7533 Romney lambs, born in 1991 on six farms in the southern North Island of New Zealand, were studied as part of an investigation into the feasibility and implications of breeding sheep whose lambs require minimal anthelmintic treatment to maintain acceptable performance when exposed to challenge by nematode parasites ("resilience"). They were the progeny of 123 different sires. All flocks involved were genetically linked by the use of reference sires so that analyses could be carried out across farms. After weaning, ram lambs were mustered on 4 or 5 occasions for anthelmintic treatment. Treatment was administered on an individual basis as dictated by body condition and degree of breech#x2010;soiling ("selective drenching"). Drench requirement data including age at first drench (normalised by transformation to include the 5 of animals not requiring drench treatment), 5 binary scores denoting whether or not a lamb was drenched by Time 1, Time 2 #x2026; Time 5 ("BY1, BY2#x2026; BY5"), and total number of drench treatments used over the 4#x2010;month study period (range 0#x2013;5), were subsequently used to assess resilience in these lambs. Their weaning weight, post#x2010;weaning liveweight gain, autumn liveweight, hogget liveweight, hogget fleece weight, and dag score were also examined. In ewe lambs, weaning weight, post#x2010;weaning liveweight gain, autumn liveweight, hogget liveweight, hogget fleece weight, dag score, faecal consistency score, and faecal worm egg count were examined before or after an extended period of exposure to natural nematode challenge without anthelmintic treatment. Date of birth, age of dam, and birth rank all significantly affected resilience traits in male lambs, largely as a result of their influence on weaning weight. Heritabilities of the traits proved to be relatively low (in the range 0.05#x2013;0.14). Nevertheless, genetic correlations between resilience traits in male progeny and production traits in their female half sibs showed that sires whose male progeny required fewer drench treatments to maintain acceptable condition (i.e. were more resilient to nematode challenge), had female progeny which had significantly higher than average growth rates and significantly lower than average breech#x2010;soiling (dags) under challenge (genetic correlations of 0.34#x2013;0.68;P 0.05). Genetic correlations between resilience traits in male progeny and hogget fleece weight in their female half sibs were favourable but not significant. Results suggest that selection of lambs for any of the resilience traits examined simply on the basis of individual performance is unlikely to be practical because of low heritabilities. However, selection using breeding values based on progeny#x2010;testing of sires may provide worthwhile progress in a breeding programme. To select for resilience using a progeny#x2010;testing approach, the simplest of the traits is probably BY2 or age at first drench. The consequences of progeny#x2010;test selection with a series of index combinations as predictors are presented. Testing 20 sons on normalised age at first drench, BY2, and dagscore, with autumn liveweight records on 20 daughters following a programme of minimal drenching, would achieve 1.9 times faster genetic progress for resilience per generation than performance#x2010;test selection on males for drenching frequency during the study period. There were no significant genetic correlations between resilience traits in male progeny and resistance to the establishment of nematode burdens in their female half sibs (as assessed by faecal egg counts). Thus reduced drench requirements in lambs are unlikely to be associated with genetic gains in achieving reduced faecal worm egg counts, unless a combined breeding objective is established.
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