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Dicamba[1]
Names
Preferred IUPAC name
3,6-Dichloro-2-methoxybenzoic acid
Other names
3,6-Dichloro-o-anisic acid
Dianat
Identifiers
3D model (JSmol)
ChEBI
ChEMBL
ChemSpider
ECHA InfoCard 100.016.033 Edit this at Wikidata
KEGG
UNII
  • InChI=1S/C8H6Cl2O3/c1-13-7-5(10)3-2-4(9)6(7)8(11)12/h2-3H,1H3,(H,11,12) checkY
    Key: IWEDIXLBFLAXBO-UHFFFAOYSA-N checkY
  • InChI=1/C8H6Cl2O3/c1-13-7-5(10)3-2-4(9)6(7)8(11)12/h2-3H,1H3,(H,11,12)
    Key: IWEDIXLBFLAXBO-UHFFFAOYAV
  • Clc1ccc(Cl)c(c1OC)C(=O)O
Properties
C8H6Cl2O3
Molar mass 221.03 g·mol−1
Appearance White crystalline solid
Density 1.57
Melting point 114 to 116 °C (237 to 241 °F; 387 to 389 K)
"low"[2]
Solubility in acetone 810 g/L
Solubility in ethanol 922 g/L
Hazards
GHS labelling:[3]
GHS05: CorrosiveGHS07: Exclamation mark
Danger
H302, H318, H412
P273, P280, P305+P351+P338
Flash point 199 °C (390 °F; 472 K)
Related compounds
Related compounds
 Tricamba
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).

Dicamba (3,6-dichloro-2-methoxybenzoic acid) is a selective systemic herbicide first registered in 1967.[4] Brand names for formulations of this herbicide include Dianat, Banvel, Diablo, Oracle and Vanquish. This chemical compound is a chlorinated derivative of o-anisic acid.[5] It has been described as a "widely used, low-cost, environmentally friendly herbicide that does not persist in soils and shows little or no toxicity to wildlife and humans."[6]

Despite its success in improving crop yields, dicamba has attracted controversy. According to the United States Environmental Protection Agency (EPA), dicamba's primary ecological risk is for non-target terrestrial plants from exposure through spray drift, whereby dicamba inadvertently migrates to non-targeted neighboring areas, damaging those plants.[7][8]

In 2016, dicamba was approved for use in the United States over GMO dicamba-resistant crops created by Monsanto. Dicamba came under significant scrutiny due to its tendency to spread from treated fields into neighboring fields, causing severe damage.[9] The controversy led to litigation, state bans and additional restrictions over dicamba use.

Use as an herbicide

[edit]
Dicamba use in the U.S. in 2019. Usage has substantially increased since dicamba was approved for use over dicamba-resistant GMO crops in 2016.

Dicamba is a selective and systemic herbicide that kills annual and perennial broadleaf weeds.[10] Its primary commercial applications are weed control for grain crops and turf areas. It is also used to control brush and bracken in pastures, as well as controlling legumes and cacti. In combination with a phenoxy herbicide or with other herbicides, dicamba can be used for weed control in range land and other noncrop areas (fence rows, roadways, railways,pipelines, transmission lines and wastage). Dicamba is toxic to conifer species but is in general less toxic to grasses.[2] Dicamba is a synthetic auxin that functions by increasing plant growth rate, leading to senescence and cell death.[2][11]

The growth regulating properties of dicamba were first discovered by Zimmerman and Hitchcock in 1942.[12] Soon after, Jealott's Hill Experimental Station in England was evaluating dicamba in the field. Dicamba has since been used for household and commercial weed control.

Increasing use of dicamba has been reported with the release of dicamba-resistant genetically modified plants by Monsanto. In October 2016, the EPA launched a criminal investigation into the illegal application of older, drift prone formulations of dicamba onto these new plants.[13][14] Older formulations have been reported to drift after application and affect other crops not meant to be treated.[15][16] A less volatile formulation of dicamba made by Monsanto, designed to be less prone to vaporizing and inhibit unintended drift between fields, was approved for use in the United States by the EPA in 2016, and was commercially available in 2017.[17] As a result, the use of dicamba in U.S. agriculture rose sharply from approximately 8,000,000 pounds (3,600,000 kg) in 2016 to 30,000,000 pounds (14,000,000 kg) in 2019, according to the U.S. Geological Survey.[18]

Drift

[edit]

Dicamba came under scrutiny due to its reputation for drifting from treated fields onto neighboring crops.[19][20] In 2011, the European Food Safety Authority identified dicamba's potential for long-range transport through the atmosphere as a critical area of concern.[21] In 2022, the United States Environmental Protection Agency identified spray drift as the primary ecological risk for dicamba due to its potential effects on non-target terrestrial plants.[8] Dicamba is also available in a drift-resistant formulation, which is less likely to affect neighboring fields.

Toxicology

[edit]

Humans

[edit]

In 2022 the EPA identified potential occupational risks to handlers mixing and loading dry flowable formulations for application to sod and field crops.[8] The Agency did not identify dietary, residential, aggregate, or post-application risks of concern.[8]

Increased lung and colon cancer rate ratios and positive exposure–response patterns were reported for dicamba, in a review of data gathered in the National Institutes of Health's Agricultural Health Study.[22] The Cross-Canada Study of Pesticides and Health found that exposure to dicamba increased the risk of non-Hodgkins lymphoma in men.[23]

Mammals

[edit]

Dicamba has low toxicity by ingestion and inhalation or dermal exposure (oral LD50 in rats: 757 mg/kg body weight, dermal LD50 in rats: >2,000 mg/kg, inhalation LC50 in rats: >200 mg/L). In a three-generation study, dicamba did not affect the reproductive capacity of rats.[24]

When rabbits were given doses of 0, 0.5, 1, 3, 10, or 20 (mg/kg)/day of technical dicamba from days 6 through 18 of pregnancy, toxic effects on the mothers, slightly reduced fetal body weights, and increased loss of fetuses occurred at the 10 mg/kg dose. U.S. Environmental Protection Agency (EPA) has set the NOAEL for this study at 3 (mg/kg)/day.[24]

In dog tests, some enlargement of liver cells has occurred, but a similar effect has not been shown in humans.[24]

Aquatic animals

[edit]

