Herbicides can be classified several ways, including by weed control spectrum, labeled crop usage, chemical families, mode of action, application timing/ method, and others.

For this publication, herbicides will be grouped according to mode and site of action, which are also important in understanding herbicide resistance in weeds.

Contact herbicides kill only the plant parts contacted by the chemical, where-as systemic herbicides are absorbed by the roots or foliage and translocated (moved) throughout the plant. Herbicide activity can be either selective or non-selective. Selective herbicides are used to kill weeds without significant damage to desirable plants. Nonselective herbicides kill or injure all plants present if applied at an adequate rate.

Boom Sprayer

Herbicides provide a convenient, economical, and effective way to control weeds.

Herbicide mode and Site of Action

To be effective, herbicides must (1) adequately contact plants, (2) be absorbed by plants, (3) move within the plants to the site of action without being deactivated, and (4) reach toxic levels at the site of action. The term “mode of action” refers to the sequence of events from absorption into plants to plant death, or, in other words, how an herbicide works to injure or kill the plant. The specific site the herbicide affects is referred to as the “site or mechanism of action.” Understanding herbicide mode of action is helpful in knowing what groups of weeds are killed, specifying application techniques, diagnosing herbicide injury problems, and preventing herbicide-resistant weeds.

A common method of grouping herbicides is by their mode of action. Although a large number of herbicides are available in the marketplace, several have similar chemical properties and herbicidal activity. Herbicides with a common chemistry are grouped into “families.” Also, two or more families may have the same mode of action, and thus can be grouped into “classes.” Table 2 lists several groups of herbicides and information related to their mode of action.

The following section provides a brief overview of herbicide functions in the plant and associated injury symptoms for each of the herbicide classes found in Table 2.

Table 2. Important herbicide groups and examples for agronomic and horticultural crops, turf, forestry, and industrial areas in Pennsylvania.
Mode of Action (Class) Site of Action WSSA Group Family Active Ingredient Trade Name(s)*
*Only selected trade names appear. Certain active ingredients may have other trade names or be contained in prepackaged mixtures.
Plant growth regulators (PGR)
IAA like 4 phenoxy 2,4-D various
MCPP (mecoprop)

4 benzoic acid dicamba Banvel, Clarity, Distinct, Vanquish
caroxylic acid (pyridines)
Stinger, Lontrel, Transline
auxin transport inhibitors 19 phthalamates
component of Distinct, Status
Amino acid biosynthesis inhibitors ALS enzyme
Glean, Telar
Permit, Sandea, SledgeHammer
Autumn, Equip
Cimarron, Escort
Matrix, Resolve
Harmony Gt
triazolopyrimidine (sulfonamides)
EPSP enzyme
amino acid derivative (glycines)
Roundup, Touchdown, Accord, Honcho, many others
Fatty acid (lipid) biosynthesis inhibitors ACCase enzyme 1 aryloxyphenoxy propionates
Acclaim, Puma
Assure II
Select, Prism
Poast, Vantage
Seedling growth inhibitors (root and shoot) microtuble inhibitors 3
Sonalan, Curbit
Prowl, Pre-M, Pendulum
Barricade, Endurance
Treflan, Tri-4
benzoic acids
cell wall biosynthesis inhibitors 20 nitriles dichlobenil
21 benzamides isoxaben
Seedling growth inhibitors (shoot) unknown
acetochlor Harness, Surpass, Topnotch
Frontier, Outlook
Dual, Pennant
lipid synthesis inhibitors
Eptam, Eradicane
cell division inhibitors
Photosynthesis inhibitors (mobile 1) photosystem II
5 triazines ametryn
atrazine Atrazine
Milo Pro
Lexone, Sencor
(mobile 2) photosystem II 7 ureas diuron
Photosynthesis inhibitors (nonmobile; "rapid-acting")
photosystem II
benzothiadiazoles bentazon Basagran
phenyl-pyridazines pyridate Tough
Cell membrane disrupters
PPO enzyme
diphenyl ethers
Reflex, Flexstar
Authority, Spartan
photosystem I
Gramoxone, Boa
Pigment inhibitors (bleaching) diterpenes (carotenoid biosynthesis) 13 isoxazolidinones
4-HPPD enzyme 27
triketones mesotrione Callisto
tembotrione Laudis
topramezone Impact
Phosphorylated amino acid (N-metabolism disrupters)
GS enzyme 10
amino acid derivatives (phosphinic acids)
Liberty, Finale, Rely, Ignite
Unknown ?
? dazomet
pelargonic acid
cinnamon oil

citric acid

clove oil
corn gluten mean

thyme oil

vinegar (acetic acid)

