Glufosinate is an effective postemergence herbicide, and overreliance on this herbicide for weed control is likely to increase and select for glufosinate-resistant weeds. Common assays to confirm herbicide resistance are dose–response and molecular sequencing techniques; both can require significant time, labor, unique technical equipment, and a specialized skillset to perform. As an alternative, we propose an image-based approach that uses a relatively inexpensive multispectral sensor designed for unmanned aerial vehicles to measure and quantify surface reflectance from glufosinate-treated leaf disks. Leaf disks were excised from a glufosinate-resistant and glufosinate-susceptible corn (Zea mays L.), cotton (Gossypium hirsutum L.), and soybean [Glycine max (L.) Merr.] varieties and placed into a 24-well plate containing eight different concentrations (0 to 10 mM) of glufosinate for 48 h. Multispectral images were collected after the 48-h incubation period across five discrete wave bands: blue (475 to 507 nm), green (560 to 587 nm), red (668to 682 nm), red edge (717 to 729 nm), and near infrared (842 to 899 nm). The green leaf index (GLI; a metric to measure chlorophyll content) was utilized to determine relationships between measured reflectance from the tested wave bands from the treated leaf disks and the glufosinate concentration. Clear differences of spectral reflectance were observed between the corn, cotton, and soybean leaf disks of the glufosinate-resistant and glufosinate-susceptible varieties at the 10 mM concentration for select wave bands and GLI. Leaf disks from two additional glufosinate-resistant and glufosinate-susceptible varieties of each crop were subjected to a similar assay with two concentrations: 0 and 10 mM. No differences of spectral reflectance were observed from the corn and soybean varieties in all wave bands and the GLI. The leaf disks of the glufosinate-resistant and glufosinate-susceptible cotton varieties were spectrally distinct in the green, blue, and red-edge wave bands. The results provide a basis for rapidly detecting glufosinate-resistant plants via spectral reflectance. Future research will need to determine the glufosinate concentrations, useful wave bands, and susceptible/resistant thresholds for weeds that evolve resistance.

Poppy (also common poppy or corn poppy; Papaver rhoeas L., PAPRH) is one of the most harmful weeds in winter cereals. Knowing the precise and accurate location of weeds is essential for developing effective site-specific weed management (SSWM) for optimized herbicide use. Among the available tools for weed mapping, deep learning (DL) is used for its accuracy and ability to work in complex scenarios. Crops represent intricate situations for weed detection, as crop residues, occlusion of weeds, or spectral similarities between crop and weed seedlings are frequent. Timely discrimination of weeds is needed, because postemergence herbicides are used just when weeds and crops are at an early growth stage. This study addressed P. rhoeas early detection in wheat (Triticum spp.) by comparing the performance of six DL-based object-detection models focused on the “You Only Look Once” (YOLO) architecture (v3 to v5) using proximal RGB images to train the models. The models were assessed using open-source software, and evaluation offered a range of results for quality of recognition of P. rhoeas as well as computational capacity during the inference process. Of all the models, YOLOv5s performed best in the testing phase (75.3%, 76.2%, and 77% for F1-score, mean average precision, and accuracy, respectively). These results indicated that under real field conditions, DL-based object-detection strategies can identify P. rhoeas at an early stage, providing accurate information for developing SSWM.

High crop densities are valuable to increase weed suppression, but growers might be reluctant to implement this practice due to increased seed cost. Because it is also possible to lower planting densities in areas with no or low weed interference risk, the area allocated to each planting density must be optimized considering seed cost and productivity per plant. In this study, the growth and yield of maize (Zea mays L.), cotton (Gossypium hirsutum L.), and soybean [Glycine max (L.) Merr.] were characterized in response to low planting densities and arrangements. The results were used to develop a bioeconomic model to optimize the area devoted to high- and low-density plantings to increase weed suppression without increasing seed cost. Physiological differences seen in each crop varied with the densities tested; however, maize was the only crop that had differences in yield (per area) between densities. When a model to optimize low and high planting densities was used, maize and cotton showed the most plasticity in yield per planted seed (g seed−1) and area of low density to compensate for high-density area unit. Maize grown at 75% planting density compared with the high-planting density (200%) increased yield (g seed−1) by 229%, return by 43%, and profit by 79% while decreasing the low-density area needed to compensate for high-density area. Cotton planted at 25% planting density compared with the 200% planting density increased yield (g seed−1) by 1,099%, return by 46%, and profit by 62% while decreasing the low-density area needed to compensate for high-density area. In contrast, the high morphological plasticity of soybean did not translate into changes in area optimization, as soybean maintained return, profit, and a 1:1 ratio for area compensation. This optimization model could allow for the use of variable planting at large scales to increase weed suppression while minimizing costs to producers.

