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Cole crops including broccoli and collard contribute more than $119 million to Georgia’s farm gate value yearly. To ensure maximum profitability, these crops must be planted into weed-free fields. Glyphosate is a tool often used to help achieve this goal because of its broad-spectrum activity on weeds coupled with the knowledge that it poses no threat to the succeeding crop when used as directed. However, recent research suggests that with certain soil textures and production systems, the residual soil activity of glyphosate may damage some crops. Therefore, field experiments were conducted in fall 2019 and 2020 to evaluate transplanted broccoli and collard response to glyphosate applied preplant onto bare soil and what practical mitigation measures could be implemented to reduce crop injury. Herbicide treatments consisted oGf 0, 2.5, or 5 kg ae ha−1 glyphosate applied preplant followed by 1) no mitigation measure, 2) tillage, 3) irrigation, or 4) tillage and irrigation prior to transplanting broccoli and collard by hand. When no mitigation was implemented, the residual activity of glyphosate at 2.5 and 5.0 kg ae ha−1 resulted in 43% to 71% and 79% to 93% injury to broccoli and collard transplants, respectively. This resulted in a 35% to 50% reduction in broccoli marketable head weights and 63% to 71% reduction in collard leaf weights. Irrigation reduced visible damage by 28% to 48%, whereas tillage reduced injury by 43% to 76%, for both crops. Irrigation alleviated yield losses for broccoli but only tillage eliminated yield loss for both crops. Care must be taken when transplanting broccoli and collard into a field recently treated with glyphosate at rates ≥2.5 kg ae ha−1. Its residual activity can damage transplants with injury levels influenced by glyphosate rate, and tillage or irrigation after application and prior to planting.

Georgia vegetable growers produce more than 27% of the nation’s fresh-market cucumbers. To maximize yields and profit, fields must be weed-free when planting. Limitations with current burndown herbicide options motivated academic, industry, and U.S. Department of Agriculture partners to search for new tools to assist growers. One possibility, glufosinate, controls many common and troublesome weeds, but its influence on cucumber development through residual activity when applied before or at planting is not understood. Thus, four different studies were each conducted two to four times from 2017 to 2020 to determine 1) transplant cucumber response to preplant glufosinate applications as influenced by rate, overhead irrigation, and interval between application and planting; and 2) seeded cucumber response to preemergence (PRE) glufosinate applications as influenced by rate, overhead irrigation, and planting depth. Glufosinate applied at 330, 660, 980, and 1,640 g ai ha−1 the day before transplanting caused 11% to 53% injury on sandy, low organic matter soils. Cucumber vine lengths and plant biomass were reduced up to 28% and 46%, respectively, with the three highest rates. Early-season yield (harvests 1 to 4) noted a 31% to 60% yield loss with glufosinate at 660 to 1,640 g ha−1 with similar trends observed with total yield (11 to 13 harvests). Irrigation (0.75 cm) after application and before transplanting reduced injury to less than 21%, eliminated vine length and biomass suppression except at the highest rate, and eliminated yield loss. Extending the interval between glufosinate application and transplanting from 1 to 4 d was not beneficial, and further extending the interval to 7 d significantly reduced injury half the time. When applied PRE to seeded cucumber and combining the data across locations, glufosinate caused less than 7% injury even at 1,640 g ha−1. Seeded plant vine lengths, biomass, and marketable yield were not influenced by the PRE application, and neither irrigation nor planting depth influenced seeded crop response to glufosinate.

