New Zealand Journal of Agricultural Research abstracts
Correlated responses to selection for or against facial eczema susceptibility
in Romney sheep in New Zealand
C. A. MORRIS
N. C. AMYES
N. R. TOWERS
C. WESSELINK
AgResearch
Ruakura Agricultural Research Centre
Private Bag 3123
Hamilton, New Zealand
Abstract 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
non-significant 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.
Keywords facial eczema; sheep; Pithomyces chartarum;
selection; susceptibility; genetic change
New Zealand Journal of Agricultural Research, 1999, Vol. 42: 475-481
0028-8233/99/4204-0475 $7.00/0 (c) The Royal Society of New Zealand
1999
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