Genetics of resilience to nematode parasites in young Romney sheep)#x2014;use of weight gain under challenge to assess individual anthelmintic treatment requirements

摘要

The performance of 2997 male Romney lambs, sired by 102 rams and born in spring 1992 on four farms in southern North Island, was measured under a selective anthelmintic treatment regime to obtain genetic information on the resilience of grazing lambs to nematode challenge. The basis on which selective treatment decisions were made was modified from that of an earlier study (which used farmer assessment of body condition and dag score) in an endeavour to make the procedure more objective. On the first two drenching occasions after weaning, only those lambs whose liveweight gain was below an acceptable threshold, defined using data from flock mates, received anthelmintic treatment (selective drenching). A resilience score for each animal was derived from drench requirement data either as age at first drench (normalised to include animals never drenched) (NAD), or as a binary code denoting whether or not the animal had been drenched by the end of the selective drenching regime (BY2). Productivity, breech soiling (dags), and faecal worm egg count (FEC) were measured in female lambs (half sibs of the recorded males) on each of the farms following an extended period of grazing without drench treatment. This enabled genetic correlations between resilience (NAD or BY2), and productivity under challenge, dag score, or resistance to nematode infection to be assessed without being confounded by the variable drench treatment histories of the male lambs. Heritability estimates of NAD and BY2 (0.058 #xB1; 0.018 and 0.033 #xB1;0.016 respectively) were lower than those calculated in the earlier study, but nevertheless indicated that genetic factors were involved. In line with the earlier study, genetic correlations between NAD or BY2 in male progeny and liveweight gain or dag score under challenge in their half sisters were relatively strong and favourable (0.55#x2013;0.73, ignoring sign). The results confirm that breeding for resilience, using either approach, should ultimately lead to flocks whose lambs are able to maintain both better growth rates and lower dag scores under nematode challenge, and thus require a lower frequency of anthelmintic treatment. However, as a consequence of low heritabilities, genetic progress in such a breeding programme could be expected to be relatively slow. Genetic correlations between NAD or B Y2 in male lambs and loge(FEC + 100) in their half sisters (0.01 #xB1; 0.23 and #x2010;0.14 + 0.29 respectively) were in line with earlier results in suggesting that resilience to nematode challenge and resistance to nematode infection in grazing Romney lambs are essentially independent traits. This indicates that, in order to capture the potential benefits of both traits, each will have to be selected for independently, or an appropriate selection index developed.