Herbicides are valuable tools in agriculture; however, off-field non-target terrestrial plants (NTTPs) can potentially be inadvertently exposed via spray drift.  The current risk assessment framework for NTTPs is a three-step process involving (1) toxicity evaluation of standard test plants directly oversprayed with a range of exposure levels (rates) to determine effects endpoints corresponding to regulatory levels of protection, (2) if standard exposure estimates are exceeded, generation of a downwind drift exposure curve based on predictive modeling or deposition data, and (3) if needed, determination of a conservative buffer distance at the intersection of the toxicity endpoint and the drift curve.  In this presentation, we share a refined framework, in which plants are exposed to spray drift deposits under representative field conditions to generate a more realistic exposure scenario than is possible with the current framework.  The experimental design consisted of sequential rows of sensitive plants placed at distances downwind from the spray path of an outdoor herbicide application.  Interspersed with the plants in each row were horizontal deposition collectors and vertical airborne drift collectors.  Herbicides were sprayed at maximum target application rates and wind speeds at or above those specified on product labels, using the EPA Spray Drift Reduction Technologies protocol.  Toxic effects were measured against untreated controls for two herbicides based on the two most sensitive plant species in their respective standard EPA guideline vegetative vigor studies.  This enabled direct measurement of a “No Observed Effects Distance” (“NOED”) for each herbicide/plant combination from an actual spray drift event using a low-drift nozzle.  The deposition and airborne drift collectors provided a two-vector profile of the drifting spray cloud encountered by the plants at each distance that, when coupled with the effects data, gave additional insight into the nature of the interaction between spray droplets and plant structural features.

Higher tier framework for determining appropriate buffer distance to non-target plants

254th National Meeting (2017)

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