Genetic gain per year in clean fleece weight was estimated in a flock under partial selection for that trait by comparing the progeny of 1+,-year-old rams with the progeny of 3+-year-old rams. Three mathematical models were fitted to the data. Model 1 included years, ram ages, type of birth, age of dam and sex as main effects, plus all possible two-way interactions. Model 2 was as model 1, but it included also the sire selection intensity as a linear covariate. Model 3 was as model 1 with the addition of sires as a random effect nested within years and ram ages. The three models fitted gave similar results. The estimate of genetic gain in clean fleece weight was 0.12 kg per year and was statistically significant (PC 0.05) when models 1 and 2 were fitted, while it bordered the 5% level of significance (P � 0.07) when model 3 was fitted. The present estimate of genetic gain is somewhat greater than the one observed in other experiments, even though 70% of the rams were rejected visually before selection for clean fleece weight took place. The results are discussed, and the limitations of the experimental design adopted for the experiment are stressed.
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.
Divergence over time in flocks selected for clean fleece weight and for fecundity is measured by gene frequency changes at the R-r-i blood group, haemoglobin, and transferrin loci. No unequivocal effect of selection is demonstrable. Problems of interpretation even in flocks of known history are discussed.
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