On This Page

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• Introduction

• What are Herbicides?

• How do Herbicides Work?

• How are Herbicides Applied?

  • Basal Bark​

  • ​Cut Stump and Injection​

  • Wet-blading

  • Guns and Booms

• How Toxic are Herbicides?

• How is Toxicity Calculated?

• How is Toxicity Data used in Risk Assessment?

• Recommended Reading

• References

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Introduction

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Large scale right-of-way vegetation management plans are necessary to ensure reliable power for both residential and industrial customers. Integrated vegetation management plans are applied to rights-of-way in changing landscapes to determine the most effective treatment techniques and tools in each unique situation [1]. Prior to the 1940s mechanical methods were essentially the only tools available, but today there are multiple techniques that can be used in combination for improved results [Jump to IVM Page] [2].

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A main technique employed in IVM is ecological control. Once a dense cover of low-growing plants, such as shrubs and grasses, is established it becomes difficult for tree saplings to succeed and even for seeds to begin growing [2,3]. Over time the number of problem trees (i.e. trees that can touch or fall on power lines) should diminish. Herbicides are a tool that can be used to promote and maintain desirable plant communities on power line rights-of-way [2,3]. There are many different ways that herbicides can be applied [Jump to Herbicide Application Section] and many different types of herbicides [Jump to Garlon Page] [4,5]. Understanding how these herbicides behave in the environment, before use, is critical to determining if they are effective and to ensure the environment is protected from harm. [Top of Page]

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What are Herbicides?

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The products bought in small volumes by home users, or in large quantities by farmers and vegetation managers are what most people think of as herbicides. Although they may be talked about as a single substance, herbicide products are not sold as pure active ingredient. Instead they are mixed with other ingredients that each have a specific function in the product. Each specific combination of ingredients is referred to as a formulation by manufacturers and scientists. The ingredients can be mixed together in different amounts so that each formulation has slightly different characteristics or properties [6].

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The active ingredient is the part of the herbicide mixture that kills the plant. This makes up only a small portion of the formulation. A technical grade active ingredient is formulated before sale. Adjuvants, or additives, are included in the formulation to improve the activity of the herbicide or to improve application characteristics. They may improve the function of the active ingredient, improve absorption into the plant, help prevent drift, increase rainfastness, decrease photodegradation, increase stability in storage, increase safety and ease of handling, improve mixing of oily chemicals with water, or prevent clumping of powders and granules [5,6]. Surfactants are common adjuvants that are used to decrease surface tension of herbicide sprays. Decreased surface tension allows droplets to spread out on leaves, rather than beading, which increases absorption and reduces waste. They may also remove the waxy layer on leaf surfaces and therefore increase absorption [7]. There is often an adjuvant, such as water or vegetable oil, used to dilute the active ingredient. Essentially adjuvants improve effectiveness of the herbicide action, or change the nature of the mixture to make application and handling more efficient [5,6].

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The active ingredient in a herbicide will have a common, or chemical, name but will be sold under various product names [5]. For example the chemical triclopyr [Jump to Garlon Page] is sold under the product name GarlonTM RTU, Forestry GarlonTM XRT, GarlonTM 3A, GarlonTM 4, GarlonTM 4 Ultra, Forestry GarlonTM 4, Access, Battleship, Chaser®, ConfrontTM, CrossbowTM, Furflon D, Furflon II, Grandstand® R, PasturegardTM, PathfinderTM II, Redeem® R&P, RemedyTM, RemedyTM RTU, RemedyTM Ultra, TurflonTM, Amine, and TurflonTM Ester [8]. Each of these products will contain triclopyr but will have different concentrations of active ingredient and adjuvants[8].

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Household example of formulations

OFF!® insect repellent products are a common household example of formulations. OFF!®Deep Woods® Sportsmen and OFF!® Deep Woods® Insect Repellent Dry are sold as different products because they have different formulations. Sportsmen® contains 30% DEET, while Insect Repellent Dry® contains 25% DEET. DEET is the active ingredient that repels biting insects. The “other ingredients”, or adjuvants, have a specific function in the products. For example, some adjuvant is involved in providing the non-greasy feel of OFF!®’s “powder-dry technology that dries on contact”, while other ingredients include ethyl alcohol, butane, corn starch, propane, isobutane, magnesium carbonate and N,N-Diethyl-m-toluamide (quantities vary by product) which preform other functions [9,10,11]. [Top of Page]

 

How do Herbicides Work?

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To be effective a herbicide must come into contact with a plant, be absorbed by the plant either by leaves, roots or bark, move to the target site and reach toxic levels at the target site. If a herbicide does not accomplish these four things it will have no effect on plants [5].

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The toxicity of herbicides to plants often varies based on plant physiology (basic plant functions), morphology (plant structures) and metabolism (ability to deactivate or activate the herbicide by breaking it down). Differences in plant structure may change how herbicides contact the plant or effect whether or not it is absorbed. Changes to plant physiology may effect how herbicides move through the plant to the target site (translocation) and its ability to impact the target site [5]. Herbicides often fit with a target site much as a key fits a lock. If the lock (plant) has changed then the key (herbicide) will not work. For example, conifers are often more resistant to herbicides than deciduous trees due to physiological, morphological and metabolic differences.

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The metabolic activity of a plant also determines herbicide effectiveness. Some plants are able to rapidly metabolize and deactivate the herbicide. For example, even though Roundup (glyphosate) works on all plant types, Roundup Ready soybeans are resistant because they are able to break it down before permanent damage is caused. Their resistance is due to an overproduction of an enzyme that metabolizes glyphosate, compared to other plants found in nature [4,5]. Caffeine tolerance in humans works in a similar way. Some people get jittery after only a single cup of coffee, while others are seemingly unaffected by three. This is due to differences in caffeine metabolism. Interestingly caffeine is actually toxic to humans (ex: change to heart rate) however we are able to metabolize it before lasting affects occur.

