Feasibility and implications of breeding sheep for resilience to nematode challenge.

There are potentially 2 different types of host response which could be targeted when seeking to breed sheep which require minimal frequency of anthelmintic treatment to maintain acceptable health and productivity in the face of nematode challenge. These are "resistance" (the ability to suppress the establishment and/or subsequent development of a parasite infection), and "resilience" (the ability to maintain relatively undepressed production while subjected to parasite challenge). Most studies have concentrated on resistance, and used faecal worm egg count (FEC) as the selection criterion. However, in Romney sheep in New Zealand, it has become apparent that "resistance to infection" does not equate to "disease resistance" in the strict sense. Although genetically low FEC Romney lambs have been shown to have significantly reduced burdens of the most important nematode species, they nevertheless appear to have no significant production advantages over their higher FEC counterparts when all are grazed together under the same larval challenge. Furthermore there is evidence of an unfavourable association between FEC and dagginess (soiling of the breech area) suggesting 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 of reduced pasture contamination. This has led us to investigate resilience as a possible alternative breeding option to resistance. Over a period of 3 years approximately 14,000 progeny of 213 different rams were tested for resilience (assessed in terms of drench requirements using 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 varied between flocks and years possibly as a result of differences in the severity of challenge. Nevertheless, results suggested that progeny-test selection for resilience would lead to lambs with higher growth rates and lower dags-scores when left undrenched on infective pasture. In contrast to some Australian results we found no positive association between resilience to mematode challenge and resistance to infection. If the potential benefits of both resistance and resilience are to be captured our results suggest that an index selection procedure will be needed.