Plant Growth Regulators (PGRs)

These herbicides are effective on annual and perennial broadleaf plants and usually have no activity on grasses or sedges, except at high application rates. They produce responses similar to those of natural, growth-regulating substances called auxins. Application of artificial auxins, such as 2,4-D, upsets normal growth as follows:

  • Cells of leaf veins rapidly divide and elongate, while cells between veins cease to divide. This results in long, narrow, strap-like young leaves.
  • Water content increases, making treated plants brittle and easily broken.
  • Cell division and respiration rates increase, and photosynthesis decreases. Food supply of treated plants is nearly exhausted at their death.
  • Roots of treated plants lose their ability to take up soil nutrients, and stem tissues fail to move food effectively through the plant.

The killing action of growth-regulating chemicals is not caused by any single factor but results from the effects of multiple disturbances in the treated plant.

Injury Symptoms

Broadleaf plant leaves become crinkled, puckered, strap shaped, stunted, and malformed; leaf veins appear parallel rather than netted, and stems become crooked, twisted, and brittle, with shortened internodes. If injury occurs in grasses (e.g., corn), new leaves do not unfurl but remain tightly rolled in onion-like fashion, and stems become brittle, curved, or crooked, with short internodes. A lesser effect in corn is the fusion of brace roots, noticed later in the season.

Amino Acid Biosynthesis Inhibitors

These herbicides are effective mostly on annual broadleaves, while a few in this large group have activity on grasses, nutsedge, and/or perennial plants. (Glyphosate [Roundup], for example, is a broad-spectrum herbicide and has activity on all types of plants.) These herbicides work by interfering with one or more key enzymes that catalyze the production of specific amino acids in the plant. When a key amino acid is not produced, the plant’s metabolic processes begin to shut down. The effect is like that of an assembly line worker not doing his or her job. Different herbicides affect different enzymes that catalyze the production of various amino acids, but the result is generally the same—the shutdown of metabolic activity with eventual death of the plant.

Injury Symptoms

Plants that are sensitive to these herbicides stop growth almost immediately after foliar treatment; seedlings die in two to four days, established perennials in two to four weeks. Plants become straw colored several days or weeks after treatment, gradually turn brown, and die.

Fatty Acid (Lipid) Biosynthesis Inhibitors

These herbicides are rapidly absorbed by grasses and are translocated to the growing points, where they inhibit meristematic activity, stopping growth almost immediately. They have no activity on broadleaf plants and are most effective on warm-season grasses such as Johnsongrass, shattercane, corn, fall panicum, giant foxtail, and crabgrass. Cool-season grasses such as quackgrass, annual and perennial ryegrass, orchardgrass, timothy, and small grains are not as sensitive as the warm-season grasses. Some of these herbicides are weaker on perennial species than other products. They are frequently referred to as “post-grass” herbicides.

Injury Symptoms

Growing points are killed first, resulting in the death of the leaves’ inner whorl. Older, outer leaves of seedlings appear healthy for a few days, and those of perennials for a couple of weeks, but eventually they also wither and die. After several weeks, the growing points begin to rot, allowing the inner leaves to be pulled out of the whorl. Sensitive grasses commonly turn a purplish color before dying.

Seedling Growth Inhibitors (Root and Shoot)

Herbicides in this group prevent cell division primarily in developing root tips and are effective only on germinating, small-seeded annual grasses and some broadleaves.

Injury Symptoms

Seeds of treated broadleaved plants germinate, but they either fail to emerge or emerge as severely stunted seedlings that have thickened, shortened lower stems, small leaves, and short, club- shaped roots. Seedlings of tap-rooted plants, such as soybeans and alfalfa, are usually not affected, nor are established plants with roots more than a couple of inches deep.