Dicamba and its derivatives are practically nontoxic to aquatic invertebrates. Studies suggest that dicamba should be considered to be a potential endocrine disruptor for fish at environmentally relevant concentrations.[25]

Birds and bees

[edit]

The 2022 EPA draft ecological risk assessment identified potential adverse effects to birds, and bee larvae for all dicamba uses.[8]

Genetically modified crops

[edit]

The soil bacterium Pseudomonas maltophilia (strain DI-6) converts dicamba to 3,6-dichlorosalicylic acid (3,6-DCSA), which lacks herbicidal activity. The enzymes responsible for this first breakdown step comprise a three-component system called dicamba O-demethylase.[6]

In the 2000s, Monsanto incorporated one component of the three enzymes into the genome of soybean, cotton, and other broadleaf crop plants, making them resistant to dicamba.[6] Monsanto has marketed their dicamba resistant crops under the brand name Xtend.[26]

Farmers have expressed concern about being forced to grow resistant crops as protection against drifting dicamba.[26]

Resistance

[edit]

Herbicide resistance after the introduction of resistant crops is a common concern with herbicide.[26][27] In the laboratory, researchers have demonstrated weed resistance to dicamba within three generations of exposure.[26] This effect is illustrated by glyphosate-resistant crops (marketed as 'Roundup Ready').[26][27][28] The weed species Amaranthus palmeri and Bassia scoparia developed resistance to dicamba already in the 1990s.[29]

Dicamba's HRAC resistance class is Group I (Aus), Group O (Global), Group 4 (numeric).[30][31]

Environmental fate

[edit]
Pathway for biodegradation of dicamba

Dicamba is poorly soluble in water.[2] A study conducted from 1991 to 1996 by the U.S. Geologic Survey found dicamba in 0.13% of the groundwaters surveyed. The maximum level detected was 0.0021 mg/L.[32]

It is biodegraded by both aerobic and anaerobic bacteria to 3,6-dichlorosalicylic acid. This conversion is catalyzed by a dicamba monooxygenase, which hydroxylates the methyl group of dicamba. The hydroxymethylated derivative hydrolyzes readily to an inactive dichlorosalicylic acid or its conjugate base. Modification of the gene encoding this enzyme is one strategy toward dicamba-resistant crops.[33] The dichlorosalicylic acid is far less toxic than dicamba, which is already rather low.[34][35]

Use on dicamba-tolerant crops

[edit]

Complaints against dicamba accelerated after the United States EPA approved a Monsanto-created soybean which could tolerate it in 2016.[36] The soybean was a part of Monsanto's Xtend products.[36] Dicamba was approved by the EPA for "over-the-top" (OTT) use on those dicamba-tolerant soybean and cotton crops.[8] In 2017 and again in 2018, EPA amended the registrations of all OTT dicamba products following reports that farmers had experienced crop damage and economic losses resulting from spray drifting.[8]

Arkansas and Missouri banned the sale and use of dicamba in July 2017 in response to complaints of crop damage due to drift.[37] Monsanto responded by arguing that not all instances of crop damage had been investigated and a ban was premature.[38] Monsanto sued the state of Arkansas to stop the ban, but the case was dismissed in February 2018.[39] It has also been acknowledged that the use of dicamba had increased since 2017.[40][41]

In June 2020, the 9th U.S. Circuit Court of Appeals blocked sales of three dicamba-based herbicides in the United States, finding that the Environmental Protection Agency "substantially understated risks that it acknowledged and failed entirely to acknowledge other risks."[42][43][8] The EPA's Office of the Inspector General concluded that the EPA had deviated from typical procedures in its 2018 decision despite the best efforts of EPA's career scientists and managers to recommend a different approach that was scientifically, procedurally, and legally sound.[8]

On 8 June 2020, the EPA clarified that existing stocks of the dicamba-based pesticides bought before 3 June 2020 may be used according to their previous labels until 31 July 2020.[44] In October 2020 the EPA issued a decision on the registration application of three dicamba-based products, Xtendimax, Engenia, and Tavium. They approved of their use from 2021 to 2025 with some additional changes, including labeling restrictions.[45]

Despite the control measures implemented by the EPA in 2020, the 2021 incident reports showed little change in the number, severity, and/or geographic extent of dicamba-related incidents.[8] In March 2022 and in February 2023, EPA approved additional labeling to further restrict use of OTT dicamba to reduce the likelihood of volatility and offsite movement of OTT dicamba by avoiding application on days with high temperatures.[8] The restrictions apply to Minnesota, Iowa, Illinois, Indiana, and South Dakota.[8]

Lawsuits

[edit]

In February 2018, it was reported that numerous farmers from 21 states had filed lawsuits against Monsanto alleging that dicamba damaged their crops, with the most prominent cases coming from Missouri and Arkansas.[46] By August 2019, more lawsuits were filed, alleging that dicamba had damaged crops, gardens, and trees of neighbors of the farmers who used it.[40][41]

On 27 January 2020, the first trial concerning dicamba-related products began in Cape Girardeau, Missouri.[47][48] The lawsuit involves a peach farmer who alleged that dicamba-based herbicides caused significant damage to his crops and trees.[49] It had also been filed in November 2016, when dicamba was still owned by Monsanto.[50][51][52] On 14 February 2020, the jury involved in the lawsuit ruled against dicamba owner Bayer and its co-defendant BASF and found in favor of the peach grower, Bader Farms owner Bill Bader.[53] Bayer and BASF were also ordered to pay Bader $15 million in damages.[54] On 15 February 2020, Monsanto and BASF were ordered to pay an additional $250 million in punitive damages.[55][56] Court documents revealed Monsanto had used dicamba drift as a sales pitch to convince farmers to buy their proprietary dicamba-resistant seeds or face devastated crops.[57]

On 17 February, it was announced that dicamba would face many more lawsuits.[58] On 26 February, the Peiffer Wolf Carr & Kane Law Firm announced that after the Bader verdict, more than 2,000 U.S. farmers hired the law firm to represent them in upcoming lawsuits.[59]

In June 2020, Bayer agreed to a settlement of up to $400 million for all 2015–2020 crop year dicamba claims, not including the $250 million judgement.[60] On 25 November 2020, U.S. District Judge Stephen Limbaugh Jr. reduced the punitive damage amount in the Bader Farms case to $60 million.[61]

