Herbicide Mode of Action
Fabián D. Menalled
Cropland Weed Specialist
Dept. Land Resources and Environmental Sciences
Montana 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 species
But 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
Phloem
(Roundup)
Foliar Contact
(Gramoxone)
Xylem and
Phloem
Root Contact
(Treflan)
(Banvel, Tordon)
Xylem
(Spike)
Mode of Action
(Amino Acid Biosynthesis Inhibitors)
Site of Action (EPSPS inhibitor)
Chemical Family (Glyicines)
Active Ingredient (Glyphosate)
Commercial Products (Roundup, Durango)
Mode of Action
• Sequence of events from
absorption of the herbicide
into the plant until the plant
dies
Herbicide Mode of Action
Movement
Absorption
Contact
Site of
Action
Toxicity
CO2 + H2O Sugar + O2
Classification by Mode of Action
1)
2)
3)
4)
5)
6)
7)
8)
Plant Growth Regulators
Amino Acid Biosysthesis Inhibitors
Lipid Biosynthesis Inhibitors
Cell Division Inhibitors
Photosynthesis Inhibitors
Cell Membrane Disrupters
Pigment Inhibitors
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
Examples of PGR
Chemical Family
Phenoxy acetic acids
Common Name
2,4-D
Trade Name
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
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
PGR Symptoms
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
Photo: Kansas State University Extension
Puckered soybeans from Tordon
Tordon 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+)
Examples of Amino Acid Synthesis Inhibitors
Chemical Family
Sulfonylureas
Imidazolinones
Amino acid derivates
Common Name
chlorsulfuron
Trade Name
Glean, Telar
thifensulfuron
Harmony GT
nicosulfuron
Accent
imazamethabenz Assert
imazapic
Plateau
imazamox
Raptor
glyphopste
Roundup, Glyphomax,
Rodeo, and others
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 of
youngest 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
chlorosis
stem proliferation
shortened
internodes
Roundup on Azalea
Yellowing 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 herbicide
Note 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 nonselective 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 plant
Banvel-resistant
kochia
Where do Resistant Weeds
Come From?
It’s all about selection…..
One in one million, billion, trillion….?
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?