Safe Herbicide Use

Use herbicides only when necessary, only at recommended rates and times of application, and only for those crops and uses listed on the label.

Safe Herbicide Use

Correct use is essential to ensure that chemical residues on crops do not exceed the limits set by law. Recommended herbicides do not generally injure people, livestock, wildlife, or crops if used properly and if recommended precautions are observed. However, any herbicide is potentially dangerous if improperly handled or used.

Follow these basic pesticide safety procedures:

  • Make sure that you are familiar with current federal and state pesticide laws and regulations and that you have a license, if required.
  • Avoid drift of spray or dust that may endanger other crops or animals. Cover feed pans, troughs, and watering tanks in livestock areas; protect beehives.
  • To protect yourself and others, follow all safety precautions on the label. Know and observe the general rules for safe pesticide use, and record the date, time, location, and amount of each pesticide used.
  • Wear protective clothing and use protective equipment according to instructions on the pesticide label.
  • Never eat, drink, or smoke while applying pesticides.
  • Avoid spilling spray materials on skin or clothing. If such an accident occurs, wash immediately with soap and water.
  • Bathe after applying pesticides and change into freshly laundered clothing. Wash clothing after applying pesticides, keeping in mind that, until laundered, such clothing must be handled according to the same precautions as the pesticide itself. Wash pesticide-contaminated clothing apart from other laundry, and take care in disposing of the wash water.
  • Store pesticides in their original containers in a locked, properly marked cabinet or storeroom, away from food or feed.
  • Do not store herbicides with other pesticides; avoid the danger of cross-contamination.
  • Be sure to triple-rinse all empty containers before recycling (in a special recycling program only through the Pa. Dept. of Agriculture; this is different from typical household curbside recycling programs) or disposing of them in an approved landfill.
  • If you suspect poisoning, contact your nearest Poison Control Center, hospital emergency room, or physician. Take the pesticide label and, if possible, the MSDS sheet with you and give it to the attending physician.

Safety Equipment

Always wear the proper safety equipment when working with herbicides or other pesticides.


When used properly and in accordance with the use restrictions on the product’s label, herbicides sprayed on plants usually are not toxic to livestock. Animals can be poisoned by consuming unused herbicides left in open containers or by drinking water contaminated with herbicides.

Certain unpalatable or poisonous plants treated with herbicides may become more attractive as forage to livestock. Make sure livestock cannot get to poisonous plants that have been sprayed with herbicides.

The nitrate content of several kinds of weeds may increase after they have been sprayed with 2,4-D, Clarity, or similar herbicides. Livestock grazing on these treated plants may become ill. Remove all animals from sprayed areas for several days, or until it has rained or the weeds have died.

Game and Fish

Controlled spraying may benefit wildlife by maintaining desirable cover. Herbicides recommended for control of aquatic weeds usually have beneficial results for fish populations. Be sure to properly apply these herbicides. Do not drain or flush equipment where chemicals may wash into ponds or streams, and do not leave open containers where curious animals might find them.

Crop Safety

Farmers are occasionally concerned about possible herbicide injury to crops. Most injuries of this kind are caused by misuse, contaminated equipment, or drift. Unfavorable weather conditions combined with herbicide residues from a previous crop planting can potentially injure crops.

Cleaning Contaminated Equipment

Sprayer cleanout is necessary to prevent crop injury from spray contamination and to preserve the life of the sprayer. Cleaning is very important, especially when using a sprayer in different types of crops. Many herbicides, even at low concentrations, may have the potential to injure crops for which they are not labeled. Sprayers used to apply 2,4-D–type herbicides can be used to apply other chemicals before crops are planted or before crop plants emerge, but this equipment must be thoroughly cleaned before applications are made on emerged crops (except grasses). Ester formulations are harder to remove than amine or salt formulations.