References

[edit]
[edit]
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Dicamba

View on Grokipedia
from Grokipedia
Dicamba is a synthetic auxin herbicide classified as a benzoic acid compound with the chemical formula C₈H₆Cl₂O₃, functioning by disrupting plant growth hormones to selectively control broadleaf weeds in pre- and post-emergent applications.[1][2] Developed in the early 1960s and first registered for agricultural use in the United States in 1967, it targets weeds through enhanced cell division and elongation, leading to uncontrolled growth and plant death.[3][4] Its widespread adoption accelerated with the commercialization of dicamba-tolerant genetically engineered soybean and cotton varieties in the mid-2010s, enabling over-the-top applications to combat glyphosate-resistant weeds in row crops.[5] However, empirical observations and field studies have documented extensive off-target movement via particle drift and volatilization, particularly under temperature inversions and with certain formulations, resulting in damage to sensitive non-target vegetation, reduced pollinator activity on affected weeds, and economic losses for farmers.[6][7][8] This has prompted multiple U.S. Environmental Protection Agency label restrictions, a 2020 registration cancellation for certain uses, and ongoing proposals for renewed conditional approvals amid debates over formulation volatility and application stewardship.[9][5]

Chemical Properties and Mechanism of Action

Molecular Structure and Synthesis

Dicamba, systematically named 3,6-dichloro-2-methoxybenzoic acid, possesses the molecular formula C₈H₆Cl₂O₃ and a molar mass of 221.04 g/mol.[1] Its core structure consists of a benzene ring with a carboxylic acid substituent at the 1-position, a methoxy group (-OCH₃) at the 2-position, and chlorine atoms at the 3- and 6-positions.[1] This configuration positions dicamba as a derivative of benzoic acid, with halogen and alkoxy substitutions that confer its chemical properties.[10] The molecular architecture of dicamba exhibits similarity to natural auxins, such as indole-3-acetic acid, particularly in the benzoic acid moiety that facilitates interaction with plant auxin receptors.[11] Dicamba was first described in 1958 by researchers at the Jealott's Hill Experimental Station in the United Kingdom, with Velsicol Chemical Corporation subsequently acquiring the patent rights for its development and commercialization.[12] Initial synthesis involved multi-step processes starting from chlorinated anilines or benzoic acid precursors, incorporating diazotization, hydrolysis, and methoxylation reactions to introduce the key functional groups.[13] Commercial manufacturing of dicamba typically employs optimized routes from 2,5-dichloroaniline, involving nitration, reduction, diazotization, and Sandmeyer-type reactions followed by methoxylation and hydrolysis to yield the free acid.[14] For practical application, the dicamba acid is converted into various salts to improve solubility and handling; common formulations include the dimethylamine (DMA) salt, diglycolamine (DGA) salt, isopropylamine (IPA) salts, and potassium salt.[2] These salts are prepared by neutralization of the acid with the respective amine or base, resulting in water-soluble products suitable for formulation into herbicides.[2]

Biochemical Mode of Action

Dicamba acts as a synthetic auxin herbicide by mimicking the natural plant hormone indole-3-acetic acid (IAA), binding to auxin receptors such as the TIR1/AFB family proteins in susceptible plants.[15][16] This interaction destabilizes Aux/IAA repressor proteins, derepressing ARF transcription factors and leading to overexpression of auxin-responsive genes that regulate cell division, elongation, and differentiation.[15] The resulting hormonal imbalance triggers uncontrolled growth responses, including excessive proton pump activation in cell membranes, which promotes rapid cell expansion and tissue deformation, ultimately causing plant death through resource depletion and vascular disruption.[17] The herbicide's selectivity primarily targets dicotyledonous (broadleaf) weeds over monocotyledonous (grass) crops due to differences in metabolic detoxification rates. Monocots efficiently metabolize dicamba via cytochrome P450-mediated hydroxylation and subsequent glycosylation or conjugation, rendering it inactive more rapidly than in dicots, where slower metabolism allows accumulation and sustained receptor binding.[18][19] This differential metabolism prevents significant physiological disruption in tolerant grasses while exploiting the slower degradation in sensitive broadleaves.[20] Laboratory studies demonstrate rapid onset of symptoms following dicamba exposure, with epinasty—downward curling and twisting of leaves and petioles—appearing within hours due to ethylene biosynthesis induced by auxin excess.[21] Further progression includes stem elongation anomalies and chlorosis within 24 to 48 hours, confirming the herbicide's disruption of auxin homeostasis at the molecular level through direct empirical observation of gene expression changes and morphological assays in model plants like Arabidopsis.[16][22]

Historical Development

Discovery and Initial Commercialization

Dicamba, a benzoic acid derivative classified as a synthetic auxin herbicide, was first described scientifically in 1958 as part of research into plant growth regulators capable of selectively disrupting broadleaf weeds.[23] Velsicol Chemical Corporation acquired the patent for the molecule and led its early development, focusing on formulations that mimicked natural auxins to induce uncontrolled growth and death in susceptible plants.[23] The compound's synthesis involved substituting methoxy and dichloro groups on the benzoic acid backbone, building on prior auxin discoveries from the 1940s by researchers like Zimmerman and Hitchcock, though dicamba's specific structure and herbicidal potential were refined in U.S. laboratories during the late 1950s.[12] The herbicide received its initial U.S. registration from the Department of Agriculture in 1962, marking the start of commercial availability under products like Banvel, marketed by Velsicol for targeted weed control.[24] Early applications emphasized non-crop and specialty uses, including suppression of broadleaf weeds in turfgrass, forestry sites, pastures, rangelands, and rights-of-way such as roadsides and utility corridors, where its systemic action allowed for effective control of perennials like thistles and brush without severely impacting grasses.[25] These formulations, often applied at rates of 0.5 to 2 pounds active ingredient per acre, were valued for their postemergence efficacy in cool-season applications, minimizing risks to desirable vegetation when used according to label guidelines.[26] By the 1970s and early 1980s, dicamba expanded into row crop agriculture, particularly for postemergence broadleaf control in corn, soybeans, and cotton, prior to the advent of herbicide-tolerant genetically modified varieties.[27] Combinations with 2,4-D became a standard practice in regions like the Midwest, where they provided broad-spectrum weed management in cornfields, with dicamba rates typically around 0.25 to 0.5 pounds per acre to enhance control of species resistant to 2,4-D alone.[27] This period saw growing adoption in conventional farming systems, driven by dicamba's complementarity to preemergence herbicides and its role in integrated programs, though exact national usage volumes remain sparsely documented before comprehensive tracking in the 1990s.[28]

