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Herbicide Mode-Of-Action Summary

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.

I. Foliar Applied Herbicides

A. Downwardly Mobile Herbicides [Symplastically Translocated (leaf to growing points)]

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).

1. Auxin Growth Regulators

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.

2. Amino Acid Inhibitors (Aromatic)

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.

3. Amino Acid Inhibitors [Branched-chain (AHAS/ALS)]

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.

Imidazolinones

Sulfonylureas

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.

Sulfonanilides

Selective soil or foliar applied for control of annual broadleaf weeds in corn or soil applied treatments in soybeans.

4. Chlorophyll/Carotenoid Pigment Inhibitors

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.

5. Grass Meristem Destroyers (Lipid Biosynthesis Inhibitors)

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.

Aryloxyphenoxypropionates

B. Non Translocated (Contact Herbicides)

Cell Membrane Destroyers

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.

Bipyridyliums

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.

Diphenyl ethers (nitrophenyl ethers)

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.

Other postemergence herbicides

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.

C. Upwardly Mobile Only Herbicides (Apoplastically Translocated)

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.

Triazines

II. Soil Applied Herbicides

Cell Division Inhibitors

Root Inhibitors

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.).

Dinitroanilines (Dinitrobenzenamines)

Miscellaneous Herbicides

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.

2. Shoot Inhibitors

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.

Colorado State University

Contact your local county Extension office through our County Office List.

Russian Knapweed – 3.111

by K.G. Beck * (11/13)

Quick Facts…

  • Russian knapweed is a creeping perennial that reproduces from seed and vegetative root buds.
  • Russian knapweed emerges in early spring, bolts in May to June, and flowers through the summer into fall.
  • Russian knapweed is toxic to horses.
  • The key to Russian knapweed control is to stress the weed and cause it to expend nutrient stores in its root system.
  • The best management plan includes cultural controls combined with mechanical and/or chemical control techniques.
Figure 1. Russian knapweed rosette emergence in early spring.
Figure 2: Russian knapweed shoot and leaves; note hairs and lobed leaves.
Figure 3. Russian knapweed flower; note smooth papery bracts that lack any spines.

Russian knapweed (Acroptilon repens) is a creeping, herbaceous perennial of foreign origin that reproduces from seed and vegetative root buds. Shoots, or stems, are erect, 18 to 36 inches tall, with many branches. Lower leaves are 2 to 4 inches long and deeply lobed (Figure 1). Upper leaves are smaller, generally with smooth margins, but can be slightly lobed (Figure 2). Shoots and leaves are covered with dense gray hairs. The solitary, urn-shaped flower heads occur on shoot tips and generally are 1/4 to 1/2 inch in diameter with smooth papery bracts. Flowers can be pink, lavender or white (Figure 3). Russian knapweed has vertical and horizontal roots that have a brown to black, scaly appearance, especially apparent near the crown.

The weed forms dense, single species stands over time due to competition and allelopathy (biochemicals it produces that inhibit the growth of other plants). A 2008 survey conducted by the Colorado Department of Agriculture showed Colorado with more than 132,400 acres infested with Russian knapweed (Figure 4). Russian knapweed is toxic to horses.

Phenology, Biology and Occurrence

Russian knapweed emerges in early spring, bolts in May to June (elevation dependent) and flowers through the summer into fall. It produces seeds sparingly, approximately 50 to 500 per shoot. Seeds are viable for two to three years in soil. Its primary method of reproduction is from vegetative propagation, with seed of secondary importance. Roots from a recently established plant expand rapidly and may cover up to 12 square yards in two growing seasons.

Russian knapweed is native to southern Ukraine, southeast Russia, Iran, Kazakhstan and Mongolia. It grows on clay, sandy or rocky prairies and sunny meadows; on saline soils; or clay, rocky or sandy shores of lakes and rivers; and on rocky and clay slopes of hills and bottomlands. It is a weed of cultivated land, dry pastures and degraded noncropland (waste places) in its native land. Russian knapweed grows in most western states. In Washington, it is common on heavier, often saline soils of bottomlands and grows in pastures, hayfields, grainfields and irrigation ditches. In Colorado, Russian knapweed is not restricted to certain soils and occurs in pastures, agronomic crops, roadsides, waste places and rangeland. Stands may survive 75 years or longer.

Management

Like other creeping perennials, the key to Russian knapweed control is to stress the weed and cause it to expend nutrient stores in its root system. An integrated management plan should be developed that places continual stress on the weed. Currently, the best management plan includes cultural control combined with mechanical and/or chemical control techniques. A single control strategy, such as mowing or a herbicide, usually is not sufficient.

Russian knapweed typically invades degraded areas, dominating the plant community and desirable plants (e.g. perennial grasses). Seeding competitive, perennial grass species (cultural control) after Russian knapweed has been stressed by other control measures (set-up treatments) is essential. Set-up treatments may include chemical or mechanical methods.

Figure 4. 2008 distribution of Russian knapweed in Colorado.