The following cleaning procedure is recommended for all herbicides unless the label specifies a different cleaning procedure:

  1. Add one-half tank of fresh water and flush tanks, lines, booms, and nozzles for at least 5 minutes using a combination of agitation and spraying. Rinsate sprayed through the booms is best sprayed onto croplands to avoid accumulation of pesticide-contaminated rinsate. Thoroughly rinse the inside surfaces of the tank, paying particular attention to crevices and plumbing fixtures.
  2. Fill the tank with fresh water and add one of the cleaning solutions below, or a commercially available tank cleaner, and agitate the solution for 15 minutes. Add one of the following to each 50 gallons of water to make a cleaning solution: (a) 2 quarts of household ammonia (let stand in sprayer overnight for growth regulator herbicides such as 2,4-D and Clarity) or (b) 4 pounds of trisodium phosphate cleaner detergent. Operate the spray booms long enough to ensure that all nozzles and boom lines are filled with the cleaning solution. Let the solution stand in the system for several hours or overnight. Agitate and spray the solution onto an area suitable for the rinsate solution.
  3. Add more water and rinse the system again by using a combination of agitation and spraying. Remove nozzles, screens, and strainers and clean separately in a bucket of cleaning agent and water.
  4. Rinse and flush the system once again with clean water.


Drift is the movement of any pesticide through the air to areas not intended for treatment. During application, droplet or particle drift occurs as spray droplets or dust particles are carried by air movement from the application area to other places. Vapor drift takes place after application as herbicides evaporate (volatilize) and yield fumes (gases) are carried on wind currents and deposited on soils or plants in untreated areas.

Drift may injure sensitive crops, ornamentals, gardens, livestock, wildlife, or people and may contaminate streams, lakes, or buildings. It may contaminate crops and cause illegal or intolerable residues. Excessive drift may mean poor performance in the desired spray area because the application rate is lower than expected.

Highly active chemicals present the greatest drift hazard because extremely small amounts can cause severe problems. For example, growth regulator herbicides such as 2,4-D, dicamba, and picloram at a rate of 1 ounce per acre can deform sensitive crops such as tobacco, grapes, or tomatoes.

Vapor drift from Command (clomazone) that has not been incorporated can cause bleaching of chlorophyll in sensitive plants within a quarter mile of application. Vapor drift problems can often be avoided by using nonvolatile formulations. Essentially, no vapor drift hazard is involved in the use of amine formulations of 2,4-D. Soil incorporation of Command and a microencapsulated formulation greatly reduces vapor loss of this herbicide.

Particle drift depends on the size of the particle or droplet, and droplet size depends on pressure and nozzle design. Very small particles of fog or mist present the greatest drift hazard. To minimize particle drift, calibrate equipment to create droplets about the size of light rain. Most nozzles can be adjusted to a pressure that permits droplet formation as a result of surface tension. If nozzles are operated at this pressure, a minimum of mist-sized droplets will be formed. For some nozzles, this pressure may be as little as 15 psi; for others, it may be 30 psi.

The distance particles will drift increases with the height of release. Wind velocities usually are lower close to the ground. Therefore, sprays should be released as close to the soil surface or vegetation as adequate coverage permits.

Pesticide Drift

Pesticide drift is influenced by wind, air temperature, boom height, and spray droplet size.

Drift hazard usually is minimized if prevailing winds are blowing away from sensitive crops, but a sudden shift in wind direction could result in serious damage. If possible, do not apply pesticides when wind speed is greater than 5 mph.

High temperatures increase the loss of volatile herbicides. Esters of 2,4-D rapidly evaporate at temperatures above 800°F. The use of such ester formulations should be restricted to fall, winter, and early spring because sensitive plants are not present and lower temperatures reduce vapor drift hazard.

