What happens to herbicides after they are applied? This two-part series discusses environmental factors and herbicide properties that influence the environmental fate of several herbicides used on range, pasture and natural areas .
The ideal herbicide is one that controls the targeted invasive plants for a desired period of time and then rapidly degrades or breaks down in soil to naturally occurring compounds.
Understanding how long an herbicide remains active in soil is very important. This determines the length of time invasive plant seedlings can be controlled and also influences how long a land manager must wait before seeding susceptible desirable plants on a treated site.
Herbicides vary in their potential to dissipate in the environment. As soon as an herbicide is applied to its target, several processes immediately begin to remove the compound from the site of application. This dissipation refers to the degradation, movement or immobilization of an herbicide in the environment or plant, and is the process called environmental fate (Figure 1).
Part I: Environmental Factors
Environmental factors that influence herbicide fate in the environment include rainfall, sunlight intensity, microbial populations, and soil texture, organic matter, and chemistry (pH). Once an herbicide is applied it is intercepted by plants or contacts the soil surface directly. Herbicides applied to plant foliage are absorbed by the plant itself, washed off the foliage by precipitation and onto the soil, undergo photodecomposition on plant surfaces or the soil surface, or volatilize into the air.
Herbicides that fall directly on the soil surface or are washed onto the soil may be degraded or transported within soil. Degradation processes include biological degradation by soil microorganisms and abiotic chemical and photochemical transformations. Transport of herbicides within the soil can occur downward into the soil profile (leaching), across the soil surface (runoff), or into the air (volatilization).
Soil is one of the most important environmental factors influencing dissipation and movement of an herbicide following application. Soil serves as both a physical and temporal trap, filtering and slowing movement of the herbicide following rainfall events. It also serves as a chemical trap since soil has both positive and negative charged particles, which can bind herbicides. Soil texture, pH and microbial composition are important factors that influence herbicide persistence. These factors are described below.
Soil texture and composition is determined by the relative amounts of sand, silt, and clay in the soil as well as by the organic-matter content (Figure 2). In general, herbicides may persist longer in medium to fine textured soils (loam, clay loam and clay) with an organic matter content of more than 3 percent because more of the herbicide can bind to soil and organic matter particles. Coarse- to medium-textured soils (sands and sandy loams) with a lower organic matter content (less than 3 percent), are less likely to retain herbicides. Herbicidal activity and persistence are influenced by an herbicide’s ability to bind to soil (adsorption) or move through the soil column. Soil structure, or the arrangement of soil particles into units called soil aggregates, and porosity can also influence the ability of water and herbicides to move in soil.
Soil pH is an important chemical property influencing persistence of some herbicides, including those in the sulfonylurea family (e.g. metsulfuron*, sulfometuron). Degradation rates of this herbicide family slows as soil pH increases above about pH 7.0. Soil pH levels below 6.0 allow a more rapid dissipation of this herbicide family. The influence of soil pH on herbicides in the growth regulator family (e.g. Milestone®, Vastlan™, Garlon® 4 Ultra, or Transline® specialty herbicides) is minimal.
Soil microorganisms are one of the most important pathways responsible for the breakdown of many herbicides including Milestone, Transline, Tordon® 22K, Vastlan, and Garlon 4 Ultra specialty herbicides, 2,4-D and others. The type of microorganisms (fungi, bacteria, protozoans, etc.) and their relative numbers determine how quickly decomposition occurs. Microorganisms require certain environmental conditions for optimal growth and utilization of an herbicide. Factors affecting microbial activity are moisture, temperature, pH, oxygen, and mineral nutrient supply. In general, a warm, moist, porous, fertile soil with a near-neutral pH is most favorable for microbial growth, which accelerates breakdown of herbicides that are susceptible to microbial degradation.
*Metsulfuron is one of the active ingredients in Opensight® specialty herbicide and sulfometuron is the active ingredient in Oust herbicide.
Climatic factors involved in herbicide degradation include moisture, temperature, and sunlight. Herbicide degradation either by microbial or chemical processes generally increase as temperature and soil moisture increase.
Rainfall amount, duration, frequency and timing following herbicide application can influence herbicide persistence and downward movement in the soil profile. High rainfall immediately following application may move the herbicide deeper in the soil profile (leaching) where microbial activity may be less (Figure 4). Other factors that influence how far an herbicide will move in the soil profile include herbicide-soil binding properties, soil physical characteristics, rainfall frequency and intensity, herbicide concentration, and soil moisture content at time of herbicide application. In general, herbicides that are less soluble in water and strongly attracted to soil particles remain in the upper several inches of the soil profile, particularly in dry years.
