Genetic parameters for lamb birth weight in spring and autumn lambing

Genetic parameters for birth weight were estimated for two sets of spring-born and one set of autumn-born lambs of the same breed composition and genetic origin using a model that included random additive, additive maternal, permanent environmental maternal, litter and residual effects. Litter effects were significant for all data sets, indicating that within-year effects of common environment and (or) non-additive genotype were important for birth weight and should be included in genetic evaluation programmes. Estimates of variance components for autumn-born lambs differed significantly from those obtained from spring-born lambs or from the combined data. In contrast, estimates obtained for the two sets of spring-born lambs did not differ from those obtained when data on all spying-born lambs were pooled. The estimate of h(2) for birth weight in spring-born lambs was over twice as large as that obtained for autumn-born lambs (0.26 v. 0.12). In contrast, the across-year repeatability of ewe effects was much larger in autumn lambs (0.41 v. 0.26) as was the full-sib littermate correlation (0.73 v. 0.49). Mean birth weights were also 0.4 to 0.6 kg less in autumn lambs. These results suggest that negative effects on birth weight that have been reported for lambs gestated during summer in equatorial and subtropical regions are also manifested (albeit at reduced levels) in less extreme climates (in this case, 37 degrees, 15' N latitude and 654 m elevation) and that maternal variation exists in the ability of the ewe to maintain adequate lamb birth weights in autumn lambing systems.