→Toxicology: Add summary to the lead of the EPA's assessment of ecological risks, as well as clarification that only potential occupational risks to human health have been identified
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| Solubility = "low"<ref name=Ullmann/> |
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| Solubility2 = 810 g/L |
| Solubility2 = 810 g/L |
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'''Dicamba''' (3,6-dichloro-2-methoxybenzoic acid) is a selective systemic [[herbicide]] first registered in 1967.<ref name="US-EPA-reregistration-2006">{{cite web|url=http://archive.epa.gov/pesticides/reregistration/web/pdf/dicamba_red.pdf|title=Reregistration Eligibility Decision for Dicamba and Associated Salts - US EPA|date=8 June 2006}}</ref> Brand names for formulations of this herbicide include '''Dianat''', '''Banvel''', '''Diablo''', '''Oracle''' and '''Vanquish'''. This chemical compound is a [[organochlorine compound|chlorinated]] [[derivative (chemistry)|derivative]] of [[o-Anisic acid|''o''-anisic acid]].<ref>{{cite web|title=Dicamba (Banvel) Herbicide Profile 10/83, Pesticide Management Education Program|website= Cornell University|url=http://pmep.cce.cornell.edu/profiles/herb-growthreg/dalapon-ethephon/dicamba/herb-prof-dicamba.html}}</ref> |
'''Dicamba''' (3,6-dichloro-2-methoxybenzoic acid) is a selective systemic [[herbicide]] first registered in 1967.<ref name="US-EPA-reregistration-2006">{{cite web|url=http://archive.epa.gov/pesticides/reregistration/web/pdf/dicamba_red.pdf|title=Reregistration Eligibility Decision for Dicamba and Associated Salts - US EPA|date=8 June 2006}}</ref> Brand names for formulations of this herbicide include '''Dianat''', '''Banvel''', '''Diablo''', '''Oracle''' and '''Vanquish'''. This chemical compound is a [[organochlorine compound|chlorinated]] [[derivative (chemistry)|derivative]] of [[o-Anisic acid|''o''-anisic acid]].<ref>{{cite web|title=Dicamba (Banvel) Herbicide Profile 10/83, Pesticide Management Education Program|website= Cornell University|url=http://pmep.cce.cornell.edu/profiles/herb-growthreg/dalapon-ethephon/dicamba/herb-prof-dicamba.html}}</ref> 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."<ref name="Weeks"/> |
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Despite its success in improving crop yields, dicamba has attracted controversy. Accordingto 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.<ref>{{Cite web |date=February 16, 2023 |title=Dicamba |url=https://www.epa.gov/pesticides/dicamba |website=[[United States Environmental Protection Agency]]}}</ref><ref name=":0">{{Cite web |date=February 16, 2023 |title=Dicamba |url=https://www.epa.gov/pesticides/dicamba |archive-url=https://web.archive.org/web/20231223145749/https://www.epa.gov/pesticides/dicamba |archive-date=December 23, 2023 |website=[[United States Environmental Protection Agency]]}}</ref> |
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In 2016, dicamba was approved for use over [[Genetically modified organism|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.<ref name=":1">[https://www.theguardian.com/us-news/2020/mar/30/monsanto-crop-system-damage-us-farms-documents Revealed: Monsanto predicted crop system would damage US farms] The Guardian, 2020</ref> The controversy led to litigation, state bans and additional restrictions over dicamba use. |
In 2016, dicamba was approved for use in the United States over [[Genetically modified organism|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.<ref name=":1">[https://www.theguardian.com/us-news/2020/mar/30/monsanto-crop-system-damage-us-farms-documents Revealed: Monsanto predicted crop system would damage US farms] The Guardian, 2020</ref> The controversy led to litigation, state bans and additional restrictions over dicamba use. |
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== Use as an herbicide == |
== Use as an herbicide == |
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[[File:Dicamba USA 2019.png|thumb|Dicamba use in the |
[[File:Dicamba USA 2019.png|thumb|Dicamba use in the US in 2019. Usage has substantially increased since dicamba was approved for use over dicamba-resistant GMO crops in 2016.]] |
