Herbicide Mode of Action

Fabián D. Menalled

Cropland Weed SpecialistDept. Land Resources and Environmental

SciencesMontana State University

menalled@montana.edu

Why Understand Herbicide Mode of Action?

• Better understanding of how herbicides perform

• Improve herbicides performance

• Diagnosing herbicide injury

• Prevent and manage herbicide resistance

Some Things to Remember….

• Photosynthesis (food)

• Pigments (energy/light capture)

• Respiration (energy)

• Amino acids (proteins/growth)

• Lipids (cell membranes)

• Mitosis (cell division)

Why do we use herbicides?

• Use herbicides to achieve your goal – Reduce the impact of invasive species

– Secure the presence of targeted speciesBut not all herbicides are

equal!

Herbicide Classification - Selectivity -

• Selective: controls or suppresses one

species of plant without seriously affecting

the growth of another plant species

– 2,4-D

• Nonselective: control plants regardless of

species

– Roundup

Site of Absorption and Translocation

Foliar Contact

(Gramoxone)

Root Contact

(Treflan)

Phloem

(Roundup)

Xylem

(Spike)

Xylem and Phloem

(Banvel, Tordon)

Commercial Products (Roundup, Durango)

Mode of Action (Amino Acid Biosynthesis Inhibitors)

Site of Action (EPSPS inhibitor)

Chemical Family (Glyicines)

Active Ingredient (Glyphosate)

Mode of Action •Sequence of events from

absorption of the herbicide into the plant until the plant dies

Contact

AbsorptionMovement

Site of Action

Herbicide Mode of Action

Toxicity

CO2 + H2O Sugar + O2

Classification by Mode of Action

1) Plant Growth Regulators2) Amino Acid Biosysthesis Inhibitors3) Lipid Biosynthesis Inhibitors4) Cell Division Inhibitors5) Photosynthesis Inhibitors6) Cell Membrane Disrupters7) Pigment Inhibitors8) Unknown mode of action

Plant Growth Regulators

• Synthetic auxins (regulate plant growth)

• Affect several plant processes such as cell division, cell enlargement, protein synthesis and respiration

• Act by upsetting the normal hormonal balance in plants

Plant Growth Regulators

• Herbicide uptake is primarily through the foliage but root uptake is possible

• Translocate in both xylem and phloem

• Effective on perennial and annual broadleaf weeds

• Selectively kill broadleaf plants

• Injury may occur in grasses

Chemical Family

Common Name Trade Name

Phenoxy acetic acids 2,4-D 2,4-D, Campaign,

Crossbow,

Landmaster BW,

others

2,4-DB Butyrac

MCPA MCPA, others

Benzoic acid dicamba Banvel, Clarity

pyridines clopyralid Curtail, Transline

fluroxypyr Starane

picloram Tordon

Examples of PGR

More examples of PGR

• Transline & Curtail: Clopyralid

• Milestone: Amynopiralid

• Weedmaster: 2,4-D + Dicamba

• Grazon P&D: Triclopyr + Picloran

PGR Symptoms• Most obvious on newly developing leaves

• Abnormal growth resulting in twisting stems

• Stems swelling due to rapid cell division

• Leaves on broadleaf plants exhibit cupping, crinkling, strapping, or drawstring affect

• Symptoms on grass plants include leaf rolling, crinkling, brace root fusion and malformation.

• Flower sterility and missing grain in crops

Parallel veination due to 2,4-D

Photo: HMOA and Crop Injury Symptoms.

Univ. of Minnesota Extension

PGR Symptoms

•

Leaf cupping caused by dicamba

Photo: HMOA and Crop Injury Symptoms.

Univ. of Minnesota Extension

PGR Symptoms

•

Puckered soybeans from Tordon

Photo: Kansas State University Extension

Tordon runoff into soybean fieldTordon runoff into soybean field

Photo: Kansas State University Extension

Growth Regulator Use Concerns

• Herbicide resistance

• Drift and injury to nontarget plants

• Carryover: Tordon

• Groundwater Contamination:

Tordon

Questions, so far?

