Responses of romney sheep to selection for resistance or susceptibility to nematode infection

Divergent breeding lines of Romney sheep, selected as lambs for consistently low or high faecal worm egg count (FEC) following natural multispecific challenge by nematode parasites, have been maintained at Wallaceville Animal Research Centre since 1979. From the start of the trial until 1992, 821 and 736 experimental lambs in lines selected for increased resistance (R) and increased susceptibility (S), respectively, were generated, with the use of 44 different sires. In order to assess genetic responses to selection, FEC and productivity data were analysed using restricted maximum likelihood procedures. By 1988 the two lines had diverged in average loge (FEC+100) by 0.67 log units, and by 1992 divergence had increased to 1.48 log units, representing 2.90 genetic standard deviations of divergence. No significant correlated responses were observed in live weights. However, by 1992 the mean score for breech soiling (dags) in the R line was proportionally 0.41 greater than in the S line (P < 0.001). Yearling fleece weight was proportionally 0.15 greater in S-line than in R-line animals when grazed together under identical levels of challenge (P < 0.001.) but the lines did not differ significantly in fleece weight when they were grazed apart. The R line had significantly higher reproductive success than the S line (P < 0.05), as measured by lambs weaned per ewe mated (1.01 v. 0.92, respectively). It is concluded that selective breeding can change the degree of resistance or susceptibility to nematode infection in Romney sheep, and selection for productivity needs to be continued at the same time in any practical breeding programme. © 1997 British Society of Animal Science.