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Dicamba is a selective and systemic herbicide that kills annual and perennial broadleaf weeds.<ref name=ppdb>{{cite web |title=Dicamba (Ref: SAN 837H) |url=http://sitem.herts.ac.uk/aeru/ppdb/en/Reports/213.htm |website=Pesticide Properties DataBase |publisher=University of Hertfordshire}}</ref> 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.<ref name=Ullmann>Arnold P. Appleby, Franz Müller. "Weed Control, 2" in ''Ullmann's Encyclopedia of Industrial Chemistry'' 2011, Wiley-VCH, Weinheim. {{doi|10.1002/14356007.o28_o01}}</ref> |
Dicamba is a selective and systemic herbicide that kills annual and perennial broadleaf weeds.<ref name=ppdb>{{cite web |title=Dicamba (Ref: SAN 837H) |url=http://sitem.herts.ac.uk/aeru/ppdb/en/Reports/213.htm |website=Pesticide Properties DataBase |publisher=University of Hertfordshire}}</ref> 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.<ref name=Ullmann>Arnold P. Appleby, Franz Müller. "Weed Control, 2" in ''Ullmann's Encyclopedia of Industrial Chemistry'' 2011, Wiley-VCH, Weinheim. {{doi|10.1002/14356007.o28_o01}}</ref> |
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Increasing use of dicamba has been reported with the release of dicamba-resistant [[genetic modification|genetically modified]] plants by [[Monsanto]]. In October 2016, the [[Environmental Protection Agency|EPA]] launched a criminal investigation into the illegal application of older, drift prone formulations of dicamba onto these new plants.<ref>{{cite news|last1=Weinraub|first1=Mark|title=U.S. agency searches for proof of criminal use of herbicide dicamba|url=https://www.reuters.com/article/usa-soybeans-dicamba-idUSL1N1CV2EG|access-date=29 October 2016|publisher=[[Reuters]]|date=25 October 2016}}</ref><ref>{{cite news|title=EPA Probes Dicamba Use- Federal Search Warrants Issued in Missouri |url=http://kticradio.com/agricultural/epa-probes-dicamba-use-federal-search-warrants-issued-in-missouri/|access-date=4 October 2017|date= 25 October 2016|publisher=[[KTIC Radio]]}}</ref> Older formulations have been reported to drift after application and affect other crops not meant to be treated.<ref name='stl_today1'>{{cite news|last1=Gray|first1=Bryce|title=Suspected illegal herbicide use takes toll on southeast Missouri farmers|url=https://www.stltoday.com/business/local/suspected-illegal-herbicide-use-takes-toll-on-southeast-missouri-farmers/article_af161843-b6cf-5939-beba-fc23585e8478.html|access-date=16 August 2016|work=[[St Louis Post-Dispatch]]|date=5 August 2016}}</ref><ref name="stl_today2">{{cite news|last1=Gray|first1=Bryce|title=Illegal herbicide use may threaten survival of Missouri's largest peach farm|url=https://www.stltoday.com/business/local/illegal-herbicide-use-may-threaten-survival-of-missouri-s-largest/article_c4a4a96b-aba3-5e48-83b5-a546f5a9b8b1.html|access-date=16 August 2016|publisher=St Louis Post-Dispatch|date=14 August 2016}}</ref> 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.<ref>{{cite news|url=https://www.stltoday.com/business/local/epa-approves-monsanto-s-less-volatile-form-of-dicamba-herbicide/article_391a036d-fc66-55e3-804c-0f261d03ac11.html|work=St. Louis Post-Dispatch|last=Gray|first=Bryce|date=9 November 2016|title=EPA approves Monsanto's less-volatile form of dicamba herbicide|access-date=24 June 2017}}</ref> As a result, the use of dicamba in US agriculture rose sharply from approximately {{convert|8000000|lb|kg}} in 2016 to {{convert|30000000|lb|kg}} in 2019, according to the [[US Geological Survey]].<ref>{{cite web |url=https://water.usgs.gov/nawqa/pnsp/usage/maps/show_map.php?year=2019&map=DICAMBA&hilo=L |title=Estimated Agricultural Use for Dicamba, 2019 |author=US Geological Survey |date=2021-10-12 |access-date=2021-12-27 }}</ref> |
Increasing use of dicamba has been reported with the release of dicamba-resistant [[genetic modification|genetically modified]] plants by [[Monsanto]]. In October 2016, the [[Environmental Protection Agency|EPA]] launched a criminal investigation into the illegal application of older, drift prone formulations of dicamba onto these new plants.<ref>{{cite news|last1=Weinraub|first1=Mark|title=U.S. agency searches for proof of criminal use of herbicide dicamba|url=https://www.reuters.com/article/usa-soybeans-dicamba-idUSL1N1CV2EG|access-date=29 October 2016|publisher=[[Reuters]]|date=25 October 2016}}</ref><ref>{{cite news|title=EPA Probes Dicamba Use- Federal Search Warrants Issued in Missouri |url=http://kticradio.com/agricultural/epa-probes-dicamba-use-federal-search-warrants-issued-in-missouri/|access-date=4 October 2017|date= 25 October 2016|publisher=[[KTIC Radio]]}}</ref> Older formulations have been reported to drift after application and affect other crops not meant to be treated.