Dicamba. Dicamba (3,6-dichloro-2-methoxybenzoic acid) is a selective systemic herbicide first registered in 1967.  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.  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."

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. 

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 damage. The controversy led to litigation, state bans and additional restrictions over dicamba use.

Use as an herbicide 

Dicamba is a selective and systemic herbicide that kills annual and perennial broadleaf weeds. 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, and wastage).  Dicamba is toxic to conifer species but is in general less toxic to grasses.
Dicamba is a synthetic auxin that functions by increasing plant growth rate, leading to senescence and cell death.

The growth regulating properties of dicamba were first discovered by Zimmerman and Hitchcock in 1942.  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. Older formulations have been reported to drift after application and affect other crops not meant to be treated. 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. As a result, the use of dicamba in US agriculture rose sharply from approximately  in 2016 to  in 2019, according to the US Geological Survey.

Drift 
Dicamba came under scrutiny due to its reputation for drifting from treated fields onto neighboring crops. In 2011, the European Food Safety Authority identified dicamba's potential for long-range transport through the atmosphere as a critical area of concern. 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. Dicamba is also available in a drift-resistant formulation, which is less likely to affect neighboring fields.

Toxicology

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

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. The Cross-Canada Study of Pesticides and Health found that exposure to dicamba increased the risk of non-Hodgkins lymphoma in men.

Mammals 
Dicamba has low toxicity by ingestion and inhalation or dermal exposure (oral  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.

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.

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

Aquatic animals 
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.

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

Genetically modified crops 
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.

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. Monsanto has marketed their dicamba resistant crops under the brand name Xtend.

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

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

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

Environmental fate 

Dicamba is poorly soluble in water. 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.  

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. The dichlorosalicylic acid is far less toxic than dicamba, which is already rather low.

Use on dicamba-tolerant crops 
Complaints against dicamba accelerated after the United States EPA approved a Monsanto-created soybean which could tolerate it in 2016. The soybean was a part of Monsanto's Xtend products. Dicamba was approved by the EPA for "over-the-top" (OTT) use on those dicamba-tolerant soybean and cotton crops. 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.

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

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." 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.

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.  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.

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. 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. The restriction apply to Minnesota, Iowa, Illinois, Indiana, and South Dakota.

Lawsuits 
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. By August 2019, more lawsuits were filed, alleging that dicamba had damaged crops, gardens, and trees of neighbors of the farmers who used it.

On 27 January 2020, the first trial concerning dicamba-related products began in Cape Girardeau, Missouri. The lawsuit involves a peach farmer who alleged that dicamba-based herbicides caused significant damage to his crops and trees. It had also been filed in November 2016, when dicamba was still owned by Monsanto. 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. Bayer and BASF were also ordered to pay Bader $15 million in damages. On 15 February 2020, Monsanto and BASF were ordered to pay an additional $250 million in punitive damages. 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.

On 17 February, it was announced that dicamba would face many more lawsuits. 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.

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. On 25 November 2020, U.S. District Judge Stephen Limbaugh Jr. reduced the punitive damage amount in the Bader Farms case to $60 million.

References

External links 

 Appendix E: Herbicide Information, US Department of Agriculture
 Chemical Fact Sheet, Speclab.com
 Dicamba Technical Fact Sheet – National Pesticide Information Center
 Dicamba General Fact Sheet – National Pesticide Information Center
 Dicamba Pesticide Information Profile – Extension Toxicology Network
 Monsanto's Xtend Crop System Product Page
 EPA Dicamba Reregistration Eligibility Decision

Auxinic herbicides
Benzoic acids
Chloroarenes
Salicylyl ethers
Systemic herbicides