Grass seeds germinate, but generally fail to emerge. Injured seedlings have short, club-shaped roots and thickened, brittle stem tissue. Seedlings die from lack of moisture and nutrients because of the restricted root system.

Seedling Growth Inhibitors (Shoot)

Herbicides in this class are most ef- fective on annual grasses and yellow nutsedge. They are sometimes referred to as “pre-grass” herbicides. Depending on the product, some will control small-seeded annual broadleaves. These herbicides cause abnormal cell development or prevent cell division in germinating seedlings. They stop the plant from growing by inhibiting cell division in the shoot and root tips while permitting other cell duplication processes to continue. Then follows a slow decline in plant vigor.

Injury Symptoms

Germinating grasses normally do not emerge. If they do, young leaves fail to unfold, resulting in leaf looping and an onion-like appearance. The tip of the terminal leaf becomes rigid, not free flapping (flag like). The leaves of broad-leaved plants turn dark green, become wrinkled, and fail to unfold from the bud. The roots become shortened, thickened, brittle, and club like.

Photosynthesis Inhibitors (mobile)

These herbicides are effective primarily on annual broadleaves, while some provide control of grasses as well. Photosynthesis-inhibiting herbicides block the photosynthetic process so captured light cannot be used to produce sugars. In the presence of light, green plants produce sugar from carbon dioxide and water. Energy is needed for carbon, hydrogen, and oxygen atoms to rearrange and form sugar. To supply this necessary energy, electrons are borrowed from chlorophyll (the green material in leaves) and replaced by electrons split from water. If chlorophyll electrons are not replaced, the chlorophyll is destroyed and the plant’s food manufacturing system breaks down. The plant slowly starves to death due to lack of energy.

As soil-applied treatments, these herbicides permit normal seed germination and seedling emergence, but cause seedlings to lose their green color soon afterward. With the seeds’ food supply gone, the seedlings die. These herbicides are more effective on seedling weeds than on established perennial weeds. Herbicides such as prometon (Primitol) and tebuthiuron (Spike) are considered soil sterilants. Soil sterilants are nonselective chemicals that can kill existing vegetation and keep the soil free from vegetation for one or more years.

Injury Symptoms

In broadleaved plants, early seedling growth appears normal, but shortly after emergence (when energy reserves in cotyledons are depleted), leaves become mottled, turn yellow to brown, and die. In most cases, the oldest leaves turn yellow on the leaf margins first, the veins remain green, and eventually the plant turns brown and dies. Herbaceous and woody perennials starve very slowly because they have large energy reserves in roots or rhizomes to live on while photosynthesis is inhibited. The herbicide may have to effectively inhibit photosynthesis for a full growing season to kill trees or brush. This kind of death may be slow, but it is certain.

Photosynthesis Inhibitors (Nonmobile; “Rapid-Acting”)

Herbicides in this group have activity on primarily annual and some perennial broadleaves and are applied to the plant foliage. The mode of action is the same as the mobile photosynthesis inhibitors.

Injury Symptoms

Their activity within the plant is similar to that of the mobile photosynthesis inhibitors, except the injury occurs at the site of contact, causing “leaf burning” and eventual death of the plant.

Cell Membrane Disrupters

These herbicides control mostly broadleaves. Certain products have some activity on grasses, and paraquat (Gramoxone) provides broad-spectrum control of many different species.

These herbicides are referred to as contact herbicides and they kill weeds by destroying cell membranes. They appear to burn plant tissues within hours or days of application. Good coverage of the plant tissue and bright sunlight are necessary for maximum activity. The activity of these herbicides is delayed in the absence of light.

Injury Symptoms

All contact herbicides cause cellular breakdown by destroying cell membranes, allowing cell sap to leak out. Effected plants initially have a “water-soaked” appearance, followed by rapid wilting and “burning,” or leaf speckling and browning. Plant death occurs within a few days.

Pigment Inhibitors

These herbicides provide control of many annual broadleaves and some grasses. These products are referred to as “bleachers” since they inhibit carotenoid biosynthesis or the HPPD enzyme by interfering with normal chlorophyll formation.

