Along with other pesticides , herbicides help keep our green spaces safe for sports and they play an important safety role in industrial settings — for example, by keeping telephone and power lines free from damaging weed growth. This white-flowered weed can grow up to six feet tall and has been found in Quebec, British Columbia and Ontario — including in gardens and along roadsides in Toronto and Ottawa. Herbicides can help control its spread in urban areas and prevent its toxic sap causing painful burns and rashes on humans.
Herbicides have proven to be the most effective way to combat the weed with the least environmental impact. Farmers can also grow herbicide-tolerant soybeans and corn, which were introduced in Canada in 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. Use herbicides only when necessary, only at recommended rates and times of application, and only for those crops and uses listed on the label. 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. 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. 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. 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.
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:. 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 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. 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. 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:. 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. 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. 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. 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. 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. 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 LD 50 , 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. The probable lethal dose of a highly toxic herbicide for a pound person is a few drops to 1 teaspoon.
The probable lethal dose of a moderately toxic herbicide for a pound person is 1 teaspoon to 1 ounce. The signal word on the label reads "Warning.
The probable lethal dose of a slightly toxic herbicide for a pound person is 1 ounce to 1 pint or 1 pound. The signal word on the label reads "Caution. The probable lethal dose of an almost nontoxic herbicide for a pound person is more than 1 pint or 1 pound.
Prepared by Dwight D. Lingenfelter, agronomy extension associate in weed science, and Nathan L. Hartwig, professor emeritus of weed science. Let's Stay Connected. By entering your email, you consent to receive communications from Penn State Extension.
View our privacy policy. Thank you for your submission! Home Introduction to Weeds and Herbicides. Introduction to Weeds and Herbicides. Weeds are plants whose undesirable qualities outweigh their good points. In This Article. What Are Weeds and Their Impacts? There are numerous definitions of a weed. Some common definitions include: a plant that is out of place and not intentionally sown a plant that grows where it is not wanted or welcomed a plant whose virtues have not yet been discovered a plant that is competitive, persistent, pernicious, and interferes negatively with human activity No matter which definition is used, weeds are plants whose undesirable qualities outweigh their good points, at least according to humans.
Characteristics of Weeds There are approximately , species of plants worldwide; of those, about 3 percent, or 8, species, behave as weeds. Weeds possess one or more of the following characteristics that allow them to survive and increase in nature: abundant seed production rapid population establishment seed dormancy long-term survival of buried seed adaptation for spread presence of vegetative reproductive structures ability to occupy sites disturbed by humans Abundant Seed Production Weeds can produce tens or hundreds of thousands of seeds per plant, while most crop plants only produce several hundred seeds per plant.
The following are some examples of approximate numbers of seeds produced per weed: giant foxtail, common ragweed, purslane, lambsquarters, pigweed, Since most weeds deposit their seeds back to the soil, seed numbers in the soil increase rapidly from year to year if the weeds are not managed. Rapid Population Establishment Most weeds can germinate and become established relatively quickly.
Seed Dormancy Dormancy is basically a resting stage or a temporary state in which the weed seeds do not germinate because of certain factors.
Long-Term Survival of Buried Seed If conditions are adequate, buried weed seeds have the potential to remain viable for 40 years or more. Adaptation for Spread Weeds have certain mechanisms for easy dispersal of seeds. Vegetative Reproductive Structures Most perennial weeds possess special vegetative structures that allow them to reproduce asexually and survive.
These perennial structures contain carbohydrates food reserves, sugars , have numerous buds in which new plants can arise, and include the following: stolons --aboveground, horizontal stems that root at the nodes e. Ability to Occupy Disturbed Sites Weeds are very opportunistic. Problems with Weeds Weeds are troublesome in many ways. Costs of Weeds Weeds are common on all million acres of U.
Benefits of Weeds Despite the negative impacts of weeds, some plants usually thought of as weeds may actually provide some benefits, such as: stabilizing and adding organic matter to soils providing habitat and feed for wildlife providing nectar for bees offering aesthetic qualities serving as a genetic reservoir for improved crops providing products for human consumption and medicinal use creating employment opportunities Weeds have a controversial nature.
Weed Ecology and Biology Only about 40 percent of the weeds found in the United States are native, while the remaining 60 percent are considered exotic or imported. Origin of Weeds Weeds are found throughout the world. The following are some examples of weeds and their origins: United States --common and giant ragweed, common milkweed, fall panicum, common cocklebur, poison ivy, marestail horseweed , nightshade, wild or common sunflower, and wild onion South America --pigweed species and prickly sida Europe --quackgrass, chickweed, Canada thistle, common lambsquarters, common purslane, wild garlic, and yellow foxtail Asia or Africa --Johnsongrass, wild carrot, giant foxtail, velvetleaf, kudzu, and witchweed Questions and Answers Regarding Concerns about Nonnative, Invasive Plants Adapted from Swearingen, J.