Drift control should be considered with each pesticide application. You can prevent severe drift problems by

  • using sprayer nozzles especially designed for drift reduction;
  • using low volatile or nonvolatile formulations;
  • using low spray-delivery pressures (15–30 psi) and nozzles with a larger orifice;
  • using drift-inhibiting adjuvants in the spray mixture when spraying under less-than-ideal conditions;
  • using nozzles that allow for lowered boom height;
  • avoiding application of volatile chemicals at high temperatures;
  • spraying when wind speed is low (less than 5 mph) or when the wind is blowing away from areas that should not be contaminated;
  • spraying during the early morning or evening hours when there is usually less wind;
  • leaving border areas unsprayed if they are near sensitive crops.

Evaluating Herbicide Injury

Insects, diseases, severe weather (hail, lightning, drought, flooding), fertilizer burn, and nutrient deficiencies are among the causes of symptoms often attributed to herbicide injury. Cool, wet weather can increase the potential for injury, particularly with preemergence herbicides. When evaluating crop injury, careful consideration of the following will help you diagnose the problem:

  1. 1. What is the pattern in the field of plant injury or uncontrolled weeds?
    • A pattern of injury that starts on one side of an area and diminishes gradually and uniformly away from that area is typical of application drift.
    • A pattern of injury occurring in irregular patches that follow air drainage could indicate herbicide volatilization and movement of vapors.
    • Strips of injured areas or surviving weeds at predictable intervals indicate possible skipping or overlapping application.
    • Poor control at the edges of a field can result from only half coverage by the last nozzle on the boom and/or more sunlight availability along the edge of the field.
    • Injury limited to the end rows or ends of the field is usually due to overlapping applications or high herbicide rates in the turnaround areas at the ends of the rows.
    • A definite break between the normal or uninjured part of the field and the rest of the field usually indicates some major difference in soil type or pH between the two sides.
    • A pattern of obvious overapplication as indicated by bare ground (both crop and weeds killed), followed by improved crop survival and appearance with good weed control, followed by lack of crop injury or weed control, indicates inadequate or poor agitation in the sprayer tank. The evidence is even stronger if this pattern repeats itself at intervals that correspond to each new load.
  2. What is the history of the problem area—fertility program, cropping sequence, land preparation, soil pH, soil texture and organic matter, and seed source?
  3. What was the temperature, moisture, rainfall, and prevailing wind at and immediately following herbicide application?


The residual life or length of time an herbicide persists in the soil is the length of time it can be expected to control weeds. Residual toxicity, if not considered, may injure the next crop planted in a herbicide-treated field.

Inactivation, breakdown, and disappearance of herbicides are influenced by the following factors.

Microbial Degradation

Microorganisms feed on all types of organic matter, including organic herbicides. Microbial degradation is the primary means of herbicide breakdown. Some herbicides are more readily attacked by microorganisms than others, often because of minor differences in chemical structure that permit rapid decomposition in some cases and block decomposition in others. Soil temperature, aeration, pH levels, organic matter, and moisture levels favorable for microbial growth promote rapid herbicide breakdown. Microbial degradation takes place primarily in the top foot of soil, where microbial activity is the greatest.

Chemical Degradation

Herbicides may be inactivated upon reaction with salts, acids, and other substances in the soil. These reactions are affected by the same environmental factors that influence microbial breakdown. Chemical degradation can occur anywhere in the soil profile and is the primary process responsible for herbicide dissipation below the top foot of soil, where microbial activity is limited or nonexistent.


Water moving over the surface of a field or treated area can carry herbicide with it. The greatest loss of herbicide occurs when the herbicide is applied to the soil surface and is washed off by the first rain after application. If the herbicide is incorporated or leached into the soil with light rains or irrigation, most loss occurs only with erosion after the herbicide is adsorbed to soil particles.


Water carries herbicides into and ultimately out of the root zone. The portion lost to leaching depends on soil texture, herbicide solubility, and amount and intensity of rainfall. As a rule, herbicides leach most from sandy soils and least from clay soils or soils high in organic matter.


After application, herbicides may become adsorbed (bound) to clay and organic matter particles. The extent of adsorption increases as the percentage of organic matter and/or clay increases. Adsorption reduces the amount of chemical available to plants and slows leaching. Herbicides are then degraded by various means.


