Genetic and phenotypic trends and parameters in reproduction, greasy fleece weight and liveweight in Merino lines divergently selected for multiple rearing ability

Genetic and phenotypic trends and parameters were estimated for reproduction, fleece weight and liveweight in a South African Merino population, divergently selected from 1986, either for (H line) or against (L line) maternal multiple rearing ability. Annual reproduction, ewe greasy fleece weight and pre-joining liveweight data were recorded on 809 Merino ewes, from 1986 to 2002. Phenotypic trends indicated divergence in reproduction traits between the H and L lines. The direct additive variance ratio (h2 ± s.e.) for day of lambing was 0.08 ± 0.02. Estimates of h2 for reproduction traits were: 0.10 ± 0.02 for number of lambs born per ewe; 0.04 ± 0.02 for number of lambs weaned per ewe; and 0.04 ± 0.02 for weight of lamb weaned per ewe, corrected for the gender of the lamb. Corresponding h2 estimates for annual production were 0.57 ± 0.06 for greasy fleece weight and 0.48 ± 0.06 for ewe liveweight at joining. Service sire only exerted a significant (P