<ref name='stl_today1'>{{cite news|last1=Gray|first1=Bryce|title=Suspected illegal herbicide use takes toll on southeast Missouri farmers|url=https://www.stltoday.com/business/local/suspected-illegal-herbicide-use-takes-toll-on-southeast-missouri-farmers/article_af161843-b6cf-5939-beba-fc23585e8478.html|access-date=16 August 2016|work=[[St Louis Post-Dispatch]]|date=5 August 2016}}</ref><ref name="stl_today2">{{cite news|last1=Gray|first1=Bryce|title=Illegal herbicide use may threaten survival of Missouri's largest peach farm|url=https://www.stltoday.com/business/local/illegal-herbicide-use-may-threaten-survival-of-missouri-s-largest/article_c4a4a96b-aba3-5e48-83b5-a546f5a9b8b1.html|access-date=16 August 2016|publisher=St Louis Post-Dispatch|date=14 August 2016}}</ref> 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.<ref>{{cite news|url=https://www.stltoday.com/business/local/epa-approves-monsanto-s-less-volatile-form-of-dicamba-herbicide/article_391a036d-fc66-55e3-804c-0f261d03ac11.html|work=St. Louis Post-Dispatch|last=Gray|first=Bryce|date=9 November 2016|title=EPA approves Monsanto's less-volatile form of dicamba herbicide|access-date=24 June 2017}}</ref> As a result, the use of dicamba in US agriculture rose sharply from approximately {{convert|8000000|lb|kg}} in 2016 to {{convert|30000000|lb|kg}} in 2019, according to the [[US Geological Survey]].<ref>{{cite web |url=https://water.usgs.gov/nawqa/pnsp/usage/maps/show_map.php?year=2019&map=DICAMBA&hilo=L |title=Estimated Agricultural Use for Dicamba, 2019 |author=US Geological Survey |date=2021-10-12 |access-date=2021-12-27 }}</ref> |
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== Drift == |
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⚫ | Dicamba came under scrutiny due to its reputation for [[pesticide drift|drifting]] from treated fields onto neighboring crops.<ref name="dtn2020">{{cite web |title=Dicamba Know-How: Seven ThingstoKnow Before You Spray Dicamba in 2020 |url=https://www.dtnpf.com/agriculture/web/ag/crops/article/2020/04/03/seven-things-know-spray-dicamba-2020 |publisher=Progressive Farmer |access-date=10 July 2020}}</ref><ref>{{cite news |last=Pollack |first=Andrew |date=25 April 2012 |title=Dow Corn, Resistant to a Weed Killer, Runs into Opposition |newspaper=[[The New York Times]] |url=https://www.nytimes.com/2012/04/26/business/energy-environment/dow-weed-killer-runs-into-opposition.html?_r=0 |access-date=1 August 2016 |archiveurl=https://archive.today/20130104103413/http://www.nytimes.com/2012/04/26/business/energy-environment/dow-weed-killer-runs-into-opposition.html?_r=0 |archivedate=4 January 2013}}</ref> In 2011, the [[European Food Safety Authority]] identified dicamba's potential for long-range transport through the atmosphere as a critical area of concern.<ref>{{Cite web |date=2011-01-14 |title=Conclusion on the peer review of the pesticide risk assessment of the active substance dicamba {{!}} EFSA |url=https://www.efsa.europa.eu/en/efsajournal/pub/1965 |access-date=2023-12-23 |website=www.efsa.europa.eu |language=en |doi=10.2903/j.efsa.2011.1965}}</ref> 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.<ref name=":0" /> Dicamba is also available in a drift-resistant formulation, which is less likely to affect neighboring fields. |
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Dicamba came under scrutiny due to its |
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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.<ref>{{cite news|last1=Tatge|first1=Jason|title=Protecting Your Business with Ag Data: What Dicamba Can Teach Us (Guest Column) {{!}} PrecisionAg|url=http://www.precisionag.com/systems-management/data/protecting-your-business-with-ag-data-what-dicamba-can-teach-us-guest-column/|access-date=12 November 2017|work=PrecisionAg|date=9 November 2017}}</ref> A lower volatility formulation, M1768, was approved by the EPA in November 2016.<ref>{{citation | title=Final Registration of Dicamba on Dicamba-Tolerant Cotton and Soybean | url=https://www.regulations.gov/contentStreamer?documentId=EPA-HQ-OPP-2016-0187-0959&contentType=pdf|date=9 November 2016}}</ref> However, this formulation has not been evaluated by experts outside of Monsanto.<ref>{{cite news|url=https://www.reuters.com/article/us-usa-pesticides-dicamba-insight/scant-oversight-corporate-secrecy-preceded-u-s-weed-killer-crisis-idUSKBN1AP0DN|title=Scant oversight, corporate secrecy preceded U.S. weed killer crisis|date=9 August 2017|newspaper=Reuters}}</ref> |
