What does herbicide do?

Using Herbicide In Gardens – When And How To Use Herbicides

There are times when the only way to get rid of a stubborn weed is to treat it with an herbicide. Don’t be afraid to use herbicides if you need them, but try other control methods first. Pulling, hoeing, tilling and digging will often take care of weed problems without the need for chemical sprays. Let’s learn more about using herbicide in gardens.

What are Herbicides?

Herbicides are chemicals that kill plants or prevent them from growing. Their method of killing plants is as varied as the plants they kill. The first step in understanding herbicides is to read the label. Labels tell you how to use herbicides safely and effectively. It is illegal to use herbicides for any purpose or by any method other than as indicated on the label.

Here are some tips to help you use herbicides safely and effectively:

  • Avoid using herbicides on windy days and near bodies of water.
  • Always wear a protective mask, gloves and long sleeves.
  • Make sure children and pets are indoors when you spray herbicides.
  • Buy only as much herbicide as you need and store it in a safe place, out of the reach of children.

Types of Herbicides

Herbicides can be divided into two main categories: selective and non-selective.

  • Selective herbicides kill certain types of weeds while leaving other plants unharmed. The herbicide label lists the target weeds as well as garden plants that are unaffected.
  • Non-selective herbicides, as the name implies, can kill almost any plant. Selective herbicides are useful when treating weeds in lawns and gardens. Non-selective herbicides make it easy to clear an area when starting a new garden.

Selective herbicides can be further divided into pre-emergent and post-emergent herbicides.

  • Pre-emergent herbicides are applied to the soil, and they kill young seedlings soon after they emerge.
  • Post-emergent herbicides are usually applied to the foliage where they are absorbed into the plant tissue.

The type determines when to apply an herbicide. Pre-emergents are usually applied in late winter or early spring, while post-emergents are applied in spring after the weeds begin to grow.

When using herbicide in gardens, take care to protect the plants you don’t want to kill. If you have identified your weed, you may be able to find a selective herbicide that will kill the weed without harming garden plants. Those containing glyphosate are good herbicides for hard to control plants and unidentified weeds because they kill most plants. Protect the other plants in the garden by making a cardboard collar to fit around the weed before applying the herbicide.

Note: Chemical control should only be used as a last resort, as organic approaches are more environmentally friendly.

Dawn, noon or midnight: when to spray herbicides?

The time of day when you spray often makes a significant difference to herbicide efficacy. That’s one of the overall findings from a recent Alberta project. The sometimes-surprising results are leading the project team to explore the reasons behind the data and to look at a possible tool to help producers make time-of-day spraying decisions.

The idea for this project was sparked by the growing use of night spraying. “GPS-guided autosteer has given farmers the ability to spray during the night. So we wondered, is night spraying as effective as daytime spraying?” says Ken Coles, the general manager of Lethbridge-based Farming Smarter.

Coles led the three-year project (2012 to 2014), which was funded by the Alberta Canola Producers Commission and Alberta Barley Commission. The project compared early morning (4 a.m. to 5 a.m.), midday (noon to 1 p.m.), and night (midnight to 1 a.m.) timings for herbicide applications. This small plot, replicated research project included a pre-seed burndown study and an in-crop study, and involved a range of herbicide groups.

The pre-seed burndown study, which took place at Lethbridge, evaluated herbicide efficacy for controlling natural weed infestations in the plots. The treatments involved glyphosate and several products that were becoming popular as tank mixes with glyphosate (see table).

The in-crop study was carried out at Lethbridge by Farming Smarter, at Bonnyville by the Lakeland Applied Research Association, and at Falher by the Smoky Applied Research and Demonstration Association. The crops included wheat, peas, Liberty Link (LL) canola, and Roundup Ready (RR) canola. They were seeded on two seeding dates each year to try to make sure the herbicide applications would occur in range of weather conditions. Simulated weeds were seeded in the plots: tame mustard for a broadleaf weed, and tame oats for a grassy weed. Herbicides from various groups were compared (see table).

Highlights of findings

“Herbicides are registered to work in a wide range of conditions so I didn’t really expect to see a tremendous difference in efficacy between the different timings. But what I started to see right away was often the early morning application was the least consistent of any timing,” Coles says.

“Having grown up in southern Alberta, I found that particularly interesting because it is almost inbred in our culture that we wake up really early to spray, to beat the wind. But the project’s results show the early morning timing is often coming at a cost for herbicide efficacy.”

In both the burndown and in-crop studies, the most effective timing was usually midday, followed by midnight. Coles says, “Since night spraying was usually more effective than dawn, night spraying could be a good option when daytime opportunities for spraying are limited.”

For canola, Liberty and Roundup (Vantage Plus Max II) usually performed best at midday and worst in the early morning. He notes, “I was surprised that Roundup had a strong time-of-day effect. We expected that for Liberty – it is well known that you should spray Liberty in the heat of the day because it needs the heat to activate it as a contact herbicide.”

The story was somewhat different for wheat and peas. “We didn’t see nearly as strong a correlation with time-of-day on the wheat herbicides; generally, they worked the best under most conditions. The pea herbicides, Odyssey and Select, tended to work better when sprayed at night.”

So Coles suggests a general guideline would be to spray wheat in the early morning, canola in the middle of the day, and peas at night.

Generally, broadleaf weeds tended to be more sensitive to the time-of-day effect than grassy weeds.

On grass control, Coles points to another interesting finding: “Liberty is known for not having the greatest control of grassy weeds, but when we sprayed Liberty at night, its grass kill improved. This type of information could be helpful when dealing with herbicide-resistant weeds. Let’s say you’ve got Group 1 and 2-resistant wild oats and you don’t have a lot of options left to kill them. Spraying Liberty at night on Liberty canola might help keep those resistant wild oats under control.”

Looking into results

The results showed some strong patterns overall, but not every site in every year followed the general trends. To better understand the reasons behind the results, Coles and his team looked closely at the weather data for the trials because of the profound effect weather can have on herbicide efficacy.

“Usually, our conditions were pretty dry, so during the day, the temperature rose and the humidity dropped. Then at night, the temperature dropped and the humidity rose. But, for instance, if we had a rain event with lots of moisture and maybe no wind, then that pattern really got jumbled up. It was harder to predict the results when more moisture was present,” Coles says.

The project team was able to identify the likely causes for some of the plot results that bucked the general trends. For instance, the results suggest that plants stressed by very dry soil conditions might have reduced herbicide translocation, resulting in poorer herbicide efficacy. Moisture stress may also change a plant’s form and structure, causing such things as leaf rolling or thickening of the protective waxy covering on the leaf surface – changes that could reduce the amount of herbicide entering the plant. It’s also likely that a heavy rainfall event shortly after spraying in one of the experiments washed away the herbicides, so those applications were almost totally ineffective.

The link between time of day and changing temperature and humidity conditions got Coles interested in Delta T. “Delta T is the wet bulb temperature minus the dry bulb temperature. The dry bulb is just a regular thermometer, and the wet bulb is essentially a thermometer with a wet sock on it. It measures the evaporative cooling effect, which is the same effect as if you jump out of the shower and stand in front of a fan – you feel cold.”

Delta T can be used to determine if conditions are optimum for spraying. The general guideline is that Delta T should be between two and eight or 10 (the upper limit depends on whether the spray is fine or coarse).

“When I mapped all of the Delta T values for all of our data, the poorest herbicide performance was between zero and two,” Coles says. So he dug a little deeper into Delta T.

According to Coles, very little academic research has been done on Delta T for spraying decisions. And most of that research has been in Australia, where growers use Delta T mainly to determine if conditions are too hot and dry for spraying. As Delta T rises above 10, the air gets very hot and dry, causing spray droplets to evaporate faster and volatile pesticides to vaporize faster, so herbicide effectiveness tends to be reduced.

If Delta T is below two, then the air is very moist. In the Australian literature, the reason given for not spraying when Delta T is below two is that the high relative humidity causes the spray droplets to be very slow to evaporate. So fine droplets tend to last a long time, increasing the risk of spray drift if a temperature inversion occurs.

A temperature inversion is when the air near the ground is cooler than the air above it. To check for an inversion, compare the temperature at the top of the crop canopy with the temperature at about eight to 10 feet above the canopy. The air in an inversion is very stable because the lower, cooler air is denser than the warmer air above. So there’s no vertical movement of air parcels; the airflow is horizontal. Although coarse spray droplets will fall to the surface fairly quickly, fine droplets will take a long time to fall and may float for long distances.