What Are Native Species? What Are Invasive Plants? How Are Invasive Plants Introduced? How Do Invasive Plants Spread? Annuals Annuals are generally divided further into summer annual and winter annual weeds. Biennials Biennial weeds grow from seed anytime during the growing season. Perennials Perennial weeds live for more than two years and can be divided into two groups: simple and creeping. Weed Management Techniques Since weeds are so prevalent in many areas of the landscape, management techniques are necessary to maintain order.
Prevention Preventative methods are used to stop the spread of weeds. Cultural Cultural and crop management techniques provide a healthy crop to best compete with weeds.
Mechanical Mechanical or physical techniques either destroy weeds or make the environment less favorable for seed germination and weed survival. Tine weeders and cultivators can be used to control weed seedlings. Biological Biological weed control involves the use of other living organisms, such as insects, diseases, or livestock, for the management of certain weeds.
Chemical Herbicides can be defined as crop- protecting chemicals used to kill weedy plants or interrupt normal plant growth. Herbicide mode and Site of Action To be effective, herbicides must: adequately contact plants be absorbed by plants move within the plants to the site of action without being deactivated 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.
Table 2. Important herbicide groups and examples for agronomic and horticultural crops, turf, forestry, and industrial areas in Pennsylvania. Certain active ingredients may have other trade names or be contained in prepackaged mixtures. 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. 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.
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. 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.
Injury Symptoms Growing points are killed first, resulting in the death of the leaves' inner whorl. 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.
Seedling Growth Inhibitors Shoot Herbicides in this class are most ef- fective on annual grasses and yellow nutsedge. Injury Symptoms Germinating grasses normally do not emerge. Photosynthesis Inhibitors mobile These herbicides are effective primarily on annual broadleaves, while some provide control of grasses as well.
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. 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.
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. Injury Symptoms All contact herbicides cause cellular breakdown by destroying cell membranes, allowing cell sap to leak out. Pigment Inhibitors These herbicides provide control of many annual broadleaves and some grasses.
Injury Symptoms Symptoms are very evident and easy to identify. Phosphorylated Amino Acid Nitrogen metabolism Disrupters This herbicide provides broad-spectrum control of most annual grasses and broadleaves and some perennials. Injury Symptoms Injury is similar to that of the cell membrane disrupter herbicides. Unknown Herbicides This category contains miscellaneous products for which the mode of action and family are unknown. Herbicide Resistance A number of weed species that were once susceptible to and easily managed by certain herbicides have developed resistance.
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.
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.
Emulsifiable concentrates EC or E Liquid formulations with an active ingredient that is dissolved in one or more petroleum-based solvents. Emulsifiable gels EG or GL Herbicides that traditionally are emulsifiable liquids formulated as gels. Many herbicides that are used in one year of a typical cropping rotation have the potential to damage crops grown in other years of that rotation.
Fortunately, in an average year these herbicides are broken down degraded into harmless compounds by a combination of biological and chemical processes and do not cause any problems for the crops. From the moment a herbicide enters the soil system, it begins to break down. Most of this degradation is due to the action of microscopic living organisms. These organisms break down the organic material in the soil to provide the energy they need to live it is their food supply.
When a herbicide is added to their environment, it also becomes a potential food supply. Some herbicides are also prone to chemical reactions which alter their structure and render them non-phytotoxic. These degradation processes all depend upon soil temperature and moisture levels. They increase in direct proportion to soil temperature, and begin when the moisture level rises above the wilting point.
Extra moisture increases the rate of degradation but not by as much as higher temperatures. All other things being equal, imidazolinones will be more persistent on acid soils and sulphonyl ureas on alkaline soils. Triazines are very slightly more persistent on alkaline soils. Any paddock where any of these products were applied in June or later especially in a year with low rainfall after application should be regarded as having potentially damaging levels of residues, and so should areas of earlier applications if there were significant periods when the soil surface was dry.
Triazine residues will vary in the damage they cause according to the seasonal conditions. Residue effects will be much less when the season start is uniform and rainy compared to dry. Root disease will exacerbate the effect of triazine residues as the young seedlings cannot grow away from the residues which are concentrated in the cultivation layer.
Sulfonyl urea and imidazolinone residues are less affected by soil moisture as they are more soluble. The first effect of the sulfonyl ureas is to prune roots. Be careful in duplex soils that have sand over alkaline clay.
The sulfonyl ureas can leach down to the clay where they will be more persistent due to the high pH. Herbicide drift is the movement of pesticide away from the target area in the atmosphere.
The three main forms of drift are droplet drift, vapour drift and particulate drift. Droplet drift is the main cause of off-target damage. Spray emerging from a boom breaks up into droplets of varying size. Larger droplets fall onto the target area, while the smallest droplets may remain in the air.
