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MCPA
Structural formula of MCPA
Ball-and-stick model of the MCPA molecule
Names
Preferred IUPAC name
(4-Chloro-2-methylphenoxy)acetic acid
Other names
2-(4-Chloro-2-methylphenoxy)acetic acid
4-Chloro-o-tolyloxyacetic acid
MCPA
Identifiers
3D model (JSmol)
ChEBI
ChEMBL
ChemSpider
ECHA InfoCard 100.002.146 Edit this at Wikidata
KEGG
UNII
  • InChI=1S/C9H9ClO3/c1-6-4-7(10)2-3-8(6)13-5-9(11)12/h2-4H,5H2,1H3,(H,11,12) checkY
    Key: WHKUVVPPKQRRBV-UHFFFAOYSA-N checkY
  • InChI=1/C9H9ClO3/c1-6-4-7(10)2-3-8(6)13-5-9(11)12/h2-4H,5H2,1H3,(H,11,12)
    Key: WHKUVVPPKQRRBV-UHFFFAOYAG
  • Cl-C1=CC=C(OCC(=O)O)C(C)=C1
Properties
C9H9ClO3
Molar mass 200.62 g·mol−1
Appearance White to light brown solid
Density 1.18-1.21 g/cm3
Melting point 114 to 118 °C (237 to 244 °F; 387 to 391 K)
825 mg/L (23 °C),[1]
amine salt[which?]: 866 g/L
ester[which?]: 5 mg/L
Hazards
Safety data sheet (SDS) External MSDS
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).

MCPA (2-methyl-4-chlorophenoxyacetic acid) is a widely used phenoxy herbicide introduced in 1945. It selectively controls broad-leaf weeds in pasture and cereal crops. The mode of action of MCPA is as an auxin, which are growth hormones that naturally exist in plants.[2][3]

History

[edit]

In 1936 investigations began at ICIs Jealott's Hill research center into the effects of auxins on plant growth looking specifically for a way to kill weeds without harming crops such as wheat and oats. William Templeman found that when indole-3-acetic acid (IAA), the naturally occurring auxin, was used at high concentrations, it could stop plant growth. In 1940, he published his finding that IAA killed broadleaf plants within a cereal field.[4][5] Templeman and the ICI group were searching for compounds with similar or greater selective activity than IAA or 1-naphthaleneacetic acid in inhibiting the growth of weeds while not adversely affecting the growth of cereal crops. They synthesized MCPA from the corresponding phenol by exposing it to chloroacetic acid and dilute base in a straightforward substitution reaction:[6]

2-methyl-4-chlorophenol + ClCH2CO2H + base → MCPA + base·HCl (hydrochloric acid)

By the end of 1941 it was clear to the Templeman group that MCPA was one of the most active compounds tested but other auxin herbicides including 2,4-D were also effective. This work took place during World War II and was a case of multiple discovery. Four groups worked independently in the United Kingdom and the United States: the ICI team; Philip S. Nutman and associates at Rothamsted Research in the UK; Franklin D. Jones and associates at the American Chemical Paint Company; and Ezra Kraus, John W. Mitchell, and associates at the University of Chicago and the United States Department of Agriculture. All four groups were subject to wartime secrecy laws and did not follow the usual procedures of publication and patent disclosure, although ICI did file an application relating to both MCPA and 2,4-D on 7 April 1941 in the UK. In December 1942, following a meeting at the Ministry of Agriculture the Rothamsted and ICI workers pooled resources and Nutman moved to Jealott's Hill to join the ICI effort.[5] The first publications about this group of herbicides were by other workers who were not the original inventors: the precise sequence of discovery events has been discussed.[7] MCPA was first reported in the open scientific literature by Slade, Templeman and Sexton in 1945.[8] ICI's decision to commercialize MCPA (rather than 2,4-D, for example) was influenced by the fact that ICI had access to 2-methyl-4-chlorophenol and following extensive field trials the material was first made available to UK farmers in 1946, as a 1% dust.[5]

Mode of action

[edit]

MCPA acts by mimicking the action of the plant growth hormone auxin, which results in uncontrolled growth and eventually death in susceptible plants, mainly dicotyledons.[3] It is absorbed through the leaves and is translocated to the meristems of the plant. Uncontrolled, unsustainable growth ensues, causing stem curl-over, leaf withering, and eventual plant death.

