Genetic parameters and trends for lamb survival and birth weight in a Merino flock divergently selected for multiple rearing ability.

Data of 5,390 Merino lambs born from 1986 to 2007 (approximately 6.9 generations) were used to derive genetic parameters and trends for age-specific and overall lamb survival on the underlying scale, as well as for lamb birth weight, using Gibbs sampling. The majority of lambs were descended from lines that were divergently selected for the ability of ewes to rear multiples. The line selected in the upward direction was denoted as the high line (H line), whereas the line selected in the downward direction was the low line (L line). Analyses included the covariance between direct and maternal genetic effects, except where it was not estimable owing to small direct additive variance components, a high incidence of lambs surviving, or both. Direct heritability estimates were 0.02 for lamb survival at birth, 0.12 for lamb survival from birth to tail docking, 0.39 for lamb survival from docking to weaning, 0.28 for overall lamb survival, and 0.17 for birth weight. Corresponding estimates for the maternal genetic effect were 0.26, 0.14, 0.16, 0.14, and 0.29. Dam permanent environmental variance ratios were, respectively, 0.14, 0.09, 0.05, 0.07, and 0.07. Estimates of the direct-maternal genetic correlation were -0.60 for lamb survival from docking to weaning, -0.61 for overall lamb survival, and -0.15 to -0.23 for birth weight. Genetic, maternal genetic, and dam permanent environmental correlations between lamb birth weight and overall or age-specific lamb survival were not different from zero. Expressed relative to overall means, annual direct genetic change in the H line amounted to -0.01% per annum for lamb survival at birth, 0.52% per annum for lamb survival from birth to docking, and 1.3% per annum for lamb survival from docking to weaning. Corresponding trends in the L line were, respectively, -0.01, -0.42, and 0.042% per annum. Maternal genetic trends amounted to, respectively, 0.23, -0.11, and -0.24% per annum in the H line and -0.78, -0.50, and -0.16% per annum in the L line. It was concluded that sustained genetic progress in lamb survival is feasible if directed selection is applied to a correlated trait such as the ability of ewes to rear multiples.