Response to partial selection on clean fleece weight in South Australian strong-wool Merino sheep. V. Estimation of genetic gain by contemporary comparison of progeny of sires two years apart in age

Abstract

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 ÿ 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.