The mode-of-action is the overall manner in which a herbicide affects a plant at the tissue or cellular level. Herbicides with the same mode-of- action will have the same translocation (movement) pattern and produce similar injury symptoms. Selectivity on crops and weeds, behavior in the soil and use patterns are less predictable, but are often similar for herbicides with the same mode-of-action. This publication organizes herbicides into those which are applied to foliage (many of these are applied to soil as well) and those herbicides applied almost strictly to soil. The foliar applied groups are then divided into three categories according to movement through the plant:
Each translocation group is subdivided into mode-of-action groups which are further categorized by herbicide chemistry group. Strictly soil applied herbicides are divided into mode-of-action and then into herbicide chemistry groups.
Plants are complex organisms with well-defined structures in which
multitudes of vital (living) processes take place in well ordered and
integrated sequences. Plants are made up of organs (root, stem, leaf,
and flower); organs consist of tissues (meristems, conducting,
photosynthetic, structural); and tissues are made up of cells. Plant
cells contain subunits including walls, membrane systems (golgi, plasma
membrane, nuclear membrane, endoplasmic reticulum) and organelles
(mitochondria, nucleus, chloroplasts), and undifferentiated cytoplasm.
Some vital metabolic plant processes include photosynthesis (capture of light energy and carbohydrate synthesis), amino acid and protein synthesis, fat (lipid) synthesis, pigment synthesis, nucleic acid synthesis (RNA - DNA essential to information storage and transfer), respiration (oxidation of carbohydrate to provide CO2 and usable energy), energy transfer (nucleic acids) and maintenance of membrane integrity. Other vital processes include growth and differentiation, mitosis (cell division) in plant meristems, meiosis (division resulting in gamete and seed formation), uptake of ions and molecules, translocation of ions and molecules, and transpiration. One or more of the vital processes must be disrupted in order for a herbicide to kill a weed.
Symptoms are evident on new growth first. Pigment loss (yellow or white), stoppage of growth, and distorted (malformed) new growth are typical symptoms. Most injury appears only after several days or weeks. Plants die slowly. Herbicides in this group are usually molecular (non- charged) at low pHs found in the cell walls and negatively charged at higher pHs encountered in the cytoplasm of leaf sieve cells of the phloem (the ionization inside the cytoplasm of the phloem accounts for trapping and movement of these herbicides).
Soil activity varies from almost none to long residual depending on herbicide and dose.
Auxin growth regulator herbicides are used for control of annual, simple perennial, and creeping perennial broadleaves in grass crops (corn, small grains, sorghum, turf, pastures, sodded roadsides and rangeland) and in non-crop situations. All are organic acids which take on a negative charge after ionization of acids and salts. Esters are hydrolyzed to acids or salts in both plants and soils. Injury to off-target vegetation is a major problem associated with these herbicides.
Common Trade Name Name
Phenoxyaliphatic Acid Herbicides
2,4-D 2,4-DB MCPP (mecoprop) MCPA 2,4-DP (dichlorprop)
Benzoic Acids
dicamba BANVEL/ CLARITY/ VANQUISH/ VETERAN
Picolinic Acids (Pyridines) and Relatives
picloram TORDON clopyralid STINGER/ LONTREL triclopyr GARLON/ TURFLON fluroxypyr STARANE
Glyphosate and sulfosate are the compounds with this mode of action. Uses are limited to foliar applications only, since these chemicals are rapidly inactivated in the soil. Symptoms include yellowing of new growth and death of treated plants in days to weeks. These relatively nonselective compounds control annual grasses, annual broadleaves, johnsongrass, quackgrass, yellow nutsedge, cool season pasture and turf grasses, cattail, Canada thistle, hemp dogbane, Jerusalem artichoke, poison ivy, and multiflora rose. Glyphosate tolerant cultivars of soybeans (Roundup Ready), corn, and other crops are currently being marketed. Corn and other glyphosate tolerant crops are being tested for future release.
Common Trade Name Name
glyphosate ROUNDUP ULTRA/ RODEO/ACCORD sulfosate TOUCHDOWN
Several groups of different chemistry have this same mode of action. Shoot meristems cease growth; yellow, pink and purple symptoms appear; roots tend to develop poorly; and the secondary roots are shortened and all nearly the same length producing a "bottlebrush" appearance. Complete symptom development is very slow and requires two to three weeks or more. Late postemergence applications of some of these herbicides used on corn may result in malformed (bottle shaped) ears.
