In Georgia plasticulture vegetable production, a single installation of plastic mulch is used for up to five cropping cycles over an 18-mo period. Preplant applications of glyphosate and glufosinate ensure fields are weed-free before transplanting, but recent data suggest that residual activity of these herbicides may pose a risk to transplanted vegetables. Glyphosate and glufosinate were applied preplant in combination with three different planting configurations, including 1) a new plant hole into new mulch, 2) a preexisting plant hole, 3) or a new plant hole spaced 15 cm from a preexisting plant hole (adjacent). Following herbicide application, overhead irrigation was used to remove residues from the mulch before punching transplanting holes for tomato, cucumber, or squash. Visible injury; widths; biomass; and yield of tomato, cucumber, or squash were not influenced by herbicide in the new mulch or adjacent planting configurations. When glyphosate was applied at 5.0 kg ae ha−1 and the new crop was planted into preexisting holes, tomato was injured by 45%, with reduced heights, biomass, and yields; at 2.5 kg ae ha−1 injury of 8% and a biomass reduction was observed. Cucumber and squash were injured by 23% to 32% by glyphosate at 5.0 kg ae ha−1, with reductions in growth and early-season yield; lower rates did not influence crop growth or production when the crop was placed into a preexisting plant hole. Glufosinate applied at the same rates did not affect tomato growth or yield when planted into preexisting plant holes. Cucumber, when planted into preexisting plant holes, was injured by 43% to 75% from glufosinate, with reductions in height and biomass, and yield losses of 1.3 to 2.6 kg ai ha−1; similar results from glufosinate were observed in squash. In multi-crop plasticulture production, growers should ensure vegetable transplants are placed a minimum of 15 cm away from soil exposed to these herbicides.

The 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.

Research was conducted from 2013 to 2015 across three sites in Mississippi to evaluate corn response to sublethal paraquat or fomesafen (105 and 35 g ai ha−1, respectively) applied PRE, or to corn at the V1, V3, V5, V7, or V9 growth stages. Fomesafen injury to corn at three d after treatment (DAT) ranged from 0% to 38%, and declined over time. Compared with the nontreated control (NTC), corn height 14 DAT was reduced approximately 15% due to fomesafen exposure at V5 or V7. Exposure at V1 or V7 resulted in 1,220 and 1,110 kg ha−1 yield losses, respectively, compared with the NTC, but yield losses were not observed at any other growth stage. Fomesafen exposure at any growth stage did not affect corn ear length or number of kernel rows relative to the NTC. Paraquat injury to corn ranged from 26% to 65%, depending on growth stage and evaluation interval. Corn exposure to paraquat at V3 or V5 consistently caused greater injury across evaluation intervals, compared with other growth stages. POST timings of paraquat exposure resulted in corn height reductions of 13% to 50%, except at V7, which was most likely due to rapid internode elongation at that stage. Likewise, yield loss occurred after all exposure times of paraquat except PRE, compared with the NTC. Corn yield was reduced 1,740 to 5,120 kg ha−1 compared with the NTC, generally worsening as exposure time was delayed. Paraquat exposure did not reduce corn ear length, compared with the NTC, at any growth stage. However, paraquat exposure at V3 or V5 was associated with reduction of kernel rows by 1.1 and 1.7, respectively, relative to the NTC. Paraquat and fomesafen applications near corn should be avoided if conditions are conducive for off-target movement, because significant injury and yield loss can result.

A telephone survey was conducted with growers in Iowa, Illinois, Indiana, Nebraska, Mississippi, and North Carolina to discern the utilization of the glyphosate-resistant (GR) trait in crop rotations, weed pressure, tillage practices, herbicide use, and perception of GR weeds. This paper focuses on survey results regarding herbicide decisions made during the 2005 cropping season. Less than 20% of the respondents made fall herbicide applications. The most frequently used herbicides for fall applications were 2,4-D and glyphosate, and these herbicides were also the most frequently used for preplant burndown weed control in the spring. Atrazine and acetochlor were frequently used in rotations containing GR corn. As expected, crop rotations using a GR crop had a high percentage of respondents that made one to three POST applications of glyphosate per year. GR corn, GR cotton, and non-GR crops had the highest percentage of growers applying non-glyphosate herbicides during the 2005 growing season. A crop rotation containing GR soybean had the greatest negative impact on non-glyphosate use. Overall, glyphosate use has continued to increase, with concomitant decreases in utilization of other herbicides.

Conservation tillage systems, used successfully by cotton producers on the Texas southern High Plains, have facilitated the development of new weed problems including horseweed (Conyza canadensis) and Russian thistle (Salsola iberica). Studies were conducted in 2004 and 2005 near Lubbock, TX to evaluate winter weed control with (2,4-dichlorophenoxy)acetic acid (2,4-D), dicamba, and diflufenzopyr plus dicamba. All of these herbicides have current registration restrictions limiting their use in cotton. Cotton response studies were initiated in 2003 and repeated in 2004 and 2005 to evaluate cotton injury and yield from dicamba (0.14 and 0.28 kg ai/ha), diflufenzopyr plus dicamba (0.10 and 0.20 kg/ha), and 2,4-D (0.56 and 1.12 kg/ha) applied 4, 2, and 1 wk before planting (WBP); and to determine the minimum interval between application of these herbicides and cotton planting without affecting yield. 2,4-D controlled both horseweed and Russian thistle, and could be applied as close as 2 WBP without injuring cotton. However, dicamba as well as diflufenzopyr plus dicamba were less effective on horseweed and Russian thistle as size increased and both injured cotton regardless of interval between application and planting.