Inversions tend to be strongest and deepest just before sunrise, so they may be a factor in the generally poorer performance of the early morning applications in the project. Coles explains, “We always talk about not spraying when there’s a temperature inversion because of the risk of spray drift. But it’s also possible that inversion issues are resulting in poorer weed control on our own fields – basically not all of our fine droplets are hitting our targets. If that is the case, that might explain why we’re not getting as good control in the early morning: we have less spray coverage, especially with the finer droplets which are sometimes more easily absorbed in plants.”

However, Coles thinks the effects of early morning weather conditions on plant physiology might be even more important than the effects of early morning inversions on spray coverage.

One physiological factor could be that most metabolic process in plants increase with warming temperatures. So, as the day warms up from the relatively cool conditions at dawn, herbicides tend to become more biologically active. (However, if conditions get too hot, then plants will start to reduce their metabolic activity, slowing the rate of translocation and metabolism.)

Coles also suspects evaporative cooling could be important. “Early morning is when we have the highest humidity and the lowest temperatures. Then the wind comes in with really dry air and it dries off the plants really quickly. So my premise is that the evaporative cooling effect is sucking a lot of heat energy out of the plant, which stresses the plant. And a stressed plant is not going to uptake herbicide.”

From his initial look at Delta T, Coles thinks it might be a useful tool to help farmers make time-of-day decisions on herbicide spraying. Small hand-held units are available for measuring Delta T, so it’s easy to do. “It looks like avoiding spraying if Delta T is between zero and two is more important ,” Coles says. “I think I would go somewhat higher than 10 and be comfortable.” However, he’d like to see more research on Delta T, especially on why spraying is less effective when Delta T is below two.

“We don’t have it all figured out, but it has definitely taken us onto an interesting path trying to understand the why,” Coles says. “And the more we know about herbicide effectiveness, the better job we can do at managing herbicides to increase yields, decrease weed seeds in the seed bank, and deal with herbicide resistance before it becomes an even bigger issue.”

When to Apply Weed Control

Product packages usually provide a general guide to application timing. For example, they’ll tell you approximately when you need to apply a crabgrass preventer. However, regional and year-to-year weather variations can make correct timing tricky. Fortunately, there are some easy ways to time your applications to get the most out of them. The best-known method is to time crabgrass preventers for your lawn according to forsythia bloom. This popular landscape shrub is grown for its striking yellow bloom in early spring. When most trees and shrubs are still bare, you can’t miss forsythia when it flowers. When the blossoms start to drop to the ground, it’s time to put down the crabgrass preventer. For garden beds, the timing depends on the primary target weed. If you are dealing with cool season weeds, such as henbit or chickweed, then apply a garden weed preventer as soon as soil can be worked in spring. However, summer weeds like crabgrass, foxtail, or spurge are your bane (as they are for most gardeners) time the application with forsythia bloom, as you would do with a lawn application. Resist the urge to apply too soon; weed preventers have a limited life span in soil, and the sooner you apply them, the sooner they wear out. 8-12 weeks is the typical length of effectiveness.

How to Apply Weed Preventer Most residential weed preventers are granular products, meaning that you apply them with a spreader. It’s important to apply at the rate stated on the product package. Also, granular products usually don’t work well unless they’re watered in with a half-inch or so of water. If no rain is expected in the week or so after application, consider irrigating to activate the weed preventer.

What to Apply to Prevent Weeds For lawns, there are several synthetic weed preventers that work well, and are sold under recognized brand names such as Bayer, Scotts, and many others. These are typically combined with fertilizer so that both can be applied at once, saving you a separate application for fertilizer. For garden beds, these products often are sold strictly as weed preventers (i.e. no fertilizer is included). Preen is one widely used brand for garden beds. An organic weed preventer option is corn gluten, which is nearly as effective as conventional products. Many brands of synthetic weed preventers also have corn gluten weed preventers in their product lines.

Precautions Weed preventers stop seeds from germinating. That means you should not apply them anywhere you want to plant flower seeds during the following 3 months, or in lawns where you’ve just reseeded (or plan to). However, you can use weed preventers in spring and plant seed in fall without any trouble. Remember that weed preventers don’t kill weeds that are already growing. Perennial weeds such as wiregrass, dandelions and many others will return each year unless you physically remove them or spray with a herbicide such as glyphosate (Roundup).

  • By Eric Liskey

What is an herbicide?
An herbicide is a chemical used to kill or inhibit the growth of weeds and other unwanted plant pests. There are two kinds of herbicides. Species-specific herbicides are designed to kill a specific kind of plant only, leaving other desirable plants unharmed. Non-specific herbicides kill every plant they contact.

How do herbicides work?
Every herbicide works in one of two ways. The first way is by contact, wherein the herbicide kills only the parts of the plant they are directly applied to. Herbicides that work through contact are considered fast-acting. The second way is by systemic action, wherein the herbicide is absorbed into the plant either by its roots or waxy surface, and then moves throughout the entire plant shutting down all of its functions. Herbicides that work systemically are slow-acting.

When should you use an herbicide?
An herbicide is appropriate any time that you have pest plants or weeds that are competing with desired plants or crops for space, water, light, and nutrients. They are often used in lawn and garden care to achieve a desired aesthetic effect. They are also used in farming where crop productivity is decreased by certain plant pests.

How are herbicides applied?

Most herbicides come in granule or liquid form to be mixed with water in a one gallon sprayer or two gallon sprayer. The herbicide you choose will have very specific instructions on the label as to how the chemical should be mixed and handled. The label will also tell you how much solution should be used for the size area to be treated. Always read the entire label before applying an herbicide or other lawn and garden product, and pay particular attention to the instructed use of safety clothing such as gloves and eyewear!

Herbicides we recommend:
Roundup QuikPRO – 5 x 1.5 oz. packs
Roundup QuikPRO – 6.8 lb. jug
Ranger Pro Herbicide – 2.5 Gallons

View our entire line of Lawn and Garden Pest Control Products

Related Article:

Weed Control and Herbicides

Additional Resources:

  • Weed Identification – Univeristy of California Statewide IPM Program
  • Broadleaf and Grass Weed Seedling Identification Keys – University of Minnesota Extension Service
  • Encycloweedia – California Department of Food and Agriculture
  • Weed Photos – Weed Science Society of America
  • Invasive and Noxious Weeds – US Department of Agriculture
  • Plant Fact Sheets and Guides – US Department of Agriculture
  • Weed Information – University of California, Davis
  • Creating a Weed Management Plan for Your Organic Farm – Penn State Extension
  • Weed Management on Organic Farms – NC State University Cooperative Extension
  • Paraquat Dichloride: One Sip Can Kill – US Environmental Protection Agency

Lawn Weed Control:

  • Guide to Healthy Lawns – Univeristy of California Statewide IPM Program
  • Weed Management in Lawns – Univeristy of California Statewide IPM Program
  • Control of Annual Grassy Weeds in Lawns – Colorado State University Extension
  • Guidelines for Herbicide Use – Weed Control Handbook – Global Invasive species Team
  • Keep Pets Safe Around Pesticides (p.5-Weeds) – Oregon State University Extension
  • Greenscaping: The Easier Way to a Greener, Healthier Yard – EPA
  • Moss and Algae Control In Lawns – Clemson University Cooperative Extension Service
  • Controlling Moss in Lawns – Oregon State University Extension Service
  • Weed Information – University of California, Davis
  • Lawn and Garden (Videos and Resources) – Environmental Protection Agency (EPA)

Selecting Herbicides:

  • Herbicides – Univeristy of California Statewide IPM Program
  • Weed Control Methods Handbook – Global Invasive Species Team
  • Understanding Herbicides – University of Illinois
  • Weed Susceptibility to Herbicides – University of California, Riverside Extension
  • Choosing the Right Herbicide – University of Tennessee Extension
  • Herbicide Mode-Of-Action Summary – Purdue University Extension
  • Are Herbicides Safe to Use in My Pond? – Oklahoma Cooperative Extension

Alternative Methods:

  • Non-Toxic Weed Control – Bio-Integral Resource Center
  • Hand Weeding – Univeristy of California Statewide IPM Program
  • Corn Gluten – VA Soil and Water Conservation District
  • Weed Control Handbook – Global Invasive Species Team
  • Commonly Encountered Pests and How to Deal With Weeds – MA Department of Agricultural Resources


A herbicide is a chemical used to kill or otherwise manage certain species of plants considered to be pests. Plant pests, or weeds, compete with desired crop plants for light, water, nutrients, and space. This ecological interaction may decrease the productivity and yield of crop plants, thereby resulting in economic damage. Plants may also be judged to be weeds if they interfere with some desired aesthetic effect, as is the case of weeds in lawns.