Basal bark treatment of invasive trees is an approach designed to limit damage to non-target vegetation in the vicinity, but non-target injury is still documented. No study of basal bark treatments has examined the release of herbicide residues from roots of treated plants and resulting non-target impacts. Studies were conducted in Alaska interior and coastal boreal forests on basal bark treatments with aminopyralid and triclopyr on active-growth and dormant invasive chokecherry (Prunus padus L.). The study assessed non-target damage and soil herbicide residue using a combination of visual evaluations, bioassays, and soil residue analyses. Non-target damage from herbicide residues were identified in 40% of treatments containing aminopyralid with triclopyr, 60% of treatments containing aminopyralid alone, and 5% of treatments containing only triclopyr. Laboratory studies of aminopyralid treatments to saplings isolated the effects of herbicide exudation from roots, which was found to be significant, and the magnitude was dependent on dose. Herbicide soil residues in field and laboratory experiments were quantified with analytical detection and plant bioassays. Aminopyralid soil residues were identified in 57% of field treatments receiving 8 to 60 ml of herbicide solution (2% ai) and 70% of laboratory treatments receiving 10 µl of herbicide solution (2% to 16% ai). Triclopyr residues were found from one field treatment following dosage with 28 ml of herbicide solution (18.5% ai). Anatomically, plants grown in soils associated with herbicide-treated trees, both in the field and lab, grew less dry mass than non–herbicide treated controls. This study provides the first evidence that root exudation of herbicide following basal bark treatments contributes to non-target damage of adjacent vegetation and to accumulation of soil herbicide residues. This is an important new factor for integrated pest management within basal bark treatment systems and has implications for other herbicide application types such as injections and frill, as well as determining whether root exudation is species or herbicide specific.

Comparing fitness of herbicide-resistant and herbicide-susceptible weed biotypes is important for managing herbicide resistance. Previous research suggests there is little to no fitness penalty from amplification of the 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) gene (a mechanism of glyphosate resistance) in Palmer amaranth (Amaranthus palmeri S. Watson) in controlled studies in the greenhouse or growth chamber. A field study was conducted in North Carolina at three locations naturally infested with A. palmeri to determine vegetative, reproductive, and germination fitness of plants with and without EPSPS amplification grown season-long with cotton (Gossypium hirsutum L.). Seed number was not correlated with EPSPS copy number. However, when plants were binned into two groups, those having an EPSPS copy number ≥2 (relative to reference genes) and those having an EPSPS copy number <2, plant fresh weight and seed number were 1.4 and 1.6 times greater, respectively, for plants with fewer than 2 EPSPS copies. Amaranthus palmeri height and seed germination, and yield of cotton, did not differ when comparing the two binned groups. These data suggest that A. palmeri plants with EPSPS amplification are relatively less fit in the absence of glyphosate, but this reduced fitness does not translate into differences in interference with cotton.

Alfalfa (Medicago sativa L.) hectares in Michigan are declining partly due to reliance on corn (Zea mays L.) silage as a continuous feed source. By interseeding corn and alfalfa, farmers can replace the low alfalfa yield in the establishment year with corn silage while simultaneously establishing alfalfa. A randomized split-block field study was conducted in East Lansing, MI, over 3 yr (2019 to 2021) to determine the critical period of weed control (CPWC) in the interseeded corn and alfalfa system using two corn hybrids with differing leaf architecture (pendulum vs. upright). Whole plots were assigned to corn hybrids interseeded with alfalfa, and subplots were assigned to a surrogate weed, Japanese millet [Echinochloa esculenta (A. Braun) H. Scholz], for the duration of competition treatments. Weed-free and weedy plots were included as controls. At the end of the interseeding year, corn was harvested, while alfalfa was harvested the following year. The CPWC is made up of two components: the critical timing of weed removal (CTWR) and the critical weed-free period (CWFP). Corn hybrid had no impact on the CTWR or CWFP for interseeded corn or alfalfa. Averaged across hybrids, the CTWR was 303 growing degree days (GDD), and CWFP was estimated to be greater than the study duration. The CTWR in the first cutting of alfalfa was estimated to be 369 GDD. The CWFP was estimated to be 394 GDD for a 5% acceptable yield loss for the first alfalfa cutting. Identification of the CPWC in the interseeded system will increase adoption and interest in other interseeded systems that can mitigate potential negative environmental and economic impacts of monoculture agriculture.