Sunlight, the ultraviolet (UV) portion, can be an important factor for degrading herbicides through a process called photodecomposition. If an herbicide absorbs light in the UV range, the molecular structure is changed when chemical bonds are broken. All herbicides are susceptible to photodecomposition to some degree, but the effect is greater in certain herbicide families than others. For most herbicides applied on range, pasture and natural areas, some loss may occur when herbicides are applied on plant leaf surfaces during dry, sunny conditions. However, once soil contact is made, losses due to photodecomposition are small.
Part 2: Herbicide Properties
Herbicide molecules are eventually decomposed to carbon dioxide (CO2) water, and salts through photochemical, chemical, or biological (microbial) reactions. When an herbicide degrades, it usually yields several compounds (metabolites), each of which has its own chemical properties including toxicity, adsorption capacity, and resistance to degradation. For example, aminopyralid (Milestone® specialty herbicide) produces no metabolites other than CO2 and ammonia (NH3), and has a low bioaccumulation potential. Chemical properties that influence herbicide degradation in the environment include the following parameters:
One of the most important factors determining herbicide fate in the environment is its solubility in water. Water-soluble herbicides generally have low adsorption capacities, and are consequently more mobile in the environment and more available for microbial metabolism and other degradation processes. Table 1 shows solubility for seven different herbicides.
Adsorption is the attraction of ions or molecules to the surface of a solid is shown as a Koc value. After application many herbicides adsorb to clay and organic-matter fractions of soils with relatively poor adsorption to sand and silt soil fractions. Thus, the extent of herbicide adsorption increases as the percentage of organic matter and/or clay content increases. Herbicides with a high Koc value bind more tightly to soil. For example, glyphosate has a Koc of 24,000 and is tightly bound in most soils. Table 1 shows Koc values for seven different herbicides.
The average length of time (days, weeks, months) it takes an herbicide to reach one-half of the originally applied dose is an herbicide half-life. This value is determined by the herbicide molecule's susceptibility to chemical or microbial alteration or degradation. Half-life of a particular herbicide can vary significantly depending on soil characteristics, weather (especially temperature and soil moisture), and vegetation at the site. An herbicide’s chemical structure dictates how the herbicide will degrade in soil. Some herbicides are rapidly decomposed by microorganisms if the right populations are present and if soil conditions are favorable for their growth. For example, 2,4-D, Milestone, Tordon 22K, and Transline specialty herbicides are decomposed by microorganisms. The chemical structure of 2,4-D, allows microbes to quickly detoxify the molecule into inactive metabolites. Table 1 shows half-life values for seven different herbicides.
Photodecomposition (degradation of an herbicide by sunlight)
Sunlight intensity varies with numerous factors including latitude, season, time of day, weather, pollution, and shading by soil, plants, litter, etc. Studies of the photodecomposition of herbicides are often conducted using UV light exclusively. It is important to remember that UV light is present on cloudy days and can photodegrade herbicides sensitive to UV light. For most range, pasture and natural area herbicides degradation by sunlight is relatively minor importance in field applications.
Vapor Pressure (volatility)
The vapor pressure of an herbicide determines its volatility, a process whereby an herbicide changes from a liquid or solid to a gas. Volatile herbicides (those with higher vapor pressures) generally dissipate more rapidly than herbicides with lower vapor pressures. Volatilization increases with temperature and moisture. Volatilization is more common if an herbicide is applied to a inert surface likes rocks or gravel. Most herbicides are relatively nonvolatile under normal field-use conditions. Dicamba is an example of an herbicide that can volatilize under warm to hot temperatures.
Table 1. Solubility, adsorptive potential (Koc) and half-life are three important factors influencing herbicide degradation and movement in soil. Values for seven different herbicides are shown .
 The larger the number the more soluble the herbicide is in water
 The larger the number the tighter the herbicide binds to soil.
 Majority of values obtained from Herbicide Handbook, Tenth Edition. WSSA.
