Download Crop Profiles for North Carolina Agriculture

Herbicides
Stephen J. Toth, Jr.
Department of Entomology
North Carolina State University
Wayne G. Buhler
Department of Horticultural Science
North Carolina State University
Photograph from University of Illinois
Weeds
• Compete with desirable plants
for water, nutrients, light and
space
• Contaminate crop at harvest
• Harbor pest insects, mites,
vertebrates or plant disease
agents
• Release toxins in the soil which
may inhibit crop growth
Photographs by David
and Dale Monks.
Stages of Plant Development
• Seedling - small, vulnerable plants
• Vegetative - rapid growth and production of
stems, roots and foliage
• Seed production - energy is directed toward
the production of seed
• Maturity - little or no energy production
Weeds Annual Plants
Lambsquarters
• Complete all four stages of
growth in one year
• There are many annual
weeds, including pigweed,
crabgrass, lambsquarters,
morningglory, cocklebur and
henbit
Tall morningglory
Photographs by Kathy Kalmowitz and David Monks.
Weeds - Biennial
Plants
• Complete the seedling and
vegetative stages of growth in
first year; seed production and
maturity stages are completed
in second year
• Common biennial weed is
wild carrot (also known as
Queen Anne’s lace)
Photograph courtesy of the University of Illinois.
Queen Anne’s Lace
Weeds - Perennial Plants
• May or may not complete all
four stages of growth in first
year, then repeat vegetative,
seed and maturity stages for
several more years
• Seed production and maturity
stages may be delayed for
several years
• Examples: Johnsongrass,
Bermudagrass, pokeweed
Bermudagrass
Photograph by David Monks.
Weed Classification - Grasses
• Have only one leaf as
they emerge from seed
• Leaves are narrow and
upright with parallel
veins
• Examples are Johnsongrass and crabgrass
Johnsongrass
Photograph by Dale Monks.
Weed Classification - Broadleaves
• Have two leaves as
they emerge from seed
• Leaves broad with
netted veins
• Usually have taproot
and coarse root system
• Examples: pigweed,
cocklebur, dandelion
and poison ivy
Cocklebur
Photograph by David Monks.
Weed Classification - Sedges
• Similar to grasses, but have
triangular (3-sided) stems
• Often listed on pesticide
labels as grasses, but
certain herbicides will
control grasses and not
sedges (i.e., Poast)
• Examples are nutsedges
Yellow nutsedge
Photograph by David Monks.
Factors Affecting Weed Control
• Growing points - contact herbicides do not
reach those sheathed or below soil surface
• Leaf shape - herbicides run off of narrow
upright leaves; held by broad, flat leaves
• Wax and cuticle - thick wax/cuticle layer
prevents herbicide entry; waxy surface forms
droplets which run off leaves
• Leaf hairs - dense layer hold droplets away
from leaf; thin layer holds herbicide longer
Factors Affecting Weed Control
• Plant size and age - young, rapidly-growing
plants more susceptible to herbicides
• Deactivation - certain plants can deactivate
herbicides and are less susceptible; basis for
herbicide selectivity, can cause resistance
• Plant life cycle - seedlings very susceptible to
most weed control methods; vegetative and
early bud stages susceptible to translocated
herbicides; timing important
Weed Control Methods
• Cultivation - traditional weed
Farmer cultivating beans
control method; may increase
erosion, spread weeds/diseases
• Planting timing - planting date
delayed to avoid or remove weeds
• Mulching - keeps light from weed
seedlings
• Mowing - often used in orchards;
prevents erosion
• Others - nurse crops, fire and
flooding (in rice)
Photograph by Bill Tarpenning, USDA/ARS.