The direct effect of Palmer amaranth, Amaranthus palmeri Watson, on cotton growth and development is well documented, but its indirect effect through harboring feeding insects is less understood. Palmer amaranth emerged with cotton and remaining in the field for 30 days increased tarnished plant bug, Lygus lineolaris (Palisot de Beauvois), populations compared with a weed-free system. Weedy systems noted up to 49% more damaged terminals than weed-free systems, with cotton yield decreasing as damaged terminals increased at one of two locations. Thrips (Thysanoptera: Thripidae) populations were effectively controlled with Aeris® (Bayer, St. Louis, MO) seed treatment (imidacloprid + thiodicarb at 0.375 mg active ingredient per seed), but there was no correlation between thrips infestations and increasing damaged cotton terminals. However, Aeris seed treatment significantly reduced the occurrence of damaged cotton terminals. In a second experiment, Palmer amaranth infesting an area adjacent to a weed-free cotton field had maximum damaged terminals of 51% on the cotton row proximal to the weedy area, with the distal cotton row (44 m away) having 8% terminal damage. Cotton yield significantly decreased as damaged terminals increased. A final bioassay experiment further evaluated the influence of seed treatment on tarnished plant bug feeding impacting cotton seedlings. With Aeris seed treatment, tarnished plant bug mortality was 97%, compared with 37% for nontreated seed. Results suggest tarnished plant bug infestations increased where Palmer amaranth was present in cotton fields. Additionally, greater Palmer amaranth infestations led to an increase in damaged cotton terminals and lower yields.

Glyphosate and paraquat are effective preplant burndown herbicide options for multicrop vegetable production that uses plastic mulch, but problematic weeds such as wild radish, cutleaf evening primrose, annual morningglory, or horseweed may not be adequately controlled with these herbicides alone. The herbicides 2,4-D and dicamba could help control these troublesome weeds prior to planting if they can be removed from plastic mulch and thus avoid crop damage. Treatments included 2,4-D (1,065 and 2,130 g ae ha−1) and dicamba (560 and 1,120 g ae ha−1) applied broadcast over plastic mulch a day before transplanting. Just before transplanting, treatments received either 0.76 cm of water via overhead irrigation or no irrigation. Plastic mulch samples were collected at application and planting to determine herbicide presence using analytical techniques, and cantaloupe and zucchini squash were subsequently transplanted on the plastic beds. Analytical ultra-high performance liquid chromatography revealed that 88% to 99% of the initial herbicide concentration was present at crop planting when irrigation was not implemented. At most, a 1/50 rate of dicamba and a 1/500 rate of 2,4-D was present at planting when overhead irrigation was applied prior to transplanting. Maximum cantaloupe and squash injury from 2,4-D with irrigation was 10% and did not influence plant growth, biomass, or yield. For dicamba with overhead irrigation, cantaloupe injury was 35%, vine lengths were reduced by 24%, and maturity was delayed, whereas squash injury ranged from 9% to 12%, with no influence on growth or yield. Without irrigation to wash herbicides from the mulch prior to planting, 60% to 100% injury of both crops occurred with both herbicides. Zucchini squash was more tolerant to dicamba than cantaloupe. Results demonstrated that 2,4-D can be adequately removed from the surface of plastic mulch with irrigation, whereas a single irrigation event was not sufficient to remove dicamba.

Planting cole crops and leafy greens in plastic mulch free of summer and winter annual broadleaf weeds is challenging. Because these crops are often grown as a second or third crop on mulch, weeds emerge in previously punched plant holes, tears in plastic, and row middles. Without the ability to use tillage and with limited herbicide options available for weed control, achieving a weed-free planting window is not often feasible. Additional herbicide options are needed, but their interaction with plastic mulch must be understood. Therefore, research has determined the persistence of preplant applications of 2,4-D tank-mixed with glyphosate applied over plastic mulch. Analytical laboratory analyses of plastic samples from field experiments, in conjunction with bioassays using broccoli (Brassica oleracea var. botrytis L.) and collard (Brassica oleracea var. viridis L.), evaluated herbicide dissipation. Analytical studies determined that 0.5 cm of irrigation after herbicide application and 1 day before planting removed 99% of 2,4-D, and 100% of glyphosate from the plastic mulch. Waiting an additional 14 days after application and irrigation further reduced the amount of 2,4-D on the plastic mulch 88% to 95%. For the field bioassay, preplant applications of 2,4-D tank-mixed with glyphosate resulted in 7% or less visual broccoli or collard injury without influencing crop growth, biomass, early season yield, or total yield as long as the mulch was washed with 0.5 cm of irrigation before planting. These studies also demonstrated there were no differences between the 1× and 2× use rates with respect to all response variables measured. Results suggest that 2,4-D and glyphosate can be effectively removed from the surface of plastic mulch with irrigation or rainfall before planting broccoli and collard.