Common Trade Name Name
imazquin SCEPTER imazethapyr PURSUIT imazapyr ARSENAL/ CHOPPER
Sulfonylurea herbicides are applied preplant incorporated, preemergence, and postemergence at doses of 0.5 to 6 ounces active ingredient per acre. This herbicide group provides selective control of wild garlic and Canada thistle in small grains; broadleaf weeds in soybeans; johnsongrass, shattercane, quackgrass and wirestem muhly in corn; and weeds in conifers, hardwoods and pastures. Several compounds are used for general vegetation control on non-crop sites. High soil pH greatly increases persistence since only biodegradation takes place at higher soil pHs. At soil pHs below 6.8, chemical degradation occurs in addition to biodegradation and speeds inactivation. Sulfonylurea tolerant soybeans are available to farmers.
Common Trade Name Name
chlorimuron CLASSIC chlorsulfuron GLEAN/ TELAR nicosulfuron ACCENT primisulfuron BEACON thifensulfuron HARMONY PINNACLE tribenuron EXPRESS sulfometuron OUST metsulfuron ALLY halosulfuron PERMIT/ MANAGE
Selective soil or foliar applied for control of annual broadleaf weeds in corn or soil applied treatments in soybeans.
flumetsulam BROADSTRIKE
Vivid white new growth, sometimes tinged with pink or purple, characterize the symptoms associated with the pigment inhibitors. New growth initially appears normal except for the conspicuous lack of green and yellow pigments. Uses include, selective weed control in soybeans and cotton, poison ivy control, general vegetation control and aquatic weed control.
Amitrole is the only compound of this group which moves well in the symplast, however other compounds in the group show initial movement into shoot tips causing new growth to be devoid of green and yellow pigments.
Common Trade Name Name
clomazone COMMAND amitrole AMITROL-T norflurazon ZORIAL/ SOLICAM fluridone SONAR
All provide the same symptoms on grass species; namely discoloration and disintegration of meristematic tissue at and above the nodes, including nodes of rhizomes. Leaves yellow, redden and sometimes wilt. Seedling grasses tend to lodge by breaking over at the soil. These herbicides have the potential to be used for selective removal of most grass species from any non-grass crop. There is also some selectivity among grass species (particularly with the aryloxyphenoxypropionates in cool season grasses). The grass meristem destroyers should be used early postemergence on seedling grasses, and postemergence but before the boot stage (the seedhead detectable in the top leaf sheath) on established perennial grasses. Mixing with postemergence broadleaf herbicides frequently results in reduced grass control. When used under less than ideal conditions (no-till, open crop canopies and drought) two applications per season are frequently required.
These compounds are more active postemergence (foliar) than soil applied. At normally used postemergence doses, soil activity is marginal or lacking.
Common Trade Name Name
fenoxaprop WHIP/ HORIZON/ OPTION/ ACCLAIM fluazifop-P FUSILADE 2000/ FUSILADE DX quizalofop ASSURE II
Cyclohexanediones
clethodim SELECT sethoxydim POAST/ POAST PLUS
Compounds in this group result in rapid disruption of cell membranes and very rapid kill. The bipyridyliums and the diphenyl ethers penetrate into the cytoplasm, cause the formation of peroxides and free electrons (light is required) which destroy the cell membranes almost immediately. Herbicidal oils dissolve membranes directly. Rapid destruction of cell membranes prevents translocation to other regions of the plant. Severe injury is evident hours after application, first as water-soaked areas which later turn yellow or brown. Maximum kill is attained in a week or less. Partial coverage of a plant with spray results in spotting and/or partial shoot kill. New growth on surviving plants will be normal in appearance. Foliar activity alone can provide only shoot kill.
These foliar applied, strongly cationic, relatively toxic herbicides are used postemergence only. Extremely strong binding to clay prevents activity for weed control or leaching in the soil. Only shoot kill can be expected. Liquids with suspended colloids (muddy water, slurry fertilizers) cause inactivation. These herbicides are used for general shoot kill in numerous situations including burn down in conservation tillage systems and preharvest desiccation. Diquat is used for control of aquatic weeds.
Common Trade Name Name
paraquat GRAMOXONE diquat DIQUAT/REWARD
These herbicides have both foliar and soil activity. They mostly control broadleaves. Acifluorfen is labeled for postemergence applications to soybeans, peanuts, and rice. Fomesafen and lactofen are similar to acifluorfen. Although bronzing or burning of soybean leaf tissue is evident after application, yield is rarely affected. Oxyfluorfen is used preemergence for cole crops and postemergence for mint, onions and conifer nurseries. This herbicide group is relatively unaffected by soil texture and organic matter.
