GENETICS OF RESILIENCE TO NEMATODE PARASITES IN ROMNEY SHEEP

7533 Romney lambs, born in 1991 on six farms in the southern North Isl and of New Zealand, were studied as part of an investigation into the feasibility and implications of breeding sheep whose lambs require min imal anthelmintic treatment to maintain acceptable performance when ex posed to challenge by nematode parasites (''resilience''). They were t he progeny of 123 different sires. All flocks involved were geneticall y linked by the use of reference sires so that analyses could be carri ed 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-s oiling (''selective drenching''). Drench requirement data including ag e at first drench (normalised by transformation to include the 5% of a nimals not requiring drench treatment), 5 binary scores denoting wheth er or not a lamb was drenched by Time 1, Time 2 ... Time 5 (''BY1, BY2 ... BY5''), and total number of drench treatments used over the 4-mon th study period (range 0-5), were subsequently used to assess resilien ce in these lambs. Their weaning weight, post-weaning liveweight gain, autumn liveweight, hogget liveweight, hogget fleece weight, and dag s core were also examined. In ewe lambs, weaning weight, post-weaning li veweight gain, autumn liveweight, hogget liveweight, hogget fleece wei ght, dag score, faecal consistency score, and faecal worm egg count we re 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 rang e 0.05-0.14). Nevertheless, genetic correlations between resilience tr aits in male progeny and production traits in their female half sibs s howed that sires whose male progeny required fewer drench treatments t o maintain acceptable condition (i.e. were more resilient to nematode challenge), had female progeny which had significantly higher than ave rage growth rates and significantly lower than average breech-soiling (dags) under challenge (genetic correlations of 0.34-0.68; P < 0.05). Genetic correlations between resilience traits in male progeny and hog get fleece weight in their female half sibs were favourable but not si gnificant. Results suggest that selection of lambs for any of the resi lience traits examined simply on the basis of individual performance i s unlikely to be practical because of low heritabilities. However, sel ection using breeding values based on progeny-testing of sires may pro vide worthwhile progress in a breeding programme. To select for resili ence using a progeny-testing approach, the simplest of the traits is p robably BY2 or age at first drench. The consequences of progeny-test s election with a series of index combinations as predictors are present ed. Testing 20 sons on normalised age at first drench, BY2, and dagsco re, with autumn liveweight records on 20 daughters following a program me of minimal drenching, would achieve 1.9 times faster genetic progre ss for resilience per generation than performance-test selection on ma les for drenching frequency during the study period. There were no sig nificant genetic correlations between resilience traits in male progen y and resistance to the establishment of nematode burdens in their fem ale 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 breedi ng objective is established.