Abstract
AbstractThe loss of methyl bromide led vegetable growers to rely more heavily on herbicides to control weeds. Although herbicides can be effective, limited options in vegetable production challenge growers. Identifying new, effective tools to be applied over plastic mulch before planting, for improved weed control with minimal crop injury, would be beneficial. The objective of these experiments was to evaluate the persistence of preplant applications of glyphosate (1,125 or 2,250 g ae ha−1) plus 2,4-D (1,065 or 2,130 g ae ha−1) or dicamba (560 g ae ha−1) over plastic mulch, using analytical techniques and subsequent yellow squash and watermelon response. Glyphosate and 2,4-D were not analytically detected at damaging concentrations on plastic mulch when at least 3.5 cm of rainfall was received after application and before planting. In addition, bioassay results showing less than 10% visual injury for either squash and watermelon, with no growth or yield suppression observed, supported analytical results. In contrast, dicamba concentrations on plastic mulch, regardless of rainfall amount or time between application and planting, remained at damaging levels. Squash yields were reduced by dicamba applied 1 to 30 d before planting, whereas watermelon was more resilient. 2,4-D plus glyphosate applied preplant over plastic mulch can provide an additional herbicide option for vegetable growers. More research is needed to understand the impact of residual activity of 2,4-D when transplants land directly in holes in plastic mulch at the time of application. The relationship of dicamba with plastic mulch is complex, because the herbicide cannot be easily removed by rainfall. Thus, dicamba should not be included in a weed management system in plasticulture vegetable production.
Highlights
The loss of methyl bromide from the market place more than a decade ago led to a shift in weed control practices for plasticulture vegetable production systems (Culpepper et al 2009; Eure and Culpepper 2017; Stevens et al 2016)
Dicamba at 560 g ha−1, 2,4-D at 1,065 and 2,130 g ha−1, and glyphosate at 1,125 and 2,250 g ha−1 are theoretically equivalent to 56,000; 106,500; 213,000; 112,500; and 225,000 μg ae m−2 applied to plastic mulch, respectively
The exponential decay equation accurately described the removal of all three herbicides from the surface of the mulch with first-order rate constants (k) of 0.34 for dicamba, 0.24 and 0.17 for the 1× and 2× rates of 2,4-D, and 0.32 and 0.32 for the 1× and 2× rates of glyphosate (Table 3; Figures 1 and 2)
Summary
The loss of methyl bromide from the market place more than a decade ago led to a shift in weed control practices for plasticulture vegetable production systems (Culpepper et al 2009; Eure and Culpepper 2017; Stevens et al 2016). When glyphosate or paraquat was applied alone, 80% to 81%, 56% to 60%, and 55% to 74% control of wild radish, cutleaf evening primrose, and horseweed was observed, respectively (Culpepper et al 2005; Eubank et al 2008) To control these problematic weeds and start the season with minimal weed competition, additional herbicidal tools are needed. Synthetic auxin herbicides have been available for 70 years, interest in using these chemistries has been renewed, due to formulation improvements along with the introduction of 2,4-D and dicamba-tolerant agronomic crops (Anonymous 2017, 2018a, 2018b; Johnston et al 2018) The use of these compounds to control many troublesome weeds has been documented for decades. The objective of this experiment was to determine, using analytical and bioassay techniques, the potential for applying 2,4-D or dicamba over plastic mulch before transplanting yellow squash and watermelon
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