Asulam residues in dandelion flowers after a herbicide treatment

The application of asulam-based herbicides on grassland could explain the finding of sulfanilamide in some Swiss honeys. In fact, this antibiotic belonging to the sulfonamides is produced during the breakdown of asulam. To assess the risks related to the use of these herbicides, a permanent meadow with a high proportion of dandelions was sprayed before or at the beginning of dandelion flowering. Asulam as well as sulfanilamide residues were analyzed in dandelion flowers by liquid chromatography. Very high asulam contents were observed in flowers collected just after the herbicide application. However, asulam residues in the flowers collected one or more weeks after the herbicide application were much lower, independently of the time of spraying. We also found sulfanilamide in dandelion flowers, but at relatively low levels. Our results support the hypothesis that asulam is involved in the contamination of honey by sulfanilamide and justify a modification of the registration conditions for asulam-based herbicides in Switzerland: these must be applied only on grassland free of flowering plants.