Pesticide Toxicity Profile: Chlorophenoxy Herbicides
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Pesticide Toxicity Profile: Chlorophenoxy Herbicides

   

Pesticide Toxicity Profile: Chlorophenoxy Herbicides1

Frederick M. Fishel2

This document provides a general overview of human toxicity, provides a listing of laboratory animal and wildlife toxicities and a cross reference of chemical, common and trade names of the chlorophenoxy herbicides registered for use in Florida.

General

The chlorophenoxy herbicides are a group of older herbicides, with their discovery and use dating back to the 1940s. They are used singly or in combination with other active ingredients to control broadleaf weeds in turfgrass, pasture, rights-of-way, corn, soybean (preplant only), small grains, and fence rows. Current chlorophenoxy herbicides registered for use in Florida include 2,4-D, 2,4-DB, MCPA, and mecoprop (MCPP). Malformation, twisting, and curvature of plant foliage are typical symptoms of the chlorophenoxy herbicides. These herbicides are absorbed by the plant and translocated to the younger tissues. Perennial weed control is better achieved in fall, when large amounts of nutrients are being transported to the roots. Numerous studies have been conducted to determine the mechanism of action of the chlorophenoxy herbicides, but the exact mode remains very elusive. It is known that the chlorophenoxy herbicides are auxin mimics causing hyperplastic growth.

2,4-D

There are approximately 100 commercial 2,4-D products registered for use in Florida, packaged singly or in combination with other active ingredients. The first 2,4-D product was registered for use in the U.S. in 1948. Various products are formulated as emulsifiable acids, amine salts, mineral salts, and esters. Amine forms of 2,4-D are soluble in water and present the least volatility hazard of causing harm to desirable vegetation. Esters, in contrast to amine formulations, are essentially insoluble in water and relatively volatile. Their vapors may kill or injure susceptible plants (Figures 1 - 3), and where the vapors are confined, they may inhibit the germination of seed. Concern for drift is a primary reason for their regulation by the Florida Department of Agriculture and Consumer Services (EDIS Document SS-AGR-12, Florida's Organo-Auxin Herbicide Rule - 2005 ). Esters are more likely to cause injury to crop plants as compared to amine formulations. The ester formulations are generally preferred for controlling difficult vegetation, such as woody species. Regardless, the rates for essentially all 2,4-D formulations are expressed as acid equivalents in pounds per gallon. There are numerous turfgrass fertilizers impregnated with 2,4-D available to homeowners.

a)

b)

c)

Figure 1. Ester vapors may kill or injure susceptible plants.

MCPA

MCPA has many of the same use patterns as 2,4-D and most products containing MCPA are premixed with 2,4-D and other herbicides. It is also available in amine and ester formulations. It is used as a specialty herbicide in the U.S. on a small scale. MCPA is less injurious to cereal grain crops and has been labeled for weed control use in small-seeded legumes, such as clover. It was first registered in the U.S. in 1973.

2,4-DB

2,4-DB was first registered in the U.S. in 1958. It is registered for use as a foliar treatment to peanut and soybean. It is also formulated as amines and low-volatile esters. 2,4-DB is not highly phytotoxic, per se; however, it undergoes beta-oxidation within plants and soils to form 2,4-D. Some plants are able to make this conversion rapidly, resulting in injury or control, while others, such as peanut and soybean, react slowly making them more tolerant of the effects.

MCPP

MCPP is an abbreviation for mecoprop. It acts relatively slowly compared to the other chlorophenoxy herbicides. In most cases, it is prepackaged with other herbicidal active ingredients.

