Agronomy 317 - Principles of Weed Science

Herbicide Mode of Action


Synthetic Auxins

Herbicide Resistance Action Committee (HRAC) Group - O

Weed Science Society of America (WSSA) Group - 4

These herbicides generally mimic auxin, a plant growth hormone. They are often called growth regulators because they upset the natural hormone balance in the plant. The synthetic auxins are used primarily to control broadleaf weeds in grass crops, pastures, and turf. They are some of the most widely used herbicides in turfgrass, especially for home use. Some herbicides in this chemistry suppress difficult to manage perennial broadleaves, such as Canada thistle and field bindweed. Many provide good brush control and are also used for controlling woody plants in forest management and noncropland sites. Some are used for controlling certain aquatic weeds.

2,4-D and MCPA were developed in 1942 in the United States and United Kingdom, respectively. They revolutionized weed control when they were introduced shortly after World War II and provided momentum for the development of the pesticide industry. These herbicides were quickly accepted by growers because they were inexpensive and easy to apply in low water volumes. The inorganic salt herbicides that preceded them were inexpensive, but the had to be applied in large volumes of water and often provided poor weed control.

Most synthetic auxins are readily absorbed through both roots and foliage and are translocated in both the xylem and phloem. However, they are primarily foliar applied and symplastically translocated. They can be applied to the soil, but symplastic movement in roots is limited, generally making these compounds less effective as soil treatments. Translocation of foliar-applied treatments is more restricted in grasses than in susceptible broadleaves.

Synthetic auxin herbicides are not metabolically stable in plants and are converted to several different products. They cause injury because plants cannot readily control the concentration of synthetic auxins applied in relatively high doses like they can control natural auxin levels.

The killing action of growth-regulating chemicals is not caused by any single factor, but by the disruption of several growth processes in susceptible plants. They affect proteins in the plasma membrane, interfere with RNA production, and change the properties and integrity of the plasma membrane. Protein synthesis and RNA concentration increase as persistent auxin-like molecules prevent normal and necessary fluctuation in auxin levels required for proper plant growth. Cell proliferation increases and the plant's vascular system becomes blocked because excessive cell division and resulting growth crushes the vascular transport system. Tissues that are undergoing active cell division and growth are particularly susceptible to injury.

Injury symptoms include growth and reproduction abnormalities, especially on new growth. Broadleaf species exhibit stem and petiole twisting (epinasty), leaf malformations (parallel venation, leaf strapping, and cupping), undifferentiated cell masses and adventitous root formation on stems, and stunted root growth. Grass plants exhibit rolled leaves (onion leafing), fused brace roots, leaning stems, and stalk brittleness. Synthetic auxins can distrupt reproductive process resulting in sterile or multiple florets and nonviable seed production. Death may not occur for several weeks, but symptoms show up on young growth almost immediately after application.

Drift injury on sensitive plants is often a problem when using these herbicides, especially turf applications near gardens and trees.

Families

Phenoxyacetic acids

Benzoic acids

Pyridine-Carboxylic acids


Phenoxyacetic acids (Phenoxys)

Site of Action

Auxin-binding sites. They affect the hormone balance in the plant by mimicking auxin.

History

The oldest of the organic herbicides, these chemicals were developed in the early 1940's.

Representative Herbicides

Common name Trade name
2,4-D Various
2,4-DB Various
2,4-DP (dichloroprop) Various
2,4,5-T Banned in the U.S.
2,4,5-TP Banned in the U.S.
MCPA Various
MCPB Various
MCPP (mecoprop) Various

Crop Use

Grass crops (corn, small grains, and grain sorghum), pastures, rangeland, and turf. Control of brush and some aquatic weeds.

Application Target

Mostly used as foliar applications, but there are some soil applications.

Translocation Type

Symplastically translocated

Weed Spectrum

Primarily control broadleaf weeds, both annual and perennial.

Selectivity

Broadleaves rapidly absorb and translocate these herbicides.

Grasses without elongating stems show little or no symptoms. They rapidly metabolize phenoxys to irreversible non-toxic molecules.