Weeds compete with crops for soil moisture, along with other resources, which can impact the germination, growth, and seed production of weeds; however, this impact has not been systematically recorded and synthesized across diverse studies. To address this knowledge gap, a global meta-analysis was conducted using 1,196 paired observations from 86 published articles assessing the effect of water stress on weed germination, growth characteristics, and seed production. These studies were conducted and published during 1970 through 2020 across four continents (Asia, Australia, Europe, and North America). Imposed water stress was expressed as solution osmotic potential (ψsolution), soil water potential (ψsoil), or soil moisture as percent field capacity. Meta-analysis revealed that water stress inhibits weed germination, growth, and seed production, and the quantitative response intensified with increasing water stress. A ψsolution greater than −0.8 MPa completely inhibits germination of both grass and broadleaf weeds. A ψsolution from −0.09 to −0.32 MPa reduces weed germination by 50% compared with the unstressed condition. Moderate soil water stress, equivalent to 30% to 60% field capacity, inhibits growth characteristics (branches or tillers per plant, leaf area, leaves per plant, plant height, root, and shoot biomass) by 33% and weed seed production by 50%. Severe soil water stress, below 30% field capacity, inhibits weed growth by 51% and seed production by 88%. Although water stress inhibits weed growth, it does not entirely suppress the ability to germinate, grow, and produce seeds, resulting in weed seedbank accumulation. This creates management challenges for producers, because weed seeds can survive in the soil for many years, depending on weed species and environmental conditions. Quantitative information compiled in this meta-analysis can be instrumental to model the weeds’ multidimensional responses to water stress and designing integrated weed management strategies for reducing the weed seedbank.

Recent innovations in 3D imaging technology have created unprecedented potential for better understanding weed responses to management tactics. Although traditional 2D imaging methods for mapping weed populations can be limited in the field by factors such as shadows and tissue overlap, 3D imaging mitigates these challenges by using depth data to create accurate plant models. Three-dimensional imaging can be used to generate spatiotemporal maps of weed populations in the field and target weeds for site-specific weed management, including automated precision weed control. This technology will also help growers monitor cover crop performance for weed suppression and detect late-season weed escapes for timely control, thereby reducing seedbank persistence and slowing the evolution of herbicide resistance. In addition to its many applications in weed management, 3D imaging offers weed researchers new tools for understanding spatial and temporal heterogeneity in weed responses to integrated weed management tactics, including weed–crop competition and weed community dynamics. This technology will provide simple and low-cost tools for growers and researchers alike to better understand weed responses in diverse agronomic contexts, which will aid in reducing herbicide use, mitigating herbicide-resistance evolution, and improving environmental health.

Two species of cleavers, Galium aparine L. and Galium spurium L., are known to inhabit croplands in western Canada. The latter is the more abundant of the two species. An increased abundance of these species over the most recent decades warrants a more comprehensive understanding of their developmental phenology and the cause of this increase. This study aimed to identify the base temperature and emergence characteristics of Galium spp. across different populations from western Canada. A thermal gradient plate experiment was conducted using five G. spurium populations collected from various Saskatchewan (SK) and Alberta (AB) locations. One known G. aparine reference sample was also included. A common garden experiment was conducted using the six Galium spp. populations to determine emergence characteristics. The base germination temperature identified was 2 C for all populations of G. spurium and 4 C for G. aparine. The median germination temperature for G. aparine was 8.34 C, whereas G. spurium had a similar median germination temperature of 6.5 C. Despite similar germination characteristics, the field emergence study revealed differences between populations’ initiation of emergence (150 to 250 growing degree days [GDD]) and time to 50% emergence (275 to 470 GDD) in spring. Highly variable emergence among years and populations within the year (200 to 600 GDD in 2013 and 100 to 200 GDD in 2014) were observed during fall, probably due to differences in moisture availability. Cumulative emergence among populations in fall was very low (1% to 9%) compared with spring emergence (2% to 17%). Overall, this study provides evidence for a low base temperature and differences in emergence periodicity among populations, both of which may be significant factors contributing to the seasonal success of this species.

