Assessing Effects of Herbicides on Reproduction of Non-Target Terrestrial Plants in the Field: DOs and DON´Ts

To assess potential risks of herbicide use for non-target terrestrial plants (NTTPs), routine testing in form of standardized lower tier studies is conducted. In these studies, effects on vegetative parameters (e.g. shoot weight, emergence, survival) are measured in the greenhouse. Higher tier testing may be needed to assess whether the identified risks may occur under more realistic conditions in the field and whether the herbicide potentially affects reproduction of plants. We developed a method for conducting higher tier field studies with NTTPs (Isemer et al., 2020. A Field Study Method as a Potential Higher Tier Option to Refine Herbicide Risk Assessment for Nontarget Terrestrial Plants IEAM 16, pp 691-705) which allows for assessing and comparing effects on reproductive and vegetative parameters.

Throughout the growing season 2017 (May to September), the sulfonyl-urea herbicide iofensulfuron-sodium was tested on 11 plant species sown as a seed mixture for establishing artificial plant communities on a test acre in Germany. Three herbicide rates were applied, and vegetative parameters (vegetation cover, dry biomass of total vegetation, phytotoxicity, BBCH stage) were assessed. In order to measure effects on reproduction, fully ripened seeds were collected from the plants in the field during the last two months of the study and germination of the F1 generation was assessed in the greenhouse. Other reproduction measurements (e.g. number of flowers, yield) were regarded as less suitable because recording of these parameters in the field was proven to be infeasible in our study design. Furthermore, F1 seed germination is identified as an important parameter for plant species population dynamics. Suitability of the used test species for reproduction testing was judged by their seed´s availability in the test plots, collectability, seed handling and ability to germinate in the lab. In order to compare effects of different growing conditions, the germination tests were done in petri dishes on filter papers, in pots filled with soil as well as in the dark or in a dark/light regime, respectively. Results indicated that growing conditions need to be carefully considered when conducting germination experiments in order to not tamper effects of the herbicide treatment. In conclusion, vegetative endpoints were lower than reproductive endpoints confirming that reproduction is not generally the more sensitive parameter.