The phenoxybutyrics, 2,4-DB and MCPB can be used as selective postemergence control of annual broadleaf weeds in certain annual broadleaf crops, such as soybean, peanuts, and seedling forage legumes. Susceptible species have enzymes that convert these molecules to the active herbicidal products, 2,4-D and MCPA, through beta oxidation. Older forage legumes are more susceptible than younger plants because the older plants have a more developed and efficient beta oxidation system that can break 2,4-DB into 2,4-D, which is immediately toxic.

Reaction in Soils and the Environment

Potentially mobile because they dissociate in the soil solution to form negatively charged ions that are not attracted to soil particles.

Leaching is minimal because of rapid degradation and plant uptake.

Microbial breakdown in warm, moist soils. Rate of breakdown increases with increased temperature, moisture, pH and organic matter content.

Half life in soil 5 to 21 days (2,4-D - 10 days)

Symptomology

Dicots Epinasty (stem twisting and leaf malformations), parallel leaf venation, leaf strapping, and cupped leaves.

Grass crops - Onion leafing, abnormal brace root development, and stalk bending and brittleness.

Injury Pictures


Benzoic Acids

Site of Action

Auxin-binding sites. They affect the hormone balance in the plant by mimicking auxin.

History

Developed in the 1960's and 70's.

Representative Herbicide

Common name Trade name
dicamba Banvel, Clarity, others

Crop Use

Grass crops (corn, small grains, and grain sorghum), pastures, rangeland, and turf. Brush control.

Application Target

Both foliar- and soil-applied

Translocation Type

Symplastically translocated

Weed Spectrum

Primarily control broadleaf weeds, both annual and perennial.

Selectivity

Absorbed and translocated rapidly in dicots, but not in grasses.

Weed control spectrum similar to 2,4-D, but it is more effective on perennial weeds. Strong on smartweed family, but weak on mustards.

Reaction in Soils and the Environment

Rapidly degraded in soils (half-life less than 14 days).

Breakdown is primarily due to volatization losses and microbial degradation.

Highly mobile in soils, especially on sandy soils, but leaching potential is low to medium due to rapid degradation.

Persist longer under conditions of low rainfall or soil moisture.

Symptomology

Dicots - Epinasty and leaf cupping

Grasses - Onion leafing, abnormal root development, and stalk brittleness if applied during stem elongation.

Dicamba injury is similar to that caused by phenoxy acid herbicides, but broadleaf plants may exhibit more cupping than strapping of leaf tissues.

Injury Pictures

 


Pyridine-Carboxylic Acids

Site of Action

Auxin-binding sites. They affect the hormone balance in the plant by mimicking auxin.

History

Developed in the 1960's and 70's

Representative Herbicides

Common name Trade name
clopyralid Stinger, Reclaim
picloram Tordon
triclopyr Garlon, Remedy, Crossbow

Crop Use

Grass crops (corn, small grains, and grain sorghum), pastures, rangeland, and turf. Noncropland broadleaf and woody species control.

Application Target

Both foliar- and soil-applied

Translocation Type

Symplastically translocated

Weed Spectrum

Primarily control broadleaf weeds, both annual and perennial.

Selectivity

Absorbed and translocated rapidly in dicots, but not in grasses.

Little activity on grasses or mustards.

Often used for brush and Canada thistle control.

Reaction in Soils and the Environment

Degraded by photodegradation and by microbial breakdown (clopyralid only by microbial breakdown). They are degraded more slowly than the phenoxies or benzoics.

Half life in soil clopyralid and triclopyr 30 to 90 days; picloram 90 days

Picloram is water soluble, loosely absorbed, and susceptible to leaching. It can also carryover to affect succeeding crops. Because mobility in soil is great, picloram was one of the first herbicides designated as a restricted-use pesticide. This herbicide must be used carefully to avoid off-site injury and environmental contamination.

Symptomology

Dicots - Epinasty and leaf cupping

Grasses - Onion leafing, abnormal root development, and stalk brittleness if applied during stem elongation.

Injury Pictures


Agronomy 317 - Principles of Weed Science
Authored by Dr. Lance R. Gibson
Copyright 2001 Iowa State University. All rights reserved.
Revised: July 23, 2004 .