Relevance to Field Applications
The question of how long an herbicide remains active in the soil system and how far it can move downward in a soil system is based on multiple factors, both chemical and environmental. Field studies are conducted on herbicides to measure their movement and dissipation under various conditions. For example, field studies conducted on Tordon® 22K and Milestone® specialty herbicides indicate that under dryland conditions dissipation of Milestone is relatively rapid, and the majority of both herbicides remains in the top 6 to 18 inches of soil. In these studies, Tordon 22K was applied at the maximum spot treatment rate of 2 quarts per acre in western Montana. In Manitoba, Canada, Milestone was applied at the maximum broadcast use rate of 7 fluid ounces per acre. The percent herbicide remaining in different portions of the soil profile and depth herbicide moved is shown in Figure 2. Neither Milestone nor Tordon 22K were detected in ground or surface water in these studies.
Figure 2. Movement and degradation of herbicides applied at field sties in Manitoba, Canada (Milestone® specialty herbicide) and Western Montana (Tordon® 22K specialty herbicide). Figures show the percent of herbicide remaining at various depths within the soil profile under different field conditions 90 days after application (90 DAA).
IN SUMMARY, THERE ARE GENERAL CONCLUSIONS THAT CAN BE MADE REGARDING FATE OF HERBICIDES IN THE ENVIRONMENT BASED ON FIELD STUDIES CONDUCTED ACROSS THE UNITED STATES AND CANADA.
- Water (rainfall or irrigation) is an important environmental factor regulating herbicide movement in soil. Precipitation is limited on most lands in the western United States reducing the potential for downward movement of an herbicide in the soil system.
- Soil is an important environmental factor influencing dissipation and movement of an herbicide following application. Soil serves as both a physical, temporal, and chemical trap, filtering and slowing movement of the herbicide following rainfall events.
- Herbicide properties that are most important in influencing herbicide degradation following application include solubility, absorption, and half-life. The majority of herbicides applied for noxious weed control are soluble in water, have a relatively low adsorption potential (with exception of glyphosate), and a moderate to short half-life in soil.
- In general, the majority herbicides applied to range, pasture and natural areas remain in the top 6 to 18 inches of soil if they applied under non-irrigated conditions.
- Improve your skills and expand your understanding of the science behind selecting, applying, and assessing the effects of herbicides. Explore TechLine’s “Understanding Herbicides” series. http://techlinenews.com/herbicides/
You might also like these articles from TechLine’s “Understanding Herbicides” series.
Curran, WS. Persistence of Herbicides in Soil. Pennsylvania State Univ. Extension Agronomy Facts Pub. #36. Accessed 29 April 2015. http://pubs.cas.psu.edu/freepubs/pdfs/uc105.pdf
Dow AgroSciences. Personal communication on aminopyralid movement in soil in Manitoba, Canada.
Helling CS, PC Kearney, and M Alexander. 1971. Behavior of Pesticides in Soil. Adv. Agron. 23:147-240
Herbicide Handbook. 2014. Weed Science Society of America. Champaign Ill. P. 500.
Murray TR. 1999. Turfgrass Herbicide Mode of Action and Environment Fate. Univ. of Georgia. College of Ag and Environ. Sci. Accessed 29 April 2015. http://www.commodities.caes.uga.edu/turfgrass/georgiaturf/WeedMngt/weedcontrol/TURFMOA.PDF
Tu M, C Hurd, and JM Randall. 2001. Weed Control Methods Handbook. Tools and techniques for use in natural areas. Chapter 6. The Nature Conservancy. pp 6.1-6-13.
Watson VJ, PM. Rice, and EC. Monnig. 1989. Environmental fate of picloram used for roadside weed control. J. Environ. Qual. 18:198-205.
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Milestone and Vastlan are not registered for sale or use in all states. Contact your state pesticide regulatory agency to determine if a product is registered for sale or use in your state.
Milestone and Opensight: When treating areas in and around roadside or utility rights-of-way that are or will be grazed, hayed or planted to forage, important label precautions apply regarding harvesting hay from treated sites, using manure from animals grazing on treated areas or rotating the treated area to sensitive crops. See the product label for details.
State restrictions on the sale and use of Garlon 4 Ultra, Milestone, Opensight and Transline apply. Consult the label before purchase or use for full details. Always read and follow label directions.
Tordon 22K is a federally Restricted Use Pesticide.
Some states require an individual be licensed if involved in the recommendation, handling or application of any pesticide. Consult your local Extension office for information regarding licensing requirements.