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Dicamba formulations, including those registered in the late 2010s, can be especially prone to [[Volatility (chemistry)|volatility]], [[Inversion (meteorology)|temperature inversions]] near ground level, and drift.<ref name="dtn2020">{{cite web |title=Dicamba Know-How: Seven Things to Know Before You Spray Dicamba in 2020 |url=https://www.dtnpf.com/agriculture/web/ag/crops/article/2020/04/03/seven-things-know-spray-dicamba-2020 |publisher=Progressive Farmer |access-date=10 July 2020}}</ref> |
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== Toxicology == |
== Toxicology == |
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=== Mammals === |
=== Mammals === |
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Dicamba |
Dicamba has low toxicity by ingestion and inhalation or dermal exposure (oral {{LD50}} in rats: 757 mg/kg body weight, dermal LD<sub>50</sub> in rats: >2,000 mg/kg, inhalation LC<sub>50</sub> in rats: >200 mg/L). In a three-generation study, dicamba did not affect the reproductive capacity of rats.<ref name="Cornell-ExToNet" /> |
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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. [[United States Environmental Protection Agency|U.S. Environmental Protection Agency]] (EPA) has set the [[NOAEL]] for this study at 3 (mg/kg)/day.<ref name="Cornell-ExToNet" /> |
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. [[United States Environmental Protection Agency|U.S. Environmental Protection Agency]] (EPA) has set the [[NOAEL]] for this study at 3 (mg/kg)/day.<ref name="Cornell-ExToNet" /> |
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=== Aquatic animals === |
=== Aquatic animals === |
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Dicamba |
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.<ref>{{cite journal|last1=Zhu|first1=L|last2=Li|first2=W|last3=Zha|first3=J|last4=Wang|first4=Z|title=Dicamba affects sex steroid hormone level and mRNA expression of related genes in adult rare minnow (Gobiocypris rarus) at environmentally relevant concentrations.|journal=Environmental Toxicology|date=2015|volume=30|issue=6|pages=693–703|doi=10.1002/tox.21947|pmid=24420721|bibcode=2015EnTox..30..693Z|s2cid=45373092|url=http://ir.rcees.ac.cn/handle/311016/32753}}</ref> |
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=== Birds and bees === |
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The 2022 EPA draft ecological risk assessment identified potential adverse effects to birds, and bee larvae for all dicamba uses.<ref name=":0" /> |
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== Genetically modified crops == |
== Genetically modified crops == |
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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.<ref name="Weeks">{{cite journal |last1=Behrens |first1=M. R. |last2=Mutlu |first2=N. |last3=Chakraborty |first3=S. |last4=Dumitru |first4=R. |last5=Jiang |first5=W. Z. |last6=Lavallee |first6=B. J. |last7=Herman |first7=P. L. |last8=Clemente |first8=T. E. |last9=Weeks |first9=D. P. |year=2007 |title=Dicamba Resistance: Enlarging and Preserving Biotechnology-Based Weed Management Strategies |url=http://digitalcommons.unl.edu/cgi/viewcontent.cgi?article=1031&context=plantscifacpub |journal=Science |volume=316 |issue=5828 |pages=1185–8 |bibcode=2007Sci...316.1185B |doi=10.1126/science.1141596 |pmid=17525337 |s2cid=7093076}}</ref> |
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.<ref name="Weeks">{{cite journal |last1=Behrens |first1=M. R. |last2=Mutlu |first2=N. |last3=Chakraborty |first3=S. |last4=Dumitru |first4=R. |last5=Jiang |first5=W. Z. |last6=Lavallee |first6=B. J. |last7=Herman |first7=P. L. |last8=Clemente |first8=T. E. |last9=Weeks |first9=D. P. |year=2007 |title=Dicamba Resistance: Enlarging and Preserving Biotechnology-Based Weed Management Strategies |url=http://digitalcommons.unl.edu/cgi/viewcontent.cgi?article=1031&context=plantscifacpub |journal=Science |volume=316 |issue=5828 |pages=1185–8 |bibcode=2007Sci...316.1185B |doi=10.1126/science.1141596 |pmid=17525337 |s2cid=7093076}}</ref> |