Amino Acid Synthesis Inhibitors

• Prevent synthesis of certain amino acids produced

by plants but not animals

• Excellent foliar and root absorption

• Broad weed spectrum

• Translocates to shoot and root new growth in both

xylem and phloem

• Plants stop growing shortly after application

• Plant death may be slow (10 days+)

Chemical Family Common Name Trade Name

Sulfonylureas chlorsulfuron Glean, Telar

thifensulfuron Harmony GT

nicosulfuron Accent

Imidazolinones imazamethabenz

Assert

imazapic Plateau

imazamox Raptor

Amino acid derivates glyphopste Roundup, Glyphomax, Rodeo, and others

Examples of Amino Acid Synthesis Inhibitors

More Examples of Amino Acid Synthesis Inhibitors

• Amber: Triasulfuron• Cimarron, Escort: Metsulfuron• Journey: Imazapic + glyphosate:

Chlorosis of New Growth on Tansy Mustard

Plateau Injury Symptoms

Stunting

Chlorosis ofyoungest tissue

EPSP Inhibitors(Glyphosate)

• Tightly adsorbed and inactive in soil

• Phloem translocated

• Inhibits EPSP enzyme responsible for production of aromatic amino acids phenylalinine, tyrosine and tryptophan

• Very nontoxic

Gradual Death from Roundup Treatment

Glyphosate (Roundup) Injuries

chlorosisshortened internodes

stem proliferation

Roundup on AzaleaYellowing of new growth

Strapped leaves on a maple due to glyphosate

Mimics 2,4-D and other hormone-like herbicides

Questions, so far?

Photosynthesis Inhibitors• Control annual or perennial grasses or broadleaves

• Shut down the photosynthetic process

• Slow starvation of the plant

• However, the plant experiences a more rapid death be due to the production of secondary toxic substances

• Injury symptoms: yellowing (chlorosis) of leaf tissue followed by death (necrosis) of the tissue

Photosynthesis Inhibitors• Controls big sage, shinnery

and other oaks, tarbush and creosote bush

• Sagebrush thinning and brush sculpting programs

• Rangeland, pastures, clearings for wildlife and other non-cropland areas

tebuthiuron

Photosynthesis Inhibitors• Injury symptoms:

– Only occur after the cotyledons and first leaves emerge

(do not prevent seedlings from germinating or

emerging)

– yellowing (chlorosis) of leaf tissue followed by death

(necrosis) of the tissue

– Older and larger leaves affected first: they take up

more of the herbicide-water solution as they are the

primary photosynthetic tissue of the plant

Injury from PSII herbicideNote the interveinal chlorosis

Cell Membrane Disruptor

• Postemergence contact herbicides

• Little soil activity

• Activated by exposure to sunlight to form oxygen compounds such as hydrogen peroxide

• These oxygen compounds destroy plant tissue by rupturing plant cell membranes

• Perennial weeds usually regrow because there is no herbicide movement to underground root or shoot systems

Cell Membrane Disruptor

• Controls weeds in just 24 to

48 hours

• Broad-spectrum and non-

selective control of grasses,

broadleaf weeds and sedges

• Cheatgrass, kochia, Russian

thistle, annual mustards

• No residual effect

Cell Membrane Disruptor, Injuries

Rapid browning (necrosis) of plant tissue

Paraquat injury on corn leaves

Why Understand Herbicide Mode of Action?

• Better understanding of how herbicides perform

• Improve herbicides performance

• Diagnosing herbicide injury

• Prevent and manage herbicide resistance

Herbicide Resistance is NOT due to:

1. Sprayer skips or

plugged nozzles

2. Weather problems that

cause poor control

3. Plants that are ‘naturally tolerant’ to the

herbicide

4. Genetic changes caused by the herbicide

Herbicide Resistance is:The ability of a plant to survive and

reproduce after treatment with a dose

of herbicide that would normally kill

the plantBanvel-

resistant kochia

Where do Resistant Weeds Come From?

One in one million, billion, trillion….?

It’s all about selection…..

Herbicide Resistance

• Selection intensity– Herbicide efficacy

• Length of soil residual period

– Number of herbicide applications / year

Selection Pressure is Affected by:

Herbicide Quality

“Better” herbicide = more

chance of resistance

Is herbicide resistance a

problem in range and wildlands?

Resources• University of Minnesota:

– Herbicide Mode of Action and Injury Symptoms (http://www.extension.umn.edu/distribution/cropsystems/DC3832.html)

• Kansas State University:

– Herbicide Mode of Action – (http://www.oznet.k-state.edu/library/crpsl2/c715.pdf)

• Montana State University:

– Preventing and Managing Herbicide-resistant Weeds in Montana (http://www.montana.edu/wwwpb/pubs/mt200506.html)

Questions?