Variation of pesticide concentration in sheep dips operated according to traditional and revised methods

Objective To quantify stripping in traditional dipping operations and to re vise dipping methods, based on prediction of stripping so that a more stabl e concentration of pesticide in the dipwash is achieved. Design and methods Plunge and shower dips were operated sequentially accord ing to traditional and revised dipping instructions, Dips were operated by continuous and intermittent replenishment. Samples of mixed dipwash were co llected periodically and assayed for pesticide (diazinon) concentration. Results Diagrammatic representations of pesticide concentration versus numb er of sheep dipped indicated traditional dipping leads to wide variations i n the concentration of pesticide in dipwash during dipping. Intermittent re plenishment led to a 'saw-tooth' pattern in the pesticide concentration. Tr aditional continuous replenishment (using the starting concentration of pes ticide) indicated both the rate and extent of stripping was higher in showe r dipping. If sufficient sheep were dipped, equilibrium was reached between the rate of pesticide replenishment and removal. An alternative method of dip operation by continuous replenishment, using a low starting concentrati on of pesticide and a replenishment concentration high enough to offset the pesticide loss through stripping resulted in a more stable concentration o f pesticide in the dip. Conclusion Revision of dipping instructions can lead to exposure of sheep t o stable concentrations of stripping pesticide during dipping.