Toxicity

The chlorophenoxy herbicides are regarded as a family of relatively safe pesticides in terms of acute, chronic, and long-term effects. For acute oral, dermal, and inhalation effects, they are classified Category III or IV. As single ingredients, they are not classified by the EPA as restricted use pesticides. Some products carry the signal word, "DANGER" on their labels due to corrosiveness and causing irreversible eye damage. 2,4-D has not been classified as a human carcinogen by the EPA Cancer Peer Review Committee. Acute symptoms of 2,4-D toxicity can be fatigue and weakness and perhaps nausea. Symptoms vary with the different commercial products because of the specific amounts and types of additives such as surfactants and solvents. 2,4-D has a very limited ability to cause teratogenic effects and extensive testing for mutagenicity has found it to be non-mutagenic. 2,4-DB is placed in the following acute toxicity categories: oral III; dermal III; inhalation IV; eye irritation III; and, dermal irritation IV. EPA determined that there is reasonable certainty that no harm to any population subgroup will result from aggregate exposure to 2,4-DB when considering dietary exposure. 2,4-DB does not pose great ecological concerns; it is only slightly toxic to fish. MCPA is classed category III or IV, depending on form. Three of its four forms are classified as strong eye irritants. Acute and chronic dietary risk is not a concern. All available evidence indicates that MCPA does not cause cancer. It appears that the compound poses little mutagenic risk to humans. Ecologically, MCPA is the most toxic to birds, but only considered to be moderately toxic. MCPP has a low acute toxicity to test animals. It is irritating to skin and eyes, causing redness and swelling and can cause cloudy vision. MCPP is a teratogen in rats at moderate to high doses. Studies show that MCPP may be mutagenic at very high doses. It is not clear if occupational exposures cause cancer. There is no information available for its chronic and reproductive effects. Mammalian toxicities for the chlorophenoxy herbicides are shown in Table 1. Table 2 lists the toxicities to wildlife by the common name of the pesticide. Table 3 provides a cross listing of many of the trade names that these products are registered and sold by in Florida.

Additional Information

Tables

Table 1. Chlorophenoxy herbicide mammalian toxicities (mg/kg of body weight).

Common name

Rat oral LD50

Rabbit dermal LD50

2,4-D

500 - 949

---

2,4-DB

>2,000

>10,000

MCPA

900 - 1160

>4,000

MCPP

1,000 - 1,166

>2,000

Table 2. Chlorophenoxy herbicide wildlife toxicity ranges.

Common name

Bird acute oral LD50 (mg/kg)*@

Fish (ppm)**

Bee†
2,4-D

PNT

PNT

PNT
2,4-DB

PNT

ST

PNT
MCPA

MT

PNT

PNT
MCPP

ST

PNT

PNT
*Bird LD50: PNT = >2,000; ST = 501 - 2,000; MT = 51 - 500; HT = 10 - 50; VHT = <10.
**Fish LC50: PNT = >100; ST = 10 - 100; MT = 1 - 10; HT = 0.1 - 1; VHT = <0.1.
†Bee: HT = highly toxic (kills upon contact as well as residues); MT = moderately toxic (kills if applied over bees); PNT = relatively nontoxic (relatively few precautions necessary).

Table 3. Cross reference list of common, trade and chemical names of chlorophenoxy herbicides.

Common name


 Trade names*


 Chemical name
2,4-D


 Weedar®, Weedone®


 2,4-dichlorophenoxyacetic acid
2,4-DB


 Butoxone®, Butyrac®


 4-(2,4-dichlorophenoxy)butyric acid
MCPA


 Many


 4-chloro-o-tolyloxyacetic acid
MCPP


 Many


 Potassium (RS)-2-(2-methyl-4-chlorophenoxy)propionate
*Does not include manufacturers' prepackaged mixtures.

Footnotes

1. This document is PI-83, one of a series of the Pesticide Information Office, Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida. Original publication date October 2005. Visit the EDIS Web Site at http://edis.ifas.ufl.edu.

2. Frederick M. Fishel, Associate Professor, Agronomy Department, and Director, Pesticide Information Office; Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida, Gainesville, FL 32611.

The use of trade names in this publication is solely for the purpose of providing specific information. UF/IFAS does not guarantee or warranty the products named, and references to them in this publication does not signify our approval to the exclusion of other products of suitable composition. Use pesticides safely. Read and follow directions on the manufacturer's label.

The Institute of Food and Agricultural Sciences (IFAS) is an Equal Opportunity Institution authorized to provide research, educational information and other services only to individuals and institutions that function with non-discrimination with respect to race, creed, color, religion, age, disability, sex, sexual orientation, marital status, national origin, political opinions or affiliations. For more information on obtaining other extension publications, contact your county Cooperative Extension service.

U.S. Department of Agriculture, Cooperative Extension Service, University of Florida, IFAS, Florida A. & M. University Cooperative Extension Program, and Boards of County Commissioners Cooperating. Larry Arrington, Dean.


Copyright Information

This document is copyrighted by the University of Florida, Institute of Food and Agricultural Sciences (UF/IFAS) for the people of the State of Florida. UF/IFAS retains all rights under all conventions, but permits free reproduction by all agents and offices of the Cooperative Extension Service and the people of the State of Florida. Permission is granted to others to use these materials in part or in full for educational purposes, provided that full credit is given to the UF/IFAS, citing the publication, its source, and date of publication.