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In the 2000s, [[Monsanto]] incorporated one component of the three enzymes into the genome of [[soybean]], |
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.<ref name="Weeks" /> {{anchor|Xtend}}Monsanto has marketed their dicamba resistant crops under the brand name [[Xtend (seed)|Xtend]].<ref name="NPR1">{{cite news |last=Charles |first=Dan |date=1 August 2016 |title=Crime in the Fields: How Monsanto And Scofflaw Farmers Hurt Soybeans in Arkansas |newspaper=[[NPR]] |url=https://www.npr.org/sections/thesalt/2016/08/01/487809643/crime-in-the-fields-how-monsanto-and-scofflaw-farmers-hurt-soybeans-in-arkansas |access-date=1 August 2016}}</ref> |
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Farmers have expressed concern about being forced to grow resistant crops as protection against drifting dicamba.<ref name="NPR1" /> |
Farmers have expressed concern about being forced to grow resistant crops as protection against drifting dicamba.<ref name="NPR1" /> |
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== Resistance == |
== Resistance == |
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[[Herbicide resistance]] after the introduction of [[Genetically modified crops|resistant crops]] is a common concern with herbicide.<ref name="NPR1" /><ref name="Take">{{cite web |date=2 August 2016 |title=Business: Monsanto's Superweeds Saga Is Only Getting Worse |url=https://www.yahoo.com/news/monsanto-superweeds-saga-only-getting-worse-000354904.html |access-date=3 August 2016 |publisher=[[Yahoo]], TakePart.com}}</ref> In the laboratory, researchers have demonstrated weed resistance to dicamba within three generations of exposure.<ref name="NPR1" /> This effect is illustrated by [[Glyphosate#Emergence of resistant weeds|glyphosate]]-resistant crops (marketed as '[[Roundup Ready]]').<ref name="NPR1" /><ref name="Take" /><ref>[http://www.scientificamerican.com/media/inline/a-hard-look-at-3-myths-about-genetically-modified-crops_3.jpg The Rise of Superweeds] scientificamerican.com</ref> The weed species ''[[Amaranthus palmeri]]'' and ''[[Bassia scoparia]]'' developed resistance to dicamba already in the 1990s.<ref>{{cite journal |last1=Cranston |first1=Harwood J. |last2=Kern |first2=Anthony J. |last3=Hackett |first3=Josette L. |last4=Miller |first4=Erica K. |last5=Maxwell |first5=Bruce D. |last6=Dyer |first6=William E. |year=2001 |title=Dicamba resistance in kochia |journal=Weed Science |volume=49 |issue=2 |pages=164 |doi=10.1614/0043-1745(2001)049[0164:DRIK]2.0.CO;2}}</ref> |
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== Environmental fate == |
== Environmental fate == |
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[[File:DicambaDeMeth.svg|thumb|right|Pathway for biodegradation of dicamba|422px]] |
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⚫ | Dicamba is poorly soluble in water.<ref name=Ullmann/> 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.<ref>{{cite journal |last1=Kolpin |first1=Dana W. |last2=Barbash |first2=Jack E. |last3=Gilliom |first3=Robert J. |title=Pesticides in Ground Water of the United States, 1992–1996 |journal=Groundwater |date=November 2000 |volume=38 |issue=6 |pages=858–863 |doi=10.1111/j.1745-6584.2000.tb00684.x|bibcode=2000GrWat..38..858K }}</ref> |
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It is biodegraded by both aerobic and anaerobic bacteria to 3,6-[[dichlorosalicylic acid]]. This conversion is catalyzed by a [[Dioxygenase|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.<ref>{{cite journal |doi=10.1016/j.jmb.2009.07.021 |title=Crystal Structure of Dicamba Monooxygenase: A Rieske Nonheme Oxygenase that Catalyzes Oxidative Demethylation |date=2009 |last1=Dumitru |first1=Razvan |last2=Jiang |first2=Wen Zhi |last3=Weeks |first3=Donald P. |last4=Wilson |first4=Mark A. |journal=Journal of Molecular Biology |volume=392 |issue=2 |pages=498–510 |pmid=19616011 |pmc=3109874 }}</ref> The dichlorosalicylic acid is far less toxic than dicamba, which is already rather low.<ref>{{cite journal | last1=Krueger | first1=James P. | last2=Butz | first2=Robert G. | last3=Atallah | first3=Yousef H. | last4=Cork | first4=Douglas J. | title=Isolation and identification of microorganisms for the degradation of Dicamba | journal=Journal of Agricultural and Food Chemistry | volume=37 | issue=2 | date=1989 | issn=0021-8561 | doi=10.1021/jf00086a057 | pages=534–538}}</ref><ref>{{cite journal | last1=Milligan | first1=Peter W. | last2=Häggblom | first2=Max M. | title=Biodegradation and Biotransformation of Dicamba under Different Reducing Conditions | journal=Environmental Science & Technology | volume=33 | issue=8 | date=1999-04-01 | issn=0013-936X | doi=10.1021/es981117e | pages=1224–1229| bibcode=1999EnST...33.1224M }}</ref> |
