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

Genetic selection for or against susceptibility to facial eczema (FE) in Ro mney sheep began in 1975, with the establishment of a resistant (R) selecti on flock, a susceptible (S) selection flock, and later (in 1982) a control (C) flock. For all but the initial years, rams were identified by performan ce 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 teste d at each dose rate. Results are reported here up to the 1997-born lamb cro p. Mixed-model animal-model methods were used to determine the direct respo nses to selection, expressing results as breeding values for log, GGT. Corr elated responses were monitored in lambs for weights, fleece weight, and su rvival, and in ewes for reproductive traits. Up to 1997, a 9-fold differenc e in GGT concentration, 3 weeks after challenge, was achieved between the R and S lines. Selection line differences in correlated traits were signific ant (P < 0.01) for weaning and January weights (with the R line 5-6% lighte r than the S line), whilst the R line yearlings produced 8% heavier fleeces (P < 0.05). Ewe reproductive differences among lines were small, and nonsi gnificant for the overall trait (numbers of lambs weaned per ewe mated), bu t there were significant ram differences in fertility among lines, with hig hest fertility in the R line. Using a multi-variate restricted maximum like lihood analysis, genetic correlations of lamb production traits with log, G GT were consistent in sign with the selection-line responses, i.e., for ani mals which were more resistant (with lower GGT), both weaning and autumn we ights decreased slightly, and both yearling weight and fleece weight increa sed slightly.