After 9.6 generations of selection, yearling ewe greasy fleece weight has increased by about 1kg (2 phenotypic standard deviations). Further evidence that the traditional method of assessing cumulative selection pressure is biased upwards is presented. In comparison to the 4.8kg of cumulative selection pressure applied, a 1kg response corresponds to a realized heritability of about 0.2, which is somewhat less than the commonly assumed value of 0.3. Reasons for this discrepancy are discussed. No evidence of a decline in response to selection could be found. Increased liveweight contributed significantly towards the extra wool produced. Selected flock breeding ewes had improved annual fleece producion of 1kg, despite producing a greater number of lambs. Physiological studies indicate selection flock sheep have lower levels of blood plasma urea, possibly induced through an increased rate of urea clearance in the urine. Comparisons of dry matter digestibility in the control and selected flocks produced conflicting results.

Hugh T Blair

Proceedings of the World Congress on Genetics Applied to Livestock Production, Volume XII. Biotechnology, selection experiments, parameter estimation, design of breeding systems, management of genetic resources., , 215–220, 1986
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