Commercial use

[edit]
US Geological Survey estimate of MCPA use in the USA, 1992 to 2017

MCPA is used as an herbicide, generally as its salt or esterified forms. Used thus, it controls broadleaf weeds, including thistle and dock, in cereal crops and pasture. It is selective for plants with broad leaves, and this includes most deciduous trees. Clovers are tolerant at moderate application levels. It is currently classified as a restricted use herbicide in the United States: its use is mapped by the US Geological Survey, whose data show consistent use from 1992, with a small recent decline in the ten years to 2017, the latest date for which figures are available. The compound is now used almost exclusively in wheat.[9]

Its toxicity and biodegradation are topics of current research. One formulation is described by its manufacturer as "designed for specific markets that require the safest possible phenoxy product, primarily for use in the Pacific Northwest".[10] Though not extremely toxic,[11] it has been determined that MCPA can form complexes with metal ions and thereby increase their bioavailability,[12] and there is also work being done to utilize this ability.[13]

Chemical use

[edit]

Because it is inexpensive, MCPA is used in various chemical applications. Its carboxylic acid group allows the formation of conjugated complexes with metals (see above). The acid functionality makes MCPA a versatile synthetic intermediate for more complex derivatives.[14]

Brand names

[edit]

The following commercial products contain MCPA:[11]

  • Agritox, Agroxone, Chiptox, Chwastox, Cornox, Methoxone, Rhonox, Spurge Power, Tigrex, Verdone Extra (UK), Weed-Rhap, Weed'n'Feed, Weed-B-Gone, Zero Bindii & Clover Weeder (Aus), Jolt (Aus), BIN-DIE (Aus), Maatilan MCPA, K-MCPA, Hedonal, Basagran (Finland), and others.

Degradation in soil

[edit]

Since MCPA is extensively used in the USA, the extensively dispersed MCPA and its biological and photochemical metabolites might be deemable as environmentally hazardous. However, current studies show that there is no resistance of MCPA to degrade in soil.

Behaviors in soil

[edit]

MCPA herbicide is usually sprayed to the soil surface and plant leaves in its water solution, sometimes with additional surfactant. MCPA in soil can be absorbed by plant roots, and translocated in phloem to leaves and stems. The MCPA residue left in soil typically has a half-life of 24 days.[15] However, the degradation rate depends on environmental conditions, such as temperature and soil moisture.[16] MCPA is rather mobile in soil, and not strongly adsorbed to soil particles, with Kf = 0.94 and 1/n = 0.68 of Freundlich adsorption.[15][16]

Environmental risks

[edit]

Wide usage of MCPA as an herbicide raises concern of environmental risks, so considerable research has been done in recent decades to evaluate the environmental risk of MCPA. MCPA can be moderately toxic to mammal and aquatic organisms, and relatively less toxic to birds.[17] MCP (4-chloro-2-methylphenol) is the intermediate in the synthesis of phenoxy herbicides, and is also the metabolite of MCPA degradation. It has been estimated that a total of 15000 tons of MCP were produced in 1989 in the EU.[18] MCP is considered very toxic to aquatic organisms. However, the concentration of MCPA and MCP detected in water and soil are lower than the predicted no-effect levels of all environmental compartments, and considered to present low potential risk.[18][19]

The carboxyl group of MCPA can form conjugated complex with metals as a ligand.[20] In the general pH range of aqueous environments, the MCPA-metal complex has higher solubility than metal ions. MCPA may be environmentally hazardous by affecting the mobility and bio-availability of heavy metals such as cadmium and lead. The acid functionality makes MCPA a versatile synthetic intermediate for more complex derivatives[21]

-COOH + M+ → -COOM + H+

Bio-degradation

[edit]
Bio-degradation of MCPA in soils

The MCPA can be degraded biologically in soils by plants and microorganisms. The major metabolite of MCPA degradation is MCP (4-chloro-2-methylphenol). The pathway could be the cleavage of the ether linkage, yielding MCP and acetate acid. Another pathway could be the hydroxylation of the methyl group, yielding cloxyfonac (4-Chloro-2-hydroxymethylphenoxyacetic acid). Recent studies have demonstrated that biological degradation of MCPA is enzymatically catalyzed by an α-ketoglutarate-dependent dioxygenase encoded by the tfdA gene of soil microorganisms. Soil indigenous bacteria that carry the tfdA gene could use MCPA as the sole source of carbon.[22][23]

Photo-degradation

[edit]
Oxidation of MCPA by hydroxyl radicals
Oxidation of MCPA by positive holes h+

