Alternative replenishment regimens to maintain diazinon concentration during shower, plunge and immersion cage dipping of Merino sheep

Abstract

Constant replenishment of a 1000-L Buzzacott shower dip sump with diazinon solution at concentrations 100, 200, 300 or 500 mg/L failed to maintain the initial charge rate of 100 mg/L when short-wool Merino sheep were showered for 12 min. According to these regimens, by the completion of dipping the third pen of sheep working concentrations had dropped to 11, 29, 35 and 51 mg/L for sheep treated with replenishment solutions of 100, 200, 300 and 500 mg/L, respectively. These results suggest that the working concentration of diazinon sheep dips may be increased and maintained by raising the concentration of the replenishment solution. On average, wool samples taken from these sheep immediately after dipping contained 207, 453, 473 and 730 mg/kg, respectively. Comparison of dip concentration and wool residues indicated a linear relationship. This suggests that the capacity for Merino wool to absorb diazinon had not yet reached saturation. Compounding the difficulties with the reliability of constant replenishment shower dipping, current label directions for reinforcement and topping-up of diazinon sheep dips are unclear and often misinterpreted. A simpler and far more reliable strategy that ensured that each pen of sheep was treated similarly was tested. In this scheme, sufficient fresh dip wash (540 mg/L) from the replenishment tank was transferred to the dip sump to restore the initial sump volume after showering of each pen of sheep. This scheme reliably restored the sump concentration to the current nominal initial charge rate (100 mg/L) and was by far the easiest and most practical replenishment scheme tested. This should be the preferred strategy for shower dipping. Working concentrations of diazinon in a 5000-L mobile plunge dip charged at 100 mg/L and constantly replenished with 200 mg/L solution were maintained close to the nominal application rate of 100 mg/L. Current product label directions for reinforcement and topping-up of plunge dips were shown to be suitable for immersion cage dips operated according to these principles.