Clearly, the designation of plants as weeds involves a human judgment. However, in other times and places weeds may be judged to have positive values. For example, in large parts of North America, the red raspberry (Rubus strigosus) is widely considered to be one of the most important weeds in forestry. However, this species also has positive attributes. Its fruits are gathered and eaten by people and wildlife. This vigorously growing plant also provides useful ecological services. For example, it binds soil and helps prevent erosion, and takes up nutrients from the soil, which might otherwise be leached away by rainwater because there are so few plants after disturbance of the site by clear-cutting or wildfire. These ecological services help to maintain site fertility.

Still, it is undeniable that in certain situations weeds exert a significant interference with human purposes. To reduce the intensity of the negative effects of weeds on the productivity of desired agricultural or forestry crops, fields may be sprayed with a herbicide that is toxic to the weeds, but not to the crop species. The commonly used herbicide 2,4-D, for example, is toxic to many broad-leaved (that is, dicotyledonous) weeds, but not to wheat, maize or corn, barley, or rice, all of which are members of the grass family (Poaceae), and therefore monocotyledonous. Consequently, the pest plants are selectively eliminated, while maintaining the growth of the desired plant species.

Modern, intensively managed agricultural systems have an intrinsic reliance on the use of herbicides and other pesticides. Some high-yield varieties of crop species are not very tolerant of competition from weeds. Therefore, if those crops are to be successfully grown, herbicides must be used. Many studies have indicated the shorter-term benefits of herbicide use. For example, studies of the cultivation of maize in Illinois have demonstrated that the average reduction of yield was 81% in unweeded plots, while a 51% reduction was reported in Minnesota. Yields of wheat and barley can be reduced by 25-50% as a result of competition from weeds. To reduce these important, negative influences of weeds on agricultural productivity, herbicides are commonly applied to agricultural fields. As noted above, the herbicide must be toxic to the weeds, but not to the crop species.



Chemical weed control has been used for a very long time: sea salt, industrial by-products, and oils were first employed. Selective control of broad-leaved weeds in fields of cereal crops was discovered in France in the late 1800s, and this practice soon spread throughout Europe. Sulfates and nitrates of copper and iron were used, and sulfuric acid proved even more effective. Application was by spraying. Soon sodium arsenite became popular both as a spray and as a soil sterilant. On thousands of kilometres of railroad right-of-way, and in sugarcane and rubber plantations in the tropics, the hazardous material was used in tremendous quantities, often resulting in the poisoning of animals and occasionally humans.

Sinox, the first major organic chemical herbicide, was developed in France in 1896. In the late 1940s new herbicides were developed out of the research during World War II, and the era of the “miracle” weed killers began. Within 20 years over 100 new chemicals were synthesized, developed, and put into use. Chemical weed control superseded both plant-disease and insect-pest control in economic impact. In particular, the year 1945 was key to the development of selective chemical weed control. Introduced then were 2,4-D (2,4-dichlorophenoxyacetic acid), 2,4,5-T (2,4,5-trichlorophenoxyacetic acid), and IPC (isopropyl-N-phenylcarbamate)—the first two selective as foliar sprays against broad-leaved weeds, the third selective against grass species when applied through the soil.

The new herbicides were revolutionary in that their high toxicity allowed for effective weed control at dosage rates as low as one to two kilograms per hectare (one or two pounds per acre). This contrasted with carbon bisulfide, borax, and arsenic trioxide, which were required at rates of up to 2,242 kilograms per hectare (one ton per acre), and with sodium chlorate, required at rates of around 112 kilograms per hectare (100 pounds per acre). However, some of those early herbicides, including 2,4,5-T, were later deemed unsafe for humans and the environment and were discontinued in many countries. Effective herbicides have continued to be developed, and some, such as glyphosate, are widely used around the world.

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Since the mid-1980s, certain agricultural plants, known as herbicide-resistant crops (HRCs), have been genetically engineered for resistance to specific chemical herbicides, notably glyphosate. These genetically modified organisms (GMOs) enable effective chemical control of weeds, since only the HRC plants can survive in fields treated with the corresponding herbicide. Such crops have been especially valuable for no-till farming, which helps prevent soil erosion. However, because these crops encourage increased application of chemicals to the soil rather than decreased application, they remain controversial with regard to their environmental impact and general safety. In addition, in order to reduce the risk of selecting for herbicide-resistant weeds, farmers must use multiple diverse weed-management strategies.

genetically modified cornGenetically modified corn (maize).© S74/.com

Triazine Herbicides in Conservation Tillage

Triazine herbicides are particularly well suited for conservation tillage because they provide foliar and residual control of a broad spectrum of weeds. Atrazine, simazine, and metribuzin are used in corn, atrazine and propazine in sorghum, metribuzin in soybean, and simazine reduces tillage required for weed control in many perennial and tree crops. Atrazine is also used extensively in chemical fallow cropping systems in rotations involving corn, sorghum, and wheat. Cyanazine was also used extensively in corn and cotton until 2002.

Triazine herbicides such as atrazine and cyanazine are not tightly adsorbed to surface crop residue, allowing rainfall to wash intercepted herbicide into the soil. Low vapor pressures also avoid excessive vapor losses of residue-intercepted triazine herbicides. When atrazine was applied to corn-stalk residue, 52% of the herbicide washed off the stalk residue by the first 0.5 cm of simulated rainfall (Martin et al., 1978). After 3.5 cm of rain, 89% of the intercepted atrazine had washed off the residue. Similarly, in another study (Baker and Shiers, 1989), 75% of applied cyanazine washed off corn-stalk residue with 0.7 cm of simulated rain, and an additional 11% was recovered from the residue.

Even when rainfall washes herbicides intercepted by the crop residue into the soil, some herbicides may be less effective because of altered distribution within the soil. Weeds may germinate under crop residue and escape contact with herbicides as they emerge. If an herbicide must be shoot adsorbed, weeds may not be controlled. Because triazines are root absorbed and relatively stable in the soil, they can kill small weed seedlings after emergence as roots grow to and encounter the herbicides in the soil. This property has made triazines highly popular in conservation tillage, used either alone or in combination with shoot-adsorbed, grass-controlling herbicides. Weeds escaping a shoot-adsorbed herbicide due to interception by crop residue can be controlled by residual activity of the triazines.

With conventional tillage, conditions for weed germination and growth are relatively similar each year; weeds emerging prior to crop planting are killed by tillage and the soil surface is devoid of crop residue. With conservation tillage, differing weather conditions each year have a much greater impact in changing weed germination and growth patterns; weeds germinating prior to planting often must be controlled by herbicides. Stage of growth and species mix of these early germinating weeds vary from year-to-year, depending on weather. Surface crop residue reduces soil temperatures and delays weed seed germination. In the absence of a tillage operation, which stimulates more uniform weed seed germination, weed seed germination in conservation tillage is often delayed and more sporadic (Fawcett, 1987). The triazines are popular in conservation tillage due to their consistent performance under a wide variety of environmental, soil, and surface crop residue conditions and their residual soil activity, which controls late-germinating weeds.

Surface-applied herbicides require timely rainfall to incorporate them into the soil prior to weed germination. Timely rains after application often are more important in no-till systems than with tillage; weeds may have germinated (but not emerged) several days prior to planting and herbicide application, and thereby escape foliar nonselective herbicides. By the time rainfall activates the chemical, these weeds may be too large to control. Mechanical controls such as rotary hoeing and cultivation may be difficult or impossible in no-till due to heavy crop residue. The early preplant herbicide application program was developed to eliminate the weed control limitations of no-till systems and to allow growers more time flexibility to apply herbicides. Using the preplant program, residual herbicides are applied up to several weeks prior to planting, well before most weeds emerge. Early application allows more time for rains to occur before weed germination, reducing chances for dry-weather herbicide failure. Often, the need for a foliar nonselective herbicide, such as paraquat or glyphosate, is eliminated, as weeds are killed before or during emergence.

