Fate of the Benzimidazole Antiparasitics Flubendazole and Fenbendazole in Manure and Manured Soils

AbstractThe fate of flubendazole and fenbendazole in manure and manured soils was investigated under laboratory and field conditions. In pig manure, both 14C‐labeled benzimidazoles disappeared slowly. After a 102 day incubation period, extractable fractions contained 72% flubendazole or 80% fenbendazole of the radioactivity initially applied. The latter was accompanied by 4% of the corresponding metabolite fenbendazole‐sulfoxide. Non‐extractable residues amounted to 24 and 13%, respectively. On this basis, test manures with 7 day aged benzimidazole residues were prepared. Mobility tendencies differed for clay and sand soils as well as for standard and test‐manure application. Regarding KOC > 1100 L/kg, however, criteria for potential leachers were not fulfilled. The metabolic fate of flubendazole was predominated by the occurrence of the parent compound while fenbendazole was accompanied by fenbendazole‐sulfoxide. In clay soil samples after standard application, DT50 values were 174 and 54 days, respectively. Mineralization and formation of non‐extractable residues were of minor relevance. For fenbendazole, these processes were intensified after test‐manure application. Due to enhanced formation of fenbendazole‐sulfoxide, fenbendazole‐sulfone, and non‐extractable residues, DT50 thus dropped to 9 days. Similar mobility and degradability tendencies were also found under field conditions. In the sand soil, however, the metabolic dynamics decelerated due to its lower microbial activity.