Herbicides

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

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 and 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. Lambsquarters Weeds -Annual Plants • 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.

5. 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

6. Weeds - Perennial Plants Bermudagrass • 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 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 Yellow nutsedge • 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 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 Farmer cultivating beans • 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) 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 • Nonselective: 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 • 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)

15. 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

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

17. 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: inhibit root and shoot growth; imazapyr (Arsenal) and imazaquin (Scepter) • Sulfonylureas: inhibit root and shoot growth; nicosulfuron (Accent) and primisulfuron (Beacon)

19. 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)

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

22. 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. (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.