Injury Symptoms

Symptoms are very evident and easy to identify. Effected plants either do not emerge or emerge white or bleached and eventually die. Older leaf tissue is affected first.

Phosphorylated Amino Acid (Nitrogen metabolism) Disrupters

This herbicide provides broad-spectrum control of most annual grasses and broadleaves and some perennials. It affects growth by disrupting nitrogen metabolism, thus interfering with other plant processes. It is a contact herbicide with slight translocation throughout the plant. Good spray coverage and sunlight are important for maximum efficacy.

Injury Symptoms

Injury is similar to that of the cell membrane disrupter herbicides. Sensitive plants show “leaf burning,” yellowing and browning, and eventual death after a week or so. Perennials generally take longer for symptoms and death to occur.

Unknown Herbicides

This category contains miscellaneous products for which the mode of action and family are unknown. Dazomet (Basamid) and metam (Vapam) are considered soil fumigants. These products are applied to the soil and covered with a gas-tight tarp; there, they are converted to gases and penetrate the soil to kill weeds, diseases, and nematodes. Endothall (Aquathol) is used for aquatic weed control. Fosamine (Krenite) is used in noncrop areas to control perennial weeds and brush. Other compounds such as pelargonic acid (Scythe), a fatty-acid herbicide, and clove oil and vinegar are contact, nonselective, broad-spectrum, foliar-applied products that are sometimes used for weed control in organic crop production settings. However, because they basically “burn” only the plant tissue they contact, there is potential for plant regrowth.

Selective Herbicides

Selective herbicides control weeds without causing injury to the crop or other desirable plants.

Herbicide Resistance

A number of weed species that were once susceptible to and easily managed by certain herbicides have developed resistance. These weeds are no longer controlled by applications of previously effective herbicides. As a result of repeatedly using a certain type of herbicide on the same land, many different species of weeds have developed resistance to these chemicals. Currently, about 180 weed species (more than 300 weed biotypes) worldwide are resistant to about ten different herbicide families. It is believed that within any population of weeds, a few plants have sufficient tolerance to survive any herbicide that is used. Since only the survivors can produce seed, it is only a matter of time until the population of resistant weeds outnumbers the susceptible type. Depending on the herbicide family and weed species, resistance can occur within 5 to 20 years. Certain precautions, such as tank-mixing, crop rotations, and a combination of weed management techniques, must be taken to prevent resistance.

Growers, consultants, and those working with herbicides to manage weeds should know which herbicides are best suited to combat specific resistant weeds. The Weed Science Society of America (WSSA) developed a grouping system to help with this process. Herbicides that are classified as the same WSSA group number kill weeds using the same mode of action. WSSA group numbers can be found on many herbicide product labels and can be used as a tool to choose herbicides in different mode of action groups so mixtures or rotations of active ingredients can be planned to better manage weeds and reduce the potential for resistant species. Refer to Table 2 (pp. 12–14) for WSSA mode of action group numbers and corresponding herbicides.

Times of Application

The following terms describe herbicides based on when they are applied:

  • Preplant incorporated: applied to soil and mechanically incorporated into the top 2 to 3 inches of soil before the crop is planted
  • Preplant: applied to soil before the crop is planted
  • Preemergence: applied after the crop is planted but before it emerges
  • Postemergence: applied after crop emergence

Although these terms normally refer to application in relation to crops, they may also imply application in relation to weeds. Always be certain whether reference is being made to the crop or to the weed. In no-till situations, it is possible for an herbicide application to be preplant or preemergence to the crop but postemergence to weeds. Some herbicides must be preplant or preemergence to the weed for maximum activity.

Methods of Application

The following terms refer to the ways herbicides can be applied:

  • Broadcast: applied over the entire field
  • Band: applied to a narrow strip over the crop row
  • Directed: applied between the rows of crop plants with little or no herbicide applied to the crop foliage
  • Spot treatment: applied to small, weed-infested areas within a field

Product Formulations

Herbicides are not sold as pure chemicals, but as mixtures or formulations of one or more herbicides with various additives. Adjuvants (surfactants, emulsifiers, wetting agents, etc.) or various diluents may increase the effectiveness of a pure herbicide. The type of formulation determines toxicity to plants, uniformity of plant coverage, and stability in storage. Herbicides are formulated to permit uniform and easy application as liquid sprays or dry granules.

