A comparative study of rac- and S-metolachlor toxicity to Daphnia magna

Racemic product of metolachlor contains two R enantiomers and two S enantiomers. S-metolachlor is now widely used instead of rac-metolachlor because the former is more effective in herbicidal activity than the latter. The acute and chronic toxicities of rac- and S-metolachlor to Daphnia magna were determined and compared in this study. The acute 24-h LC50s for rac- and S-metolachlor to D. magna were 69.4 and 51.2 mgL(-1), respectively, indicating that the acute toxicity of S-metolachlor is slightly higher than that of rac-metolachlor. In chronic test, lowest-observed-effect concentration (LOEC), no-observed-effect concentration (NOEC), days to first brood, length, longevity, number of broods per female, number of young per female, and the intrinsic rate of natural increase (r) were determined. The results showed that the LOEC and NOEC of rac-metolachlor were 0.01 and 0.001 mgL(-1), while those of S-metolachlor were 0.5 and 0.1 mgL(-1), respectively. The first brood day of D. magna was not affected by rac- or S-metolachlor. Longevity and the number of broods per female were significantly (P<0.05) affected when rac-metolachlor concentration was higher than 1.0 mgL(-1) but were not significantly affected until S-metolachlor concentration was higher than 10 mgL(-1). The length was affected at the same concentration of rac- and S-metolachlor. Number of brood per female was significantly reduced when rac-metolachlor concentration was higher than 0.01 mgL(-1) but not significantly reduced until S-metolachlor concentration was higher than 0.5 mgL(-1). The r values were significantly reduced when the rac-metolachlor concentrations were above 0.01 mgL(-1) but not reduced only when S-metolachlor concentration was higher than 0.5 mgL(-1). The results of chronic toxicity indicated that rac-metolachlor was significantly (P<0.05) more toxic than S-metolachlor to D. magna.