In an Iowa study at nine locations, traditional no-till corn herbicide programs using foliar nonselective herbicides combined with residual herbicides were compared with early preplant herbicide programs (Fawcett et al., 1983). Traditional programs averaged 86% weed control, while all early preplant programs averaged 92% weed control. Because of its residual activity and broad spectrum of control, atrazine is one of the most effective herbicide alternatives applied early preplant. The postemergence activity of atrazine provides control of small emerged weeds from no-till planting-time treatments, often eliminating the need for nonselective herbicides.

In the western United States and other arid regions of the world, fallowing land for 1 year or a portion of a year stores some soil moisture, so water availability is sufficient to facilitate germination and better growth of grain crops the following year. However, weeds must be controlled during the fallow period to prevent evapotranspiration water losses. Repeated tillage had been traditionally used to control weeds. However, tillage increases water and wind erosion, increases evaporation losses, disturbs wildlife habitat, and expends extra fuel and labor.

Triazine herbicides have been integral components in the development of chemical fallow systems. Atrazine is used during the fallow period for weed control in wheat–sorghum–fallow, wheat–corn–fallow, and wheat–fallow–wheat rotations. Atrazine’s low cost and broad spectrum weed control have made these fallow rotations profitable in areas where grain production otherwise would not be economically feasible. Greater water storage with chemical fallow, compared with conventional tillage fallow, has increased profitability and reduced risk associated with grain production in the Great Plains of the United States (Norwood, 1994).

Conversion from conventional tillage to conservation tillage involves considerable operator learning and crop production risk. Farmers reluctant to change to conservation tillage consistently rank concern about weed control as their primary reason for not converting to conservation tillage. Farmers who have successfully converted to conservation tillage have relied on many years of research and have invested many years of experience on their own farms. Confidence in the consistent weed control provided by triazine herbicides has encouraged these farmers to make a major management change and has allowed them to reap the economic and environmental benefits of the crop production system. If triazine herbicides were not available, major changes in weed control programs for conservation tillage would be necessary, increasing yield risk and uncertainty and hindering grower acceptance of conservation tillage. Thus, adoption of conservation tillage would be slowed or perhaps reversed.

What are herbicides?

Herbicides are a type of pesticide used to control weeds so that crops can flourish. Weeds are the most significant pests for most agricultural crops because they aggressively compete for vital nutrients, space, water and sunlight. Herbicides have been used widely in Canadian farming to control weeds since the second world war, allowing farmers to harvest more crops, safely and efficiently.

Plant science has used biotechnology to develop herbicide-tolerant crops, such as canola. With herbicide-tolerant crops, farmers can use herbicides to control weeds without harming their crop. This means the crop is protected without farmers having to plough the fields, which leaves nutrients and water in the soil, increases soil fertility and reduces soil erosion.

In urban settings, herbicides help control weeds that could otherwise destroy lawns, gardens, parks and sports fields. 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.

How Herbicides Work

All pesticides work by disrupting some natural mechanism within the biology of the targeted plant, insect or animal species. Most of these natural or man-made chemicals kill their targets. Some protect crops or livestock by repelling pests. The ultimate goal for pesticide researchers is to find chemicals that kill or repel the target pests without affecting other organisms in the environment or humans.

Herbicides – chemicals that kill weeds – are the most widely used pesticides in farming. Each year, they account for about 70 percent of all agricultural pesticide use in the U.S.

In the 60s and 70s, many of today’s common herbicides were introduced. Scientists could document that the chemical worked, but they often didn’t understand how the pesticide worked.

Modern herbicides can be grouped by first by the way they act, then by the way they’re used, and then by how they kill the weeds they’re meant to kill.

First, herbicides are active in the weed either through contact or in a systemic way.

Contact herbicides destroy only the plant tissue that is in contact with the chemical. Generally, these are the fastest acting herbicides. They are less effective on perennial plants because they can grow new tops from their roots, tubers or rhizomes.

Systemic herbicides can move through the target plant. So, if the herbicide is applied to the tips, it can then move to the roots, and vise versa. These herbicides can control perennial plants and are ultimately more effective than contact herbicides, but they are slower acting.

Second, herbicides can be categorized by their use.

Spray applied herbicides are usually contact chemicals and they are mixed with liquid and sprayed on the field.

Soil applied herbicides are injected into the ground where the roots can take up the chemical. There are three types of soil applied herbicides. First, “pre-plant incorporated herbicides” (as the name suggests) are applied to the ground before the crops are planted. Second, “pre-emergent herbicides” are applied to the soil before the crop germinates and emerges from the ground; the goal here is to kill weeds also before they emerge. Third, “post-emergent herbicides” are applied after the crop has emerged.

Third, herbicides can be classified by their mechanism of action. In other words, herbicides work on different enzymes, proteins or biochemical steps.

  • The synthetic auxin class herbicides were some of the first chemical herbicides in the 40s and 50s. The widely-used 2, 4-D is a synthetic auxin. These chemicals work on broadleaf or dicot plants by mimicking plant hormones. They make the plant grow uncontrollably, breaking down critical structures like the cell walls. Other growth inhibitors include Banvel, Tordon and Paramount.
  • Photosystem II inhibitors reduce the flow of electrons from one chemical to another during the process of converting light energy into food through photosynthesis. Atrazine and other trazine herbicides, as well as urea derivatives (like diuron) are of this type. They also work against broadleaf or dicot plants. These chemicals don’t break down in the environment readily, and so have been linked to problems of groundwater contamination.
  • EPSPS inhibitors kill all kinds of plants (grasses and broadleaves) by disrupting the plant’s ability to synthesize critical amino acids like tryptophan. Roundup or glyphosate is perhaps the best-known EPSPS inhibitor on the market. Liberty herbicide inhibits the glutamine synthesis pathway. These chemicals break down when they reach the soil, and their use has exploded after seed companies introduced GMO (genetically modified organism) versions of crops that can resist the herbicides. Roundup Ready versions of corn, soybeans, cotton and other crops have come to dominate the market.
  • ALS inhibitors also attack a plant’s ability to synthesize important amino acids, and they affect both grasses and broadleaves. Common trade names include Arsenal, Pursuit, Ally, Beacon, Peak, Everest and Python. Since the plants can’t produce critical amino acids, they slowly starve to death and the chemicals can inhibit the production of DNA. However, the ALS chemical reaction pathway exists only in plants, so these chemicals are thought to be among the safest to humans and animals.
  • ACCase inhibitors kill only grasses. ACCase is an enzyme that is needed in the first steps of the lipid formation process in grasses. Lipids are used in the formation of cell membranes, so the chemicals break down cellular structures. Trade names include Discovery, Hoelon, Acclaim, Fusilade and Select.

There are other classes of herbicides that inhibit the growth of weed seedlings and disrupt cell membranes in other ways.

All herbicides affect the environment and humans in some form, although there is a lot of debate over how significant and damaging the effects of individual compounds are. The problems with herbicides can range from skin rashes to death. For instance, phenoxy herbicides are often contaminated with dioxins and research has suggested that exposure to dioxin can cause a rise in cancer risk. Triazine exposure has been implicated in an increased risk of breast cancer, but there’s debate over a direct causal relationship. Other studies suggest that both herbicides and insecticides could result in Parkinson’s disease. And the herbicide Paraquat – often used to kill marijuana and coca plants – has also been linked to Parkinson’s.

Written by Bill Ganzel, the Ganzel Group. First published in 2009. A partial bibliography of sources is here.

2, 4-D

Trends in glyphosate herbicide use in the United States and globally

Annual agricultural glyphosate use volumes in the nine EPA pesticide use reports issued between 1997 and 2007 exceed NASS annual totals for the same years by 20–70 %, largely because EPA had access to multiple data sources that made it possible to estimate the volume of glyphosate applied on all crops, as well as non-crop use patterns (e.g., pasture and range uses). NASS estimates, on the other hand, were limited in any given year to the crops surveyed in a particular year, and NASS never or rarely surveys pesticide use on crops grown on limited acreage. The differences are largest in the first two decades of glyphosate use (through 1995), and reflect the array of glyphosate uses not covered in NASS, crop-by-crop pesticide use surveys. But as total agricultural use rises sharply post-1996 in the wake of the introduction of GE-HT crops, glyphosate use on the major GE crops (maize, soybeans, cotton) is fully captured in NASS, EPA, and USGS data. Differences in agricultural use estimates between the datasets all but disappear by 2007 (NASS, 184.2 million pounds glyphosate use; EPA mid-range, 182.5; USGS, 183.2; , Additional file 1: Table S18).