Some everyday household products are formulated similarly to herbicide products. These similarities will be noted in the sections below.

Emulsifiable concentrates (EC or E) are liquid formulations with an active ingredient that is dissolved in one or more petroleum-based solvents. An emulsifier is added to cause oil to form tiny globules that disperse in water. The formulation then will mix readily with water for proper application. Emulsifiable concentrates usually contain between 2 and 8 pounds of active ingredient per gallon. Dual II Magnum, Pennant, Acclaim, and Prowl are generally emulsifiable herbicide formulations. (Household product with similar formulation—Pine-Sol.)

Emulsifiable gels (EG or GL) are herbicides that traditionally are emulsifiable liquids formulated as gels. The gels typically are packaged in water-soluble bags (WSB) and are stable at temperatures ranging from –20 to 500°C. In addition, the gelling process reduces the need for nonaqueous solvents, compared to standard nonaqueous EC-type formulation processes. Currently, few herbicides are formulated as gels.

Wettable powders (WP or W) are finely ground, dry particles that may be dispersed and suspended in water. They contain from 25 to 80 percent active ingredient. Suspensions of wettable powders appear cloudy. Wettable powders are nearly insoluble and require agitation to remain in suspension. Atrazine, Kerb, and Dacthal are formulated as wettable powders. (Household products with similar formulation—cocoa mix and flour.)

Soluble liquid (S) and soluble powders (SP) dissolve in water to form a true solution. Once the soluble liquid or powder is dissolved, the spray mixture requires no additional mixing or agitation. Few herbicides are available as solubles because most active ingredients of herbicides are not very soluble in water. 2,4-D amine and Roundup are examples of soluble liquid herbicide formulations. (Household products with similar formulation—grape juice concentrate and Kool-Aid mix.)

Dry flowables (DF), also called water-dispersible granules (WDG or WG) or dispersible granules (DG) are wettable powders formed into prills so they pour easily into the sprayer tank without clumping or producing a cloud of dust. Nearly insoluble, they require agitation to remain in suspension. Many herbicides are now formulated in this fashion. Atrazine, Accent, Gallery, and Pendulum are examples of products formulated as water-dispersible granules. (Household products with similar formulation—grits and dry milk.)

Flowables (F or FL), suspension concentrates (SC), and aqueous suspension (AS) are finely ground, wettable powders or solids already suspended in a liquid carrier so they can be poured or pumped from one tank to another. They usually contain at least 4 pounds of active ingredient per gallon of formulation. Flowables are nearly insoluble in water and require agitation to remain in suspension. Suspoemulsion (SE) is a combination formulation of an SC and an oil-based emulsion (E). Atrazine, Princep, and Callisto are formulated as flowables or SCs. (Household products with similar formulation—Pepto-Bismol and V8 vegetable juice.)

Microencapsulated (ME or MT) and capsule suspension (CS) herbicides are en- cased in extremely small capsules that can be suspended in a liquid carrier and pumped and applied with normal equipment. Microencapsulated formulations are nearly insoluble in water and require agitation to remain in suspension. Micro-Tech, Prowl H2O, and Command are formulated in microcapsules, allowing the active ingredient to be slowly released over a period of time. This extends the soil activity and improves weed control. (Household product with similar formulation—older versions of Contac cold capsules.)

Granules (G) are formulated with a premixed carrier that contains a low per- centage of active ingredient. The carrier may be fertilizer, clay, lime, vermiculite, or ground corn cobs. These herbicides are applied directly (dry) to the soil without further dilution. The performance of granulated herbicides compared with that of sprayable formulations varies with the herbicide. Granular forms generally require more rainfall for activation than do sprayable formulations. Granule herbicides are used often in turf and ornamental settings. Some examples include Balan and Ronstar. (Household products with similar formulation—cat litter and Grape-Nuts cereal.)