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Herbicides are categorized on a spectrum of selectivity. A non-selective or broad spectrum herbicide will be toxic to  many types of plants (trees, grasses, forbs, etc). A selective herbicide will only be effective on specific types of plants. For example, the herbicide triclopyr is effective on broadleaved plants but not grasses or sedges, while fenoxoprop-P is effective on grasses but not broad leaved plants [4,5,8]. [Top of Page]

 

How are Herbicides Applied?

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Herbicide application can be foliar (applied to leaves), cut stump (applied to a tree stump following cutting) and basal bark (applied to the bark of small trees). Applications can be at high volumes (many square km) or low volumes (spot treatments). Specific techniques are discussed below [12].

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Basal Bark

For basal bark applications a worker on foot with a backpack sprayer and wand, will spray the bark of small trees (5-15 cm diameter) with herbicide. The herbicide will be absorbed into the bark and move throughout the tree (translocate), ensuring that the roots are also killed. This prevents suckering regrowth [12]. A video showing basal bark application of Garlon RTU can be found on youtube (Garlon RTU, Dow AgroSciences) at

https://www.youtube.com/watch?v=CEU_VSWmC1w&t=111s, minute 1:19 – 1:48.

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Backpack spray treatments can be used for soil, foliar or basal bark applications. This is a low-volume highly targeted technique as workers are capable of treating a single plant at a time [12,13].

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Images and video by University of Saskatchewan research team

  

Cut Stump and Injection

Cut-Stump techniques deliver low volume, highly selective treatments as they target single trees. After a tree is cut down, herbicide is applied to the stump. The herbicide will then move down into the roots, damaging the root system and preventing regrowth.  

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For injection techniques either a cut is made in the tree bark and herbicide is sprayed in, or herbicide can be directly stabbed into the bark with a lance like machine [12,13].

 

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Cut surface treatments – hack and squirt. Images by

Steven Manning, Nashville, Tennessee [12].

 

Wet-blading

Wet-blade treatments apply the herbicide as vegetation is being cut. As the mower or brusher blade cuts through the plants herbicide is wiped or sprayed on. The herbicide then moves through the remainder of the plant to the roots where it prevents regrowth. This technique applies mid to low volume levels of herbicide, depending on the equipment being used. It is generally a less targeted technique [12].

 

Vehicle Mounted Spray Guns and Booms

Tanks and spray guns can be mounted on ATVs, skids and trailers and can be used to spot treat individual trees or small areas of vegetation from a greater distance, due to greater pressurization, than a backpack and wand allow.   

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Boom sprayers (horizontal bars with a row of nozzles) can be mounted on trucks, tractors, ATVs, helicopters or airplanes. Booms are used to apply herbicide evenly along a swath of land [12].  Vehicles allow for greater distances to be covered than can be traversed on foot.

 

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Roadside and turf sprayers are often standard work trucks

custom fit with booms and guns on the right hand side. They are most

often used on road sides to maintain ditches. Image by Art Gover [12].

[Top of Page]

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How Toxic are Herbicides?

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The chemicals in herbicides have been designed to be toxic to plants, but benign to other organisms such as mammals (including humans), birds and fish. The main processes in plants that are disrupted by herbicides include photosynthesis, cell growth, and enzyme function. Photosynthesis only occurs in plants. Cell growth in plants is vastly different than cell growth in other organisms. Often the enzymes disrupted by herbicides are not found in other organisms, therefore there is nothing for the herbicide to bind to or act on. However, as with all things, if enough of a herbicide is ingested it can have negative effects on non-target organisms [4,5]. This is often summarized as “the dose makes the poison”.

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The toxicity of active ingredients to non-target organisms (i.e. not plants) is always tested before products can be sold on the market. Adjuvants are often not tested and their toxicity to non-target organisms do not have to be disclosed [14]. After treatment with various herbicidal active ingredients and formulations [Jump to definitions], tests looking at soil microbe and fungi composition and activity (necessary for microbial degradation), found that the addition of adjuvants could both increase and decrease microbial biomass. The changes were mostly dependent on soil properties such as soil type, pH and original microbial community composition, rather than concentration of herbicide or adjuvants applied. Changes observed in the study were often minimal, being detected only at 10-100 times the recommended rate of application [18]. However, in the case of aquatic organisms such as fish, frogs and aquatic invertebrates, studies have found that some surfactants can be more toxic than some pure active ingredients [15,16]. Moore et al. (2011) found that in the original Roundup® formulation, the surfactant MON 0818 essentially caused 100% of the deaths observed in larval frogs tests (lab tests using 4 wild South Carolina frog species and 2 commercially purchased species) [15]; meaning the store bought product studied was more toxic to frogs than the pure active ingredient. However, not all adjuvants are intrinsically toxic. For example, canola oil is a commonly added non-toxic ingredient in herbicides. [Top of Page]

 

How is Toxicity Calculated?

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A common unit for comparing toxicity is the LD50 – the dose which is lethal to 50 percent of the organisms tested. Because smaller animals (ex: a rat) are affected by smaller doses, while larger animals (ex: a horse) need a larger dose, the LD50 is expressed in mg/kg. That is milligram of herbicide given per kilogram body weight. The most common test species for determining toxicity to humans and other mammals is the lab rat. Lab rats have been well studied, and accurate comparisons can be made between their responses and those found in other mammals.