Herbicide Characteristics
• Foliage-absorbed: herbicide kills all foliage contacted
• Root-absorbed: herbicide is applied to the soil
• Contact: herbicide must be applied directly to weeds, little
or no movement to underground or shaded part of weed
• Translocated: herbicide is absorbed and moves throughout
the living portion of the plant (weed)
• Selective: differences between plant and weed (foliar
characteristics, deactivation) makes herbicide selective
• Nonselective: herbicide kills most plants, must be applied
in absence of desirable plants
Herbicide Characteristics
• Persistent: herbicide remains for weeks or months
• Nonpersistent: herbicide remains for hours or days
• Pre-plant incorporated (PPI): herbicide application made
prior to planting and mixed into the soil
• Preemergence: herbicide is applied after planting, but
prior to crop and, generally, weed emergence
• Postemergence: herbicide application occurs after crop
emergence (also can refer to after weed emergence)
Inorganic Herbicides
• Romans used brine and a mixture of salt and ashes to
sterilize land in biblical times (nonselective herbicides)
• Copper sulfate used in grain fields to kill weeds in 1896
• Sodium arsenite solutions used as herbicides from 1906
to 1960
• Sodium chlorate used for nonselective weed control for
the last 50 years
• A few inorganic herbicides are still in use, but are being
replaced by organic herbicides because of persistence of
inorganic herbicides
Organic Herbicides
Arsenicals:
• Inhibit the growth of weeds
• Not as toxic to mammals (humans) as inorganic forms
of arsenic
• Not used in agriculture as widely as in the past
• Example is monosodium methanearsonate (MSMA)
Additional Organic Herbicides:
• Numerous classes of herbicides with several modes
of actions
Organic Herbicides Arranged by Mode of Action:
Growth Inhibitors (Soil Applied)
• Acetanilides: inhibit root growth;
alachlor (Lasso), metolachlor (Dual)
• Dinitroanilines: inhibit root
and shoot growth; benefin
(Balan), oryzalin (Surflan),
pendimethalin (Prowl) and
trifluralin (Treflan)
• Thiocarbamates: inhibit root
and shoot growth; pebulate
(Tillam), thiobencarb (Bolero)
Organic Herbicides Arranged by Mode of Action:
Growth Inhibitors (Soil & Foliar Applied)
• Imidazolinones: inhibit
root and shoot growth;
imazapyr (Arsenal) and
imazaquin (Scepter)
• Sulfonylureas: inhibit
root and shoot growth;
nicosulfuron (Accent)
and primisulfuron
(Beacon)
Organic Herbicides Arranged by Mode of Action:
Growth Inhibitors (Foliar Applied)
• Phosphono amino acids:
inhibit shoot growth;
glyphosate (Roundup)
• Phenoxy proprionic acids:
inhibit shoot growth;
fluazifop-butyl (Fusilade)
• Cyclohexones: inhibit
root and shoot growth;
sethoxydim (Poast)
Organic Herbicides Arranged by Mode of Action:
Photosynthesis Inhibitors (Soil & Foliar Applied)
• Triazines: atrazine (Aatrex), metribuzin (Sencor) and
simazine (Princep)
• Phenylureas: linuron (Lorox)
• Uracils: terbacil (Sinbar)
Organic Herbicides Arranged by Mode of Action:
Photosynthesis Inhibitors (Foliar Applied)
• Benzothiadiazoles:
bentazon (Basagran)
• Phthalic acids:
endothall (Aquathol)
Organic Herbicides Arranged by Mode of Action:
Cell Membrane Disrupters
• Bipyridyliums: paraquat
• Diphenyl ethers: acifluorfen (Blazer)
Organic Herbicides Arranged by Mode of Action:
Plant Growth Hormones
• Phenoxy acids: affect cellular division and metabolism;
2,4-D and 2,4,5-T
• Benzoics: resemble growth hormones; dicamba (Banvel)
• Pyridinoxy acids: hormone herbicides; picloram (Tordon)
References
• Ware, G. W. An Introduction to Herbicides.
Radcliffe’s IPM World Textbook.
(http://ipmworld.umn.edu/chapters/wareherb.htm)
• Ware, G. W. 1994. The Pesticide Book. 4th
edition. Thomson Publications, Fresno, California.
pp. 103-125.
• U. S. EPA. 1993. Agricultural Pest Control Plant Training Manual. pp. 18-26.