Pellets (P) are like granules but are compressed into larger cylinders about 1⁄4 inch long. Herbicides formulated as pellets usually contain from 5 to 20 percent active material and are hand-applied to control clumps of brush. They also may be applied with cyclone-type spinner spreaders mounted on helicopters or aircraft to control brush in forests or permanent pastures. Pellets gradually break down from rainfall and leach into the soil for root uptake. Spike is an example of a pelleted herbicide. (Household product with similar formulation—guinea pig/ rabbit pellets.)

Premixes are not formulations, but two or more herbicide active ingredients mixed into one product by the manufacturer. The actual formulation can be any of those discussed above and commonly combines two or more herbicides that are already used together. The primary reason for using premixes is convenience. Many herbicide products are now marketed as premixes.

Trade Name and Formulation Notations

In certain publications, many herbicides are listed by trade name (or product name) and formulation (for example, Roundup 4S or Accent 75WDG). Roundup is the trade name, and 4S stands for 4 pounds of active ingredient (glyphosate) per gallon of product in a soluble (S) formulation. Accent is formulated as a water-dispersible granule with each granule (or certain unit) containing 75 percent active ingredient (nicosulfuron). The remaining parts of the formulation contain inert ingredients, which have no effect on weed control. Additional information about formulation and ingredients can be found on the product’s label and MSDS sheet.

Herbicide Spray Additives (Adjuvants)

Additives or adjuvants are substances in herbicide formulations or that are added to the spray mixture to improve herbicidal activity or application characteristics. More than 70 percent of all herbicides recommend using one or more adjuvants in the spray mixture. In general, there are two types of adjuvants: formulation and spray. Formulation adjuvants are “already in the container” from the manufacturing process. These help with mixing, handling, effectiveness, and providing consistent performance.

Effect of a Surfactant

Effect of a surfactant on the spread and penetration of spray solution across and through the leaf surface.

Spray adjuvants can be divided into special purpose adjuvants and activator adjuvants. Special purpose adjuvants include compatibility agents, buffering agents, antifoam agents, drift retardants, and others that widen the range of conditions for herbicide use. Activator adjuvants are commonly used to enhance postemergence herbicide performance by increasing herbicide activity, absorption, and rainfastness and by decreasing photodegradation. These include surfactants (i.e., “surface active agents”), crop oil concentrates, vegetable oil concentrates, wetting agents, stickers-spreaders, N-fertilizers, penetrants, and others. Commonly used surfactants are nonionic surfactants and organosilicones and are typically used at a rate of 1 quart per 100 gallons (0.25 percent v/v) of spray mixture. Crop oil concentrates are 80 to 85 percent petroleum based plus 15 to 20 percent surfactant, while vegetable oil concentrates contain vegetable or seed oil in place of petroleum oil. Oil concentrates are typically included at a rate of 1 gallon per 100 gallons (1 percent v/v) of spray mixture. In general, oil concentrates are “hotter” than surfactants, so they provide better herbicide penetration into weeds under hot/dry conditions, but they are more likely to cause greater crop injury under normal growing conditions. Nitrogen fertilizers, such as UAN (a mixture of ammonium nitrate, urea, and water) or AMS (ammonium sulfate), are used in combination with surfactants or oil con- centrates to increase herbicide activity and/or reduce problems with hard water. Many blended adjuvants are available that include various combinations of special purpose adjuvants and/or activator adjuvants.

Be sure to include the proper adjuvant(s) for the herbicide being used. Most herbicide labels specify the type and amount of additive to use. Failure to follow the recommendations can result in poor weed control or excessive crop injury.

Mixing and Applying

Be aware that improper sprayer calibration, nonuniform application, calculation errors, or use of the wrong chemicals can cause herbicide injury to the crop.

Apply only the recommended amount of herbicide. Slight increases in rates could result in crop injury or leave residues that might injure succeeding crops.

Recalibrate sprayers frequently to adjust for increased output resulting from normal nozzle wear. Be sure there is sufficient agitation in the sprayer tank to prevent settling of wettable powders, dry flowables, or flowables.

Do not stop in the field with the sprayer on, spill herbicide when loading, or dump unused herbicides into anything except a holding tank.

