Field managers have the difficult job of maintaining uniform, dense and aesthetically pleasing stands of turfgrass under conditions of very intense traffic. During the process of managing an athletic field, the application of fertilizers is essential to improve the growth and performance of the turfgrass. In addition, while we may not necessarily wish to do so, it’s often necessary to apply pesticides to control weeds, insects, or diseases that may infest our fields. In sports turf, the issue of having uniform, vigorous and well-maintained turf goes beyond just the aesthetics – it’s also a safety issue. So, while we may not wish to apply pesticides (they do, after all, cost money) they’re often necessary tools that we use in our efforts to achieve the goal of an attractive, safe playing surface.
Pesticides, by definition, are products that are designed to control something (weeds, insects, diseases). Because of this, the use of pesticides is controversial. There’s concern that pesticides may have effects that are caused by either acute or chronic exposure that may result in health problems. These concerns range from the short-term effects of exposure – including dizziness and headaches – to long-term effects like potential carcinogenicity. In some parts of the country, the application of pesticides may be restricted or banned, either on school property or in some cases to any amenity turf within a jurisdiction.
Given the concerns over the application of pesticides and potential consequences to users and to the environment, it should be the goal of every sports turf manager to apply pesticides responsibly, only when needed and in a manner that minimizes the risk of potential harm to end users and the environment.
The focus of this month’s article is on the environmental fate of pesticides after application and management strategies that can minimize the risks of off-target movement.
What happens to pesticides applied to turfgrass?
In an ideal world, a pesticide would go just to its intended target after application (or be taken up by the plant), do its job and then dissipate or break down into harmless materials. In reality, there are several competing processes that affect the fate of a pesticide. Above the soil surface, pesticides may be degraded by sunlight (a process known as photo-decomposition), be transported from the target site in flowing surface water (runoff), volatilize and be lost to the atmosphere, or be carried away by the wind (via drift).
In the soil, there may be chemical reactions that decompose the pesticide, it may bind to soil particles and organic matter, or it might leach and potentially contaminate groundwater. When bound to soil particles pesticides can be degraded by soil microbes. The importance of each of these fate processes is complicated and depends on the chemistry of the pesticide and its interaction with environmental conditions at or after application, including temperature, soil water content and soil type.
For most pesticides, the most important processes that govern fate are sorption of the pesticide to soil particles, followed by microbial degradation. However, leaching and runoff are potentially significant avenues of loss as well. The potential for leaching losses depends on the interaction between the pesticide applied and the soil type. Leaching and runoff losses are potentially higher when you use water-soluble pesticides. On the other hand, preemergence herbicides, for example, tend to be water-insoluble compounds, thus significant leaching is unlikely to occur.
Soil type also has an effect. Sandy soils will allow more rapid vertical flow of water, thus making leaching potential higher compared to a clay soil. When a particular pesticide has chemical properties suggesting a leaching hazard, there’s a section on the label (environmental hazards) that will talk about the risks and management practices to be used, or areas to avoid with the product.
Many of the pesticides that we use in turf have low leaching potential. In fact, with some of the insecticides to control grubs, it can be a challenge to get the pesticide past the thatch and into the soil. Instead of leaching, most pesticides applied to turf tend to adsorb to soil particles or organic matter associated with the soil or thatch. The extent of binding to the soil particles depends on, among other things, the chemical properties of the pesticide. Glyphosate, for example, binds very tightly to soil and is very unlikely to leach or runoff. When this adsorption occurs, the pesticide is then broken down by soil microbes (Figure 1). The pesticide is bound to the soil and there are microbes that move in the film of water surrounding the soil particle. When they encounter the pesticide, they degrade it. This microbial degradation is initially quick, but as the concentration of pesticide goes down, it takes longer for the microbes to encounter the remaining pesticide. Therefore, the degradation curve flattens out with time.
The rate at which a pesticide is broken down is referred to as the half-life – a measure of the pesticide persistence. The half-life of a pesticide varies considerably depending on product applied, temperature, soil moisture and prior pesticide usage. The longer a product persists, the more likely it is to move off-site to cause environmental contamination.
