Selection for or against facial eczema susceptibility in Romney sheep, as monitored by serum concentrations of a liver enzyme

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Abstract
Genetic selection for or against susceptibility to facial eczema (FE) was begun in Romney sheep in 1975, with the establishment of a resistant (R) selection flock, a susceptible (S) selection flock, and later a control (C) flock. For all but the initial years, rams were identified by performance testing with a sporidesmin challenge, ranking them 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; in some years R‐flock animals showing no elevation of GGT were re‐dosed later at an even higher rate. Mixed‐model methodology was used to determine responses to selection, expressing results as a breeding value for logeGGT. Analyses took account of one dose rate used in the R flock and in half of the C flock, and a second (lower) dose rate used in the remainder of the C flock and in the S flock. The heritability estimate for logeGGT was 0.45 ± 0.03. Results through to the 1993‐born lamb crop showed that the R flock became more resistant (i.e. showed reduced logeGGT) at a rate 1.77 times faster than the rate at which the S flock became more susceptible. The two flocks differed in logeGGT in 1991–93 by 2.73 ± 0.28 logeGGT units (3.03 phenotypic standard deviations or 4.52 genetic standard deviations). The C flock mean was closer to that of the S than the R flock, indicating slower progress in the S than the R flock. From the relationship between the percentage change in animals resistant and logeGGT (i.e. ‐38% per unit increase in loge GGT), it was shown that a dose which just failed to induce any R‐flock reactors would lead to all S‐flock animals reacting to the dose. Comparing annual rates of response by theoretical calculation and by best linear unbiased prediction breeding values showed that the latter were 60–70% of the theoretical value, and possible reasons for this are discussed. The realised response represents a major difference between flocks in likely liver injury from natural FE challenge in New Zealand's at‐risk regions.