Take the following steps when mixing herbicides:

  • Always be sure the sprayer has been calibrated properly for application at recommended rates.
  • Calculate the amount of herbicide to add to the sprayer tank based on the active material in each gallon of herbicide concentrate or the percentage of active ingredient of dry herbicide formulation.
  • Read and follow the instructions on the manufacturer’s label pertaining to personal hazards in handling.
  • Fill the sprayer tank with at least half the volume of water or fertilizer solution you will ultimately need.
  • Start with moderate agitation and keep it going.
  • Add compatibility agents, ammonium sulfate, or other mixing adjuvants, if needed. For maximum benefit, they must be in the solution before herbicides are added. (To determine pesticide compatibility, see the next section.)
  • If tank-mixing different types of herbicide formulations and adjuvants, be sure to add them in the following order:
    1. Add, mix, and disperse dry herbicides (wettable powders, dry flowables, or water-dispersible granules). These formulations contain wetting and dispersing agents that aid in mixing.
    2. Add liquid flowables and mix thoroughly. Liquid flowables also contain wetting and dispersing agents.
    3. Add emulsifiable concentrates or microencapsulated herbicides and mix thoroughly.
    4. Finish by adding water-soluble formulations (2,4-D amine, etc.).
    5. Add any adjuvants (surfactants, crop oil concentrates, drift inhibitors, etc.) last. Crop oils, especially, do not mix and disperse well if added first.
    6. Add the remainder of water or liquid fertilizer and maintain agitation through spraying procedure until tank is empty.

Caution: Never mix concentrated herbicides in an empty tank. Never allow a sprayer containing mixed chemicals to stand without agitation because heavy wettable powders may clog nozzles or settle into corners of the sprayer tank where they are difficult to remove.


Pesticides are not always compatible with one another or with the water or liquid fertilizer carrier. Lack of compatibility may result in the formation of a gel, precipitate, or sludge that plugs up screens and nozzles. However, extreme incompatibility may produce a settling out of material that can harden like concrete in the bottom of the tank and in hoses, pumps, and other internal parts of the sprayer. The result may be total loss of the pesticide and use of the sprayer.

Herbicides may be combined with liquid fertilizers to minimize trips over the field. However, little information exists concerning the compatibility of herbicides with specific fertilizer solutions. Herbicide-fertilizer solution combinations may form a gel or precipitate that settles to the bottom of the sprayer tank or will not flow through the sprayer equipment. Incompatibility of tank mixtures is more common with suspensions of fertilizers and pesticides.

Tank-mixing several pesticides, although convenient, may create other problems. Foliar activity may be enhanced and could result in crop leaf burn or the reduction in activity of one or more of the pesticides (“antagonism”).

To prevent the main water tank or liquid fertilizer measuring tank from becoming contaminated, commercial applicators may want to mix the herbicides and other ingredients in a separate holding tank. The herbicide mixture is then sucked into the main line as the truck tank is being filled, and thorough mixing is provided by the truck’s agitation system. Compatibility problems are more likely to result when concentrated herbicides are mixed together, so a compatibility test should be done before new mixtures are tried.

Use only labeled tank mixtures or mixtures recommended by experienced scientists whose recommendations are backed by research. For all unlabeled tank mixtures, a jar test for compatibility is strongly recommended. The compatibility of herbicide-fertilizer combinations should be tested before large batches are mixed. In some cases, adding a compatibility agent (Blendex, Combine, Unite, or comparable adjuvant) may aid in maintaining component dispersion.

The following “two-jar test” procedure may be used to test the compatibility of herbicides with one another, or herbicides and other pesticides with liquid fertilizers. Should the herbicide-carrier mixture prove compatible in this test procedure, it may be applied to the field. The following test assumes a spray volume of 25 gallons per acre. For other spray volumes, make appropriate changes to the ingredients.