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=== Soil === |
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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. The adsorption of dicamba to [[soil types|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.{{Citation needed|date=February 2011}} |
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At a level of 10 mg/kg in sandy loam soil, dicamba caused a transient decrease in [[nitrification]] 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.{{Citation needed|date=February 2011}} |
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=== Water === |
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Dicamba is |
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== Use on dicamba-tolerant crops == |
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Complaints against dicamba accelerated after the EPA approved a Monsanto-created soybean which could tolerate it in 2016.<ref name="dicambadestroy">{{Cite web |title=Dicamba Lawsuit | Crop Damage Compensation for Farmers |url=https://www.consumernotice.org/legal/dicamba-lawsuits/ |website=ConsumerNotice.org}}</ref> The soybean was a part of Monsanto's Xtend products.<ref name="dicambadestroy" /> Dicamba was approved by the EPA for "over-the-top" (OTT) use on those dicamba-tolerant soybean and cotton crops.<ref name=": |
Complaints against dicamba accelerated after the United States EPA approved a Monsanto-created soybean which could tolerate it in 2016.<ref name="dicambadestroy">{{Cite web |title=Dicamba Lawsuit | Crop Damage Compensation for Farmers |url=https://www.consumernotice.org/legal/dicamba-lawsuits/ |website=ConsumerNotice.org}}</ref> The soybean was a part of Monsanto's Xtend products.<ref name="dicambadestroy" /> Dicamba was approved by the EPA for "over-the-top" (OTT) use on those dicamba-tolerant soybean and cotton crops.<ref name=":0" /> 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.<ref name=":0" /> |
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[[Arkansas]] and [[Missouri]] banned the sale and use of dicamba in July 2017 in response to complaints of crop damage due to drift.<ref>{{cite news |last1=Gray |first1=Bryce |date=7 July 2017 |title=Missouri and Arkansas ban dicamba herbicide as complaints snowball |publisher=St. Louis Post-Dispatch |url=https://www.stltoday.com/business/local/missouri-and-arkansas-ban-dicamba-herbicide-as-complaints-snowball/article_2f0739e8-1b7f-5759-81b2-d78b7e249bac.html |access-date=10 July 2017}}</ref> Monsanto responded by arguing that not all instances of crop damage had been investigated and a ban was premature.<ref>{{cite news |last1=Pucci |first1=Jackie |date=10 July 2017 |title=Monsanto Responds to Arkansas, Missouri Dicamba Bans |publisher=Crop Life |url=http://www.croplife.com/crop-inputs/monsanto-responds-to-arkansas-missouri-dicamba-bans/ |access-date=10 July 2017}}</ref> Monsanto sued the state of Arkansas to stop the ban, but the case was dismissed in February 2018.<ref>{{cite web |last1=Nosowitz |first1=Dan |date=20 February 2018 |title=Monsanto's Lawsuit Against Arkansas for Dicamba Ban Dismissed |url=https://modernfarmer.com/2018/02/monsanto-lawsuit-arkansas-dicamba-ban/ |access-date=7 March 2018 |website=Modern Farmer}}</ref> It has also been acknowledged that the use of dicamba had increased since 2017.<ref name="dicambause" /><ref name="dicambauser" /> |
