Decay rates of organophosphate residues in the fleeces of sheep dipped for flystrike control

Residue levels of five organophosphates in the wool of sheep dipped in May and December decayed exponentially. Half‐lives were shorter by a mean of 18‐28% on the back compared with those on the sides of sheep. The mean half‐life (T/2), in days, for wool dipped in May/December was, respectively, 32/12 (diazinon), 37/12 (propetamphos), 53/23 (bromophos ethyl), 42/17 (chlorfenvinphos), and 38/12 (coumaphos). The shorter T/2s on the back of. sheep and during the summer period were attributed to degradation of the organophosphates by UV light. Bromophos ethyl and chlorfenvinphos showed the least variation in residue level throughout the fleece during the two trial periods.