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Dicamba (3,6-dichloro-2-methoxybenzoic acid) is a broad-spectrum
herbicide Herbicides (, ), also commonly known as weedkillers, are substances used to control undesired plants, also known as weeds.EPA. February 201Pesticides Industry. Sales and Usage 2006 and 2007: Market Estimates. Summary in press releasMain page fo ...
first registered in 1967. Brand names for formulations of this herbicide include Dianat, Banvel, Diablo, Oracle and Vanquish. This chemical compound is a
chlorinated In chemistry, halogenation is a chemical reaction that entails the introduction of one or more halogens into a compound. Halide-containing compounds are pervasive, making this type of transformation important, e.g. in the production of polyme ...
derivative In mathematics, the derivative of a function of a real variable measures the sensitivity to change of the function value (output value) with respect to a change in its argument (input value). Derivatives are a fundamental tool of calculus. F ...
of ''o''-anisic acid. Around 2016, dicamba's use came under significant scrutiny due to its tendency to spread from treated fields into neighboring fields, causing damage.


Use as an herbicide

Dicamba 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 Bracken (''Pteridium'') is a genus of large, coarse ferns in the family Dennstaedtiaceae. Ferns (Pteridophyta) are vascular plants that have alternating generations, large plants that produce spores and small plants that produce sex cells (eggs ...
in pastures, as well as controlling
legumes A legume () is a plant in the family Fabaceae (or Leguminosae), or the fruit or seed of such a plant. When used as a dry grain, the seed is also called a pulse. Legumes are grown agriculturally, primarily for human consumption, for livestock fo ...
and
cacti A cactus (, or less commonly, cactus) is a member of the plant family Cactaceae, a family comprising about 127 genera with some 1750 known species of the order Caryophyllales. The word ''cactus'' derives, through Latin, from the Ancient Greek ...
. In combination with a
phenoxy herbicide Phenoxy herbicides (or "phenoxies") are two families of chemicals that have been developed as commercially important herbicides, widely used in agriculture. They share the part structure of phenoxyacetic acid. Auxins The first group to be discover ...
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 Conifers are a group of conifer cone, cone-bearing Spermatophyte, seed plants, a subset of gymnosperms. Scientifically, they make up the phylum, division Pinophyta (), also known as Coniferophyta () or Coniferae. The division contains a single ...
species but is in general less toxic to grasses.Arnold P. Appleby, Franz Müller. "Weed Control, 2" in ''Ullmann's Encyclopedia of Industrial Chemistry'' 2011, Wiley-VCH, Weinheim. Dicamba is a synthetic auxin that functions by increasing plant growth rate, leading to
senescence Senescence () or biological aging is the gradual deterioration of functional characteristics in living organisms. The word ''senescence'' can refer to either cellular senescence or to senescence of the whole organism. Organismal senescence inv ...
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 Genetic engineering, also called genetic modification or genetic manipulation, is the modification and manipulation of an organism's genes using technology. It is a set of Genetic engineering techniques, technologies used to change the gene ...
plants by
Monsanto The Monsanto Company () was an American agrochemical and agricultural biotechnology corporation founded in 1901 and headquartered in Creve Coeur, Missouri. Monsanto's best known product is Roundup, a glyphosate-based herbicide, developed in th ...
. In October 2016, the
EPA The Environmental Protection Agency (EPA) is an independent executive agency of the United States federal government tasked with environmental protection matters. President Richard Nixon proposed the establishment of EPA on July 9, 1970; it be ...
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 The United States Geological Survey (USGS), formerly simply known as the Geological Survey, is a scientific agency of the United States government. The scientists of the USGS study the landscape of the United States, its natural resources, an ...
.


Resistance

Some farmers and researchers have expressed concern about
herbicide resistance Herbicides (, ), also commonly known as weedkillers, are substances used to control undesired plants, also known as weeds.EPA. February 201Pesticides Industry. Sales and Usage 2006 and 2007: Market Estimates. Summary in press releasMain page fo ...
after the introduction of resistant crops. In the laboratory, researchers have demonstrated weed resistance to dicamba within three generations of exposure. Similar herbicide resistant weeds arose after the introduction of
glyphosate Glyphosate (IUPAC name: ''N''-(phosphonomethyl)glycine) is a broad-spectrum Herbicide, systemic herbicide and Crop desiccation, crop desiccant. It is an organophosphorus compound, specifically a phosphonate, which acts by inhibiting the plan ...
-resistant crops (marketed as '
Roundup Ready Roundup Ready is the Monsanto trademark for its patented line of genetically modified crop seeds that are resistant to its glyphosate-based herbicide, Roundup. History In 1996, genetically modified ''Roundup Ready'' soybeans resistant to Roundup ...
'). Some weed species, like '' Amaranthus palmeri'', have developed resistance to dicamba. Dicamba resistance in ''
Bassia scoparia ''Bassia scoparia'' is a large annual herb in the family Amaranthaceae (''sensu lato'') native to Eurasia. It has been introduced to many parts of North America,
'' was discovered in 1994 and has not been explained by common modes of resistance such as absorption, translocation, or metabolism.