[[Arkansas]] and [[Missouri]] banned the sale and use of dicamba in July 2017 in response to complaints of crop damage due to drift.<ref>{{cite news |last1=Gray |first1=Bryce |date=7 July 2017 |title=Missouri and Arkansas ban dicamba herbicide as complaints snowball |publisher=St. Louis Post-Dispatch |url=https://www.stltoday.com/business/local/missouri-and-arkansas-ban-dicamba-herbicide-as-complaints-snowball/article_2f0739e8-1b7f-5759-81b2-d78b7e249bac.html |access-date=10 July 2017}}</ref> Monsanto responded by arguing that not all instances of crop damage had been investigated and a ban was premature.<ref>{{cite news |last1=Pucci |first1=Jackie |date=10 July 2017 |title=Monsanto Responds to Arkansas, Missouri Dicamba Bans |publisher=Crop Life |url=http://www.croplife.com/crop-inputs/monsanto-responds-to-arkansas-missouri-dicamba-bans/ |access-date=10 July 2017}}</ref> Monsanto sued the state of Arkansas to stop the ban, but the case was dismissed in February 2018.<ref>{{cite web |last1=Nosowitz |first1=Dan |date=20 February 2018 |title=Monsanto's Lawsuit Against Arkansas for Dicamba Ban Dismissed |url=https://modernfarmer.com/2018/02/monsanto-lawsuit-arkansas-dicamba-ban/ |access-date=7 March 2018 |website=Modern Farmer}}</ref> It has also been acknowledged that the use of dicamba had increased since 2017.<ref name="dicambause" /><ref name="dicambauser" /> |
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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]] |
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."<ref>{{cite news |date=4 June 2020 |title=U.S. Court Blocks Sales of Bayer Weed Killer in United States |agency=Reuters |url=https://www.nytimes.com/reuters/2020/06/04/business/04reuters-bayer-dicamba-lawsuit.html |access-date=11 June 2020}}</ref><ref>{{cite web |last1=US EPA |first1=OCSPP |date=8 June 2020 |title=Final Cancellation Order for Three Dicamba Products |url=https://www.epa.gov/ingredients-used-pesticide-products/final-cancellation-order-three-dicamba-products |access-date=11 June 2020 |website=US EPA |language=en}}</ref><ref name=":0" /> 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.<ref name=":0" /> |
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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.<ref>{{cite web |date=8 June 2020 |title=EPA Offers Clarity to Farmers in Light of Recent Court Vacatur of Dicamba Registrations |url=https://www.epa.gov/newsreleases/epa-offers-clarity-farmers-light-recent-court-vacatur-dicamba-registrations |access-date=11 June 2020 |publisher=EPA}}</ref> In October 2020 the EPA issued a decision on the registration application of three dicamba-based products, Xtendimax, Engenia, and Tavium. |
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.<ref>{{cite web |date=8 June 2020 |title=EPA Offers Clarity to Farmers in Light of Recent Court Vacatur of Dicamba Registrations |url=https://www.epa.gov/newsreleases/epa-offers-clarity-farmers-light-recent-court-vacatur-dicamba-registrations |access-date=11 June 2020 |publisher=EPA}}</ref> 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.<ref>{{cite web |date=9 November 2020 |title=EPA Approves Dicamba Pesticides Through 2025; Additional Restrictions Imposed |url=https://agrilife.org/texasaglaw/2020/11/09/epa-approves-dicamba-pesticides-through-2025-additional-restrictions-imposed/ |access-date=May 26, 2021 |website=Texas Agricultural Extension}}</ref> |
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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.<ref name=": |
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.<ref name=":0" /> 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.<ref name=":0" /> The restriction apply to Minnesota, Iowa, Illinois, Indiana, and South Dakota.<ref name=":0" /> |
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== Lawsuits == |
== Lawsuits == |
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* [http://npic.orst.edu/factsheets/dicamba_gen.pdf Dicamba General Fact Sheet – National Pesticide Information Center] |
* [http://npic.orst.edu/factsheets/dicamba_gen.pdf Dicamba General Fact Sheet – National Pesticide Information Center] |
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* [http://extoxnet.orst.edu/pips/dicamba.htm Dicamba Pesticide Information Profile – Extension Toxicology Network] |
* [http://extoxnet.orst.edu/pips/dicamba.htm Dicamba Pesticide Information Profile – Extension Toxicology Network] |
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* [ |
* [https://www.roundupreadyxtend.com/Pages/default.aspx Monsanto's Xtend Crop System Product Page] |
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* [https://web.archive.org/web/20061006100518/http://www.epa.gov/oppsrrd1/REDs/dicamba_red.pdf EPA Dicamba Reregistration Eligibility Decision] |