Velvetleaf (Abutilon theophrasti Medik.) is a troublesome broadleaf weed that competes with crops for resources such as soil moisture. Water stress can affect the ability of weed species to grow and produce seeds. The objective of this study was to determine the effect of degree of water stress on the growth and fecundity of A. threophrasti using soil moisture sensors under greenhouse conditions. Abutilon threophrasti seeds collected from a corn (Zea mays L.)–soybean [Glycine max (L.) Merr.] field were grown in silty clay loam soil, and plants were maintained at 100%, 75%, 50%, and 25% soil field capacity (FC) corresponding to no, light, moderate, and high water-stress conditions, respectively. Water was added daily to pots based on soil moisture levels detected by a Meter Group 5TM sensor to maintain the desired water-stress level required by treatment. Plants maintained at 100% FC had the maximum number of leaves (28 leaves plant−1), followed by 21 and 15 leaves plant−1 at 75% and 50% FC, respectively. Abutilon threophrasti at 100% and 75% FC achieved maximum plant height (108 to 123 cm) compared with 83 cm at 50% FC. Abutilon threophrasti maintained at 75% FC had the greatest growth index (79,907 cm3) followed by 72,197 cm3 at 100% FC and 64,256 cm3 at 50% FC. Seed production was similar at 100%, 75%, and 50% FC (288 to 453 seeds plant−1) compared with 2 seeds plant−1 at 25% FC. This is because the majority of plants maintained at 25% FC did not survive more than 77 d after transplanting. Seed germination was 96% to 100% at 100%, 75%, and 50% FC compared with 20% germination at 25% FC. Abutilon threophrasti can survive ≥50% FC continuous water-stress conditions, although with reduced leaf number, plant height, and growth index compared with 75% and 100% FC.

Site-specific weed management using open-source object detection algorithms could accurately detect weeds in cropping systems. We investigated the use of object detection algorithms to detect Palmer amaranth (Amaranthus palmeri S. Watson) in soybean [Glycine max (L.) Merr.]. The objectives were to (1) develop an annotated image database of A. palmeri and soybean to fine-tune object detection algorithms, (2) compare effectiveness of multiple open-source algorithms in detecting A. palmeri, and (3) evaluate the relationship between A. palmeri growth features and A. palmeri detection ability. Soybean field sites were established in Manhattan, KS, and Gypsum, KS, with natural populations of A. palmeri. A total of 1,108 and 392 images were taken aerially and at ground level, respectively, between May 27 and July 27, 2021. After image annotation, a total of 4,492 images were selected. Annotated images were used to fine-tune open-source faster regional convolutional (Faster R-CNN) and single-shot detector (SSD) algorithms using a Resnet backbone, as well as the “You Only Look Once” (YOLO) series algorithms. Results demonstrated that YOLO v. 5 achieved the highest mean average precision score of 0.77. For both A. palmeri and soybean detections within this algorithm, the highest F1 score was 0.72 when using a confidence threshold of 0.298. A lower confidence threshold of 0.15 increased the likelihood of species detection, but also increased the likelihood of false-positive detections. The trained YOLOv5 data set was used to identify A. palmeri in a data set paired with measured growth features. Linear regression models predicted that as A. palmeri densities increased and as A. palmeri height increased, precision, recall, and F1 scores of algorithms would decrease. We conclude that open-source algorithms such as YOLOv5 show great potential in detecting A. palmeri in soybean-cropping systems.

