Sulfonylurea Herbicides Water Quality Fact Sheet

Key Summary Point:

• Sulfonylurea herbicides represent a significant advance in protecting water resources. Contributing factors include low application rates, low toxicity, postemergence application timing, degradation in water, and monitoring information.

       A Best Management Practice (BMP) that is frequently advocated to reduce pesticide concentrations in surface water or groundwater is to reduce the amount applied. Many alternative products to sulfonylureas are used at rates 10-100 times higher. Research has shown that often between 0.5 and 2 percent of the amount of herbicide applied can be lost in surface water runoff. This means that the concentrations of some older high-use-rate herbicides detected in surface water runoff are equal to the concentration of sulfonylureas that are applied to fields at recommended weed control rates.

       Sulfonylureas are also no more toxic, and in many cases are less so, than the products they displace. So, when you combine the fact that there are much lower concentrations of sulfonylureas in surface water and that sulfonylureas are often less toxic than alternative products, it is easy to understand why sulfonylureas are important to water quality.

       Other relevant facts are when and how sulfonylureas are applied. Most sulfonylureas are applied postemergence, with several ram)fications for water quality. These products are only applied after a weed problem has been identified rather than preemergence where applications are made prior to a sign)ficant problem being diagnosed. As a result, only the parts of the field with a weed problem will be treated rather than the whole field. With postemergence applications, a sign)ficant amount of the applied herbicide lands on plants which intercept and absorb the herbicide and, therefore, render it unavailable for runoff. The weeds are treated, not the soil. Also, postemergence products are applied later than preemergence products. At this later timing, the soil is drier and rainfall is generally less. Both these conditions lessen the probability of a runoff event occurring.

       Sulfonylureas degrade in the env~ronment by two major mechanisms: microbial degradation and chemical hydrolysis. Most herbicides are degraded by microbes. Sulfonylureas are degraded by microorganisms, but they also break down in the presence of water, a process known as hydrolysis. Most other herbicides don't have this degradation mechanism. As a result, if a sulfonylurea enters surface water or groundwater, degradation will still continue. The half life of sulfonylureas (i.e., the time necessary for 50 percent to degrade) is generally no longer than other non-sulfonylurea herbicides and, in many cases, is much shorter. This is particularly true of the newer DuPont sulfonylureas which have been specifically designed to degrade faster.

       The water quality monitoring data collected on sulfonylureas confirm all the statements made above. While the amount of water quality data on sulfonylureas is limited, the concentration of sulfonylureas detected in surface water and groundwater are much lower than older higher-use-rate products. Higher-use-rate products are often detected in surface water at the edge of a treated field at concentrations in the hundreds of parts-per-billion (ppb). Sulfonylureas, on the other hand, are generally found at concentrations less than 10 ppb at the field edge. Monitoring of surface water streams and rivers as well as groundwater generally results in no detections of these products. As science advances, our ability to detect increasingly lower concentrations of chemicals in our environment also advances. In the future, as analytical detection capabilities continue to improve, it is possible there will be detections of sulfonylureas at very low levels in water. Based upon the studies already conducted, these concentrations will not cause harm to people or the environment.