MCPA also could be photochemically degraded. Two scheme pathways can be proposed for the formation of the main intermediate, MCP. One scheme is MCPA oxidation by hydroxyl radical, •OH. The hydroxyl radical adds on the ring, followed by radical transfer to the ether carbon. With oxygen present, the addition of the hydroxyl radical leads the cleavage of the ether link, yielding MCP. The other scheme is MCPA oxidation by positive electron holes h+. The positive holes h+ polarize carboxyl group, CH2-COOH bond is split to produce 4-chloro-2-methylphenylformate. With the presence of oxygen, the positive holes h+ oxidation finally yields MCP as well.[21]

References

[edit]
[edit]
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MCPA

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from Grokipedia
MCPA, or 2-methyl-4-chlorophenoxyacetic acid, is a synthetic auxin-mimicking widely used as a selective post-emergence treatment to control broadleaf annual and weeds in crops, , , and turf. With the C₉H₉ClO₃ and a molecular weight of 200.62 g/mol, it appears as a white to light brown solid with a of 114–118 °C and high in (825 mg/L at 25 °C), facilitating its systemic absorption through leaves and roots. Discovered in the early 1940s as part of research into plant growth regulators during , MCPA was patented in 1941 alongside related compounds like 2,4-D, though wartime restrictions delayed public disclosure until after 1945. (ICI) in the commercialized it that year under trade names like Methoxone, marking the start of its role in modern and agricultural productivity gains. In application, MCPA disrupts balance by overstimulating growth processes, leading to uncontrolled cell elongation, tissue deformation, and death in susceptible broadleaf species while sparing grasses like and . It is formulated as esters, amines, or sodium salts for foliar spraying and has been a staple in global agriculture since the mid-20th century, though its use is regulated due to potential environmental leaching and in and . Toxicity assessments classify MCPA as slightly toxic to mammals (EPA III), with an oral LD50 in rats exceeding 700 mg/kg, but it poses slight risks to aquatic organisms (e.g., LC50 of 117–232 mg/L for ) and moderate risks to earthworms, prompting guidelines for safe handling and application to minimize runoff into water bodies. Despite these concerns, its and selectivity continue to support its registration in many countries, with ongoing research into resistance management and ecotoxicological impacts.

Chemical Properties

Molecular Structure and Formula

MCPA, systematically named 2-methyl-4-chlorophenoxyacetic acid, possesses the molecular formula C₉H₉ClO₃ and a molecular weight of 200.62 g/mol. The molecular structure of MCPA is based on a phenoxyacetic acid backbone, consisting of a ring linked via an oxygen atom to an acetic acid moiety (-OCH₂COOH), with key substituents including a atom at the 4-position and a at the 2-position of the ring. This arrangement positions MCPA within the phenoxyacetic acid class of synthetic auxins used as herbicides. Structurally, MCPA is closely related to 2,4-D (, C₈H₆Cl₂O₃), sharing the same phenoxyacetic acid core but differing in the 2-position substituent, where MCPA features a in place of the second chlorine atom found in 2,4-D.

Physical and Chemical Characteristics

MCPA appears as a white to light brown crystalline solid in its pure form, though technical-grade material may exhibit variations in color due to impurities. The melting point of MCPA is approximately 118 °C, which influences its handling and formulation processes in agricultural products. It decomposes before reaching its boiling point, preventing vaporization under standard heating conditions and contributing to its low volatility in practical applications. Solubility of MCPA is relatively low in water, at about 825 mg/L at 20 °C for the free acid form, which limits its mobility in aqueous environments but allows for effective dispersion when formulated as salts. In contrast, it exhibits higher solubility in organic solvents, such as acetone (approximately 455 g/L at 20 °C), facilitating its incorporation into emulsifiable concentrates and other delivery systems. The (pKa) of MCPA is 3.07, characterizing it as a that partially ionizes in neutral or alkaline conditions, thereby affecting its and in and . MCPA demonstrates under neutral pH conditions, with no significant observed at 7 over extended periods at moderate temperatures. However, it undergoes in strong acidic or basic environments, which can impact its long-term storage and environmental persistence under extreme chemical stresses.

