Response to partial selection for fecundity in Merino sheep

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Abstract
Divergent selection for fecundity in a South Australian Merino flock was carried out over a 10 year period at Roseworthy Agricultural College, South Australia. Comparisons were made between two flocks of sheep in which selection was restricted to ram replacements. Analyses showed that very little response was achieved. Pooled estimates of repeatability by analysis of variance were 0.04 for lambs born per ewe joined (LBJ) and 0.14 for lambs born per ewe lambing (LBP). Pooled half-sib estimates of heritability were 0,10 for LBJ and 0.10 for LBP. The realized heritability of LBP was 0.03, averaged over age groups. The lack of response was due to limited selection pressure and the time taken for improvement to pass through the breeding flock.