Early Regulatory Approvals and Expansion

Dicamba received initial registration from the United States Environmental Protection Agency (EPA) in 1967 as a selective herbicide for broadleaf weed control in non-crop areas and certain field crops.[2] This approval followed evaluations of acute toxicity data indicating low mammalian risk, with oral LD50 values exceeding 1,000 mg/kg in rats and minimal systemic effects observed in subchronic studies.[2] The EPA classified dicamba formulations as low-toxicity products under its signal word system, requiring only cautionary labeling due to evidence of rapid metabolism and excretion in test animals, which supported its determination of negligible oncogenic or reproductive hazards at labeled rates.[2] During the 1970s and 1980s, EPA reregistration reviews incorporated residue dissipation studies demonstrating dicamba's half-life in soil averaged 15-30 days under aerobic conditions, with field trials showing detectable levels below 0.1 ppm in harvested crops within tolerances set at 0.5-3 ppm depending on commodity.[29] These empirical findings, including gas chromatography analyses from applications in ditchbanks and pastures, confirmed minimal carryover risks and low bioaccumulation potential in aquatic systems, justifying expanded labels for use in soybeans, wheat, and sorghum without exceeding established maximum residue limits.[30] Stewardship guidelines emphasized ground application and buffer zones to ensure compliance, based on wind tunnel data indicating effective droplet control under standard conditions. Label expansions in the late 1970s extended dicamba approvals to pasture, rangeland, and conservation reserve programs, enabling broadleaf suppression in forage grasses while maintaining low environmental persistence, as validated by monitoring programs detecting no significant groundwater contamination in treated watersheds.[31] Internationally, analogous registrations occurred in Canada through the Pest Management Regulatory Agency by the early 1970s and in Australia for similar non-crop and pastoral applications, reflecting shared reliance on comparable toxicology profiles and field residue data that affirmed safety margins for human and ecological exposure.[23] These milestones facilitated dicamba's integration into integrated pest management systems prior to genetically modified crop developments.

Primary Uses in Agriculture

Broadleaf Weed Control in Conventional Systems

In conventional cropping systems, dicamba is applied post-emergence to control broadleaf weeds in cereals such as corn, wheat, and sorghum, where it targets species like pigweed (Amaranthus spp.), kochia, and marestail that compete with crops during critical growth stages.[32] Typical rates range from 0.25 to 0.5 lb acid equivalent (ae) per acre for post-emergence applications in these crops, with adjustments up to 1 lb ae/acre for larger weeds or pre-plant burndown in fallow fields to ensure thorough coverage and minimize regrowth.[33] For optimal efficacy against pigweed, applications are timed when weeds are 1 to 4 inches tall, achieving control levels exceeding 90% under favorable conditions, as delayed timing reduces absorption and translocation.[34] Tank-mixing dicamba with glyphosate extends the control spectrum to include both grasses and persistent broadleaves in conventional cereal systems and fallow periods, where glyphosate alone often fails against dicot weeds.[35] Such combinations are standard in corn and wheat, applied at burndown or early post-emergence stages to suppress weed emergence and biomass accumulation. Similarly, dicamba tank-mixed with 2,4-D enhances broadleaf suppression in wheat, particularly for post-harvest or pre-plant uses in rotation systems, allowing for spectrum broadening without excessive crop injury when rates are calibrated (e.g., 0.25 lb ae/acre dicamba plus 0.5 lb ae/acre 2,4-D).[36] Field trials in conventional corn prior to the 2010s indicated that dicamba applications significantly mitigated yield losses from broadleaf weed interference, with uncontrolled competition reducing yields by 16% to 56% in Georgia studies, underscoring dicamba's role in preserving productivity through timely weed suppression.[37] In wheat systems, analogous pre-2010 evaluations confirmed dicamba's contribution to yield protection by limiting broadleaf weed density and vigor, particularly in tank-mix regimens that prevented escape of species like pigweed.[38]

Integration with Genetically Modified Crops

Genetically modified dicamba-tolerant (DT) soybean and cotton varieties incorporate the dicamba monooxygenase (DMO) gene, sourced from bacteria such as Stenotrophomonas maltophilia, which encodes an enzyme that catalyzes the hydroxylation of dicamba into the non-herbicidal metabolite 3,6-dichlorosalicylic acid, thereby conferring crop tolerance.[39][40] Monsanto (now Bayer) commercialized Roundup Ready 2 Xtend soybeans in 2016, following regulatory approvals, with initial planting on approximately 1 million acres that year.[41] Dicamba-tolerant cotton, such as Bollgard II XtendFlex varieties, was introduced concurrently, enabling combined tolerance to dicamba, glyphosate, and glufosinate.[42] These DT crops permit over-the-top (OTT) post-emergence applications of dicamba, targeting emerged broadleaf weeds without injuring the crop, in contrast to prior restrictions on ground-directed or pre-emergence use only.[43] This capability addressed glyphosate-resistant weeds, prompting rapid adoption: DT soybean acreage surged from limited planting in 2016 to over 40% of U.S. soybean acres by 2018, with dicamba use on soybeans exceeding 15 million pounds annually by 2019.[43] DT cotton adoption similarly reached 70-80% of U.S. cotton acres by 2019, per USDA surveys.[44] Accompanying this shift, herbicide manufacturers developed specialized low-volatility dicamba formulations for OTT use, including Bayer's XtendiMax with VaporGrip Technology, which employs a diglycolamine salt and volatility-reducing additives, and BASF's Engenia, utilizing a proprietary N,N-bis-(3-aminopropyl) methylamine salt to limit vaporization.[45][46] These formulations, approved by the EPA in 2016, were engineered to minimize off-target movement while maintaining efficacy against weeds like Palmer amaranth and waterhemp.[9]