Factors driving use upward

Several factors have driven the increase in glyphosate use since commercial introduction in 1974. In terms of area treated, the dominant factor has been the commercialization of GE-HT crops. Not only has glyphosate been sprayed on more hectares planted to HT crops, it has also been applied more intensively—i.e., more applications per hectare in a given crop year, and higher one-time rates of application .

In the U.S. soybean sector, the average number of glyphosate applications rose from 1.1 per crop year in 1996 to 1.52 in 2014, while the one-time rate of application rose from 0.7 kg/hectare (0.63 pound/acre) to 1.1 kg/hectare (0.98 pound/acre) in the same period (, Additional file 1: Table S2). Shifts in weed communities favoring species less susceptible to glyphosate, coupled with the emergence of first, less sensitive, and eventually glyphosate-resistant weeds drove the incremental rise in the intensity of glyphosate applications on GE-HT crops . Rising reliance on glyphosate by soybean producers in the U.S. is graphically portrayed in Fig. 1a, while Fig. 1b shows modest change during the GE era in soybean yield/acre or production per soybean seed planted. Steady increases in the number of applications of glyphosate, rate per crop year, and glyphosate’s share of overall soybean herbicide use are shown in Fig. 1c.

Other factors contributed to rising glyphosate use. These include steady expansion in the number of crops registered for use on glyphosate product labels, the adoption of no-tillage and conservation tillage systems, the declining price per pound of active ingredient (see Fig. 2b), new application method and timing options, and new agricultural use patterns (e.g., as a desiccant to accelerate the harvest of small grains, edible beans, and other crops).

The one-time average rate of glyphosate application on Kansas wheat has incrementally risen threefold, from 0.33 kg/hectare in 1993 to 0.95 kg/hectare in 2012 (, Additional file 1: Table S5). The trend toward no-till and conservation tillage systems has increased wheat farmer reliance on herbicides, including glyphosate. The average two applications in recent years on winter wheat could include a pre- or at-plant spray, an application during a summer fallow period, and/or a late-season application intended to speed up harvest operations (a so-called “harvest aid” or “green burndown” use) . The average rate per crop year—the single most important indicator of the intensity of glyphosate use—rose even more dramatically, from 0.47 kg/hectare in 1993 to 2.08 kg/hectare in 2012 (4.4-fold).

Harvest-aid uses of glyphosate have become increasingly common since the mid-2000s in U.S. northern-tier states on wheat, barley, edible beans, and a few other crops, as well as in much of northern Europe . Because such applications occur within days of harvest, they result in much higher residues in the harvested foodstuffs . To cover such residues, Monsanto and other glyphosate registrants have requested, and generally been granted, substantial increases in glyphosate tolerance levels in several crops, as well as in the animal forages derived from such crops. Table 7 provides an overview of key crops on which regulatory authorities have granted large increases in glyphosate tolerances to accommodate GE-HT crop uses, as well as harvest aid, green burndown applications. Note the 2,000-fold increase in the glyphosate tolerance on dry alfalfa hay and silage from 1993 to 2014, an increase made necessary by the approval and planting of GE-HT alfalfa. In response to the large increase in expected residues from such uses, some European countries now prohibit harvest-aid applications on food crops (e.g., Germany, since May 2014).

Table 7 Changes in selected U.S. EPA glyphosate tolerance levels (ppm)

Global use of glyphosate

Farmers worldwide applied about 51.3 million kgs (113 million pounds) of glyphosate in 1995 (, Additional file 1: Table S23). To place this volume of global glyphosate use in perspective, in just one country (the U.S.) that year, farmers applied ~60 million kgs (132 million pounds) of two herbicides (atrazine and metolachlor) on mostly one crop (maize) (, Additional file 1: Table S19).

But the scope and intensity of glyphosate use worldwide rapidly changed as GE-HT crops gained market share. There were about 1.4 billion hectares of actively farmed, arable cropland worldwide in 2014 . Across this landmass, there were an estimated 747 million kg of agricultural applications of glyphosate. Accordingly, if this volume of glyphosate had been applied evenly, about 0.53 kg of glyphosate could have been sprayed on every hectare of cropland on the planet (0.47 lbs/acre).

Glyphosate was, of course, not applied evenly on every hectare of cropland. The average rate of glyphosate applications per hectare per crop year during 2014 fell in the range of 1.5–2.0 kg/hectare . At these rates of application, the total volume of glyphosate applied in 2014 was sufficient to treat between 22 and 30 % of globally cultivated cropland. No pesticide in history has been sprayed so widely.

Since losing global patent protection around 2000, dozens of companies began manufacturing technical glyphosate, and/or formulating glyphosate products. Some two-dozen Chinese firms now supply 40 % of the glyphosate used worldwide, and export most of their annual production .

The loss of patent protection and increased generic manufacturing of glyphosate has placed downward pressure on prices since 2000 . The major manufacturer, Monsanto, has typically not competed directly or solely on price, and instead has been successful in holding or expanding market share by bundling purchase of higher-price, Monsanto brand, Roundup herbicides with the purchase of Monsanto herbicide-tolerant seeds . Especially in the U.S., this bundling strategy has been augmented by various volume incentives and discounts, special financing, rebates for purchase of other herbicides working through a mode of action other than glyphosate’s (to delay the spread of resistant weeds), and other non-price benefits tailored to appeal to large volume customers .

The diversity of global uses in agriculture and other sectors has grown over the past 40 years , making it more difficult to compile accurate global data across all glyphosate uses, especially by sector and specific use. As a result, global glyphosate use projections can only be based on industry-wide glyphosate production figures, as done from 1997–2014 in Table 4 and Additional file 1: Table S24 .

Impact of GE-HT technology

The development and marketing of GE, Roundup Ready crops fundamentally changed how crop farmers could apply glyphosate. Before RR technology, farmers could spray glyphosate prior to crop emergence, for early-season weed control, or after harvest to clean up late-season weeds. But with RR crops, glyphosate could also be sprayed 1–3 times or more after the crop had emerged, leaving the crop unharmed but controlling all actively growing weeds. This historically significant technological advance set the stage for unprecedented and rapid growth in the area planted to RR crops and sprayed with glyphosate (from usually less than 10 % of cotton, maize, and soybean acres pre-1996, to 90 % or more today) .

The interplay of various factors leading to increased glyphosate use is apparent in Fig. 2a, which shows the trend in overall glyphosate use on the key GE-HT crops in the U.S., the correlation between reductions in average price per pound and use (Fig. 2b), and rising use and the emergence of resistant weeds (Fig. 2c).

Use of glyphosate on some GE-HT crops may have declined, or may soon begin declining in some regions because (a) adoption of GE-HT soybeans, cotton, and canola has peaked in most of the countries that have embraced GE technology , and (b) farmer willingness to pay for repeat applications of glyphosate, or further increase application rates, typically declines as glyphosate-resistant weeds become well established, as they have in much of the U.S. and in Brazil and Argentina . On the other hand, GE-HT crops may move into some regions not previously planting them (e.g., China), and reductions in the price of generic glyphosate herbicides could lead to more intensive use in some countries.

In the countries that have planted the largest shares of GE-HT crops (the U.S., Argentina, and Brazil), glyphosate use rates per hectare per crop year have risen sharply since around 2000 . Worldwide on GE soybean and cotton, average total herbicide use per crop year per hectare has approximately doubled from 1996 to 2014, with the increase in glyphosate volumes applied per hectare accounting for nearly all of the per hectare increase. Maize herbicide use per hectare has risen modestly, if at all, in large part because adoption of GE-HT maize hybrids allowed farmers to reduce reliance on a half-dozen other widely used maize herbicides applied at relatively high rates (e.g., ~1 kg/hectare per crop year) .

Because GE-HT soybeans account for two-thirds of the total hectares planted to GE-HT crops worldwide, the doubling of average herbicide use per hectare of HT soybeans drives the sizable increase in overall herbicide on all GE crop hectares. There is, as well, a clear connection throughout South America in the adoption of GE-HT technology and no-tillage systems . No-till farming in South America lowers machinery and labor costs, and reduces soil erosion, but at the expense of heightened reliance on herbicides for weed control, and other pesticides to control insects and fungal pathogens.