Volatilization does occur but most pesticides are not lost this way. An easy way to avoid volatilization losses is not to apply pesticides that are known to be volatile (for example, some ester forms of broadleaf herbicides) during hot and/or dry weather.
Drift occurs when wind carries the application of the pesticide away from its intended target. Of course, an easy way to avoid drift is to not spray when it’s windy. You can also adjust spray pressure, or change nozzles in a way that makes the spray droplets larger and thus less likely to be carried away by wind.
While not an avenue of environmental fate, this can happen to pesticides and can cause controversy for the turfgrass industry. We know that most pesticides bind tightly to soil and/or organic matter and under conditions found when, for example, playing golf. We also know that the potential for the pesticide to come back off the treated surface(dislodge) and contaminate socks or clothing is low. But some pesticide can dislodge from the treated surface and, in sports where there is considerable contact with the turf surface, (football or soccer) there is an enhanced chance of this occurring (Figure 2). The amount of pesticide that dislodges is low, so the concern isn’t for effects from acute toxicity but rather long-term effects, such as potential for increased risk of certain cancers.
Assessing risk due to chronic exposure, where an organism is exposed to very small amounts of a particular substance over the course of a long period of time, is very difficult to do. Some of the pesticides we use have been implicated as being carcinogenic and when this happens, the label either changes or the product is removed from the market. But there are pesticides registered for turf that have been implicated as potentially causing chronic health concerns. An example is 2,4-D. In some locations, the use of this pesticide is banned by local ordinance. While some studies have implicated 2,4-D, the overall results are inconclusive, which is why the product remains registered.
The important thing to remember with any pesticide is to carefully follow the label to minimize the risk of exposure to the end user. In most states, you’re allowed to use a treated surface once the pesticide has dried. Current research in dislodgeable residues on athletic turf suggests that the most significant risk of exposure occurs within seven days following application of the pesticide. But this varies depending on numerous factors (product choice, weather after application, etc.) and some products will persist and present a potential hazard for a longer period. Using this as a guideline, an easy way to minimize the risk of exposure due to dislodgeable residues is to either not use or minimize pesticide use during the season of play, or schedule application in such a way as to maximize the amount of time before the field is used again. Another step to take, as long as it doesn’t decrease the effectiveness of the pesticide, is to water it in immediately after application.
Preventing unwanted environmental contamination
Factors that contribute to potential unwanted environment contamination include the product used, soil type, characteristics of the site and management practices (Table 1). If a pesticide is persistent, highly water soluble, or doesn’t bind tightly to soil, then it’s more likely to move from its intended site to cause contamination. If the pesticide is applied to sandy soil, or soil that’s low in organic matter, then the chances of leaching increase compared to application of the same product to a clay soil, or one high in organic matter. If the site of application has a shallow water table, or is near surface water, this increases the chances of contamination. Also, if certain pesticides are applied to sloping land the chances of runoff losses increase.
Management decisions can also affect pesticide fate. If a pesticide is misapplied, overapplied, or not timed correctly, then the chances of unwanted contamination can increase.
Many studies were funded at universities throughout the 1980s to the early 2000s. Most showed that leaching losses in turf were minimal. But these studies did identify that runoff losses could, in some cases, be significant. Researchers have also found that management practices that improved infiltration rates could reduce the chances of runoff loss of pesticides and fertilizers. Specifically, they found that practices such as core cultivation and vertical mowing could drastically reduce the amount of pesticide recovered from runoff water. Thus, to increase the chances that the pesticide isn’t going to cause unwanted environmental contamination due to runoff, field managers should promote practices that increase infiltration rates on the surface. The organic matter associated with turfgrass thatch and soil can then do a significant job of binding and microbes can then break down the pesticides.
Labels should also be followed exactly, including the section on environmental hazards, which will alert you to any potential problems that the product could cause. You can also do your part to reduce the use of or avoid products that have chemical or physical properties that make them more likely to cause unwanted contamination.