Correlated responses to selection for or against facial eczema susceptibility in Romney sheep in New Zealand

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Genetic selection for or against susceptibility to facial eczema (FE) in Romney sheep began in 1975, with the establishment of a resistant (R) selection flock, a susceptible (S) selection flock, and later (in 1982) a control (C) flock. For all but the initial years, rams were identified by performance testing with a sporidesmin challenge, then ranked on relative elevation of the liver enzyme, gamma glutamyltransferase (GGT), measured in serum. A different dose rate of sporidesmin was used for performance testing in the R and the S flocks, with a balanced half of the C‐flock animals being tested at each dose rate. Results are reported here up to the 1997‐born lamb crop. Mixed‐model animal‐model methods were used to determine the direct responses to selection, expressing results as breeding values for loge GGT. Correlated responses were monitored in lambs for weights, fleece weight, and survival, and in ewes for reproductive traits. Up to 1997, a 9‐fold difference in GGT concentration, 3 weeks after challenge, was achieved between the R and S lines. Selection line differences in correlated traits were significant (P < 0.01) for weaning and January weights (with the R line 5–6% lighter than the S line), whilst the R line yearlings produced 8% heavier fleeces (P < 0.05). Ewe reproductive differences among lines were small, and nonsignificant for the overall trait (numbers of lambs weaned per ewe mated), but there were significant ram differences in fertility among lines, with highest fertility in the R line. Using a multi‐variate restricted maximum likelihood analysis, genetic correlations of lamb production traits with loge GGT were consistent in sign with the selection‐line responses, i.e., for animals which were more resistant (with lower GGT), both weaning and autumn weights decreased slightly, and both yearling weight and fleece weight increased slightly.