Efficacy and Economic Benefits

Weed Control Performance Data

Field trials evaluating dicamba applications in dicamba-tolerant soybean have reported control levels of 80% to 95% for glyphosate-resistant common ragweed (Ambrosia artemisiifolia) when using single or sequential postemergence applications at labeled rates.[47] Similar efficacy has been observed against other broadleaf weeds, with university extension ratings classifying dicamba as providing good to excellent control (80-95% or higher) for species in the Amaranthus genus under optimal conditions of early application and integration with residuals. For challenging resistant weeds like Palmer amaranth (Amaranthus palmeri), controlled studies show dicamba achieving 88% or greater control when applied to plants 10-15 cm tall, particularly in mixtures with diflufenzopyr or tembotrione, though efficacy drops to 63% or less with single applications on larger plants or in high-density populations.[48][49] Dose-response data from field experiments indicate that dicamba maintains high efficacy (76-90%) against small broadleaf weeds (<10 cm) at standard rates (e.g., 0.56 kg ae/ha), with additive improvements of 14-23% when tank-mixed with ammonium sulfate for enhanced uptake.[50][51] Comparative trials post-glyphosate resistance emergence highlight dicamba's superiority for broadleaf control, offering better short-term residual activity and higher efficacy on small-seeded resistant species compared to glyphosate alone, with tank-mixtures sometimes reducing Palmer amaranth control by 14 percentage points relative to dicamba solo due to antagonism.[52][53] Meta-analytic reviews of herbicide programs underscore 80-95% seasonal control of resistant Palmer amaranth in rotations incorporating dicamba with residuals, outperforming glyphosate-reliant systems in multiple U.S. Corn Belt locations.[54]

Impacts on Crop Yields and Farm Economics

The integration of dicamba with dicamba-tolerant (DT) soybeans has facilitated effective postemergence control of broadleaf weeds resistant to glyphosate, such as Palmer amaranth and waterhemp, thereby protecting yields from potential losses that can exceed 50% in unmanaged infestations.[55] Field trials indicate that dicamba applications, often tank-mixed with glyphosate, achieve season-long weed suppression and higher soybean grain yields compared to untreated controls or delayed herbicide timings, with all dicamba-inclusive treatments outperforming weedy checks.[56][57] By 2018, DT soybean adoption reached 43% of U.S. acreage, correlating with enhanced productivity in regions facing resistant weed pressures, as farmers reported benefits from flexible over-the-top applications that minimize yield penalties from inadequate early-season control.[58] From an economic standpoint, dicamba use in DT systems yields net savings of $12–$14 per acre versus alternative programs like those relying on 2,4-D, equating to 4–7% of regional net operating revenues (e.g., 3.7% in the Corn Belt, 7.3% in the Mid-South).[59] These gains arise from lower overall herbicide expenditures, fewer postemergence passes, and reduced reliance on labor-intensive hand-weeding, which can add $10–$20 per acre in costs for conventional systems. Integrated dicamba management further supports conservation tillage, cutting fuel and machinery expenses by up to 20–30% through decreased soil disturbance, while sustaining long-term farm profitability amid rising input costs.[58] Such efficiencies have driven widespread adoption, bolstering U.S. soybean output value, which exceeded $40 billion annually in recent years, though benefits accrue primarily to stewards employing buffered formulations and precise application to mitigate off-target risks addressed elsewhere.[60]

Technical Challenges

Volatility and Drift Mechanisms

Dicamba volatility arises from its inherent vapor pressure as a weak acid herbicide, enabling post-application evaporation into the gaseous phase, particularly when dissociated from salt formulations under elevated temperatures or low humidity conditions.[46] This process is exacerbated by chemical interactions, such as hydrogen bonding that facilitates airborne persistence, and tank-mix additives like glyphosate, which promote salt bond dissociation and increase volatilization rates.[61][62] Particle drift complements volatility as a primary off-target mechanism, involving the aerodynamic transport of spray droplets, especially fine ones under windy conditions or during temperature inversions where cooler ground-level air traps and horizontally advects suspended particles and vapors. Inversions, prevalent in early morning or evening applications, reduce vertical mixing and extend drift distances by maintaining near-surface stability.[46] Application parameters critically influence both mechanisms: coarser droplet spectra from air-induction nozzles minimize particle drift compared to standard flat-fan nozzles, as demonstrated in wind tunnel tests where drift deposits decreased by factors of 2-5 times under winds of 5-10 km/h.[63] Optimal timing avoids inversion-prone periods, while buffer zones—typically 30-60 meters—intercept drifted material, though efficacy depends on wind direction and speed.[64] Mitigation in contemporary formulations targets volatility reduction through pH-buffered salts like diglycolamine (DGA), which stabilize the molecule and lower vapor pressure relative to older dimethylamine (DMA) variants, with additives achieving up to 90% decreases in measured vapor flux in controlled volatilization assays.[65] Recent studies confirm these low-volatility products exhibit field volatilization losses below 1% under standard conditions, contrasting sharply with pre-2010 formulations prone to higher emissions.[66][67]

Development of Herbicide Resistance

Herbicide resistance to dicamba evolves through natural selection, where repeated applications impose strong selective pressure on weed populations harboring rare genetic variants that confer survival advantages, such as reduced herbicide binding or enhanced detoxification. This process accelerates under high-use scenarios, as surviving individuals reproduce and propagate resistant alleles, leading to population-level shifts observable in field and laboratory settings. Empirical evidence from dose-response assays confirms resistance via metrics like increased GR50 values (herbicide rate required for 50% growth reduction), often exceeding 10-fold compared to susceptible biotypes.[68] Early detections of dicamba resistance emerged in the 1990s, with Kochia scoparia (kochia) populations in the northern Great Plains exhibiting tolerance, though mechanisms were not fully elucidated until later genomic analyses. Resistance confirmation intensified after 2016 with the commercialization of over-the-top (OTT) dicamba applications on genetically modified cotton and soybean varieties, which expanded usage and heightened selection pressure on broadleaf weeds like Palmer amaranth and waterhemp. By 2025, at least 21 weed species globally demonstrated dicamba resistance, including key U.S. problem weeds such as Amaranthus palmeri (Palmer amaranth) and Amaranthus tuberculatus (waterhemp), with field cases documented in states like Kansas (2019 for Palmer amaranth) and Illinois (2021 for waterhemp).[69][70][71][72] At the molecular level, target-site resistance (TSR) predominates in many cases, involving mutations in auxin-related genes that disrupt dicamba's interference with plant hormone signaling; for instance, a novel mutation in the GWPPV degron domain of the IAA16 gene in Kochia scoparia reduces sensitivity to synthetic auxins like dicamba. Non-target-site resistance (NTSR) mechanisms, such as cytochrome P450-mediated metabolism, also contribute, enabling weeds to degrade the herbicide before it reaches lethal concentrations, as verified through biochemical assays showing elevated detoxification enzyme activity in resistant populations. These mechanisms often co-occur, complicating control, with genomic sequencing and inheritance studies confirming dominant or semi-dominant inheritance patterns in allotetraploid weeds like Chenopodium album.[73][74][75] Integrated resistance management (IRM) strategies mitigate progression by diversifying selection pressures, including rotating herbicide modes of action (e.g., combining Group 4 auxins like dicamba with Group 5 photosystem II inhibitors or Group 15 protox inhibitors), applying full-labeled rates of residual preemergence herbicides, and maintaining weed-free seedbeds to limit seedbank replenishment. Field trials demonstrate that compliant IRM programs slow resistance evolution, with compliant cotton and soybean fields showing 20-50% lower incidence of resistant biotypes compared to monoculture dicamba systems over 5 years, while also preserving yield economics through sustained weed control efficacy.[76][77][78]

