RESPONSES OF ROMNEY SHEEP TO SELECTION FOR RESISTANCE OR SUSCEPTIBILITY TO NEMATODE INFECTION

Divergent breeding lines of Romney sheep, selected as lambs for consis tently low or high faecal worm egg count (FEC) following natural multi specific challenge by nematode parasites, have been maintained at Wall aceville Animal Research Centre since 1979. From the start of the tria l until 1992, 821 and 736 experimental lambs in lines selected for inc reased 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 analys ed using restricted maximum likelihood procedures. By 1988 the two lin es had diverged in average log (FEC+100) by 0.67 log units, and by 199 2 divergence had increased to 1.48 log units, representing 2.90 generi c standard deviations of divergence. No significant correlated respons es were observed in live weights. However, by 1992 the mean score for breech soiling (days) in the R line was proportionally 0.41 greater th an in the S line (P < 0.001). Yearling fleece weight was proportionall y 0.15 greater in S-line than in X-line animals when grazed together u nder identical levels of challenge (P < 0.001) but the lines did not d iffer 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, re spectively). It is concluded that selective breeding can change the de gree of resistance or susceptibility to nematode infection in Romney s heep, and selection for productivity needs to be continued at the same time in any practical breeding programme.