Despite gaps in publicly accessible data, the dramatically upward trajectories in glyphosate use in the U.S. and globally are unmistakable. In the pre-GE era (1974–1995) in the U.S., non-agricultural glyphosate uses accounted for ~34 to 42 % of total use. The share of total glyphosate use accounted for by the agricultural sector shifted markedly upward post-1996, starting at 66 % in 1996 and reaching 81 % 5 years later (2001) and 92 % by 2014 (, Additional file 1: Table S18).

The total volume of use and the split between agricultural and non-agricultural uses in the pre-GE era period are subject to greater uncertainty than in the 1996–2014 period. However, pre-1995 glyphosate use is minor compared to the post-GE period, when both data quantity and quality improved, especially covering applications in the U.S. and on global GE-HT hectares planted.

Figure 3 arrays milestones in the history of glyphosate discovery, commercialization, and regulation, while Fig. 4 displays key events in the history of glyphosate use and impacts.

Fig. 3

Milestones in the history of glyphosate discovery, commercialization, and regulation

Fig. 4

Milestones in glyphosate use and impacts

Rising use triggers new concerns

Driven by the growing diversity of uses and dramatic increases in volumes applied, levels of glyphosate and its primary metabolite aminomethylphosphonic acid (AMPA) have been detected in the air , soil , and water . With few exceptions though, contemporary levels of glyphosate in the air, water, and food result in typical human exposure estimates that remain well below the “levels of concern” or “Acceptable Daily Intakes” established by regulatory bodies around the world.

Still, a growing body of literature points to possible, adverse environmental, ecological, and human health consequences following exposure to glyphosate and/or AMPA, both alone and in combination with ingestion of GE proteins (e.g., EPSPS, Bt endotoxins) . Environmental studies encompass possible glyphosate impacts on soil microbial communities and earthworms , monarch butterflies , crustaceans , and honeybees .

Studies assessing possible risks to vertebrates and humans include evidence of rising residue levels in soybeans , cancer risk , and risk of a variety of other potential adverse impacts on development, the liver or kidney, or metabolic processes .

Relative toxicity and impacts

For years, glyphosate has been regarded as among the least chronically toxic herbicides for mammals, and indeed only three EPA-registered synthetic pesticides in current agricultural use have a higher chronic Reference Dose (the imidazolinone herbicides imazamox, imazethapyr, and imazapyr).

For human exposures, the U.S. EPA has set glyphosate’s daily chronic Reference Dose (cRfD) at 1.75 milligrams per kilogram of bodyweight (mg/kg bodyweight/day). The EU-set cRfD for glyphosate was recently raised from 0.3 to 0.5 mg/kg/day, 3.5-fold lower than EPA’s. A team of scientists has compiled evidence supporting the need for a fivefold reduction in the EU cRfD to 0.1 mg/kg/day , a level 17-times lower than EPA’s.

Glyphosate is a moderate dose herbicide with relatively low acute and chronic mammalian toxicity, to the extent mammalian risk is accurately reflected in required EPA toxicology studies. After an exhaustive review, however, glyphosate was classified in 2015 as a “probable human carcinogen” by the International Agency for Research on Cancer , based on increased prevalence of rare liver and kidney tumors in chronic animal feeding studies, epidemiological studies reporting positive associations with non-Hodgkin lymphoma, and strong mechanistic evidence of genotoxicity and ability to trigger oxidative stress .

The body of toxicological studies supporting glyphosate’s current EPA and EU cRfD, and hence all contemporary uses of this herbicide, dates back to the early 1970s through mid-1980s . Recent studies suggest that glyphosate in its pure form, and some formulated glyphosate end-use products, may be triggering epigenetic changes through endocrine-mediated mechanisms .

Evidence from multiple studies suggests that the kidney, and secondarily the liver, is at risk of glyphosate-triggered, or glyphosate-enhanced chronic degeneration . Industry metabolism studies in farm animals, rats and mice, and rabbits were conducted in the 1970s and 1980s, and show that in animal feeding studies, glyphosate levels in the kidney usually exceed those in the liver by three- to tenfold, and those in the liver exceed levels in other tissues by a wide margin .

The apparent tendency of glyphosate to concentrate in the kidneys, coupled with glyphosate’s action as a chelating agent, has led some scientists to hypothesize that glyphosate can bind to metals in hard drinking water, creating metallic-glyphosate complexes that may not pass normally through kidneys . For this, or other as yet unrecognized reasons, the risk of chronic kidney disease may be heightened in human and animal populations with heavy glyphosate exposure.

The IARC classification and emerging evidence relative to kidney damage and endocrine effects heightens the need for, and will complicate ongoing and future glyphosate worker and dietary-risk assessments. Annual residue tests are carried out by the U.K. Food Standards Agency (FSA). Residues of glyphosate were found in 10–30 % of grain-based samples from 2007–2013, at generally rising levels . Glyphosate and AMPA residues are present at relatively high, and rising levels (over 1 ppm) in a high percentage of the soybeans grown in the U.S., Canada, Brazil, Argentina, Paraguay, countries which account for 86.6 % of the 11.6 billion bushels of soybeans produced globally in 2014, and nearly all global trade in soybeans and soybean-based animal feeds .

Concepts, Principles, and Practices

How Herbicides Work

Herbicides kill or suppress plants by interfering with essential plant processes such as photosynthesis. All of the interactions between an herbicide and a plant from application to the final effect are referred to as the mode of action. Understanding the mode of action of an herbicide is essential in selecting the proper herbicide, diagnosing herbicide injury symptoms, preventing herbicide resistance problems, and avoiding nontarget environmental impacts.

Diagram of leaf cross-section showing mode of action of a foliar-active, systemic herbicide (e.g., atrazine) that inhibits photosynthesis. In this example, the herbicide is applied to and absorbed through the leaf, is translocated to the chloroplasts (site of action), and subsequently inhibits photosynthesis (mechanism of action).

The mode of action involves

  • contact and absorption
    contact, penetration, and movement of the herbicide into the plant through the cuticle or epidermal root tissue
  • translocation
    movement of the herbicide to the site of action
  • site of action
    specific location within the plant where the herbicide exerts toxicity at the cellular level
  • mechanism of action
    specific biochemical or biophysical process that is affected by the herbicide

The terms mode of action and mechanism of action are often used interchangeably. However, mechanism of action refers to the plant’s specific biological process that is interrupted by the herbicide, whereas mode of action is a general term referring to all of the plant-herbicide interactions.

Contact and Absorption

Herbicides must contact the plant surface to be effective. Herbicides with limited mobility that are effective at the site where they contact the plant are known as contact herbicides. Herbicides that must be absorbed and translocated to the site of action to be effective are called systemic herbicides. Contact herbicides typically affect only the portion of the plant with which they come into physical contact. Contact herbicides are fast acting, and injury symptoms can appear within hours of application. Conversely, injury symptoms from systemic herbicides can take from several days to weeks to appear, but the entire plant may eventually be killed. Soil-applied herbicides are applied to the top few inches of the soil and eventually absorbed through root tissue, whereas foliar-applied herbicides are applied to leaves or stems. Most contact herbicides are foliar-applied, whereas systemic herbicides can be either soil- or foliar-applied.

Choosing the appropriate herbicide depends upon target species biology, herbicide selectivity, application method, and site conditions. It is important to understand these factors to ensure that an effective herbicide is selected. For example, contact herbicides are most effective against annual invasive plants and in situations in which plant regrowth is not a concern. Conversely, systemic herbicides are more effective on perennial invasive plants and can limit regeneration of treated plants. Soil-applied herbicides are most effective on seedlings or germinating plants prior to their emergence above the soil. Established plants may require a foliar-applied herbicide for effective control. Mature plant tissues absorb herbicides less easily than young plant tissues due to thickening of the outer tissues in older plants.


Systemic herbicides move, or translocate, from the point of application to the site of action through either the phloem (tissue that transports sugars from the leaves to the roots), xylem (tissue that transports water from the roots to the leaves), or through both. Some herbicides move more easily and farther within plants than others.

Site of Action

To be effective, an herbicide must reach the site of action. An herbicide binds to a specific location within the plant, typically a single protein, and as a result disrupts a physiological process essential for normal plant growth and development.

