Herbicide Tank Mixtures – Potential Interactions

The toxicity of chemical mixtures can be described in one of three ways: additive, antagonistic, or synergistic. When mixture effects are equal to the sum of effects of each individual component, mixture toxicity is said to be additive. When mixture effects are greater than expected based on the sum of effects of each individual component, mixture toxicity is said to be synergistic. When mixture effects are less than expected based on the sum of effects of each individual component, mixture toxicity is said to be antagonistic.

Concerns are sometimes expressed about potential toxicity of forestry herbicide tank mixtures, which may contain two or more herbicides plus a surfactant and other inert (non-herbicidal) ingredients (e.g., defoamers). However, any effects of mixtures with additive toxicity are readily predictable based on the toxicities of individual components. This predictability allows EPA, applicators, and land managers to identify and implement mitigation measures to minimize risk of adverse effects. And when mixture effects are antagonistic, mitigation measures based on assumptions of additive effects are more than sufficiently protective. Synergistic mixtures could be problematic because effects may not be predictable and may exceed expectations, but there is considerable evidence that synergistic effects of herbicide mixtures are not an issue in actual practice.