GENETICS OF RESILIENCE TO NEMATODE PARASITES IN YOUNG ROMNEY SHEEP - USE OF WEIGHT-GAIN UNDER CHALLENGE TO ASSESS INDIVIDUAL ANTHELMINTIC TREATMENT REQUIREMENTS

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 un der a selective anthelmintic treatment regime to obtain genetic inform ation on the resilience of grazing lambs to nematode challenge. The ba sis on which selective treatment decisions were made was modified from that of an earlier study (which used farmer assessment of body condit ion and dag score) in an endeavour to make the procedure more objectiv e. On the first two drenching occasions after weaning, only those lamb s whose liveweight gain was below an acceptable threshold, defined usi ng data from flock mates, received anthelmintic treatment (selective d renching). 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 drenchin g regime (BY2). Productivity, breech soiling (dags), and faecal worm e gg count (FEC) were measured in female lambs (half sibs of the recorde d males) on each of the farms following an extended period of grazing without drench treatment. This enabled genetic correlations between re silience (NAD or BY2), and productivity under challenge, dag score, or resistance to nematode infection to be assessed without being confoun ded by the variable drench treatment histories of the male lambs. Heri tability estimates of NAD and BY2 (0.058 +/- 0.018 and 0.033 +/- 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 ma le progeny and liveweight gain or dag score under challenge in their h alf sisters were relatively strong and favourable (0.55-0.73, ignoring sign). The results confirm that breeding for resilience, using either approach, should ultimately lead to flocks whose lambs are able to ma intain both better growth rates and lower dag scores under nematode ch allenge, 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. Gen etic correlations between NAD or BY2 in male lambs and log, (FEC + 100 ) in their half sisters (0.01 +/- 0.23 and -0.14 +/- 0.29 respectively ) were in line with earlier results in suggesting that resilience to n ematode challenge and resistance to nematode infection in grazing Romn ey lambs are essentially independent traits. This indicates that, in o rder to capture the potential benefits of both traits, each will have to be selected for independently, or an appropriate selection index de veloped.