A Field Study Method as a Potential Higher Tier Option to Refine Herbicide Risk Assessment for Nontarget Terrestrial Plants

During herbicide spray application, nontarget terrestrial plants (NTTPs) growing in the off‐field area need to be protected from unacceptable effects of herbicide drift. The risk of such unintended effects is assessed in order to establish whether a particular use can be approved, possibly in combination with mitigation measures. In Europe, the risk of herbicide treatment to NTTPs is assessed on the basis of tier 2 studies done under controlled conditions in greenhouses. Following the concept of a tiered testing approach, higher tier field studies under more realistic conditions could be used to refine the risk assessment. No current guideline for conducting higher tier NTTP field studies is available. We developed an NTTP higher tier field study method done on an experimental plant community established by sowing of a seed mixture. The setup was optimized in 3 pilot field studies and subsequently used for a definitive study testing effects of the herbicide iofensulfuron‐sodium. Results show that the method can be regarded as a suitable higher tier option for assessing effects of herbicides on NTTPs. Growth of species from the soil seed bank cannot be avoided and has to be carefully considered when evaluating results. Adaptations of the study design may be necessary when testing different herbicides. Community‐level endpoints were at the same level as single‐species endpoints. Results of the field study were compared to standard greenhouse study results for the same herbicide. No observed effect rates (NOERs) in the field were about a factor of 10 higher and show that the current tier 2 risk assessment for NTTPs can be regarded as protective in this case. Whether the present field study design and the assessed endpoints can be used in higher tier risk assessment of NTTPs depends on selection of the specific protection goal and requires further discussion. Integr Environ Assess Manag 2020;16:691–705. © 2020 Bayer AG. Integrated Environmental Assessment and Management published by Wiley Periodicals, Inc. on behalf of Society of Environmental Toxicology & Chemistry (SETAC)