Is the tier-1 effect assessment for herbicides protective for aquatic algae and vascular plant communities?

In the aquatic tier-1 effect assessment for plant protection products with an herbicidal mode of action in Europe, it is usually algae and/or vascular plants that determine the environmental risks. This tier includes tests with at least 2 algae and 1 macrophyte (Lemna). Although such tests are considered to be of a chronic nature (based on the duration of the test in relation to the life cycle of the organism), the measurement endpoints derived from the laboratory tests with plants (including algae) and used in the first-tier effect assessment for herbicides are acute effect concentrations affecting 50% of the test organisms (EC50 values) and not no-observed-effect concentrations (NOECs) or effect concentrations affecting 10% of the test organisms (EC10) values. Other European legislative frameworks (e.g., the Water Framework Directive) use EC10 values. The present study contributes to a validation of the tiered herbicide risk assessment approach by comparing the standard first-tier effect assessment with results of microcosm and mesocosm studies. We evaluated EC50 and EC10 values for standard test algae and macrophytes based on either the growth rate endpoint (ErC50) or the lowest available endpoint for growth rate or biomass/yield (Er/EyC50). These values were compared with the regulatory acceptable concentrations for the threshold option as derived from microcosm and mesocosm studies. For these studies, protection is maintained if growth rate is taken as the regulatory endpoint instead of the lowest value of either growth rate or biomass/yield in conjunction with the standard assessment factor of 10. Based on a limited data set of 14 herbicides, we did not identify a need to change the current practice. Environ Toxicol Chem 2018;37:175–183

Effects of the herbicide metsulfuron-methyl on a plant community, including seed germination success in the F1 generation

A field trial was set up to simulate a field margin environment to analyze sub-lethal effects of the herbicide metsulfuron-methyl on several endpoints of non-target terrestrial plants (NTTPs). Both vegetative and reproductive endpoints were evaluated. The experiment was conducted in an experimentally established field strip with sown species. The treatments consisted of five dosages and a control: 0, 0.0097, 0.0193, 0.058, 0.174, and 0.348 gram active ingredient per hectare (g a.i./ha). The plant cover, number of (flowering) individuals per species and fruit collection were performed and estimated weekly for a period of 4 months. At the end of the growing season, the total dry biomass per species was obtained and the collected fruits were weighed, counted, and sieved to obtain the seeds. The seeds were counted and weighed as well, before they were used in a germination experiment to test the seed emergence of the F1 generation. The herbicide only affected the biomass of Matricaria recutita at the treatment levels tested (0.058 g a.i./ha and higher). Field dosages of 0.174 and 0.348 g a.i./ha differed significantly in the endpoint "plant cover" compared to lower dosages and controls. The F1 generations of Sinapis alba, Centaurea cyanus, and Phacelia tanacetifolia were particularly affected at field dosages of 0.0193 g a.i./ha and higher, showing significantly lower seed germination rates. This would imply that spray drift of metsulfuron-methyl might lead to shifts in species compositions and succession in vegetation in off-crop areas adjacent to arable fields. Conducting germination experiments is necessary to investigate a herbicide's effect on the full life cycle of plants

Mixture Toxicity of Herbicides with Dissimilar Modes of Action to Myriophyllum spicatum

Considering the vital role of rooted macrophytes in the aquatic ecosystem, validating assumptions on the interactive effects of herbicides with different modes of action at an environmentally relevant mixture ratio is necessary. We investigated the effects of diflufenican (a carotenoid biosynthesis inhibitor) and iodosulfuron-methyl-sodium (IMS; an acetolactate synthase inhibitor) in a 14-day growth inhibition experiment with Myriophyllum spicatum, wherein single compounds and their combination were tested in parallel (n = 84). The assessment was done using three different methods: significance testing, model deviation ratio (MDR), and mixture interaction factor (MIF). Interactions relative to both concentration addition and independent action were assessed via significance testing. This revealed that diflufenican and IMS acted antagonistically relative to both models for fresh weight and total shoot length (p < 0.05) and that there was slight synergism for the number of side shoots (p < 0.001) relative to concentration addition. The MDR and MIF can only assess interactions relative to the concentration addition model. According to MDR, the mixture appeared to show no interaction (neither antagonistic nor synergistic), whereas the MIF method revealed that the compounds acted antagonistically for fresh weight and that there was a slight synergism for total shoot length and number of side shoots. We conclude that inferences about mixture toxicity interactions are method- and endpoint-dependent, which can have implications for regulatory mixtures assessment. Environ Toxicol Chem 2022;00:1–12