Toxicology and Health Effects

Human Exposure and Acute Toxicity

Dicamba exposure in humans occurs mainly through dermal absorption and inhalation among agricultural applicators during mixing, loading, and spraying operations, with incidental ingestion possible via hand-to-mouth contact if hygiene protocols are not followed.[79][80] Residential or bystander exposure is minimal under typical use conditions, as dicamba formulations are designed for professional application with protective equipment. Acute toxicity profiles demonstrate low hazard potential, evidenced by rat oral LD50 values ranging from 1,707 to 2,740 mg/kg body weight, placing dicamba in U.S. EPA Toxicity Category III (slightly toxic).[81][82] Dermal LD50 exceeds 2,000 mg/kg in rabbits, indicating negligible skin penetration risk, while inhalation LC50 surpasses 3.3 mg/L air (4 hours), supporting Category IV classification (practically non-toxic).[81][83] Primary acute effects in mammalian studies include mild eye irritation, transient corneal opacity, and minor gastrointestinal symptoms at high doses, with no observed neurotoxicity or systemic organ damage at relevant exposure levels.[84] The U.S. EPA's human health risk assessments, including evaluations through the 2020 registration review docket, identify no acute dietary or occupational exposure concerns when personal protective equipment is used as directed, with margins of exposure exceeding safety thresholds by factors of 100 or more.[81] Poison control and incident databases report predominantly minor cases, such as ocular irritation or nausea from accidental splashes or ingestions, with severe outcomes rare and linked to intentional misuse or unlabeled formulations rather than standard agricultural handling.[79] Applicator biomonitoring studies confirm urinary metabolite levels below levels associated with adverse acute effects, underscoring the compound's favorable safety margin in practice.[80]

Long-Term Health Risks Including Cancer Assessments

Epidemiological studies on long-term health risks from dicamba exposure have primarily relied on the Agricultural Health Study (AHS), a prospective cohort of over 89,000 pesticide applicators in Iowa and North Carolina enrolled between 1993 and 1997, with follow-up through 2014 for cancer incidence. An initial AHS analysis published in 2006 reported elevated risks for lung cancer (relative risk [RR] 1.1 per 100 days of lifetime exposure, 95% confidence interval [CI] 1.0-1.2) and colon cancer (RR 1.1, 95% CI 1.0-1.2), but these associations were attenuated and no longer statistically significant in a 2020 updated analysis incorporating additional follow-up and refined exposure metrics.[85][86] The 2020 AHS re-analysis identified associations between dicamba use and increased risk of liver and intrahepatic bile duct cancer among ever-exposed applicators (RR 1.71, 95% CI 0.97-3.02), with a dose-response trend in the highest exposure quartile (RR 1.80, 95% CI 1.26-2.56, P trend <0.001), based on 28 exposed cases; this persisted after lagging exposure by up to 20 years. Similar patterns emerged for chronic lymphocytic leukemia (highest quartile RR 1.20, 95% CI 0.96-1.50, P trend 0.01, 93 exposed cases), though attenuated with longer lags, while myeloid leukemia showed an inverse association (highest quartile RR 0.73, 95% CI 0.51-1.03, P trend 0.01). These findings are limited by small case numbers for rare cancers, wide confidence intervals indicating imprecision, absence of dose-response for some sites, and potential confounding from co-exposures to correlated pesticides (e.g., 2,4-D, atrazine) or unmeasured factors like smoking, alcohol use, or viral hepatitis, which are not fully adjustable in self-reported data. No consistent evidence of genotoxicity or a plausible carcinogenic mechanism supports causality.[86] Regulatory assessments diverge in emphasis but prioritize controlled animal data over observational epidemiology. The U.S. Environmental Protection Agency (EPA) classifies dicamba as "not likely to be carcinogenic to humans," based on negative results from multiple chronic feeding studies in rats and mice showing no increased tumor incidence at doses up to 500 mg/kg/day, lack of genotoxic potential in validated assays, and absence of a mode-of-action for carcinogenicity; this classification was reaffirmed in the EPA's 2020 registration review, deeming no additional cancer risk quantification necessary despite AHS findings. The International Agency for Research on Cancer (IARC) has not classified dicamba, reflecting limited evidence in experimental animals and inadequate human data for positive categorization. Large cohort null findings for common cancers and reliance on animal bioassays underscore that amplified claims of definitive carcinogenicity in media and advocacy reports often overlook confounding and the absence of causal criteria like temporality, biological gradient, and specificity.[84][9][1]

