Modelling pesticide residues on greasy wool: experimental studies

Summary. A model has been fitted to results from experimental application of organophosphates, synthetic pyrethroids and insect growth regulators to sheep mostly in long wool. The model relates the rate of breakdown of pesticide to the chemical used, the method of application and the length of wool at the time of treatment. A genetic algorithm was used to combine a range of experimental results for each chemical group to determine the rate of breakdown, and also the effect of application method and length of wool on breakdown. Organophosphates break down very quickly when applied to the surface of the wool (initial half-lives of 9–12 days), but the rate of breakdown gradually slows as the proportion of pesticide near the surface of the wool decreases. When the pesticide is applied deep into the wool by hand jetting or dipping, the rate of breakdown is slower (average half-lives of 27–42 days) and more uniform. Synthetic pyrethroids applied by hand jetting or backliner to sheep with 6–9 months wool growth initially break down with half-lives of 32–39 days, but the rate of breakdown decreases to 59–215 days giving average half-lives of 48–103 days over the 3–6 months between application and shearing. Cyromazine applied by hand jetting to sheep with 6–8 months wool growth has an average half-life of 79–96 days, but less if applied by automatic jetting race. The other insect growth regulators have longer half-lives: triflumuron, off-shears backliner, 119 days; and diflubenzuron, long wool jetting on to 7–9 months wool, >119 days. The model allows for different breakdown rates due to the method of application and length of wool and for changes in the rate of breakdown between application and shearing. It can be used to estimate the expected residue on wool at any time after a specified treatment, provided the amount of chemical applied is known. Conversely, the amount applied can be estimated from the residue and treatment details.