NCASI summarizes results from studies evaluating effectiveness of BMPs for forest herbicide applications

In recent decades, herbicides have emerged as a cost-effective approach for managing competing forest vegetation. For example, by temporarily suppressing competition at stand establishment, herbicides help ensure successful regeneration and compliance with reforestation standards. Studies have shown that control of competing vegetation using herbicides often results in 30-300% increases in wood volume yield for major commercial tree species in a wide range of site conditions.

Silvicultural chemical best management practices (BMPs) have been developed as a tool for protecting water quality and aquatic biota when silvicultural chemicals, fertilizers, and pesticides are applied in forests, thereby minimizing risks of adverse impacts on biota associated with streams and other waterbodies.

NCASI recently published Technical Bulletin No. 1038, Effectiveness of BMPs for Reducing the Risk of Adverse Impacts of Herbicides on Aquatic Organisms, prepared by Dr. Vickie L. Tatum. The report summarizes results from three recent studies that evaluated effectiveness of modern forestry best management practices for protecting water quality when forest herbicides are applied. The Technical Bulletin also describes the outcome of an EPA-type preliminary risk assessment for aquatic biota using measured stream water herbicide concentrations from those studies. 

Results from the three studies indicate that, when forest herbicides are applied using forestry BMPs, there is very minimal risk of adverse impacts on water quality or aquatic biota. The abstract from the Technical Bulletin follows.

“The use of herbicides to control competing vegetation during stand establishment is a key component of intensive silviculture. When silvicultural chemicals are applied to forest land, they have the potential to impact stream water quality. However, forestry best management practices (BMPs) have been developed as the primary mechanism for achieving water quality protection from non-point source (NPS) pollutants that may result from forest management. Operational forestry herbicide applications using modern BMPs were made at three distinctly different sites in the Coastal Range of Oregon (Needle Branch), east Texas (Alto), and southwest Georgia (Dry Creek). Streamside management zones (SMZs), ranging from 12 m to 21 m, were installed as specified in each state’s BMP guidelines. No-spray zones equivalent to the SMZs were maintained at Alto and Dry Creek. At Needle Branch, an 18-m no-spray zone was observed, as specified in the Oregon Forest Practices Act guidelines, for the fish-bearing portion of the stream. In addition, half-boom spraying, while not required, was employed along the upper non-fish-bearing portion of the stream. At each study site, maximum herbicide concentrations in stream water were in the low ppb range and occurred as brief (<24 hour) pulses associated with stormwater runoff from the first few post-application storm events and declining in magnitude with successive storms. At all three sites, maximum stream water concentrations of herbicides were much lower than concentrations associated with toxicity to fish, amphibians, or aquatic invertebrates. The lowest reported concentrations affecting 50% of the test population (EC50) for some species of algae and macrophytes are below peak concentrations reported for imazapyr, hexazinone, and sulfometuron methyl at one or more sites. However, exposure durations, especially to peak concentrations, reported in these field studies were much shorter than those used in the laboratory toxicity testing upon which the EC50 values are based, and there is ample evidence that aquatic plants can tolerate much higher exposures if exposure times are short, so it’s not clear that these exposures are a concern.”

Member Company employees can access the Technical Bulletin at www.ncasi.org. Member Company employees, as well as government and academic personnel, may request a printed complimentary copy of the Technical Bulletin via email (publications@NCASI.org) or by calling (352) 331-1745.

Contact information