1. Herbicides Stephen J. Toth, Jr. Wayne G. Buhler
Photograph from University of Illinois
Stephen J. Toth, Jr. Wayne G. Buhler
Department of Entomology Department of Horticultural Science
North Carolina State University North Carolina State University

2. 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 & Dale Monks.

3. 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

4. Weeds - Annual Plants Completes all four stages of growth in one year
Lambsquarters
Weeds - Annual Plants
Completes 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.

5. Weeds - Biennial Plants
Completes 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)
Queen Anne’s Lace
Photograph courtesy of the University of Illinois.

6. 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.

7. Weed Classification - Grasses
Johnsongrass
Have only one leaf as they emerge from seed
Leaves are narrow and upright with parallel veins
Examples are Johnson-grass and crabgrass
Photograph by Dale Monks.

8. Weed Classification - Broadleaves
Cocklebur
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
Photograph by David Monks.

9. 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.

10. 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

11. 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

12. Weed Control Methods
Cultivation - traditional weed 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)
Farmer cultivating beans
Photograph by Bill Tarpenning, USDA/ARS.

13. 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
Non-Selective: herbicide kills most plants, must be applied in absence of desirable plants

14. 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
Pre-Emergence: herbicide is applied after planting, but prior to crop and, generally, weed emergence
Post-Emergence: herbicide application occurs after crop emergence (also can refer to after weed emergence)

15. Inorganic Herbicides
Romans used brine and a mixture of salt and ashes to sterilize land in biblical times (non-selective 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 non-selective 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

16. 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 MSMA (monosodium methanearsonate)
Additional Organic Herbicides:
Numerous classes of herbicides with several modes of actions

17. Acetanilides: inhibit root growth;
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)

18. Organic Herbicides Arranged by Mode of Action: Growth Inhibitors (Soil & Foliar Applied)
Imidazolinones: inhibits root and shoot growth; imazapyr (Arsenal) and imazaquin (Scepter)
Sulfonylureas: inhibits root and shoot growth; nicosulfuron (Accent) and primisulfuron (Beacon)

19. Phosphono amino acids: inhibits shoot growth; glyphosate (Roundup)
Organic Herbicides Arranged by Mode of Action: Growth Inhibitors (Foliar Applied)
Phosphono amino acids: inhibits shoot growth; glyphosate (Roundup)
Phenoxy proprionic acids: inhibits shoot growth; fluazifop-butyl (Fusilade)
Cyclohexones: inhibits root and shoot growth; sethoxydim (Poast)

20. atrazine (Aatrex), metribuzin (Sencor) and simazine (Princep)
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)

21. Benzothiadiazoles: bentazon (Basagran)
Organic Herbicides Arranged by Mode of Action: Photosynthesis Inhibitors (Foliar Applied)
Benzothiadiazoles: bentazon (Basagran)
Phthalic acids: endothall (Aquathol)

22. Bipyridyliums: Paraquat Diphenyl ethers: acifluorfen (Blazer)
Organic Herbicides Arranged by Mode of Action: Cell Membrane Disrupters
Bipyridyliums: Paraquat
Diphenyl ethers: acifluorfen (Blazer)

23. 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)

24. References
Ware, G. W. An Introduction to Herbicides. Radcliffe’s IPM World Textbook. (
Ware, G. W The Pesticide Book. 4th edition. Thomson Publications, Fresno, California. pp
U. S. EPA Agricultural Pest Control - Plant Training Manual. pp