Side effects of some of the most important sugar beet herbicides on non-target aquatic macrophyte

The widespread use of herbicides in crops can often contaminate aquatic ecosystems. Aquatic macrophytes such as duckweed (Lemna minor L.) are used as a suitable group to evaluate the environmental changes caused by herbicides. To evaluate the toxicity of six commercial herbicides including desmedipham, phenmedipham, desmedipham+phenmedipham+ethofumesate, clopyralid, quizalofop‑P‑ethyl and technical substance of chloridazon used in sugar beet fields of Iran using standard phytotoxicity test of Duckweed (Lemna minor L.), an experiment was carried out in a complete randomized design with four replications at Lorestan University in 2017. For all herbicides used, eight different concentrations of the active ingredient of each of the above-mentioned herbicides were considered as dose-response with control treatment. Toxicity evaluation was based on inhibition of relative growth rate (RGR) of duckweed after seven days. The results of the experiment showed that all the herbicides used affected the growth of duckweed. The EC50 values which derived from the log-logistic fitted curves, showed that the desmedipham+phenmedipham+ethofumesate is more toxic compared with other herbicides used in sugar beet field (EC50 = 1.04 μg/L) and caused a significant decrease in the relative growth rate of duckweed at a much lower herbicide concentration than other herbicides. Clopyralid was made less toxicity effects on duckweed than other herbicides (EC50 = 71.93 μg/L). The order of phytotoxicity of the herbicides applicable to sugar beet can be as follows: desmedipham+phenmedipham+ethofumesate > chloridazon > phenmedipham > desmedipham > quizalofop‑P‑ethyl > clopyralid. Therefore, when using these herbicides, the threshold of aquatic species tolerance such as duckweed which are vital for the formation of food chains and the functioning of aquatic ecosystems should be considered as not to reduce or destroy them.