History and Development

Discovery and Synthesis

MCPA was discovered in the early 1940s as part of research into synthetic plant growth regulators with potential herbicidal properties, conducted at the Jealott's Hill Research Station of Imperial Chemical Industries (ICI) in the United Kingdom. The work was led by William G. Templeman, who, along with colleagues including W. A. Sexton and R. E. Slade, investigated phenoxyacetic acid derivatives amid efforts to develop selective weed control agents during World War II. By late 1941, the team identified MCPA (2-methyl-4-chlorophenoxyacetic acid) as particularly effective for inhibiting broadleaf weed growth while sparing cereal crops, marking a breakthrough in phenoxy herbicide development. The initial synthesis of MCPA involved a straightforward reaction, where 4-chloro-2-methylphenol (also known as 2-methyl-4-chlorophenol) was reacted with in the presence of a base such as . This process, a variant of the , produced MCPA as the sodium salt, which could then be acidified to yield the free acid. The reaction was first detailed in by Templeman, Sexton, and colleagues in 1945, building on earlier patent filings. Key to the discovery's formal recognition was British Patent 573,929, filed on April 7, 1941, by Sexton, , and Templeman on behalf of ICI, with the complete specification lodged in 1942 and granted in 1945 due to wartime secrecy restrictions. This patent covered the use of MCPA and related compounds for weed prevention and destruction, emphasizing their selective action on plants. Early lab-scale production at ICI focused on small-batch syntheses to evaluate , involving careful control of reaction conditions like temperature and base concentration to minimize side products such as di-substituted ethers. Yield optimization efforts addressed challenges in phenol purity—obtained via chlorination of —and reaction efficiency, achieving practical lab yields sufficient for and field trials by the mid-1940s.

Commercial Introduction and Adoption

MCPA was first commercially released in 1945 in the by (ICI) under the trade name Methoxone, marking the initial market entry of this for selective . This launch followed closely on the heels of its synthesis during research efforts aimed at developing plant growth regulators. In the United States, MCPA received approval from the U.S. Department of (USDA) in the early , with initial applications focused on cereal crops such as and to target broadleaf weeds. By the 1970s, it had been formally registered under the Federal Insecticide, Fungicide, and Rodenticide Act (FIFRA), solidifying its role in American . Adoption of MCPA reached its peak during the 1960s to 1980s across and , driven by expanding cereal production and the need for effective, selective broadleaf weed management in arable farming. During this period, it contributed significantly to increased application rates, with U.S. use on major crops rising to over 90% of planted acres by the 1980s. Usage began to decline post-2010, influenced by stricter environmental regulations in regions such as the , which aimed to mitigate risks to from persistent herbicides. A key shift in adoption occurred following the phase-out of more hazardous phenoxy herbicides like 2,4,5-T in the due to contamination concerns, with MCPA continuing as a safer option for similar applications in pastures and cereals. This transition helped sustain MCPA's market presence amid evolving safety standards.

Mechanism of Action

Biochemical Mode

MCPA functions as a synthetic herbicide, mimicking the indole-3-acetic acid (IAA) to disrupt normal growth regulation in susceptible broadleaf plants. Upon absorption, primarily through foliage, MCPA is translocated via the to meristematic tissues where it interferes with auxin signaling pathways. This molecular mimicry leads to overstimulation of auxin-responsive processes, causing unregulated and elongation that ultimately results in plant tissue deformation and death. At the cellular level, MCPA binds to the receptors TIR1 and AFB (Auxin F-Box) proteins, which are F-box components of the SCF complex. This binding promotes the interaction between TIR1/AFB and Aux/IAA repressor proteins, facilitating the ubiquitination and subsequent proteasomal degradation of Aux/IAA repressors. The degradation relieves repression of auxin response factors (ARFs), leading to aberrant activation of auxin-inducible genes involved in growth and development. In susceptible plants, the elevated auxin-like activity from MCPA exceeds physiological thresholds, resulting in disrupted patterns that promote excessive and disorganized cellular proliferation rather than coordinated growth. The biochemical disruption manifests as characteristic symptoms including epinasty (downward curvature of leaves and stems), leaf cupping, and stem twisting, which appear within hours to days of exposure. These morphological abnormalities stem from imbalanced signaling that favors longitudinal cell expansion and inhibits lateral growth. Over time, the uncontrolled growth depletes resources, leading to tissue and death, typically occurring within 1 to 6 weeks depending on sensitivity and environmental conditions.