Common Trade Name Name
acifluorfen BLAZER fomesafen REFLEX lactofen COBRA oxyfluorfen GOAL
Glufosinate is applied postemergence for control of annuals prior to crop establishment, for noncrop areas and for selective directed placement in specialty crops (apples, grapes, tree nuts). There is no soil activity. The inhibition of the glutamine synthetase enzyme in the effected plant results in the decrease of several amino acids which eventually leads to cell membrane disruption and death of the cell. Symptoms of the plant include chlorosis (yellowing) followed by necrosis (dead tissue) 3 to 5 days after herbicide application. Glufosinate tolerant cultivars of rice, soybeans, and corn are being tested.
Common Trade Name Name
bentazon BASAGRAN glufosinate IGNITE/RELY/ FINALE/LIBERTY
These herbicides translocate only apoplastically. Movement is upward with the transpiration stream (water moving through the plant from the soil and evaporating into the atmosphere at the leaf surfaces).
Symptoms develop from bottom to top on plant shoots (older leaves show most injury; newer leaves least injury). Chlorosis first appears between leaf veins and along the margins which is later followed by necrosis of the tissue. Any potential control of established perennials must come from continued soil uptake and not movement downward through the plant from the shoots. Foliar activity alone can provide only shoot kill.
Herbicides in these chemical groups have excellent soil activity. Most have foliar activity as well. These herbicides are used preplant incorporated, preemergence, and to a limited extent early postemergence, for selective control of weeds in annual and established perennial crops. Crops include corn, soybeans, potatoes, celery, parsnips, carrots, cotton, alfalfa, asparagus, mint, and woody species. They are also used for brush in pastures, rangeland, and non-cropland and for general vegetation control. Soil persistence varies from weeks to months depending on compound and dose and soil pH. Soil mobility varies from low to high depending on the compound and soil characteristics.
Common Trade Name Name
atrazine AATREX/Atrazines simazine PRINCEP cyanazine BLADEX prometon PRAMITOL metribuzin SENCOR/LEXONE hexazinone VELPAR
Uracils
terbacil SINBAR bromacil HYVAR
Phenylureas
linuron LOROX/LINEX diuron KARMEX tebuthiuron SPIKE
Others (not typical)
bentazon BASAGRAN bromoxynil BUCTRIL pyridate TOUGH/LENTAGRAN
These herbicide groups have little or no foliar activity and are applied mostly preplant incorporated and preemergence for control of seedling grasses and some annual broadleaves in soybeans, peanuts, dry beans, cole crops, cotton, alfalfa, clovers, lettuce, tobacco, herbaceous ornamentals, established turf, and in woody species (nurseries, orchards, grapes, Christmas trees, etc.).
Common Trade Name Name
trifluralin TREFLAN benefin BALAN prodiamine BARRICADE/ ENDURANCE oryzalin SURFLAN pendimethalin PROWL/PENTAGON STOMP/PENDULUM ethalfluralin SONALAN
Siduron is labeled soil applied for seedling grass control in newly seeded or newly sprigged turf and established turf. It removes annual grass competition from spring established turf.
Common Trade Name Name
DCPA DACTHAL siduron TUPERSAN
The shoot inhibitors are soil applied for control of seedling grasses, some broadleaves and suppression of some perennials from tubers and rhizomes. Injury appears as malformed (twisted), dark green shoots and leaves on injured young plants. Grass crops with some tolerance to these compounds can be protected from injury with other chemicals [safeners (protectants)]. Crops include corn, large seeded legumes, small seeded legumes, beets, spinach, tomatoes, potatoes, and ornamentals.
Common Trade Name Name
EPTC EPTAM/ERADICANE butylate SUTAN+ pebulate TILLAM cycloate RO-NEET
Common Trade Name Name
acetochlor HARNESS/SURPASS/ TOPNOTCH alachlor LASSO/MICRO-TECH/PARTNER metolachlor DUAL/DUAL II propachlor RAMROD dimethenamid FRONTIER
Common Trade Name Name
bensulide BETASAN/ BENSULIDE/PREFAR napropamide DEVRINOL pronamide KERB dichlobenil CASORON dithiopyr DIMENSION
References
Herbicide Handbook Seventh Edition. Weed Science Society of America. 1994. Herbicides, absorption and translocation and their relationship to plant tolerance and susceptibility. Pg. 191-214 in S.O. Duke, (ed.). Weed Physiology, Vol. 2, Herbicide Physiology. CRC Press Inc., Boca Raton, FL.
WS-16 Weed Control Guidelines for Indiana WS-22 Herbicide List