Garlon RTU has been proposed as the primary herbicide for use on boreal Saskatchewan rights-of-way. Garlon RTU (Ready To Use) comes formulated for use and does not require any mixing. The active ingredient is triclopyr butoxyethyl ester (BEE). As a selective herbicide, Garlon RTU is applied via basal bark to directly control woody tree and shrub species [1]. [Top of Page]

Classes of Herbicides

The Weed Science Society of America (WSSA) has developed a numbered classification system for herbicides based on their mode of action (MOA). MOA refers to the overall manner in which a herbicide injures a plant. All chemicals listed in the same WSSA classification group will affect plants in the same way (i.e. will have the same MOA) [2].

Table 1: Weed Science Society of America herbicide classification with herbicide examples [2,3].

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Garlon RTU

What is Garlon RTU

Garlon RTU is a Group 4 herbicide [2,3] used to control woody brush, pine and deciduous trees [4]. RTU stands for ready to use. This indicates that dilution of herbicide before use is not necessary; the bought solution has the correct concentration and is ready to use out of the box. Garlon RTU is applied via basal bark or cut stump methods [Jump to Herbicide Application section]. These methods use low volumes of herbicides that are directly applied to the stem or stump of target individual plants [4]. After application the herbicide moves throughout the plant, killing the roots and preventing suckering [3]. A video produced by Dow AgroSciences describing the product and its use can be found on YouTube [https://www.youtube.com/watch?v=CEU_VSWmC1w&t=].

The active ingredient in Garlon RTU is triclopyr butoxyethyl ester (BEE). Garlon RTU guarantees 144 g acid equivalent per litre of product (ae/L) in the form of triclopyr BEE [1]. The rest of the product is made up of an oil and other adjuvants [Jump to general herbicide section]. While the adjuvant, or other ingredients, in Garlon RTU are proprietary information, other triclopyr BEE products are known to contain kerosene, ethylene glycol and solvent naphtha[5]. [Top of Page]

Garlon RTU is Effective On

According to the manufacturer, DowAgroSciences, Garlon RTU is effective on a variety of woody species including [4].

Alder
Aspen
Birch
Chokecherry
Cottonwood
Dogwoods
Pines
Poplar
Tamarack
Wild Rose
Willow

Other resources suggest it is effective on many annual broadleaved weeds [3]. It has little to no effect on grasses, which makes it useful for maintaining low-lying plant cover on rights-of-way, while removing trees [2]. [Top of Page]

Certification for Use in Canada

Registration No. 29334 Pest Control Products Act [6]. [Top of Page]

How Garlon RTU Works

The active ingredient triclopyr has three main forms, triclopyr butoxyethyl ester (BEE), triclopyr triethylamine salt (TEA)[7] and triclopyr acid, and is classified as a Group 4 herbicide. Group 4 herbicides mimic the plant growth hormone auxin, which regulates plant development, cell growth and cell division. Auxin is concentrated in the apical meristems, or growing parts, of plants. It tells plants to grow from the top, to bend towards the light and to grow roots downwards. The amount of auxin made by the plant is highly controlled, so that growth occurs at a measured pace [2,3].

Auxin mimicking herbicides disrupt this process by giving

plants essentially an unlimited amount of molecules that act

in the place of auxins. This tells plants to grow continually

and in an unsynchronized manner causing twisting and other

deformities. Exposed plants will grow lengthwise but will not

be strong. Eventually the cell structures will collapse and the

plant will die. Continuous growth also uses up all the plants’

energy reserves [3,4]. Triclopyr BEE gives plants growth

hormone at approximately 1000x normal auxin levels [7].

Once absorbed into the plant triclopyr BEE is transformed into triclopyr acid which is more easily spread up to leaves and down to the root system [3]. [Top of Page]

Environmental Degradation of Garlon RTU

Degradation pathways of triclopyr BEE. Half-lives are for breakdown in soil (brown), water (blue) and plants (green).

Triclopyr BEE is a compound made up of carbon, nitrogen, hydrogen, oxygen and chlorine atoms. Triclopyr BEE degrades into new compounds by incrementally losing parts of its structure. It breaks down extremely rapidly to triclopyr acid in both soil (min. 3 hrs) and water (min. 1 day)[7]. Triclopyr acid then degrades into one of two metabolites: trichloromethoxypyridine (TMP) and trichloropyridnol (TCP), with TCP being the main metabolite. A laboratory study showed that, in a soil column treated with triclopyr BEE, after 54 days 6% was triclopyr acid, 88% TCP and 7% TCP [7]. The metabolites TCP and TMP will in turn degrade to carbon dioxide (CO2), water (H2O) and other organic acids.

