Overview

MCPA is an herbicide that was registered with the EPA in 1973. MCPA acid is the parent chemical to other chemical formulations such as MCPA dimethylamine salt, MCPA sodium salt, and MCPA 2-ethylhexyl ester (EPA). It works by disrupting the normal growth of broadleaf plants, affecting both monocots and dicots. MCPA has a moderate to low toxicity for most animals, but it can affect the growth of some desirable plants.

MCPA is often formulated with other herbicides like bentazone, bromoxynil, 2,4-D, dicamba, fenoxaprop, MCPB, mecoprop, thifensulfuron, and tribenuron (EXTOXNET).

MCPA may be sold under names such as Agritox, Agroxone, Agrozone, Agsco MXL, Banlene, Blesal MC, Bordermaster, Cambilene, Cheyenne, Chimac Oxy, Chiptox, Class MCPA, Cornox Plus, Dakota, Ded-Weed, Empal, Envoy, Gordon's Amine, Kilsem, Legumex, Malerbane, Mayclene, MCP, Mephanac, Midox, Phenoxylene, Rhomene, Rhonox, Sanaphen-M, Shamrox, Selectyl, Tiller, U 46 M-Fluid, Vacate, Weed-Rhap, and Zhelan (EXTOXNET).

Chemical Description

MCPA is a chlorophenoxy compound. It is a white to light brown solid, flake, or powder. MCPA is mostly insoluble in water (EPA). It has a field half-life of 14 days to one month, and has a low affinity for soil (EXTOXNET).

MCPA is available in granular forms, emulsifiable concentrates, soluble concentrates or liquids, and wettable powders (EPA).

Uses

MCPA is used to control a wide range of broadleaf weeds. It accumulates in the meristematic tissue of plants, where growth occurs (EXTOXNET). There, it stimulates plant hormones, causing uncoordinated plant growth that disrupts both new seedlings and existing plants (EPA).

MCPA is used for weed control around a variety of agricultural crops such as alfalfa, barley, clover, flax, lespedeza, oats, grass, peas, rice, rye, sorghum, trefoil, triticale, and wheat. It is also used on sod farm turf, golf courses, public walkways, pastures, and rangeland. MCPA is used residentially on lawns (EPA). The EPA estimates that 4.6 million pounds of MCPA are used each year in agricultural and residential applications.

Human Health Effects

MCPA is readily absorbed into most plants. However, MCPA residue found in food crops fall below the EPA's level of concern (EPA).

Most formulations of MCPA are severe eye irritants. Only MCPA 2-ethylhexyl ester is believed to be a skin sensitizer (EPA). MCPA is rapidly absorbed and excreted. A study in which people were given a 5 mg dose found that all MCPA was completely excreted within five days (EXTOXNET). The International Agency for Research on Cancer identifies MCPA as a possible carcinogen (IARC).

Inhalation of MCPA can cause headache and nausea. Contact with the eyes and skin can cause redness. Symptoms of ingestion include abdominal pain, nausea, vomiting, weakness, and unconsciousness (NIOSH).

Environmental Health Effects

Although insoluble in water, MCPA can have a high mobility in soil. MCPA can negatively affect the growth of some crops, including onion, cabbage, lettuce, and turnip (EPA).

In mammals, MCPA generally has a low toxicity, and is rapidly excreted. The oral LD50 in rats is 765 mg/kg. The dermal LD50 exceeds 2000 mg/kg, and the inhalation LC50 exceeds 6.3 mg/L (EPA).

MCPA has a moderate to low toxicity to birds. The oral LC50 is 377 mg/kg in bobwhite quail (EPA). It is practically non-toxic to honey bees with an LD50 of 104 ug/bee. MCPA is slightly toxic to freshwater fish, and mostly non-toxic to other aquatic organisms. The LC50 of rainbow trout ranges from 117 mg/L to 232 mg/L (EXTOXNET).

Regulation

MCPA is a General Use Pesticide, although all product labels are required to carry the signal word "danger" because it is a severe eye irritant (EXTOXNET).

Precautionary Notes

Pregnant women should take special care to avoid exposure to MCPA. If you are using a chemical variant of MCPA, understand the personal and environmental repercussions that differ from MCPA acid. Keep MCPA away from the eyes.

References

Environmental Protection Agency. Reregistration Eligibility Decision for MCPA. (September 2004). http://www.epa.gov/oppsrrd1/REDs/mcpa_red.pdf [Accessed 7-15-10].

Extension Toxicology Network. MCPA. (June 1996). http://extoxnet.orst.edu/pips/MCPA.htm [Accessed 7-15-10].

International Agency for Research on Cancer. Agents Classified by the IARC Monographs. http://monographs.iarc.fr/ENG/Classification/ClassificationsAlphaOrder.pdf [Accessed 7-21-10].

National Institute for Occupational Safety and Health. MCPA. http://www.cdc.gov/niosh/ipcsneng/neng0054.html [Accessed 7-15-10].

National Pesticide Information Center. OSU Extension Pesticide Properties Database. (July 1994). http://npic.orst.edu/ppdmove.htm [Accessed 7-15-10].

Pesticide Action Network North America. MCPA. http://www.pesticideinfo.org/Detail_Chemical.jsp?Rec_Id=PC32901 [Accessed 7-15-10].