Comparative costs, chemical treatments and flystrike rates in mulesed and unmulesed sheep flocks as predicted by a weather-driven model

A computer model for flystrike risk was used to predict the expected costs, pesticide used and number of sheep struck, according to time of shearing and crutching, for the regions Flinders Island, Gunning and Inverell.Acomparison was carried out between mulesed and unmulesed sheep and the program optimised the preventive chemical treatments required to minimise overall costs associated with flystrike and flock treatments. This study examined cost differences between mulesed and unmulesed hoggets and ewes where the only change in management was in the method and timing of pesticide treatment.
   The model indicated that unmulesed sheep would require more frequent treatment with longer lasting pesticides. Costs associated withflystrike were estimated to increase by $220 per thousand sheep per year (ewes) or $349 (hoggets) for Flinders Island, $445 or $512 (Gunning) and $363 or $844 (Inverell). For unmulesed sheep the model indicated that dicyclanil might be required rather than cyromazine to provide acceptable flystrike control at a lower cost. Despite this increase in preventive treatment for unmulesed sheep, the predicted number of struck sheep was higher for Gunning and Inverell, but not for Flinders Island, where the model did not always require routine preventive treatment for mulesed sheep. In regions with a flystrike problem, avoiding any increase in strike after ceasing to mules was estimated to double the cost of preventive measures for most shearing dates.
   The date of shearing had a significant effect on total costs related to flystrike and in some cases shearing during the fly season increased costs and increased strike by interfering with the most efficient use of preventive treatment.
   Crutching reduced costs in some cases, but in other situations the timing of crutching interfered with the optimum timing of chemical treatment and the model sometimes predicted worse outcomes than with no crutching. The timing of shearing, crutching and treatment must be carefully managed if both costs and the number of struck sheep are to be minimised. Control of flystrike was found to be most efficient when there was a single period of high risk of strike or two equal periods of strike risk, rather than one short and one long period.