Genetically modified crops

The soil bacterium ''
Pseudomonas maltophilia '' Stenotrophomonas maltophilia'' is an aerobic, nonfermentative, Gram-negative bacterium. It is an uncommon bacterium and human infection is difficult to treat. Initially classified as ''Bacterium bookeri'', then renamed ''Pseudomonas maltophi ...
'' (strain DI-6) converts dicamba to 3,6-dichlorosalicylic acid (3,6-DCSA), which is
adsorbed Adsorption is the adhesion of atoms, ions or molecules from a gas, liquid or dissolved solid to a surface. This process creates a film of the ''adsorbate'' on the surface of the ''adsorbent''. This process differs from absorption, in which a ...
to soil much more strongly than is dicamba, but lacks herbicidal activity. Little information is available on the toxicity of this breakdown intermediate. The enzymes responsible for this first breakdown step comprise a three-component system called dicamba O-demethylase.
Monsanto The Monsanto Company () was an American agrochemical and agricultural biotechnology corporation founded in 1901 and headquartered in Creve Coeur, Missouri. Monsanto's best known product is Roundup, a glyphosate-based herbicide, developed in th ...
recently incorporated one component of the three enzymes into the genome of
soybean The soybean, soy bean, or soya bean (''Glycine max'') is a species of legume native to East Asia, widely grown for its edible bean, which has numerous uses. Traditional unfermented food uses of soybeans include soy milk, from which tofu an ...
,
cotton Cotton is a soft, fluffy staple fiber that grows in a boll, or protective case, around the seeds of the cotton plants of the genus ''Gossypium'' in the mallow family Malvaceae. The fiber is almost pure cellulose, and can contain minor perce ...
, and other broadleaf crop plants, making them resistant to dicamba. Monsanto has marketed their dicamba resistant crops under the brand name
Xtend Xtend is a general-purpose high-level programming language for the Java Virtual Machine. Syntactically and semantically Xtend has its roots in the Java programming language but focuses on a more concise syntax and some additional functionality ...
. Farmers have expressed concern about being forced to grow resistant crops as protection against drifting dicamba.


Volatilization

Dicamba came under scrutiny due to its tendency to
vaporize Vaporization (or vaporisation) of an element or compound is a phase transition from the liquid phase to vapor. There are two types of vaporization: evaporation and boiling. Evaporation is a surface phenomenon, whereas boiling is a bulk phenomeno ...
from treated fields and spread to neighboring crops. Monsanto began offering crops resistant to dicamba before a reformulated and drift resistant herbicide, which they claimed would be less likely to affect neighboring fields, had gained approval from the Environmental Protection Agency. Incidents in which dicamba affected neighboring fields led to complaints from farmers and fines in some US states. A lower volatility formulation, M1768, was approved by the EPA in November 2016. However, this formulation has not been evaluated by experts outside of Monsanto. Dicamba formulations, including those registered in the late 2010s, can be especially prone to volatility, temperature inversions near ground level, and drift.


Toxicological effects

Dicamba does not present unusual handling hazards.


Humans

Increased cancer rate ratios and positive exposure–response patterns were reported (among other Canada/US-registered pesticides) for dicamba, in a review of data gathered in the
National Institutes of Health The National Institutes of Health, commonly referred to as NIH (with each letter pronounced individually), is the primary agency of the United States government responsible for biomedical and public health research. It was founded in the late ...
's Agricultural Health Study.A Review of Pesticide Exposure and Cancer Incidence in the Agricultural Health Study Cohort
/ref> In addition, the risk of
non-Hodgkins lymphoma Non-Hodgkin lymphoma (NHL), also known as non-Hodgkin's lymphoma, is a group of blood cancers that includes all types of lymphomas except Hodgkin lymphomas. Symptoms include enlarged lymph nodes, fever, night sweats, weight loss, and tiredness. ...
in men was statistically significantly increased by exposure to dicamba, in the Cross-Canada Study of Pesticides and Health.


Mammals

It is moderately toxic by ingestion and slightly toxic by 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 The Environmental Protection Agency (EPA) is an Independent agencies of the United States government, independent executive agency of the United States federal government tasked with environmental protection matters. President Richard Nixon pro ...
(EPA) has set the
NOAEL The no-observed-adverse-effect level (NOAEL) denotes the level of exposure of an organism, found by experiment or observation, at which there is no biologically or statistically significant increase in the frequency or severity of any adverse effec ...
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.Pesticide Information Profile – Dicamba
Pesticide Management Education Program,
Cornell University Cornell University is a private statutory land-grant research university based in Ithaca, New York. It is a member of the Ivy League. Founded in 1865 by Ezra Cornell and Andrew Dickson White, Cornell was founded with the intention to teach an ...
.