Selectivity and Target Species

MCPA exhibits high selectivity for broadleaf weeds (dicotyledons) over grasses (monocotyledons), primarily due to metabolic differences, with secondary contributions from variations in uptake and translocation. In dicotyledons, MCPA is readily absorbed through foliage and roots, with efficient translocation to meristems. In contrast, monocotyledons like grasses show reduced uptake and limited translocation, but the key factor is their ability to rapidly detoxify MCPA through hydroxylation and glycosylation via enzymes, forming stable, non-toxic conjugates that prevent disruption. Dicotyledons, however, lack these efficient P450-mediated pathways or metabolize MCPA more slowly, leading to accumulation and heightened sensitivity to its auxin-mimicking effects. Broadleaves thus experience unchecked disruption of growth regulation, while grasses tolerate exposure. The primary targets of MCPA are annual and perennial broadleaf weeds in cereal crops, including thistles ( spp.), docks ( spp.), and buttercups ( spp.), where it effectively controls infestations without harming the grass crop. Non-target effects can occur if misapplied, particularly damaging dicotyledonous crops like (e.g., peas, beans), which share metabolic vulnerabilities with broadleaf weeds and may suffer growth abnormalities or yield loss from drift or over-application. Resistance to MCPA remains rare in weed populations but is emerging, with documented cases primarily involving enhanced metabolism through upregulated activity in species like Palmer amaranth () and, as of 2025, Amaranthus powellii, allowing faster detoxification and survival.

Applications and Uses

Agricultural Applications

MCPA is primarily employed as a post-emergence in crops such as , , and oats, as well as in , peas, and pastures to manage broadleaf weeds that compete with the crop for resources. It is applied selectively to these arable fields to target species like thistles, docks, and other annual and broadleaf weeds, thereby minimizing crop damage while promoting healthy growth. Typical application rates range from 0.5 to 2 kg per , depending on density and stage, with lower rates often sufficient for early-season control in and . For optimal efficacy, MCPA is timed for the active growth stage of target weeds, usually in spring from the tillering to early boot stage of the , ensuring weeds are small (up to 10-15 cm tall) for best absorption. This selectivity stems from the 's as a synthetic , which disrupts growth in susceptible broadleaf species more than in grasses like cereals. MCPA exhibits strong tank-mix compatibility with other herbicides, such as ioxynil, to broaden the spectrum of weed control in cereal fields, particularly against resistant or mixed populations of broadleaf weeds. Effective herbicide applications, including MCPA, can reduce weed competition and contribute to yield increases in cereals through integrated weed management for sustainable farming. Historically, MCPA saw limited adoption in rice paddies during the 1970s in Southeast Asia for broadleaf weed control, but its use declined due to selectivity challenges, as improper timing could cause morphological injury to rice plants despite its grass tolerance. Early formulations were applied post-transplanting at low doses, yet safer alternatives like propanil gradually replaced it to avoid crop damage.

Non-Agricultural Applications

MCPA is employed in turf management for selective control of broadleaf weeds such as and dandelions in lawns, courses, and other amenity turf areas, typically at application rates of 0.5 to 1.5 kg per to minimize impact on desirable grasses. These rates are suitable for young, actively growing weeds during spring or fall applications, with formulations like MCPA salts ensuring effective post-emergent action while allowing residential and professional use on established turf. In aquatic environments, MCPA is approved for controlling emergent broadleaf weeds in ponds, ditches, and other non-flowing water bodies, targeting species that invade shorelines without direct application to open water. Products combining with are applied at rates of 2 to 3 pints per acre in fall or winter to manage weeds like water hyacinth and smartweed, with strict adherence to label instructions to prevent contamination of potable water sources. For and rights-of-way maintenance, MCPA is used in spot treatments to suppress invasive broadleaf along roadsides, rows, under power lines, and in non-crop industrial areas, with maximum annual rates of 1.5 to 3.0 pounds acid equivalent per acre to control thistles and other perennials. These applications, often via handheld sprayers, support vegetation management plans by reducing competition from weeds without broad-spectrum harm to or utility infrastructure. Due to its high water solubility and potential for runoff, MCPA applications are restricted or prohibited in sensitive ecosystems such as natural wetlands to protect aquatic organisms and prevent leaching. Regulatory measures include mandatory vegetative buffer strips adjacent to water bodies, prohibitions on spraying during windy conditions or before forecasted rain, and exclusion from areas with habitats.

