Worm egg count is not associated with greasy fleece weight in sheep phenotypically different for resistance or resilience to gastrointestinal nematodes

A two-year cross-over experiment investigated phenotypic resistance and resilience to gastrointestinal nematodes (worms) on production characteristics in grazing sheep. On six farms, two mobs (yearling or mixed age) of 300 or more Merino ewes were chosen at shearing in 2007. Within each mob, animals were randomly allocated to receive either normal management (challenged, n=60) or worm-free treatment (n=60) involving suppressive control using combination long acting anthelmintics. Faecal worm egg count (WEC) was measured individually every two months and greasy fleece weight (GFW) at each shearing. Treatments were swapped at shearing in 2008 and continued until shearing 2009. Sheep were placed into quartiles based on either WEC(1/3) (resistance) or GFW (resilience) distributions within their treatment groups when challenged. The effect of resistance (WEC quartile) on challenged and worm-free GFW was analysed using an appropriate general linear model followed by Tukey's HSD test while the effect of resilience (GFW quartile) was analysed by intra-quartile comparison of GFW under worm-free and normal management. More resistant sheep will have a lower requirement for anthelmintic treatment but do not exhibit higher GFW in a challenged environment (Table 1). When worm-free, the most resistant quartile had lower GFW compared to the most susceptible quartile. Increased resilience was not associated with higher WEC. Phenotypic selection for high fleece weight will not increase the need for anthelmintic treatment and improve resilience without adverse effects on worm control.