When herbicides are designed today, their environmental fate is taken into consideration. In the past industry only considered how effective herbicides were at killing target plants, which led to some unexpected and unintended side effects such as bioaccumulation in wildlife. However, today thought is put into controlling potential side effects of herbicide use, and herbicides are designed in such a way that they are unlikely to accumulate in wildlife and breakdown rapidly in the environment (i.e. in soil and water). Garlon RTU breaks down through photolysis (degradation from sunlight), hydrolysis (degradation from water) and microbial degradation in soils, eventually turning into carbon dioxide, water and organic acids. So how does this happen?

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Degradation in Soil

When applied using targeted techniques such as basal bark, cut stump or point injection, nearly all of the herbicide will be absorbed by the plant. A small amount may contact the soil around the plant due to careless application or wind drift. Triclopyr is not expected to be found in the air, as it is much more likely to stay dissolved in solution. Only trace amounts will evaporate into the air [7].

Should triclopyr BEE enter the soil, either by inaccurate spraying or decomposing plant material, there are two main ways it degrades. Microbial degradation (by soil bacteria and fungi) is the main pathway, while photolysis (light degradation) is a second important pathway [7]. How quickly triclopyr breaks down in soil is dependent on factors such as climate conditions (temperature, moisture, sunlight, etc.) and soil properties (soil type, pH, clay content, organic content, etc.). The degradation of a chemical is measured as a half life. That is the time in days or hours, that it takes for 50% of the chemical to breakdown.

In the soil triclopyr BEE is rapidly degraded by sunlight (photolysis) and soil microbes (microbial degradation) to triclopyr acid. It has an half life of 3 hrs – 1 day (degrades to triclopyr acid)[7] (Table 2). The half-life of triclopyr acid in soil ranges from 1.1 to 90 days [9]. The half life depends on soil type, moisture and temperature [7]. Generally triclopyr takes longer to degrade when conditions are colder, drier, or there is less direct sunlight. One study found a half-life of 12-27 days for triclopyr acid [7], while another found half-lives of 75 and 81 days in two hill pasture sites in Oregon, USA [11]. Most pertinent, a study done on a right-of-way in Yukon Territory found triclopyr acid had a 50% dissipation time of 1 day, with 90% dissipation in 3 days [12], while a study in a Northern Ontario forest found a half-life of 2 weeks [13].

Triclopyr acid appears to have two phases of degradation. The first phase is rapid degradation, while the second is a prolonged persistence of low-level concentrations. In a Northern Ontario forest 90% of the applied triclopyr had dissipated by 4 weeks. However residue levels less than or equal to 10% of the applied triclopyr were detected from 4 – 48 weeks with little evidence of further dissipation. The measured range of residues detected was 35-55 ug/kg, thus 35-55 ug (0.035 - 0.055 mg) were still detectable in 1 kg of soil [13]. Tests on a Yukon right-of-way found that 90% of applied triclopyr dissipated in 3 days, however triclopyr acid residues were detectable for 1 year [12]. A study in the Swedish boreal forest was able to detect triclopyr residues over two years after application [13]. However, in a clear cut forest in Alaska triclopyr was below detection limits within 100 days [15].

The half-life of TCP, the main metabolite of triclopyr acid, was found to range from 30-90 days [9] in one study and 12-229 days in an other [10]. Eventually the metabolites will break down into carbon dioxide [9].

Table 2: Dissipation rates of triclopyr in soil. References are in square brackets.

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Introduction

Classes of Herb

Garlon RTU

What is GRTU

GRTU effective on

GRTU Cert

How GRTU works

Enviro degrad GRTU

Degrad soil

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Degradation in Water

In water, photolysis (break down by sunlight) is the main degradation pathway. Garlon RTU is not approved for use near water. However studies have shown that in water, triclopyr BEE degrades to triclopyr acid in approximately one day via hydrolysis (break down by water molecules) and photolysis [7,16]. In a lab test, triclopyr BEE dissolved in water and kept in the dark, had a half-life of 8.7 days (hydrolysis only)[16] (Table 3). Triclopyr acid in water, exposed to sunlight, has been found to have a half-life of 1-10 days [9], 0.08 days (lab test, natural light)[10], and 1.3 days (river water, natural and artificial light)[17]. Triclopyr acid is not affected by hydrolysis [16], with a half-life of more than 270 days when kept in the dark [10]. Factors that would change the rate of photolysis in water include cloud cover, temperature, length of daylight, water clarity, and concentration of dissolved organics (soil and plant particles) in the water [10,16].

Table 3: Dissipation rates of triclopyr acid and triclopyr TBEE in water. References are in square brackets.

Degrad water

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Toxicity

Mammals

Triclopyr is considered to be of low toxicity to humans and other mammals. Since mammals do not have the auxin growth hormone, an auxin mimic like triclopyr will not have a direct affect. However, as with all things ‘the dose makes the poison’. If excessive doses are ingested then harmful effects would be expected.