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. Nomenclature: Glyphosate; 2; 4-D; dicamba; squash; Cucurbita pepo L.; watermelon; Citrullus lanatus (Thunb.) Matsum. & Nakai

Agronomic crops engineered with resistance to 2,4-D or dicamba have been commercialized and widely adopted throughout the United States. Because of this, increased use of these herbicides in time and space has increased damage to sensitive crops. From 2014 to 2016, cucumber and cantaloupe studies were conducted in Tifton, GA, to demonstrate how auxinic herbicides (namely, 2,4-D or dicamba), herbicide rate (1/75 or 1/250 field use), and application timing (26, 16, and 7 d before harvest [DBH] of cucumber; 54, 31, and 18 DBH of cantaloupe) influenced crop injury, growth, yield, and herbicide residue accumulation in marketable fruit. Greater visual injury, reductions in vine growth, and yield loss were observed at higher rates when herbicides were applied during early-season vegetative growth compared with late-season with fruit development. Dicamba was more injurious in cucumber, whereas cantaloupe responded similarly to both herbicides. For cucumber, total fruit number and relative weights were reduced (16% to 19%) when either herbicide was applied at the 1/75 rate 26 DBH. Cantaloupe fruit weight was also reduced 21% and 10% when either herbicide was applied at the 1/75 rate 54 or 31 DBH, respectively. Residue analysis noted applications made closer to harvest were more likely to be detectable in fruit than earlier applications. In cucumber, dicamba was detected at both rates when applied 7 DBH, whereas in cantaloupe, it was detected at both rates when applied 18 or 31 DBH in 2016 and at the 1/75 rate applied 18 or 31 DBH in 2014. Detectable amounts of 2,4-D were not observed in cucumber but were detected in cantaloupe when applied at either rate 18 or 31 DBH. Although early-season injury will more likely reduce cucumber or cantaloupe yields, the quantity of herbicide residue detected will be most influenced by the time interval between the off-target incident and sampling. Nomenclature: 2,4-D; dicamba; cucumber; Cucumis sativus, L.; cantaloupe; Cucumis melo var. cantalupo; Ser.

Dicamba and 2,4-D systems control many problematic weeds; however, drift to susceptible crops can be a concern in diverse production areas. Glufosinate-based systems are an alternative, but current recommended rates of glufosinate can result in variable control. Research was conducted in 2017 and 2018 to investigate the optimum time interval between sequential glufosinate applications and determine if the addition of glyphosate with glufosinate is beneficial for controlling Palmer amaranth and annual grasses in cotton. The interval between sequential applications (1, 3, 5, 7, 10, or 14 d or no second spray) was the whole plot and herbicide option (glufosinate or glufosinate plus glyphosate) was the subplot. Combined over herbicides, Palmer amaranth 15- to 20-cm tall (at four locations) was controlled 98% to 99% with sequential intervals of 1 to 7 d compared with 70% to 88% with intervals of 10 or 14 d. Lowest biomass weight and population densities were noted with 1- to 7-d intervals. Large crabgrass 15- to 20-cm tall (at five locations) was controlled 93% to 98% with glufosinate applications 3- to 7-d apart as compared with 76% to 81% with applications 10- to 14-d apart. Lowest biomass weights were observed with 1- to 7-d intervals. When glufosinate controlled grass less than 93%, adding glyphosate was beneficial. Neither interval between sequential applications nor herbicide option influenced cotton yield. Shorter time intervals between sequential application and including glyphosate can improve the effectiveness of a glufosinate-based system in managing Palmer amaranth and large crabgrass.Nomenclature: 2,4-D; dicamba; glyphosate; glufosinate; large crabgrass, Digitaria sanguinalis (L.) Scop; Palmer amaranth, Amaranthus palmeri S. Watson; cotton, Gossypium hirsutum L. ‘DP 1646 B2XF’, ‘PHY 430 W3FE’