Because droplet drift usually disperses as it moves away from the sprayed area, the type of crop damage it causes in adjoining areas is easily recognised. That part of the sensitive crop which is closest to the sprayed area is severely damaged but damage decreases away from the severe zone. Vapour is produced by evaporation from the droplets when they leave the boom and from the target surface after spraying.
Like droplets, vapour disperses rapidly as it is carried away from the target area. The vapour will remain suspended in the air unless the contaminated air is forced back to ground level where it may damage growing plants. Vapour can drift for long distances, and the characteristic feature of vapour drift damage is that no clear damage gradients can be seen. Figure 1 and Table 1 present the ten herbicides most used on agricultural land in the U.
Glyphosate and atrazine were applied to more than double the crop field acreage than the third leading herbicide, 2,4-D, in Herbicides may cause biological impairments of water bodies if they occur in water or sediment at sufficient concentrations. Most commonly, they enter surface water in runoff or leachate, but, because they have relatively low toxicity to fish and invertebrates see Table 2.
Acute toxicity is likely only when they are deliberately or accidentally applied directly to water bodies. Direct applications may result in direct toxicity to non-target plants and animals or indirect effects due to the death and decomposition of plants.
Impairments also are more likely when herbicides are applied together or with other pesticides Streibig et. Atrazine reacts synergistically with chlorpyrifos: the mixture was seven times more toxic to an earthworm species than the two individual pesticides Lydy and Linck Atrazine also increased the effects of other pesticides in mosquito larvae and various flies Belden and Lydy , Lydy and Linck The surfactants used in herbicide solutions also can be toxic to biota and are not considered when testing active ingredients Folmar et al.
Herbicides are addressed in this module as proximate stressors. Herbicides should be a candidate cause when human sources and activities, site observations or observed effects support portions of the causal pathways see Figure 2. The conceptual diagram and other information also may be useful in Step 3: Evaluate Data from the Case.
Rather than causing direct toxicity to organisms, herbicides may contribute to other stressors e. In such cases, herbicides can be considered as part of the pathway for the proximate cause of impairment. The checklist below will help you identify key data and information useful for determining whether to include herbicides among your candidate causes. This list is intended to guide you in collecting evidence to support, weaken or eliminate herbicides as a candidate cause.
For more information on specific entries, go to the When to List tab. Figure 2. A simple conceptual diagram, depicting pathways from sources to impairments, related to herbicides. Click on diagram to enlarge. Consider listing herbicides as a candidate cause when the following sources and activities, site evidence and biological effects are present:. Forestry management practices, agricultural operations, and urban development and maintenance are all sources of herbicides that may enter surface waters and cause impairments.
Herbicides are applied to forests after harvesting to suppress brush and noncommercial trees. For that use, the rate of application may be high and exposed streams are more likely to be of higher quality than agricultural or urban streams. Conversely, agricultural operations may contribute large quantities of herbicides because they may apply herbicides multiple times per year and they may be applied by planes, addition to irrigation water or spraying onto crops see Figure 3.
Figure 3. Foliar application of herbicide on row crops. Urban land uses can contribute as homeowners and managers of parks, golf courses and other lawns use herbicides for aesthetic enhancement. Herbicides also are used on rights of way for roads, pipelines, railroads and electrical transmission lines and for control of plants in cracks in pavements.
Such urban and suburban uses are likely to contaminate storm waters. Herbicides also are directly applied to waters to control vegetation in ponds, ditches, irrigation canals and recreational waters. Such applications are sources of exposure at the point of application and downstream.
Figure 4. Agricultural ditches can transport herbicides from fields to receiving waters. Evidence of the presence of herbicides at toxic levels includes dead, deformed, chlorotic or necrotic plants, or the absence of plants from a waterbody or the riparian zone see Figure 4. Irrigation ditches and row crop farming near streams provide opportunities for herbicides to enter streams. Lakes and reservoirs used for recreation are often treated for macrophyte control as well.
Although herbicides in general have lower toxicity to animals than other pesticides, fish or invertebrate kills may be a sign of herbicide use.
For example, acrolein has been applied to irrigation ditches at levels sufficient to be acutely lethal to fish and invertebrates see acrolein in U. EPA , and if not properly applied to fields it can cause kills in receiving waters.
Kills also may be due to low dissolved oxygen DO concentrations resulting from plant materials decomposing in water. Because herbicides tend to affect plants more quickly and severely than animals, the most useful biological sign of herbicides is effects on aquatic plants Kreutzweiser et al. This trait may help distinguish the biological effects of herbicides from those of insecticides and most other toxic chemicals.
Secondary effects of herbicides are mediated by low DO concentrations from plant decomposition and changes in trophic structure due to plant community changes.
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