Environmental Fate and Impacts

Degradation and Persistence in Soil and Water

Dicamba primarily degrades in soil through microbial activity under aerobic conditions, with reported DT50 values (time for 50% dissipation) ranging from 7 to 30 days depending on soil type, moisture, and temperature.[2] In grassland soils, the half-life averages 17 days, while in forest soils it extends to 26-32 days; degradation accelerates with higher microbial populations and is slower in sterile or anaerobic environments.[2] The primary pathways involve decarboxylation and dehalogenation by soil bacteria, yielding metabolites such as 3,6-dichlorosalicylic acid.[83] Sorption of dicamba to soil particles limits its mobility, particularly through binding to clay minerals via ion exchange of its anionic form at typical soil pH levels (above pKa of 1.98), resulting in low leaching potential despite moderate water solubility (6.5 g/L).[87] Adsorption coefficients (Kd) vary from 0.25 to 0.88, influenced by clay content and organic matter, which collectively reduce vertical transport in most agricultural soils.[88] In water bodies, dicamba resists hydrolysis at neutral pH, exhibiting a half-life exceeding 1 year under dark, sterile conditions.[1] Photolysis in sunlight, however, drives faster breakdown, with laboratory studies under simulated sunlight yielding a half-life of approximately 0.77 days at pH 7, though field-adjusted values range from 38 to 105 days accounting for solar intensity and depth.[89] Runoff and monitoring data confirm limited persistence and groundwater ingress; U.S. Geological Survey assessments from 1992-1996 detected dicamba in only 0.13% of 2,305 sites, with concentrations below 1 ppb where present, aligning with models predicting negligible contamination from typical applications.[2][90]

Effects on Non-Target Plants, Wildlife, and Ecosystems

Dicamba drift has caused verifiable injury to non-target plants, primarily through off-site movement affecting sensitive broadleaf species such as soybeans, trees, and native vegetation. Between 2017 and 2020, an estimated 3.6 million acres of non-dicamba-tolerant soybeans exhibited symptoms like leaf cupping and stunting due to dicamba volatilization and particle drift.[91] In 2018 alone, USDA surveys documented damage across 4.1 million acres of soybean fields, representing about 4% of planted soybean acreage that year.[92] Tree species in Midwest and Southern regions, including those in wildlife refuges, showed widespread symptoms such as twisted leaves and dieback, with volunteer monitoring identifying 243 damage instances across 17 counties in 2019.[93] These effects stem from dicamba's auxin-mimicry mode of action, disrupting growth hormones in exposed plants at concentrations as low as 1/20,000th of typical application rates.[94] Direct toxicity to wildlife is low for most vertebrates. Dicamba salts exhibit practically non-toxic profiles to birds, with acute oral LD50 values exceeding 2,000 mg/kg in species like mallard ducks, while the acid form is slightly to moderately toxic.[79][95] Mammalian acute toxicity is moderate, with LD50 values typically above 1,000 mg/kg, indicating limited risk from direct ingestion under field conditions.[96] Extension surveys and incident reports suggest drift-related wildlife exposures are rare, with no widespread acute poisoning events documented in monitoring data.[97] Pollinator impacts are primarily indirect, mediated by drift-induced changes in forage plants rather than high direct toxicity. Field studies indicate dicamba exposure delays flowering and reduces flower production in non-target weeds, leading to decreased visitation by bees and other insects in affected plots.[98] One experiment reported up to 70% fewer insect pollinators in dicamba-exposed areas compared to controls, attributed to altered plant architecture and nectar availability.[99] However, broader population-level declines in bee colonies have not been causally linked to dicamba in longitudinal surveys, with mixed results from foraging behavior assessments showing no consistent sublethal effects at environmentally relevant doses.[100] Ecosystem-level effects remain localized, with no evidence of broad collapse from long-term monitoring. Damage to riparian and refuge habitats includes reduced broadleaf diversity from tree and shrub injury, potentially affecting dependent invertebrates and birds, but aquatic and soil communities show resilience due to dicamba's moderate persistence.[101] Illinois assessments noted limited ecological monitoring of off-target injury, emphasizing that while fruit trees and native perennials suffer acute symptoms like wilting and irregular growth, recovery occurs in surviving vegetation without cascading trophic disruptions in surveyed areas.[102][103] Drift incidence appears low relative to total applications, with state extension reports citing fewer than 5% of fields showing verifiable off-site movement in controlled surveys.[104]

Regulatory History and Legal Actions

Key EPA Registrations and Revisions

The U.S. Environmental Protection Agency (EPA) first registered dicamba in 1967 as a selective herbicide for broadleaf weed control in various crops and non-crop areas.[2][105] Initial approvals focused on directed and hooded spray applications to minimize volatility-related drift, based on assessments of its efficacy against weeds like pigweed while limiting off-target exposure through application guidelines.[106] In 2016, the EPA granted time-limited registrations for three low-volatility dicamba formulations—XtendiMax, Engenia, and FeXapan—for over-the-top (OTT) use on dicamba-tolerant (DT) soybean and cotton crops, marking the first authorization for post-emergence applications directly on growing plants.[25] These approvals, initially for two years, relied on residue data demonstrating minimal plant-back injury and efficacy trials showing effective control of glyphosate-resistant weeds, with EPA concluding that labeled use mitigated risks to non-target plants.[107] Extensions in 2017 and 2018 incorporated additional stewardship measures, such as buffer zones and trained applicator requirements, following empirical field data on drift incidents.[106] On October 27, 2020, the EPA reregistered XtendiMax and Engenia for five years and extended Tavium's registration, affirming that benefits—including enhanced weed management in DT crops and reduced reliance on alternatives—outweighed risks when mitigation measures were followed.[108][9] Key revisions included a 30-minute average wind speed cutoff not exceeding 10 mph, mandatory cutoff dates varying by crop growth stage and region to avoid late-season drift, expanded buffer zones near residential areas, and requirements for digital recordkeeping and volatility reduction nozzles, derived from risk assessments incorporating residue dissipation studies and exposure modeling.[106][9] These registrations were vacated on February 6, 2024, by the U.S. District Court for the District of Arizona in National Family Farm Coalition v. U.S. EPA, which ruled the EPA failed to adequately address procedural requirements under the Endangered Species Act and other statutes, rendering the OTT approvals unlawful despite underlying risk-benefit analyses.[106][109] The decision halted new sales and applications of the affected products, though existing stocks provisions allowed limited use under prior labels until February 2024 deadlines.[110]

Court Rulings and Ongoing Litigation (Including 2020-2025 Developments)