Some herbicides may be rapidly lost as vapors after application. Loss as vapor reduces the persistence of dinitroaniline and thiocarbamate herbicides and Command. The rate of vapor loss is influenced by soil moisture, temperature, and adsorption. Evaporation of herbicides increases as sand content, soil moisture, and soil temperature increase. Incorporation into soil immediately after application reduces this kind of loss.


Sunlight may inactivate herbicides—a factor that may contribute to a decline in effectiveness of unincorporated herbicides such as trifluralin (Treflan) and benefin (Balan). Exposure to light for two or more hours reduces the effectiveness of trifluralin and related herbicides and can be avoided by soil incorporation.

Plant Uptake

Herbicides may be absorbed by plant roots or leaves and inactivated within the plant. This effect generally accounts for a relatively small amount of herbicide removal.

Crop Removal

If a crop is harvested or removed from the treated area before rain has washed the herbicide off the foliage or before the plant has had time to metabolize the residue, the herbicide will be removed with the crop. This seldom happens because herbicides are not commonly used close to harvest. However, if grass clippings are collected shortly after treatment and used to mulch a garden, there may be enough herbicide on the grass to damage the garden plants.


Toxicity usually is measured as LD50 (lethal dose), which is the amount of a toxicant required to kill 50 percent of the test animals. The lower the LD50, the less pesticide it takes to kill the animal. As with any chemical, whether naturally occurring or synthetic, it is “the dose that makes the poison.” Below is a list of the most commonly available herbicides, as well as other commonly used substances, in order of decreasing oral toxicity.

Highly Toxic Herbicides (LD50 < 50 mg/kg)

The probable lethal dose of a highly toxic herbicide for a 150-pound person is a few drops to 1 teaspoon. The label contains the signal words “Danger/Poison” and has a skull and crossbones.

  • metham (Vapam)
  • sodium arsenitea,b

moderately Toxic Herbicides (LD50 = 50 to 500 mg/kg)

The probable lethal dose of a moderately toxic herbicide for a 150-pound person is 1 teaspoon to 1 ounce. The signal word on the label reads “Warning.”

  • bromoxynil (Buctril)
  • caffeine
  • copper sulfate (bluestone)
  • difenzoquat (Avenge)
  • diquat endothall (Aquathol, Des-i-cate)
  • gasoline
  • kerosene
  • nicotine
  • paraquat (Gramoxone)

Slightly Toxic Herbicides (LD50= 500 to 5,000 mg/kg)

The probable lethal dose of a slightly toxic herbicide for a 150-pound person is 1 ounce to 1 pint or 1 pound. The signal word on the label reads “Caution.”

  • aspirin
  • ethyl alcohol
  • sodium chloride (table salt)
  • acetochlor (Harness, Topnotch)
  • acifluorfen (Blazer)c
  • alachlor (Micro-Tech, Lasso)c
  • ametryn (Evik)c, d
  • atrazine (various)d
  • bensulide (Betasan)
  • bentazon (Basagran)
  • butylate (Sutan+)d
  • CAMA (various)
  • clodinafop-propargyl (Discover)
  • clomazone (Command)
  • clopyralid (Stinger, Transline)c
  • cloridazon (Pyramin)
  • cycloate (Ro-Neet)
  • 2,4-D (various)
  • 2,4-DB (Butyrac 200, various)
  • 2,4-DP, dichlorprop (various)
  • dicamba (Banvel, Clarity, Vanquish)
  • dichlobenil (Casoron)
  • diclofop-methyl (Hoelon)
  • dimethenamid (Frontier, Outlook)d
  • diuron (Karmex)d
  • DSMA (various)d
  • EPTC (Eptam, Eradicane)d
  • fenoxaprop-P-ethyl (Acclaim, Puma)d
  • fluazifop-P-butyl (Fusilade)
  • flufenacet (Define)
  • glufosinate (Liberty, Finale, Rely)
  • hexazinone (Velpar)d
  • linuron (Lorox)c
  • MCPA (various)d
  • MCPB (Thistrol)
  • MCPP, mecoprop (various)
  • metolachlor (Dual, Pennant)
  • metribuzin (Sencor, Lexone)
  • molinate (Ordram)
  • MSMA (various)d
  • pebulate (Tillam)
  • pinoxaden (Axial)
  • prometon (Primatol)d
  • prometryn (Caparol)d
  • propachlor (Ramrod)c
  • propanil (Stam, Stampede)
  • pyridate (Tough)
  • quizalofop-P-ethyl (Assure II)
  • sethoxydim (Poast)
  • sodium chlorated sulfentrazone (Authority)
  • tebuthiuron (Spike)
  • terbacil (Sinbar)
  • thiobencarb (Bolero)
  • topramezone (Impact)
  • tralkoxydim (Achieve)
  • triallate (Far-Go)
  • triclopyr (Garlon)d
  • vinegar (acetic acid)