The Tolerable Daily Intake (TDI) for triclopyr, that is the amount of triclopyr that can be ingested every day over a human lifetime without adverse effects, was determined to be 0.05 mg/kg bw/day (mg of herbicide ingested per kg body weight per day). This value includes a safety factor of 100, meaning that government regulations require daily food intake residues to be 100 times lower than the determined safe daily dose. This number is

based on a two generation study with rats. In this study no

negative effects were observed when rats were given daily

doses of 5.0 mg/kg bw [18]. Lab tests on rats are taken as

representative of other mammals (ex: humans, moose, deer,

wolves, etc.) so a daily dose of 5.0 mg/kg bw is generally

considered safe for otherwise healthy individual mammals [18].

Health Canada recognizes that herbicide residues such as triclopyr may end up in farmed food bought at grocery stores and have set Maximum Residue Limits (MRLs) for various pesticides. Below is a table showing acceptable residues of triclopyr for various food products (Table 4) [19].

Table 4: Maximum Residue Limits for Canadian food products as set by Health Canada [19]

Toxicity

mammals

If ingested, triclopyr is rapidly removed by the kidneys and excreted in urine [7]. The time it takes for 50% of ingested triclopyr to be eliminated (elimination half-life) is 3.6 hrs in rats, 7.2 hrs in beagle dogs, 6.3 hrs in rhesus monkeys and 5 hrs in human volunteers [9, 20]. Rats excrete 90% of ingested triclopyr unchanged in 24-72 hrs. In rats, some triclopyr is found in the fatty tissue and ovaries of females, while some is metabolized to TCP [9]. In a human volunteer study [20], only 1.65% of triclopyr BEE (Garlon 4) applied directly to the skin was absorbed over 8 hours, and 83% of the 1.65% absorbed was excreted in urine over 96 hrs with most being excreted in 12-24 hrs. The highest concentration detected in the blood was 0.08 ug/mL in one volunteer, and triclopyr was undetectable in blood after 72 hrs. No negative effects were observed. In a human volunteer study, participants given 0.5 mg/kg bw doses of triclopyr acid in apple juice, showed no adverse health effects. No triclopyr was detected in the blood after 48 hrs, and again 80% was excreted in the urine [20]. Triclopyr elimination occurred in two phases. An initial rapid phase with a half-life of 1.5 hrs (highest concentration of triclopyr in urine after 6 hrs), and a slower final phase with a half-life of 5 hrs [20]. There is a low chance of acute (single dose) toxicity in humans.

It has been estimated that only 10% of ingested triclopyr would be incorporated into deer and rabbit tissues, with the rest being excreted. For an application of 3.0 lbs/acre the estimated concentration in deer and rabbit tissue is 2.2 and 1.7 mg/kg bw respectively [7]. In general, triclopyr in forestry is considered to be of low toxicity to mammals [7,21].

There is no evidence of mutagenicity (changes to DNA) from triclopyr. Triclopyr is not classified as a cancer-causing agent by the EPA. Some developmental defects and a decrease in litter size were observed in rabbits and rats when mothers were given > 100 mg/kg bw/day, therefore triclopyr is considered to be moderately reproductively toxic [9]. [Top of Page]

Plants

The half-life of triclopyr acid in plant tissues has been reported as 3-10 days [9] and 4–291 days [7] in various studies. The half-life on fruit (cowberry) is 27 days (range: 16.5-73 days)[5].

On a Yukon right-of-way the dissipation time when triclopyr detected was at 50% of the initial concentration in willow leaves (Salix glauca) was 11.5 days. Seven days after treatment the leaves were limp and by 14 days they were drying out. After 30 days the leaves were completely dead so metabolism and breakdown of the herbicide had stopped [23]. The low quality of the leaves would likely discourage animals from grazing on them. Leaves that contain triclopyr, could potentially return triclopyr to the soil as leaves fall from trees and shrubs and decompose [23]. [Top of Page]

Fish and Aquatic Organisms

Triclopyr BEE is considered moderately to highly toxic to fish (*bluegill, rainbow trout and fathead minnow) (LC50 0.1 – 10 mg/kg), however it rapidly degrades to triclopyr acid, often in less than a day (Table 5). Triclopyr acid is considered only slightly toxic to fish*, and slightly to moderately toxic to the water flea Daphnia magna (LC50 1.7 – 12 mg/L)[7,9]. TCP is moderately toxic to fish [9]. [Explanation of LC50s]

Table 5: Available toxicity values for triclopyr acid, triclopyr BEE and Garlon RTU. References are in square brackets.

plants

fish aquat

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Integrated Vegetation Management on Boreal Saskatchewan

Rights-of-Way

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