* [https://web.archive.org/web/20061006100518/http://www.epa.gov/oppsrrd1/REDs/dicamba_red.pdf EPA Dicamba Reregistration Eligibility Decision] |
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![]() | |
Names | |
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Preferred IUPAC name
3,6-Dichloro-2-methoxybenzoic acid | |
Other names
3,6-Dichloro-o-anisic acid | |
Identifiers | |
3D model (JSmol) |
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ChEBI | |
ChEMBL | |
ChemSpider |
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ECHA InfoCard | 100.016.033 ![]() |
KEGG |
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PubChem CID |
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UNII | |
CompTox Dashboard (EPA) |
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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] | |
Solubilityinacetone | 810 g/L |
Solubilityinethanol | 922 g/L |
Hazards | |
GHS labelling:[3] | |
![]() ![]() | |
Danger | |
H302, H318, H412 | |
P273, P280, P305+P351+P338 | |
Flash point | 199 °C (390 °F; 472 K) |
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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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 derivativeofo-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 damage.[9] The controversy led to litigation, state bans and additional restrictions over dicamba use.
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, 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 US 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 US Geological Survey.[18]
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.
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]
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]
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]
The 2022 EPA draft ecological risk assessment identified potential adverse effects to birds, and bee larvae for all dicamba uses.[8]
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]
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 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.[30]
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.[31] The dichlorosalicylic acid is far less toxic than dicamba, which is already rather low.[32][33]
Complaints against dicamba accelerated after the United States EPA approved a Monsanto-created soybean which could tolerate it in 2016.[34] The soybean was a part of Monsanto's Xtend products.[34] 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.[35] Monsanto responded by arguing that not all instances of crop damage had been investigated and a ban was premature.[36] Monsanto sued the state of Arkansas to stop the ban, but the case was dismissed in February 2018.[37] It has also been acknowledged that the use of dicamba had increased since 2017.[38][39]
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."[40][41][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.[42] 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.[43]
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 restriction apply to Minnesota, Iowa, Illinois, Indiana, and South Dakota.[8]
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.[44] By August 2019, more lawsuits were filed, alleging that dicamba had damaged crops, gardens, and trees of neighbors of the farmers who used it.[38][39]
On 27 January 2020, the first trial concerning dicamba-related products began in Cape Girardeau, Missouri.[45][46] The lawsuit involves a peach farmer who alleged that dicamba-based herbicides caused significant damage to his crops and trees.[47] It had also been filed in November 2016, when dicamba was still owned by Monsanto.[48][49][50] 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.[51] Bayer and BASF were also ordered to pay Bader $15 million in damages.[52] On 15 February 2020, Monsanto and BASF were ordered to pay an additional $250 million in punitive damages.[53][54] 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.[55]
On 17 February, it was announced that dicamba would face many more lawsuits.[56] 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.[57]
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.[58] On 25 November 2020, U.S. District Judge Stephen Limbaugh Jr. reduced the punitive damage amount in the Bader Farms case to $60 million.[59]
Authority control databases: National ![]() |
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