Mechanism of Action

Herbicides can affect various sites of action within plants, and they are often categorized into different mechanisms of action based on how they work and the injury symptoms they produce.

Injury Symptoms

Amino acid synthesis inhibitors

  • block synthesis of amino acids essential for the production of new cells
  • stunted growth, leaf discoloration

Cell membrane disrupters

  • rupture plant cell membranes
  • death of plant tissue

Growth regulators

  • mimic natural growth hormones responsible for cell elongation, protein synthesis, and cell division
  • growth abnormalities: stem twisting, leaf malformations, stunted root growth

Lipid synthesis inhibitors

  • block synthesis of lipids essential for the production of new cells
  • decay, leaf discoloration

Photosynthetic inhibitors

  • block photosynthesis
  • yellowing of the leaf, death of plant tissue

Pigment inhibitors

  • inhibit synthesis of photosynthetic pigments
  • white or translucent leaves

Respiration inhibitors

  • interfere with the production of ATP (adenosine tri-phosphate), the major energy source for plants
  • defoliation, brown dessicated plant tissue

(adapted from Radosevich et al. 1997, Zimdahl 1999, Monaco et al. 2002)

Repeated use of an herbicide with the same mechanism of action can result in resistance of the plant population to that herbicide because selection pressure for the resistant portions of the population increases with each application. Using herbicides with different mechanisms of action, or combining them with other control methods, can reduce the risk of developing herbicide-resistant populations.

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Effects of Herbicides on Invasive Plants

Because herbicides are inherently toxic to plants, they are effective tools to manage undesirable plant species, but they can also have unintended, adverse effects on desirable plant species. Thus, it is important to understand the fundamentals of how herbicides affect plants as well as to focus herbicide use to meet particular invasive plant management objectives.


Plants vary in their susceptibility to different herbicides. For example, the selective herbicide 2,4-D injures or kills broadleaved plants but has little effect on grasses. Selectivity is the result of complex interactions between the plant, the herbicide, and the environment.

Factors Affecting Selectivity


  • genetic inheritance
    Members of the same plant genera typically respond in a similar manner.
  • age
    Young plants that are undergoing rapid growth have more actively growing tissues and are typically more susceptible to injury.
  • plant morphology
    Broadleaved plants can be more susceptible to herbicidal injury because they intercept more herbicide spray than grass leaves.
  • physiological and biochemical processes
    Plants that absorb and translocate herbicides readily are more susceptible.


  • formulation
    Granular formulations can improve selectivity because they are less likely than liquid formulations to drift offsite or volatilize.
  • application method
    Application methods such as spot spraying, wicking, or injection allow the applicator to select individual plants for treatment.
  • mechanism of action
    An herbicide can affect the physiologic process of some plants but not others. For example, lipid synthesis inhibitors affect only grasses.


  • soil type
    Generally, herbicides move more readily in sandy soils than in clay soils. Herbicides applied to plants growing in sandy soils may move quickly through the soil profile and affect deep-rooted plants while leaving shallow-rooted plants relatively unaffected.
  • soil moisture
    Moist soils can promote rapid plant growth, resulting in rapid herbicidal injury.
  • temperature
    Warmer temperatures can result in rapid degradation of herbicides, potentially reducing herbicidal injury.

(adapted from Radosevich et al. 1997)

Repeated use of an herbicide such as picloram can select for picloram-tolerant plant species such as hoary cress (Cardaria draba) and other members of the mustard family (Brassicaceae). Photo credit: MSNWAEP, www.forestryimages.org

Species Shift

Because of herbicide selectivity, continued use of a particular herbicide may result in a shift within a plant community from susceptible to more herbicide-tolerant species. For example, repeated use of herbicides, such as clopyralid, that select for broadleaved species can result in an increase in grasses (Tyser et al. 1998).

Removal of invasive plants from highly degraded sites can result in one undesirable species being replaced by an equally undesirable species. For example, using picloram to control spotted knapweed (Centaurea maculosa) with high canopy cover (> 60%) resulted in a plant community dominated by cheatgrass (Bromus tectorum) and Japanese brome (B. japonicus) (Kedzie-Webb et al. 2002). In these cases, revegetation with desirable and competitive plant species is often necessary (DiTomaso 2000).

Seedbank Persistence

If viable seeds remain in the soil after treatment, undesirable plants can reestablish. The relative importance of the seedbank to seedling recruitment and subsequent increase in an invasive plant population varies with the species as well as the plant community and site conditions. Depending upon the plant species, seeds can remain viable in the soil for many years. Thus, management must account for the potential of plant populations to persist even after multiple herbicide treatments. Some herbicides such as picloram can be persistent in the soil for several years after application and can control new plants germinating from seedbanks (Tu et al. 2001).

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Effects of Herbicides on Human Health and Environment

Federal laws and policies regulate many aspects of herbicides including labeling, registration, and application, but these regulations are not a substitute for a thorough knowledge of the risks associated with herbicide use. The benefits of herbicides must be weighed against the potential for exposure and impacts to human health, nontarget organisms, and the environment. Risks are always present with any herbicide use, but improper use or misapplication can increase these risks.

Herbicide Registration

The federal government, in cooperation with individual states, regulates herbicides to ensure that they do not pose unreasonable risks to human health or the environment. The EPA requires extensive test data from herbicide producers to show that products can be used without harming human health and the environment. EPA scientists and analysts carefully review these data to determine whether to register (license) an herbicide product and whether specific restrictions are necessary.

The process of registering an herbicide is a scientific, legal, and administrative procedure through which the EPA examines ingredients of the herbicide; sites or target species on which it is to be used; amount, frequency, and timing of its use; and storage and disposal practices. In evaluating an herbicide registration application, the EPA assesses a wide variety of potential human health and environmental effects associated with use of the product. The producer of the herbicide must provide data that address the following:

  1. What ingredients are in the herbicide?
  2. What is the environmental fate of the herbicide?
  3. What does the herbicide do in organisms and the environment?

Herbicide product tests follow EPA guidelines and evaluate whether an herbicide has the potential to cause adverse effects on humans, wildlife, fish, and plants, including federally listed species and nontarget organisms, or to contaminate surface water or groundwater through leaching, runoff, and spray drift. Testing is conducted on only a few faunal species of specific age and under limited environmental conditions. Care should be taken when extrapolating these data to other circumstances. Furthermore, there is essentially no testing on herbicide mixtures, and most testing is done with the technical grade of the active ingredient rather than with actual formulated products.

Read more about herbicide risk assessments:

  • EPA Assessing Health Risks from Pesticides
  • EPA Protecting Wildlife from Pesticide Risks

Read more about herbicide registration:

  • EPA Pesticide Registration Program
  • Laws Affecting EPA’s Pesticide Program
  • EPA Pesticides: Regulating Pesticides


The EPA evaluates both exposure and toxicity to determine the risk associated with use of an herbicide. People, nontarget flora and fauna, water, and soil can be exposed to herbicides during herbicide application or from subsequent offsite movement. Herbicide exposure can be minimized or avoided by following the herbicide label and understanding what happens to herbicides after application.

For animals (including humans), herbicides have three modes of entry into the body: through the skin, by swallowing, and by breathing. Exposure can occur both during and following herbicide application. Individuals who mix, load, and apply herbicides are at the greatest risk of exposure. Exposure may also occur when the labeled restricted-entry interval (REI) is not observed and people reenter an area too soon following treatment.

Once herbicides have been applied, the potential for exposure is further influenced by the many biotic and abiotic processes that affect the fate of herbicides in the environment. Some processes may move or transfer the herbicide away from the target plant to nontarget organisms while other processes degrade or break down herbicides after application.

Processes that Affect Environmental Fate of Herbicides


  • absorption
    uptake of herbicides into plants or microorganisms
  • adsorption
    binding of herbicides to soil particles
  • volatilization
    conversion of solid or liquid herbicides into a gas
  • spray drift
    airborne movement of spray droplets away from the application area
  • surface runoff
    movement of herbicides from the land into surface water or groundwater
  • leaching
    movement of herbicides into water through the soil


  • microbial breakdown
    breakdown of herbicides by microorganisms (e.g., fungi, bacteria)
  • chemical breakdown
    breakdown of herbicides by chemical reactions in the soil
  • biochemical breakdown
    breakdown of herbicides by enzymatic reactions such as metabolism within living plants
  • photodegradation
    breakdown of herbicides by sunlight

(adapted from British Columbia Ministry of Agriculture and Lands 2007)

Persistent herbicides can remain active in the environment for long periods of time, potentially causing soil and water contamination and adverse effects to nontarget organisms. In some cases, compounds that result from herbicide degradation may continue to be significantly toxic in the environment.