Formulations

Types of Formulations

MCPA, a phenoxyacetic herbicide, is formulated in several chemical forms to optimize its application, primarily as water-soluble salts or lipophilic esters for foliar spray delivery. The sodium salt and dimethylamine salt (MCPA-DMA) are the most common salt formulations, designed for high water solubility to facilitate mixing and application in aqueous sprays. These salts reduce volatility compared to the parent , minimizing vapor drift during application. Ester formulations, such as the 2-ethylhexyl ester (MCPA-EHE), enhance for improved penetration through cuticles, enabling faster absorption and efficacy on target weeds. However, esters exhibit greater volatility than salts, increasing the risk of off-target drift via , particularly under warm conditions. Liquid formulations of MCPA typically contain 50-60% by weight, often expressed as 500-600 g/L acid equivalent (a.e.) for salts and esters to ensure practical handling and dosing. For instance, MCPA-DMA is commonly formulated at 500-600 g a.e./L, while MCPA-EHE reaches 600 g a.e./L. Salt formulations offer advantages in by limiting environmental mobility through low volatility, though their hydrophilic nature may result in slower uptake by tissues compared to esters. In contrast, esters provide superior herbicidal performance via rapid foliar entry but pose higher risks of drift-related damage to non-target areas. Due to these drift concerns with esters, there has been a historical preference for and sodium salt formulations in phenoxy herbicides like MCPA since the 1980s, promoting their wider adoption for reduced off-target impacts.

Brand Names and Manufacturers

MCPA is marketed under various brand names worldwide, including historical products such as Methoxone, originally developed by (ICI), as well as Chiptox, Agroxone, Agritox, Chwastox, Cornox, Rhonox, and Tigrex. Current major manufacturers of MCPA-based herbicides include Agriscience, , and , which produce formulations for agricultural use. In the United States, products are distributed by companies like United Agri Products (UAP), while in , offers MCPA primarily in mixture formulations for enhanced efficacy. Common product examples include MCPA 750 g/L SL (soluble concentrate), such as ADAMA's MCPA 750 for broadleaf in cereals and pastures, and MCPA 500 g/kg WP (wettable powder) formulations used in similar applications. MCPA holds a notable in mixtures, often combined with mecoprop-p to broaden spectrum activity against resistant broadleaf weeds in turf and cereals.

Environmental Fate

Degradation in Soil

MCPA primarily undergoes microbial degradation in soil under aerobic conditions, where soil bacteria cleave the ether bond to form 4-chloro-2-methylphenol (CMP) as the main intermediate and , which further mineralizes to and chloride ions. This process is mediated by specialized microbial communities that express genes like tfdA for the initial cleavage step. In sterile soils lacking microbial activity, degradation is negligible, while anaerobic conditions significantly slow the process, extending persistence beyond aerobic half-lives. The half-life of MCPA in soil typically ranges from 10 to 40 days under aerobic conditions at moderate temperatures (15–25°C) and optimal moisture, though field studies report variations up to 7–60 days depending on prior exposure history. Degradation accelerates in soils previously treated with MCPA due to adaptation of degrader populations, reducing half-lives from weeks to days. Key influencing factors include soil pH, with faster breakdown at neutral to alkaline pH (6.3 and above), where half-lives can shorten to about 1 week compared to 5–9 weeks in acidic soils. Temperature positively correlates with rates, following Q₁₀ values of 2.9–3.3 between 0–29°C, while microbial activity is enhanced by adequate moisture (0.6–1.2 times field capacity) and nutrient availability. Adsorption of MCPA to is generally weak, with organic carbon-normalized coefficients (K_{OC}) of 54–118 L/kg, but binding strengthens in soils high in (OC > 2%), where it partitions preferentially to , thereby reducing and leaching potential. This sorption-desorption dynamic influences degradation kinetics, as sorbed MCPA is less accessible to microbes, potentially prolonging half-lives in low-OC sandy soils. The primary metabolite, CMP (also known as MCP), exhibits low persistence with a of about 3–4 days in aerobic soils, undergoing further microbial ring cleavage to chlorinated catechols and eventual mineralization. Minor metabolites, such as 4-chloro-2-methyl-6-nitrophenol, may form under oxidative conditions but do not accumulate significantly. Overall, these processes contribute to MCPA's moderate persistence in terrestrial environments, with complete mineralization favored in biologically active, well-aerated soils.