Dicamba was labeled in dicamba-resistant cotton (Gossypium hirsutum L.) and soybean [Glycine max (L.) Merr.] in 2017, resulting in a record number of off-target complaints. To address off-target movement via volatilization, experiments were conducted to evaluate the effectiveness of potassium tetraborate tetrahydrate (KBo) as a volatility-reducing agent (VRA) with dicamba. Low-tunnel experiments examined: (1) whether KBo functions as a dicamba VRA, (2) the relationship between KBo concentration and dicamba volatilization, (3) the effectiveness of KBo compared with potassium acetate as a VRA, and (4) the impact of KBo on dicamba volatilization with and without glufosinate. In a large-scale trial (0.4-ha plots), the effectiveness of KBo in reducing dicamba volatilization was quantified relative to a commercial dicamba application labeled for use in 2020. The addition of KBo to dicamba reduced volatility over dicamba alone and a dicamba plus potassium acetate premix. As KBo concentration increased in the dicamba spray solution, volatilization was exponentially reduced. Dicamba volatilization with the addition of KBo at 0.01 M was comparable to dicamba plus potassium acetate at 0.05 M. Potassium tetraborate tetrahydrate was more effective than potassium acetate at reducing volatility of a dicamba plus glufosinate mixture. In large-scale experiments over a 30-h period, the addition of KBo to a diglycolamine plus glyphosate mixture lowered dicamba volatilization 82% to 89% over the herbicide mixture alone. Overall, the addition of KBo to dicamba appears promising as a VRA compared with what is commercially available.

Cover crops enhance the biodiversity of cropping systems and can support a variety of useful ecosystem services, including weed suppression. In California orchards, cover crops are typically implemented as annual plants that can replace resident vegetation in orchard alleyways during the rainy winter season. Our research objective was to evaluate cover crop management factors that support a competitive, weed-suppressing cover crop in the unique orchard systems of central California. We conducted two experiments: an experiment evaluating cover crop management intensification in walnuts (Juglans regia L.) and an experiment evaluating multispecies cover crop mixes and planting date in almonds [Prunus dulcis (Mill.) D.A. Webb]. These experiments demonstrate that timely cover crop planting is important for producing an abundant cover crop, and a variety of cover crop management programs can produce weed-suppressing cover crops. However, cover crops do not result in weed-free orchards and should be considered within the context of integrated management programs. The apparent flexibility of orchard cover crop management provides an opportunity to promote other agroecosystem services, with vegetation management and weed suppression as complementary management goals.

Liverseedgrass (Urochloa panicoides P. Beauv.) is one of the most important summer grass weed species in the eastern cropping system of Australia. Experiments were conducted to evaluate the effects of temperature, salt stress, water stress, burial depth, and sorghum crop residue load on germination and emergence of U. panicoides and the performance of postemergence herbicides on this weed species. The optimal germination temperature regimes for U. panicoides were 30/20 and 35/25 C (alternating day/night temperatures), but seeds also germinated at temperatures occurring in winter, spring, and autumn in Australia (15/5, 20/10, and 25/15 C). A concentration of 48 mM sodium chloride and −0.27 MPa osmotic potential inhibited germination of U. panicoides by 50%, indicating that this weed species is not salt and drought tolerant at germination. The maximum germination was obtained for the surface seeds; a burial depth of 1.9 cm inhibited emergence by 50%. No seedlings emerged from the 12-cm depth, but about 3% of seedlings emerged from the 8 cm depth. The addition of sorghum residue amounts up to 8,000 kg ha−1 to the soil surface stimulated U. panicoides’ emergence compared with the no-residue treatment, suggesting that conservation agriculture will promote the emergence of U. panicoides. Several postemergence herbicides were found to be effective in controlling this weed species, especially when applied at an early stage. Information obtained from this study will help to develop effective and sustainable control measures for U. panicoides and other weed species with similar germination requirements.

Junglerice [Echinochloa colona (L.) Link.] is the most important grass weed species in Australian summer cropping systems. Although it is mainly a spring- and summer-emerging weed species, field observations suggest that E. colona is expanding its seasonality. A common garden experiment was conducted at the University of Queensland farm to examine the effect of planting dates on phenology, growth, and fecundity of eight populations of E. colona. All populations were planted every second month from September to July in 2019 to 2020 and 2020 to 2021. Echinochloa colona took the shortest time (4 to 6 d) to emerge when planted in November or January. However, the November population took the longest number of growing degree days to exhibit panicle emergence. In both years, populations differed in height and leaf, tiller, panicle, and seed production in response to planting times. Plants produced significantly greater biomass for the November planting (123 to 147 g plant−1) followed by the January planting and then the September planting. The March planting produced the lowest biomass. In the first year, the lowest number of seeds (3,500 seeds plant−1) was produced by the March planting; however, in the second year, similar numbers of seeds were produced by the March and July plantings. In the first year, seed production (51,000 seeds plant−1) was greatest for the November planting; however, some populations also produced a similar number of seeds for the January planting. In the second year, significantly greater seed production (111,000 seeds plant−1) was observed for the January planting compared with other planting dates. The aboveground biomass and seed production of E. colona were positively correlated. This study reveals variations among E. colona populations and suggests that although greater emphasis must be placed on controlling spring- and summer-emerging plants, management practices need to be extended throughout the year to control E. colona in southeastern Australia.