The EPA uses toxicity tests as standard reference experiments to evaluate potential harm of herbicides to animals. Various rating systems describe the relative toxicities of herbicides. The EPA has category guidelines for acute and subchronic toxicity, which are used on herbicide labels.

Toxicity tests on mammals are segregated into three categories based on the length of exposure to the pesticide:

  • acute tests
    Studies evaluate the effects of large dosages in a single exposure (dose) or short time period. Observations are conducted over a span of days to weeks.
  • subchronic tests
    Studies involve exposing the test subject to compounds repeatedly over a longer period of time (e.g., 30 to 90 days).
  • chronic tests
    Impact studies expose a test subject to a pesticide for a majority of its life span to determine the effects of long term, low level exposure. The potential for mutagenicity, carcinogenicity, hormone disruption, and developmental and reproductive effects is evaluated.

The amount of a substance to which a subject is exposed is as important as its toxicity. For example, small doses of aspirin can be beneficial to people, but at very high doses this common medicine can be deadly. A dose-response assessment involves considering the dose levels at which adverse effects are observed in non-human test subjects, and using these dose levels to calculate an equal dose in humans.

The median lethal dose (LD50) is the most commonly used index of herbicide toxicity. The LD50 is the dose that is lethal to 50 percent of the treated population (expressed as milligrams (mg) of compound ingested per kilogram (kg) of body weight). An herbicide’s toxicity to aquatic organisms is quantified with the LC50, which is the concentration of the herbicide in water required to kill half of the study animals. The LC50 is typically measured in micrograms of compound per liter of water.

The following table compares LD50 values for various products from caffeine to herbicides. Oral rat LD50 values are expressed as mg of compound per kg of body weight, and are listed from most to least toxic. These values were used to estimate LD50 in humans by converting mg/kg for rats to lb/180 lbs for humans. The column on the right shows estimated LD50 values for humans expressed in various metrics to demonstrate how much of a product would need to be consumed to be lethal.

Although some substances are more toxic pound for pound, oral exposure may be more difficult to achieve considering normal concentration and exposure to these products. For example, caffeine is nearly 10 times more toxic than dicamba herbicide when comparing milligrams of a pure substance ingested by a rat. A human would need to consume about 100 cups of coffee (containing caffeine) compared to 1 cup (0.5 pint) of formulated dicamba herbicide to reach a lethal dose. However, normal oral exposure is more likely to occur in much smaller quantities than if one were to drink a cup of the herbicide.

Comparison of Toxicity for Various Products

Chemical name


Oral Rat LD50*

(mg compound/ kg body weight)

Oral Human LD50**

(amount of compound/ 180 lb body weight)


stimulant (coffee)

about 100 cups


pain reliever

about 120 tablets


broadleaf herbicide

0.12 to 0.3 pint


broadleaf herbicide

0.5 pint


bare ground treatment

1.02 pounds

sodium chloride

condiment (table salt)

0.6 to 0.8 pounds


broadleaf herbicide


2.5 pint


bare ground treatment


2 pint


broadleaf herbicide


1.5 pounds


nonselective herbicide


2 pint


residual broadleaf herbicide

6 pint

* Acute lethal dose in mg of compound/kg of body weight for 50% of the test animals (rats); the lower the number the more toxic the substance. Adapted from WSSA 1994.

**Estimated acute lethal dose in various measures of compound for a 180 lb person; estimated from male rat data using a conversion factor for mg/kg to lbs/180lbs. These numbers are not precise but provide a relative idea of lethal doses.

> Values that are preceded by a “greater than” sign (>) mean the LD50 is higher than the quoted figures, which are the highest amounts tested.

Human Health

Measures should always be taken to minimize exposure to herbicides. Photo credit: USFWS

Following herbicide label instructions and established safety procedures minimizes herbicide exposure. Herbicide applicators generally face the greatest risk, particularly during mixing and loading. The general public can be affected by direct contact through spray drift, accidental spills, indirect contact through consumption of contaminated food or water.

People with a hypersensitivity to chemicals, or multiple chemical sensitivity, may display extreme adverse effects and should take care to avoid or reduce their exposure to herbicides. These individuals are generally aware of their sensitivities because they have reactions to a variety of natural and synthetic compounds. Posting signs in public use areas during and following herbicide application will help minimize exposure.


Herbicides can be used to reach invasive plant management objectives and achieve desired vegetation conditions. Photo credit: USFWS

Herbicides can enhance native plant communities by removing undesirable species and increasing native species. For example, treating invasive alligatorweed (Alternanthera philoxeroides) with the selective herbicide triclopyr effectively reduced the biomass and cover of alligatorweed and increased the biomass of native wetland species important to waterfowl on managed wetlands of Eufaula NWR in Alabama (Allen et al. 2007). Similarly on waterfowl production areas of Medicine Lake NWR in Montana, using the nonselective herbicide glyphosate to control Canada thistle (Cirsium arvense) reduced Canada thistle density and biomass while increasing density and biomass of desirable forbs important for nesting waterfowl (Krueger-Mangold et al. 2002).
Herbicides can also have unintended consequences for nontarget plant species, species composition, and plant species richness and diversity. For example, herbicides such as picloram that are selective for broadleaved plants can control broadleaved invasive plants such as spotted knapweed (Centaurea maculosa) and sulphur cinquefoil (Potentilla recta) and promote recolonization of native grasses. However, because of this selectivity for broadleaved species, these herbicides can promote invasion by invasive grass species and negatively impact native broadleaved plants, reducing native species richness and diversity (Tyser et al. 1998, Pokorny et al. 2004, Denny and Sheley 2006).


Use of herbicide to control encroaching woody growth increased forage for northern bobwhite. Photo credit: NC Wildlife Resources Commission

Herbicides have been designed to target biochemical processes, such as photosynthesis, that are unique to plants. Thus, they typically are not acutely toxic to animals (Tatum 2004). Some exceptions include paraquat. Data indicate that some herbicides can have a synergistic effect with commonly used insecticides when they runoff into surface waters. In addition, some herbicides can have subtle, but significant, physiological effects on animals, including developmental effects. So, as with all pesticides, the user needs to thoroughly evaluate the range on potential nontarget effects and strive to minimize these effects. In some cases, this may involve best management practices that go beyond the requirements on the pesticide label.

Herbicides have indirect effects on wildlife by altering vegetative cover and structure. Using imazapyr to control encroaching woody growth in longleaf pine stands (Pinus palustris) can increase forage for northern bobwhite (Colinus virginianus) (Welch et al. 2004). Conversely, silvicultural practices that use herbicides to eliminate competitive deciduous shrubs to promote revegetation by conifers can negatively impact songbird reproductive success (Easton and Martin 2002). In aquatic environments, treating invasive aquatic plants such as Eurasian watermilfoil (Myriophyllum spicatum) can result in massive plant die-off and decomposition. The decaying vegetation can deplete dissolved oxygen resulting in fish kills (Langeland 1998).


Herbicides can contaminate groundwater and surface water. Contamination can occur directly due to several factors including spills or leaks, improperly discarded herbicide containers, and rinsing equipment near drainage areas. Contamination can also occur due to surface runoff or leaching of herbicides. Spray drift and volatilization of herbicides can transport the chemical into the atmosphere during and after application, potentially allowing herbicides to reach surface water and groundwater via precipitation.

  • US Geological Survey – Pesticides in the Nation’s Streams and Ground Water


The effect of herbicides on soil properties, chemistry, and microbial populations depends upon herbicide concentration and characteristics, and soil type, temperature, and moisture (Haney et al. 2000). Herbicides can influence soil pH (Schreffler and Sharpe 2003) and soil microbial activity (Haney et al. 2000). Although herbicides can have direct toxic effects on soil fauna (Salminen et al. 1996), herbicides typically affect these organisms indirectly via removal of aboveground vegetation and through changes to soil decomposer community structure and reductions in nutrient cycling (Mahn and Kastner 1985, Salminen et al. 1997). Herbicides can also reduce the growth and function of mycorrhizal fungi (Vieira et al. 2007), which increase the ability of plants to absorb and translocate nutrients from the soil.

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