Almost Nontoxic Herbicides (LD50 > 5,000 mg/kg)

The probable lethal dose of an almost nontoxic herbicide for a 150-pound person is more than 1 pint or 1 pound. The signal word on the label reads “Caution.”

  • asulam (Asulox)
  • benefin (Balan)
  • benzsulfuron-methyl (Londax)
  • bromacil (Hyvar X)d
  • chlorimuron-ethyl (Classic)d
  • chlorsulfuron (Glean, Telar)
  • clethodim (Select) DCPA (Dacthal)
  • desmedipham (Betanex)
  • dithiopyr (Dimension)c
  • ethalfluralin (Sonalan)
  • ethofumesate (Prograss)
  • flucarbazone (Everest)
  • flumetsulam (Python)
  • flumiclorac (Resource)
  • fluometuron (Cotoran)
  • fomesafen (Flexstar, Reflex)
  • foramsulfuron (Option)
  • fosamine (Krenite)
  • glyphosate (Roundup, Touchdown, Rodeo, various)
  • halosulfuron (Permit, Sempra)
  • iodosulfuron (Autumn)
  • imazamethabenz (Assert)
  • imazamox (Raptor)
  • imazapic (Cadre, Plateau)
  • imazapyr (Arsenal, Chopper)d
  • imazaquin (Scepter, Image)
  • imazethapyr (Pursuit)
  • isoxaben (Gallery)
  • isoxaflutole (Balance)
  • lactofen (Cobra)
  • metsulfuron-methyl (Cimarron, Escort)
  • mesotrione (Callisto)
  • napropamide (Devrinol)
  • nicosulfuron (Accent)
  • norflurazon (Zorial, Solicam)
  • oryzalin (Surflan)
  • oxadiazon (Ronstar)d
  • oxyfluorfen (Goal)
  • pendimethalin (Prowl, Pendulum)
  • prodiamine (Barricade)
  • picloram (Tordon)
  • primisulfuron-methyl (Beacon)
  • pronamide (Kerb)d
  • prosulfuron (Peak)
  • rimsulfuron (Matrix, Resolve)
  • siduron (Tupersan)
  • simazine (Princep)
  • sodium borated sulfometuron-methyl (Oust)
  • sulfosulfuron (Maverick)
  • thifensulfuron-methyl (Harmony GT)d
  • triasulfuron (Amber)
  • trifluralin (Treflan)
  • tribenuron-methyl (Express)

Dermal response:

  1. Absorbed and poisonous
  2. Causes burns and blisters
  3. Moderately irritating
  4. Mildly irritating

Prepared by Dwight D. Lingenfelter, agronomy extension associate in weed science, and Nathan L. Hartwig, professor emeritus of weed science.

Contact Information

Dwight Lingenfelter
  • Program Development Specialist
Phone: 814-865-2242