Southern threecornerjack (Emex australis Steinh.) produces both aerial seeds on the branches and subterranean seeds at the base of the plant crown approximately 1 to 2 cm underground. Prior research indicated that seeds buried at a depth of 1 cm have a much higher probability of germinating in the following season compared with seeds on the soil surface. The current research investigated the number of subterranean seeds produced per plant, timing of subterranean seed production, and germination requirements of subterranean seeds compared with aerial seeds. This research demonstrated that E. australis plants from multiple populations produced subterranean seeds beginning at the 4-leaf development stage, before the production of aerial seeds. Individual plants produced a maximum of 30 subterranean seeds. Germination of subterranean and aerial seeds following afterripening over the summer to autumn (December to May) fallow was similar, demonstrating that the different seed types had no differences in dormancy. Further, the subterranean seeds did not have the light requirement for germination observed in aerial seeds. Subterranean seeds had 22.2% and 24.3% germination in light and dark conditions, respectively, compared with aerial seeds, which had 26.9% germination in light conditions and 12.3% germination in the dark. Production of subterranean seeds was low, given that a mature E. australis plant may produce more than 1,100 aerial seeds, but the subterranean seeds formed earlier in plant development. Recommendations for postemergence herbicides are to spray E. australis at the 2- to 4-leaf stage of growth, yet some plants may have already produced mature subterranean seeds by this stage. Further research is required to determine the impact of subterranean seeds on population ecology.

There is zero tolerance for dicamba and dicamba metabolite residue in tomato (Solanum lycopersicum L.) fruit following exposure to dicamba. Field trials were conducted in 2020 and 2021 to determine the persistence of dicamba and metabolite (5-hydroxy dicamba and 3,6-dichlorosalicylic acid [DCSA]) residue in processing tomato shoots and fruits. Dicamba was applied 49 d after transplanting at 0, 0.53, 5.3, and 53 g ae ha−1. Tomato plants were harvested 5, 10, 20, 40, and 61 d after treatment (DAT). No 5-hydroxy dicamba was recovered from any sample. In 2020, the DCSA metabolite was detected from tomato shoot tissue when dicamba was applied at the 53 g ha−1 rate at 0 (14 µg kg−1), 5 (3 µg kg−1), and 20 DAT (5 µg kg−1) and from tomato fruit tissue at 53 g ha−1 at 20 (2 µg kg−1) and 61 DAT (2 µg kg−1). In 2021, DCSA was not detected from tomato shoot or fruit tissues at any harvest date. By 5 DAT, dicamba was only detected from tomato shoot tissues treated with 53 g ha−1. At 0 DAT, dicamba residue was detectable only from tomato fruit on plants treated with 53 g ha−1. Tomato fruit dicamba residue from plants treated with 5.3 g ha−1 had a predicted peak of 19 µg kg−1 at 11.3 DAT. Tomato fruit dicamba residue from plants treated with 53 g ha−1 decreased from 164 to 8 µg kg−1 from 5 to 61 DAT. Furthermore, this study confirms that dicamba is detectable from tomato fruits at 61 DAT following exposure to 5.3 or 53 g ha−1 dicamba. Growers who suspect dicamba exposure should include tomato fruit tissue with their collected sample or sample tomato fruits separately.