In June 2020, the United States Court of Appeals for the Ninth Circuit vacated the Environmental Protection Agency's (EPA) 2018 conditional registrations for over-the-top (OTT) applications of dicamba products XtendiMax, Engenia, and Tavium, ruling that the agency had failed to adequately address volatility and drift risks under the Federal Insecticide, Fungicide, and Rodenticide Act (FIFRA), particularly regarding label compliance feasibility during the 2018-2020 growing seasons.[111] The court emphasized procedural shortcomings in the EPA's risk assessment, including insufficient evidence that updated labels would prevent off-target movement, though it did not directly adjudicate the underlying scientific validity of drift mitigation claims.[112] Following the 2020 Ninth Circuit decision, the EPA issued 2020 registrations for the same OTT dicamba products, but these were vacated on February 6, 2024, by the United States District Court for the District of Arizona in National Family Farm Coalition v. U.S. EPA, which found the agency had violated FIFRA's notice-and-comment requirements by making substantive changes to labels without proper public input, including expansions of use periods and buffer adjustments.[113] The ruling highlighted procedural lapses rather than a wholesale rejection of dicamba's efficacy data, noting prior drift incidents but deferring to the EPA on scientific evaluations unless arbitrary.[107] In response to the 2024 Arizona vacatur, the EPA issued an Existing Stocks Order on February 14, 2024, permitting the sale, distribution, and use of pre-vacatur dicamba stocks for the 2024 growing season under state-specific cutoff dates, with prohibitions on new production or import to facilitate orderly phase-out while minimizing immediate economic disruption to farmers.[114][110] On July 23, 2025, the EPA proposed unconditional registrations for new OTT uses of low-volatility dicamba formulations on dicamba-tolerant cotton and soybeans through 2031, incorporating enhanced mitigations such as mandatory 240-foot downwind buffers, approved drift-reduction and pH-buffering agents to curb volatility, bans on aerial applications, and extended application cutoffs, aiming to address prior court-cited procedural and drift concerns without revisiting core risk-benefit analyses.[5][106] Parallel to regulatory challenges, dicamba-related litigation has involved thousands of farmer claims for crop damage from drift, with Bayer (successor to Monsanto) agreeing in June 2020 to a settlement fund of up to $400 million for 2015-2020 claims alleging economic losses from non-tolerant soybean and other crop injuries.[115] A notable case, Bader Farms v. Monsanto (Eastern District of Missouri), resulted in a February 2020 jury verdict of $265 million (later reduced) against Bayer and BASF for dicamba-induced damage to a peach orchard, underscoring liability for inadequate drift warnings despite defenses based on user error.[116] As of October 2025, multidistrict litigation in the Eastern District of Missouri includes 27 pending individual and potential class actions seeking compensation for ongoing economic harms, with plaintiffs arguing persistent drift patterns despite label updates.[115][117]

Stakeholder Perspectives and Controversies

Benefits Advocated by Farmers and Industry

Farmers and agricultural industry representatives advocate for dicamba use primarily for its efficacy in controlling glyphosate-resistant weeds, such as Palmer amaranth and waterhemp, which can otherwise reduce soybean yields by up to 50 percent or more in infested fields.[55][118] Adoption of dicamba-tolerant (DT) soybean and cotton seeds reflects this perceived value, with U.S. farmers planting DT soybeans on 43 percent of soybean acreage in 2018, particularly in high-infestation states like Mississippi (79 percent) and Tennessee (71 percent).[43] In Nebraska, a 2018 survey found nearly 60 percent of DT soybean growers applied dicamba post-emergence, often in combination with glyphosate, to achieve superior broadleaf weed suppression compared to alternatives.[119] Industry analyses and farmer-reported outcomes highlight yield protection and potential gains from dicamba integration into weed management programs. Field tests by Monsanto and independent researchers demonstrated an average 5.7 bushels per acre yield increase in soybeans using dicamba-tolerant systems versus conventional glyphosate-based controls.[120] This stems from dicamba's ability to target post-emergent resistant weeds without harming DT crops, enabling timely applications that preserve crop vigor and reduce competition during critical growth stages.[6] Proponents emphasize cost efficiencies, with post-emergence dicamba programs lowering per-acre input expenses by $12–$14 in soybeans (4–7 percent of net operating revenue) and $8–$14 in cotton (5–10 percent), primarily through minimized hand-weeding labor and fewer tillage passes needed for weed escape control.[9] These savings incentivize investment in genetically modified (GM) herbicide-tolerant technologies, as DT seed systems pair with low-volatility formulations to sustain productivity amid rising resistance pressures, fostering ongoing innovation in crop protection.[9] Regarding off-target movement, farmers and industry maintain that observed drift incidents largely result from application errors or non-compliance with label instructions, rather than inherent product volatility under proper use conditions, as evidenced by elevated complaint rates tied to misuse in compliance investigations.[121] Training programs mandated for applicators are cited as effective mitigations, allowing compliant users to realize dicamba's weed control advantages without systemic risks when integrated into diversified resistance management strategies.[121]

Criticisms from Environmental Groups and Affected Parties

Environmental groups such as the Center for Food Safety and the National Audubon Society have criticized dicamba for its propensity to drift, causing widespread damage to non-target vegetation and habitats. In 2017, a survey by weed scientist Kevin Bradley at the University of Missouri estimated that dicamba drift injured approximately 3.6 million acres of soybean crops, with subsequent years seeing even higher figures, including up to 15 million acres affected in 2018 according to USDA data on sensitive soybean varieties. These groups argue that such off-target movement not only exacerbates herbicide resistance but also threatens broader ecosystems by reducing plant diversity essential for wildlife, though empirical evidence primarily documents localized rather than systemic collapses.[92][122] Affected non-dicamba-tolerant (non-DT) farmers and orchard owners have reported significant economic losses from dicamba exposure, particularly to fruit trees and specialty crops. For instance, peach growers in Missouri, such as the Flamm family, documented leaf curling and premature defoliation leading to sunburned, stunted fruit and tree mortality, with estimates of 500 to 600 trees lost in a single year from suspected drift. Similar complaints from vegetable and tree fruit producers across the Midwest and South highlight weakened trees unable to support fruit loads, resulting in yield reductions, though causation is often inferred from symptoms rather than direct residue testing in all cases.[93][123][124] Critics including Pesticide Action Network and conservation advocates have called for dicamba bans, citing risks to pollinators and habitats from reduced flowering in exposed plants. A 2025 study reported a 70% decline in insect pollinators in dicamba-treated plots compared to controls, while field observations noted delayed flowering and fewer seeds in native species like common boneset, potentially disrupting nectar sources for bees and birds. Organizations like the Center for Food Safety contend this contributes to broader pollinator declines, including honeybee colony losses, though such associations rely on correlative data from drift-affected areas rather than controlled causation across landscapes.[99][125][126]

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