Breeding for disease resistance in Merino sheep in Australia.

Breeding for disease resistance in Merino sheep in Australia has attracted considerable research and development attention. Increased labour costs, the reduced efficacy of common anthelmintics and insecticides, consumer demand for products which are free of chemicals and the poor prognosis of alternative long-term control strategies are all forcing sheep breeders to contemplate the best animal health options available, including selective breeding for resistance. The three major diseases which affect sheep production include gastrointestinal nematode parasites, flystrike (cutaneous myiasis) and footrot. Genetic improvement has been shown to be feasible in all these diseases, possibly with little adverse effect on genetic progress in other production traits. Programmes for resistance to internal parasites to be included in practical breeding programmes are now in progress. This paper deals with the incentives for focusing on the three major diseases in Merino sheep, the potential sources of genetic variation, and the means to exploit these sources of variation. The authors also highlight gains and benefits achieved in experimental selection flocks, and the difficulties and options available for commercial breeders.