Herbicide resistance has been studied extensively in agronomic crops across North America but is rarely examined in vegetables. It is widely assumed that the limited number of registered herbicides combined with the adoption of diverse weed management strategies in most vegetable crops effectively inhibits the development of resistance. It is difficult to determine whether resistance is truly less common in vegetable crops or whether the lack of reported cases is due to the lack of resources focused on detection. This review highlights incidences of resistance that are thought to have arisen within vegetable crops. It also includes situations in which herbicide-resistant weeds were likely selected for within agronomic crops but became a problem when vegetables were grown in sequence or in adjacent fields. Occurrence of herbicide resistance can have severe consequences for vegetable growers, and resistance management plans should be adopted to limit selection pressure. This review also highlights resistance management techniques that should slow the development and spread of herbicide resistance in vegetable crops.

Hydrilla [Hydrilla verticillata (L. f.) Royle] is often called the “perfect aquatic weed,” as it has numerous physiological adaptations that make it highly aggressive and competitive. Hydrilla verticillata has historically been managed effectively using fluridone; however, the overreliance on this single mechanism of action (MOA) resulted in evolved fluridone resistance in the late 1990s. Where fluridone-resistant H. verticillata populations evolved, endothall became widely used for H. verticillata control. In 2018, florpyrauxifen-benzyl, a highly active auxin-mimic herbicide, was registered for H. verticillata control, and its use has increased since its introduction. Endothall and florpyrauxifen-benzyl provide two effective MOAs for H. verticillata management, and combining these two MOAs would be an effective strategy to delay further resistance evolution. The objective of this research was to determine whether combining endothall and florpyrauxifen-benzyl would significantly impact the behavior of either herbicide in dioecious (DHV) or monoecious (MHV) H. verticillata compared with their behavior when applied alone. Endothall and florpyrauxifen-benzyl absorption and accumulation alone and in combination were measured over a 192-h time course. Translocation patterns were also determined. Herbicide accumulation in MHV and DHV was not impacted when these herbicides were applied in combination. Endothall translocation from shoots to roots in DHV was not impacted (alone = 18.7 ± 1.4%; combination = 23.2 ± 2.2%); however, endothall shoot-to-root translocation in MHV was reduced from 16.2 ± 1.3% applied alone to 2.2 ± 0.1% when applied in combination with florpyrauxifen-benzyl. Florpyrauxifen-benzyl shoot-to-root translocation was reduced in both MHV and DHV when applied in combination with endothall. Florpyrauxifen-benzyl translocation was reduced by 16- and 6-fold in DHV and MHV, respectively. These data do not suggest that there would be operational impacts from endothall and florpyrauxifen-benzyl mixtures. Still, there appear to be changes in herbicide behavior, primarily shoot-to-root translocation, when these two herbicides are applied in combination.

The adoption of dicamba-resistant cotton (Gossypium hirsutum L.) cultivars allows using dicamba to reduce weed populations across growing seasons. However, the overuse of this tool risks selecting new herbicide-resistant biotypes. The objectives of this research were to determine the population trajectories of several weed species and track the frequency of glyphosate-resistant (GR) Palmer amaranth (Amaranthus palmeri S. Watson) over 8 yr in dicamba-resistant cotton. An experiment was established in North Carolina in 2011, and during the first 4 yr, different herbicide programs were applied. These programs included postemergence applications of glyphosate, alone or with dicamba, with or without residual herbicides. During the last 4 yr, all programs received glyphosate plus dicamba. Biennial rotations of postemergence applications of glyphosate only and glyphosate plus dicamba postemergence with and without preemergence herbicides were also included. Sequential applications of glyphosate plus dicamba were applied to the entire test area for the final 4 yr of the study. No herbicide program was entirely successful in controlling the weed community. Weed population trajectories were different according to species and herbicide program, creating all possible outcomes; some increased, others decreased, and others remained stable. Density of resistant A. palmeri increased during the first 4 yr with glyphosate-only programs (up to 11,739 plants m−2) and decreased a 96% during the final 4 yr, when glyphosate plus dicamba was implemented. This species had a strong influence on population levels of other weed species in the community. Goosegrass [Eleusine indica (L.) Gaertn.] was not affected by A. palmeri population levels and even increased its density in some herbicide programs, indicating that not only herbicide resistance but also reproductive rates and competitive dynamics are critical for determining weed population trajectories under intensive herbicide-based control programs. Frequency of glyphosate resistance reached a maximum of 62% after 4 yr, and those levels were maintained until the end of the experiment.