Nutsedge species are problematic in plastic-mulched vegetable production because of the weed's rapid reproduction and ability to penetrate the mulch. Vegetable growers rely heavily on halosulfuron to manage nutsedge species; however, the herbicide cannot be applied over mulch before vegetable transplanting due to potential crop injury. This can be problematic when multiple crops are produced on a single mulch installation. Field experiments were conducted to determine the response of broccoli, cabbage, squash, and watermelon to halosulfuron applied on top of mulch prior to transplanting. Halosulfuron at 80 g ai ha–¹ was applied 21, 14, 7, and 1 d before planting (DBP), and 160 g ai ha–¹ was applied 21 DBP. In all experiments, extending the interval between halosulfuron application and planting reduced crop injury. For squash and watermelon, visual injury, plant diameters/vine runner lengths, marketable fruit weights, and postharvest plant biomass resulted in similar values when applying 80 g ha–¹ 21 DBP and with the nontreated weed-free control. Reducing this interval increased injury for both crops. Visual crop injury and yield reductions up to 40% occurred, with halosulfuron applied 14, 7, or 1 DBP in squash and 1 DBP in watermelon. Broccoli and cabbage showed greater sensitivity, with injury and plant diameter reductions greater than 15%, even with halosulfuron applied at 80 g ha–¹ 21 DBP. Experimental results confirm that halosulfuron binds to plastic mulch, remains active, and is slowly released from the mulch over a substantial period, during rainfall or overhead irrigation events. Extending the plant-back interval to at least 21 d before transplanting did overcome squash and watermelon injury concerns with halosulfuron at 80 g ha–¹, but not broccoli and cabbage. Applying halosulfuron over mulch to control emerged nutsedge before planting squash and watermelon would be beneficial if adequate rainfall or irrigation and appropriate intervals between application and planting are implemented. Nomenclature: Halosulfuron; purple nutsedge, Cyperus rotundus L.; yellow nutsedge, Cyperus esculentus L.; broccoli, Brassica oleracea var. botrytis L. ‘Emerald crown'; cabbage, Brassica oleracea var. capitata L. ‘Cheers'; squash, Cucurbita pepo L. ‘Enterprise'; watermelon, Citrullus lanatus (Thunb.) Matsum. & Nakai ‘Sangria'

The efficacy of auxinic herbicides, a valuable weed control tool for growers worldwide, has been shown to vary with the time of day in which applications are made. However, little is known about the mechanisms causing this phenomenon. Investigating the differential in planta behavior of these herbicides across different times of application may grant an ability to advise which properties of auxinic herbicides are desirable when applications must be made around the clock. Radiolabeled herbicide experiments demonstrated a likely increase in ATP-binding cassette subfamily B (ABCB)-mediated 2,4-D and dicamba transport in Palmer amaranth (Amaranthus palmeri S. Watson) at simulated dawn compared to mid-day, as dose response models indicated that many orders of magnitude higher concentrations of N-1-naphthylphthalamic acid (NPA) and verapamil, respectively, are required to inhibit translocation by 50% at simulated sunrise compared to mid-day. Gas chromatographic analysis displayed that ethylene evolution in A. palmeri was higher when dicamba was applied during mid-day compared to sunrise. Furthermore, it was found that inhibition of translocation via 2,3,5-triiodobenzoic acid (TIBA) resulted in an increased amount of 2,4-D-induced ethylene evolution at sunrise, and the inhibition of dicamba translocation via NPA reversed the difference in ethylene evolution across time of application. Dawn applications of these herbicides were associated with increased expression of a putative 9-cis-epoxycarotenoid dioxygenase biosynthesis gene NCED1, while there was a notable lack of trends observed across times of day and across herbicides with ACS1, encoding 1-aminocyclopropane-1-carboxylic acid synthase. Overall, this research indicates that translocation is differentially regulated via specific protein-level mechanisms across times of application, and that ethylene release, a chief phytotoxic process involved in the response to auxinic herbicides, is related to translocation. Furthermore, transcriptional regulation of abscisic acid involvement in phytotoxicity and/or translocation are suggested.