Three selection experiments, two involving Romneys (at Wallaceville and Ruakura) and the third involving Perendales (at Ruakura), have resulted in breeding lines showing significant divergence in faecal nematode egg count (FEC), under natural challenge. The High and Low FEC lines in each experiment experienced 18, 8 and 11 years of selective breeding respectively to 1996. The geometric means of FEC in the three experiments were 7.3, 2.0 and 4.5 times greater in the High than the Low lines, equivalent to divergences of 2.12, 0.80 and 1.27 phenotypic standard deviations. In the present analysis, the High and Low lines were compared for greasy fleece weights using both yearlings (YFW) and mixed-age ewes (EFW). Relative to the Low lines, the corresponding High lines had greater YFWs by 14.6±3.0, 7.9±2.4 and 5.3±3.1% respectively (mean of three, 9.1±1.6%), and they had greater EFWs by 14.9±1.7, -3.2±3.8 and 8.0±2.7% (mean of three, 10.9±1.3%). The conclusion is that single-trait selection for reduced FEC in Romneys and Perendales is associated with reduced greasy fleece weight, when they and less resistant animals graze together.
Proceedings of the New Zealand Society of Animal Production, Volume 57, , 26-28, 1997
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