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

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 logeGGT), 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. © 1995 Taylor & Francis Group, LLC.