  1. Add 1 pint of carrier (water or liquid fertilizer) to each of two one-quart jars. (Note: Use the same source of water that will be used for the tank mix and conduct the test at the same temperature the spray mixture will be applied.)
  2. To one of the jars, add 0.25 teaspoon (1.2 ml) of compatibility agent. To both jars, add the appropriate amount of pesticide(s), in their relative proportions, based on recommended label rates. If more than one pesticide is used, add them separately with dry formulations first, flowables next, and emulsifiable concentrates last. After each addition, shake or stir gently to thoroughly mix.
  3. When all ingredients are added, put lids on and shake both jars for 15 seconds and let stand for 30 minutes or more. Then inspect the mixture for flakes, sludge, gels, heavy oil films, or other signs of incompatibility.
    • If, after standing for 30 minutes, the components in the jar containing no compatibility agent are dispersed, the herbicides are compatible and no compatibility agent is needed.
    • If the components are dispersed only in the jar containing the compatibility agent, the herbicide is compatible only if a compatibility agent is added.
    • If either mixture separates but can be remixed readily, the mixture can be sprayed as long as good agitation is used.
    • If the components are not dispersed or show signs of incompatibility in either jar, the herbicide-carrier mixture is not compatible and should not be used.

Herbicide Selectivity

Were it not for the fact that most herbicides can be applied just before crop planting or emergence, and even over the top after crop emergence without excessive injury, herbicides would be of little value. Most of the herbicides labeled for use today will selectively remove most of the weeds without injuring the crop. Selectivity is accomplished primarily by two methods: selectivity by placement and true selectivity.

Selectivity by Placement

Selectivity accomplished by avoiding or minimizing contact between the herbicide and the desired crop is called selectivity by placement. An example is wiping or directing an herbicide such as glyphosate on a weed without exposing the desired plant. Selectivity by this means is as good as any, as long as the excess herbicide is not washed off the weeds and leached into the root zone where it might be absorbed by the root. Selectivity by placement also is accomplished when an herbicide that does not readily leach is applied to the soil surface for control of shallow-rooted weeds, but does not leach into the root zone of a more deeply rooted crop such as fruit trees or established alfalfa.

True Selectivity

Selectivity that is true tolerance as a result of some morphological, physiological, or biochemical means is referred to as true selectivity. The herbicide can be applied to the foliage of the crop or to the soil in which the crop is growing without danger of injury. Although true tolerance may be the best type of selectivity, it is not perfect. Such things as crop growth stage, cuticle thickness, hairiness of the leaf surface, location of the growing point, air temperature and humidity, spray droplet size, and the surface tension of spray droplets all can influence herbicide activity. When conditions are ideal for herbicide activity, even true selectivity may not adequately prevent crop injury.

Morphological differences include plant characteristics such as size and orientation of the leaf, waxiness or hairiness of the leaf surface, location of the growing point, and rooting depth. Generally, the more waxy or hairy the leaf surface, the more difficult it is for a foliar-applied herbicide to penetrate. The more protected the growing point (as in grasses), the less likely it is that foliar herbicides will reach the growing point. The more deeply rooted the crop is, the more difficult it is to get a soil applied herbicide to the crop roots and the less likely that there will be sufficient uptake for injury.

Physiological differences can include various processes that affect the activity and/or the breakdown of the herbicide. In certain situations, herbicides may be

  • transported differently across the plasma lemma,
  • translocated differently within the plant,
  • combined with some component within the cell wall,
  • integrated with something in the cell cytoplasm, or
  • channeled into “sinks” where the herbicide will have no effect.

These factors all can contribute to tolerance, but any one factor will seldom provide tolerance by itself.

Metabolic factors include genetic insensitivity due to an altered site of herbicide action that prevents herbicide activity. For example, Roundup Ready soybeans produce an excess of the enzyme that glyphosate (Roundup) normally inhibits, so Roundup Ready soybeans are not affected, even though normal amounts of the herbicide are absorbed by the crop plant. Corn plants metabolize and convert atrazine to an innocuous metabolite so rapidly that the herbicide does not have time to inhibit photosynthesis, which provides crop tolerance as long as the metabolic system is not overwhelmed by an excess of the pesticide or a combination of pesticides. In the case of corn treated with an organo-phosphate insecticide and followed with a post treatment of Accent, Beacon, or some other ALS-inhibiting herbicide, both the insecticide and herbicide are being metabolized by the same pathway. This pathway is unable to rapidly metabolize both the herbicide and insecticide, so corn injury may result. Metabolic insensitivity and/or the ability to metabolize the herbicide usually are the best types of true tolerance.