Mobility and Persistence in Water

MCPA exhibits moderate leaching potential in , primarily through preferential flow pathways such as macropores, which facilitate its transport to despite its moderate adsorption to particles. Field studies indicate limited vertical movement, with no leaching observed below 6 inches in most cases, though modeling estimates suggest potential concentrations of up to 0.59 µg/L under agricultural use scenarios. Actual detections in confirm this moderate risk, with concentrations ranging from 0.05 to 5.5 µg/L reported in monitoring programs across regions like and European aquifers. Runoff represents a significant transport mechanism for MCPA into surface waters, particularly following rainfall events shortly after application, where dissolved concentrations can reach peaks of 4.2 to 31 µg/L based on environmental modeling. Ester formulations, which hydrolyze rapidly to the more water-soluble MCPA acid, contribute disproportionately to runoff losses compared to salts, exacerbating contamination in high-rainfall agricultural areas. monitoring data show maximum runoff-related detections up to 18.58 µg/L, often linked to spray drift and overland flow from treated fields. In water, MCPA demonstrates variable persistence depending on environmental conditions, with aerobic degradation half-lives ranging from 13 to 236 days in systems, though field-relevant aerobic processes in surface waters can shorten this to 1-7 days when microbial activity combines with light exposure. Persistence is notably slower in anaerobic sediments, where half-lives exceed 100–1122 days due to reduced microbial breakdown. of the parent acid is minimal at neutral pH (5-7), contributing little to degradation, but photolysis in sunlit surface waters accelerates breakdown, with half-lives of 19-24 days under natural conditions. Monitoring in European rivers frequently detects MCPA at concentrations of 0.1-5 µg/L, attributed primarily to agricultural runoff during peak application seasons in spring. These detections, observed in catchments across the , , and , highlight the compound's mobility and underscore the need for targeted to protect sources.

Environmental and Health Impacts

Ecotoxicity

MCPA exhibits varying levels of across aquatic and terrestrial species, with particular sensitivity observed in non-target and macrophytes. For , such as (Oncorhynchus mykiss), the 96-hour LC50 ranges from 117 to 232 mg/L, classifying it as slightly toxic under standard exposure conditions. A 2025 study also identified that exposure to MCPA-Na causes in the intestinal tract of , leading to and increased permeability. In contrast, MCPA is highly toxic to aquatic vascular , with EC50 values below 1 mg/L; for example, the 14-day ErC50 for the submerged macrophyte is 0.243 mg/L, indicating significant risk to submerged aquatic vegetation. Algal species, however, show low sensitivity, with 120-hour ErC50 values exceeding 320 mg/L for Raphidocelis subcapitata. Birds and mammals demonstrate relatively low acute oral toxicity to MCPA. The LD50 for bobwhite quail (Colinus virginianus) is 377 mg/kg body weight, while for rats it is 962 mg/kg body weight, both indicative of moderate to low hazard in single-dose scenarios. Chronic exposure may pose risks to in these groups, though avian studies show no adverse effects at dietary levels up to 983 mg/kg; potential sublethal impacts include reduced growth and feeding rates at prolonged exposures around 50-125 mg/kg/day. Invertebrates experience moderate to low toxicity from MCPA. For honeybees (Apis mellifera), both contact and oral LD50 values exceed 200 μg/bee, suggesting low acute risk. Aquatic invertebrates, such as Daphnia magna, show low sensitivity with 48-hour EC50 >190 mg/L, though ester formulations can elevate toxicity to aquatic insects in short-term exposures. At the ecosystem level, 's herbicidal action disrupts communities, potentially altering structure and leading to indirect effects on amphibians through vegetation loss; direct to amphibians is low, with NOEC values >12 mg/L. This plant die-off can imbalance primary producer dynamics, indirectly influencing algal populations and higher trophic levels, though direct algal is minimal. Degradation products, such as MCPA conjugates, generally exhibit similar or lower profiles compared to the parent compound. Bioaccumulation potential is low due to MCPA's log Kow of 2.83 and factors (BCF) ranging from 1 to 14 in like ( carpio), resulting in minimal through food chains.

Human Health Effects

MCPA exposure in humans primarily occurs through dermal contact during herbicide mixing and application, as well as of spray drift. Acute effects of MCPA include irritation to the skin, eyes, and upon contact or . Ingestion leads to symptoms such as , , and gastrointestinal distress. In animal studies, the oral LD50 for MCPA in rats ranges from 700 to 1200 mg/kg body weight, indicating moderate . Chronic exposure to MCPA may involve potential endocrine disruption, as suggested by in vitro studies showing binding to receptors. Regarding carcinogenicity, the International Agency for Research on Cancer (IARC) classifies MCPA as Group 3, not classifiable as to its carcinogenicity to humans, due to insufficient evidence from human and animal studies. Epidemiological studies have reported limited associations between high occupational exposure to phenoxy herbicides, including MCPA, and an increased risk of among farmers, particularly older men, though the evidence is inconclusive and requires further research. A 2023 study found associations between MCPA exposure and faster motor and non-motor symptom progression in . The World Health Organization (WHO), in collaboration with the Food and Agriculture Organization (FAO), has established an acceptable daily intake (ADI) for MCPA of 0–0.1 mg/kg body weight, based on a no-observed-adverse-effect level (NOAEL) from long-term animal studies with a 100-fold safety factor.

