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.
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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
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Milestone and Vastlan specialty herbicides 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 specialty herbicides apply. Consult the label before purchase or use for full details. Always read and follow label directions.
Tordon 22K specialty herbicide 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.