Cooperative research between Colorado State University and the University of Wyoming showed that chemical set-up treatments were superior to mowing. Curtail (clopyralid + 2,4-D), Escort (metsulfuron), and Roundup (glyphosate) were used to suppress Russian knapweed. Then perennial grasses were sown in late fall as a dormant seeding. Curtail (3 quarts per acre) (A) or Escort (1 ounce/A) were applied at the bud-growth stage. Roundup was applied twice at 1 quart/A, first at the bud-growth stage and again about 8 weeks later. Curtail controlled Russian knapweed best and Roundup failed to control it. None of the herbicides injured seeded grasses. Grasses established similarly among herbicide suppression treatments, even though Russian knapweed control varied. However, where Escort or Roundup was used to suppress Russian knapweed, additional herbicide treatments would be necessary to achieve acceptable control.

Figure 5. Russian knapweed root buds on crowns in fall; note black/brown scaly appearance to root crowns—a key identifying characteristic.
Figure 6. Russian knapweed emerged rosettes in fall.

While two mowings eight weeks apart (first at bud growth stage), suppressed Russian knapweed during that year, the weed recovered vigorously the subsequent growing season. Perennial grasses established in the mowing treatments but much less than in herbicide treatments. Seeding desirable forbs and shrubs also may be effective to prevent Russian knapweed reinvasion, but research is necessary to test this hypothesis.. Two mowings per year for several years may control Russian knapweed better than in our experiments, but further research also is needed to test this hypothesis. Currently no biological control is available for this weed. Tillage often is necessary to overcome the residual allelopathic effects of Russian knapweed, but more recent research shows that an effective herbicide treatment that kills much of the root system also appears to ameliorate allelopathy.

Chemical control. In most circumstances, an herbicide alone will not effectively manage Russian knapweed. However, there may be situations where desirable plants within a Russian knapweed infestation may compete effectively with the weed if it is stressed with a single weed management technique.

Russian knapweed is controlled by Tordon 22K (picloram), Milestone (aminopyralid), Transline (clopyralid), Curtail, (clopyralid + 2,4-D), Perspective (aminocyclopyrachlor + chlorsulfuron), and Telar (chlorsulfuron). Refer to Table 1 for rates and timing recommendations and always read the herbicide label before using the product. Russian knapweed is very susceptible to fall-applied herbicides. It displays a distinct cycle of root bud development. In late summer (August into early September) Russian knapweed begins to develop buds on its roots that will emerge to form rosettes that fall or the following spring (Figures 5 and 6). Root buds continue to grow throughout the winter but once rosettes emerge in spring, remaining root buds slough off and no buds occur on roots until this cycle begins again in late summer. This active root bud growth and development in fall through winter may be the reason that Russian knapweed is susceptible to herbicides applied in fall and winter.

When integrating chemical and cultural control, avoid using herbicide rates that injure grasses because effective competition will be reduced.

Cultural control. Russian knapweed tends to form monocultures and usually eliminates other plants. Therefore, sowing desirable plant species is necessary after the weed is controlled. Smooth brome will compete with Russian knapweed. Research shows that streambank wheatgrass, thickspike wheatgrass, crested wheatgrass and Russian wildrye established after Russian knapweed was suppressed with herbicides. Sod-forming perennial grasses, like streambank or thickspike wheatgrasses, help prevent reinvasion better than bunch grasses like crested wheatgrass. More recent CSU research also shows that slender wheatgrass and western wheatgrass also compete effectively with Russian knapweed after it is suppressed.

If the Russian knapweed stand is not too old and grasses are still present, stimulating grass growth by irrigation (where possible) should increase grass competition with knapweed and keep the weed under continual stress.

Table 1. Herbicide used to control Russian knapweed.
Herbicide Rate (Product/A) Application
timing
Comments
Tordon 2 to 4 pints Spring at bud to mid-flowering growth stages; or late in fall Use higher rates for older or dense stands; late treatments in fall to dormat plants very effective
Milestone 5 to 7 fl oz Spring and summer at bud to flowering growth stages; or late in fall Use higher rate for older stands; late treatments in fall to dormant plants very effective; Milestone may be used to edge of ponds or streams
Transline 1 to 1.33 pints Spring after all shoot have emerged, bud to mid-flower growth stages; late in or fall Use higher rate for older or dense; late treatments in fall to dormant plants very effective
Curtail 3 to 4 quarts Spring after all shoots have emerged, bud to mid-flower growth stages; late in or fall Use higher rate for older or dense; late treatments in fall to dormant plants very effective
Perspective 5.5 oz Spring after shoots have emerged through the fall Late fall treatments into winter when conditions are suitable for spraying is very effective
Telar 1 oz Spring bud to flowering growth state; or late in fall Late treatments in fall to dormant plants are very effective; temporary injury to cool season grasses may occur from fall treatments

* Colorado State University Extension weed specialist and professor, bioagricultural sciences and pest management. 9/98. Revised 11/13.