulfuron. This treatment caused a significant amount of watermelon injury (30% at 2–4 WAT and 26% at 5–7 WAT), but had no effect on marketable yield (Brandenberger et al., 2005).
Cover crops have been used in numerous cropping systems to improve weed control. In addition to providing weed suppression, benefits of growing a cover crop include improved soil structure, preservation of soil moisture, erosion control, improved water infiltration, and reduced runoff (Teasdale, 1996). Teasdale and Mohler (1993) reported that cover crop residues have an influence on weed populations because of the residue proximity to the site of seed germination. Some weed seeds require light transmittance to germinate, and cover crop residue can reduce the amount of light reaching the soil. Once seeds germinate, they must emerge through a thick layer of mulch likely using much of the seedlings’ energy reserves (Teasdale and Mohler, 1993). Therefore, the cover crops can alter the microenvironment around the seed, enough to reduce or delay the weed emergence (Teasdale and Mohler, 1993).
Because of its high biomass production and allelopathic compounds, cereal rye is an excellent cover crop (Barnes and Putnam, 1987; Mwaja et al., 1995). With an average biomass of 6250 kg·ha−1 at termination, cereal rye alone can reduce palmer amaranth (Amaranthus palmeri) emergence up to 50% (Reeves et al., 2005; Webster et al., 2016). In addition, cereal rye produces allelopathic compounds that can inhibit weed growth. Although there are many benefits to growing a cover crop, they only assist in early season weed control (Teasdale, 1996). Herbicides are needed to achieve maximum weed control in a production system using a cover crop.
When cover crops are combined with herbicides, it is possible to achieve satisfactory weed control in vegetables. In a study evaluating squash (Cucurbita pepo), planted behind high biomass varieties of cereal rye, smooth crabgrass (Digitaria ischaemum) and redroot pigweed (Amaranthus retroflexus) were effectively controlled (up to 95%) when PRE herbicides were used in conjunction with the mulch (Walters et al., 2005). Benefits associated with combining PRE herbicides with mulch were demonstrated in similar studies with fresh market cucumber [Cucumis sativus (Walters et al., 2007)] and pumpkin [C. pepo (Walters et al., 2008)].
Herbicide behavior can be influenced by cover crop residues left on the soil surface (Teasdale et al., 2003). High-residue cover crops can intercept herbicides and reduce efficacy of soil-active herbicides. The herbicide’s sorption to the cover crop can render it less active or physically inhibit it from reaching the soil to control emerging weeds (Locke and Bryson, 1997). Teasdale et al. (2003) reported that hairy vetch (Vicia villosa) increased decomposition rates and initial soil solution of metolachlor. This led to reduced grass control and increased pigweed (Amaranthus sp.) emergence (Teasdale et al., 2003). Because of this issue, there is a need to evaluate PRE herbicide application methods to reduce interception and sorption by the cover crop. Cover crop management can also influence weed control. In a study conducted on palmer amaranth control in peanut (Arachis hypogaea), different cover crop management systems were tested in combination with different PRE herbicides (Dobrow et al., 2011). PRE herbicides were applied with no cover crop, with the cover crop rolled, and with the cover crop still standing. The longest period of palmer amaranth–free days occurred when the PRE herbicide was applied with the cover crop left standing.
The objective of this study was to determine if applying PRE herbicides precrimp provides greater nutsedge (Cyperus sp.) and broadleaf weed control than traditional PRE herbicide applications postcrimp in a cereal rye cover crop.
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