Aquatic animals

Dicamba was tested for acute toxicity in a variety of aquatic animals. The studies accepted by the U.S. EPA found dicamba acid and DMA salt to be practically nontoxic to aquatic invertebrates. Studies accepted by the U.S. EPA found dicamba acid to be slightly toxic to cold water fish (
rainbow trout The rainbow trout (''Oncorhynchus mykiss'') is a species of trout native to cold-water tributaries of the Pacific Ocean in Asia and North America. The steelhead (sometimes called "steelhead trout") is an anadromous (sea-run) form of the coasta ...
), and practically nontoxic to warm water fish. Recent studies suggest that dicamba should be considered to be a potential
endocrine disruptor Endocrine disruptors, sometimes also referred to as hormonally active agents, endocrine disrupting chemicals, or endocrine disrupting compounds are chemicals that can interfere with endocrine (or hormonal) systems. These disruptions can cause c ...
for fish at environmentally relevant concentrations.


Environmental fate


Soil

Dicamba is released directly to the environment by its application as an herbicide for the control of annual broadleaf weeds. It may cause damage to plants as a result of its absorption from the soil by plant roots. Dicamba is mobile in most soils and significant leaching is possible. The adsorption of dicamba to organo-clay soil is influenced by soil pH with the greatest adsorption to soil occurring in acidic soils. Dicamba is moderately persistent in soil. Its reported half-life in soil ranges from 1 to 6 weeks. Dicamba is likely to be more rapidly degraded in soils with high microbial populations, but dissipates more slowly in hardwood forests and wetlands than would be expected from the results of laboratory studies. At a level of 10 mg/kg in sandy loam soil, dicamba caused a transient decrease in
nitrification ''Nitrification'' is the biological oxidation of ammonia to nitrite followed by the oxidation of the nitrite to nitrate occurring through separate organisms or direct ammonia oxidation to nitrate in comammox bacteria. The transformation of amm ...
after two but not three weeks of incubation. The investigator determined that the decrease in nitrification is not substantial and does not suggest the potential for a prolonged impact on microbial activity. In the same study, dicamba did not affect ammonia formation or sulfur oxidation. In a more recent laboratory study, dicamba, at a concentration of 1 mg/kg soil, did not affect urea hydrolysis or nitrification in four soil types.


Water

Dicamba salts used in some herbicides are highly soluble in water. A recent study conducted from 1991 to 1996 by the U.S. Geologic Survey found dicamba in 0.13% of the ground waters surveyed. The maximum level detected was 0.0021 mg/L. The prevalence of dicamba in groundwater from agricultural areas (0.11%) did not correlate with nonagricultural urban areas (0.35%).


Legality

Arkansas Arkansas ( ) is a landlocked state in the South Central United States. It is bordered by Missouri to the north, Tennessee and Mississippi to the east, Louisiana to the south, and Texas and Oklahoma to the west. Its name is from the Osage ...
and
Missouri Missouri is a U.S. state, state in the Midwestern United States, Midwestern region of the United States. Ranking List of U.S. states and territories by area, 21st in land area, it is bordered by eight states (tied for the most with Tennessee ...
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. Complaints against dicamba accelerated after the EPA approved a Monsanto-created soybean which could tolerate it in 2016. The soybean was a part of Monsanto's Xtend products. In June 2020, the
9th U.S. Circuit Court of Appeals The United States Court of Appeals for the Ninth Circuit (in case citations, 9th Cir.) is the U.S. federal court of appeals that has appellate jurisdiction over the U.S. district courts in the following federal judicial districts: * District ...
blocked sales of three dicamba-based herbicides in the United States, finding that the
Environmental Protection Agency A biophysical environment is a biotic and abiotic surrounding of an organism or population, and consequently includes the factors that have an influence in their survival, development, and evolution. A biophysical environment can vary in scale f ...
understated risks of spraying. 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.


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 BASF Societas Europaea, SE () is a German multinational corporation, multinational chemical company and the List of largest chemical producers, largest chemical producer in the world. Its headquarters is located in Ludwigshafen, Germany. The ...
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 Punitive damages, or exemplary damages, are damages assessed in order to punish the defendant for outrageous conduct and/or to reform or deter the defendant and others from engaging in conduct similar to that which formed the basis of the lawsuit. ...
. 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


Notes


Citations


External links


Appendix E: Herbicide Information
US Department of Agriculture

Speclab.com
Dicamba Technical Fact Sheet – National Pesticide Information Center

Dicamba General Fact Sheet – National Pesticide Information Center


* ttp://www.roundupreadyxtend.com/Pages/default.aspx Monsanto's Xtend Crop System Product Page
EPA Dicamba Reregistration Eligibility Decision
{{Authority control Auxinic herbicides Benzoic acids Chloroarenes Salicylyl ethers