Regulations and Management

Global Regulatory Status

MCPA is approved for use in the European Union under Regulation (EC) No 1107/2009, with the current approval period set to expire on August 15, 2026; it is authorized in most EU member states such as Austria, Belgium, Germany, and France, as well as EEA countries including Iceland and Norway, though it is not designated as a candidate for substitution. Restrictions include prohibitions on applications near aquatic environments to mitigate drift risks, and maximum residue limits (MRLs) are established for various commodities, accessible via the EU pesticides database. In the United States, the Environmental Protection Agency (EPA) first registered MCPA in 1973 and completed its reregistration eligibility decision in 2004 as part of the Food Quality Protection Act process, confirming its safety for continued use with mitigation measures. The ongoing registration review, initiated in 2014, issued a proposed interim decision in 2019 that retained registration while imposing restrictions such as buffer zones around water bodies to reduce runoff and drift, along with limits on application methods like backpack sprayers in sensitive areas. Tolerances for MCPA residues in grains such as and are 1.0 mg/kg, with tolerances for other commodities ranging from 0.05 mg/kg in meat to 300 mg/kg in certain forages, as codified in 40 CFR §180.339. Health Canada's Pest Management Regulatory Agency (PMRA) re-evaluated MCPA in 2008, determining it eligible for continued registration with updated label precautions to address environmental risks, including buffer zones and no-spray zones near water. Subsequent reviews in the confirmed low overall risk when used according to label directions. However, as of 2025, PMRA has initiated a special review of MCPA focusing on human health risks from inhalation exposure (occupational and residential), with a proposed special review decision planned for consultation in early 2026 and no changes to approval status to date. In , the Australian Pesticides and Veterinary Medicines Authority (APVMA) permits MCPA use in various formulations for broadleaf weed control, with approvals requiring monitoring for contamination due to its mobility in . However, MCPA has been banned in several Gulf countries, including , , , , and the , primarily due to concerns over spray drift affecting non-target areas. As of 2025, no major global bans on MCPA have been implemented, though it faces ongoing regulatory scrutiny in the following the 2024 withdrawal of the proposal under the , which had aimed to reduce dependency by 50% by 2030 and potentially influence future renewals.

Risk Mitigation and Alternatives

To mitigate risks associated with MCPA application, such as drift and runoff into water bodies, best management practices include establishing buffer zones near sensitive aquatic areas as per local regulations and application methods to prevent contamination. Low-drift nozzles should be used to minimize spray drift, particularly on calm days, reducing off-target movement compared to standard nozzles. Application timing is critical; avoid spraying when heavy rain is forecast shortly after, as this can lead to and reduced efficacy. Integrated pest management (IPM) strategies for MCPA use emphasize combining chemical applications with non-chemical methods to reduce overall reliance and enhance long-term control in grasslands. with cover crops or alternative disrupts weed cycles, while mechanical practices like mowing at appropriate heights (e.g., 5-7 cm for grasses) suppress broadleaf weeds without sole dependence on herbicides. These approaches can lower MCPA application rates by 20-30% when integrated effectively. Resistance management is essential due to reports of metabolic resistance in weeds like Amaranthus powellii and hemp-nettle (), where enhanced detoxification reduces efficacy. Avoid over-reliance on MCPA by rotating with from different modes of action (e.g., Group 4 alternated with Group 9), and monitor fields for resistance through bioassays or scouting for surviving weeds post-application. Viable chemical alternatives to MCPA for broadleaf in cereals and grasslands include , which targets similar auxin-sensitive species with comparable selectivity but requires careful drift management, and fluroxypyr, effective against resistant populations like thistles. For total , mixtures of with MCPA alternatives provide broader spectrum activity while minimizing resistance buildup. As of 2025, emerging options focus on , with bioherbicides derived from microbial sources (e.g., those targeting specific enzymes) showing promise in field trials for reducing synthetic herbicide needs by 40-60% in targeted applications. Precision technologies like drone-based spraying enable site-specific delivery, using AI and to apply MCPA or alternatives only to weed-infested areas, cutting overall chemical use by up to 90% and minimizing environmental exposure.

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