FEASIBILITY AND IMPLICATIONS OF BREEDING SHEEP FOR RESILIENCE TO NEMATODE CHALLENGE

There are potentially 2 different types of host response which could h e targeted when seeking to breed sheep which require minimal frequency of anthelmintic treatment to maintain acceptable health and productiv ity in the face of nematode challenge. These are ''resistance'' (the a bility to suppress the establishment and/or subsequent development of a parasite infection), and ''resilience'' (the ability to maintain rel atively undepressed production while subjected to parasite challenge), Most studies have concentrated on resistance, and used faecal worm eg g count (FEC) as the selection criterion. However, in Romney sheep in New Zealand, it has become apparent that ''resistance to infection'' d oes not equate to ''disease resistance'' in the strict sense. Although genetically low FEC Romney lambs have been shown to have significantl y reduced burdens of the most important nematode species, they neverth eless appear to have no significant production advantages over their h igher FEC counterparts when all are grazed together under the same Lar val challenge. Furthermore there is evidence of an unfavourable associ ation between FEC and dagginess (soiling of the breech area) suggestin g that some forms of host response to nematode challenge may result in diarrhoea. Our results suggest that the main benefits of selectively breeding for low FEC are likely to be derived indirectly as a result o f reduced pasture contamination, This has led us to investigate resili ence as a possible alternative breeding option to resistance, Over a p eriod of 3 years approximately 14,000 progeny of 213 different rams we re tested for resilience (assessed in terms of drench requirements usi ng an individual ''on demand'' treatment procedure), as well as growth rate, dag-score, FEC and fleece-weight. Overall the heritabilities of various measures of resilience examined proved to be relatively low ( ranging between 0.10+/-0.03 and 0.19+/-0.04), although estimates varie d between flocks and years possibly as a result of differences in the severity of challenge. Nevertheless, results suggested that progeny-te st selection for resilience would lead to lambs with higher growth rat es and lower dag-scores when left undrenched on infective pasture. In contrast to some Australian results we found no positive association b etween resilience to nematode challenge and resistance to infection. I f the potential benefits of both resistance and resilience are to be c aptured our results suggest that an index selection procedure will be needed. Copyright (C) 1996 Australian Society for Parasitology. Publis hed by Elsevier Science Ltd.