Genetic and environmental factors affecting lamb survival at birth and through to weaning

Pedigree and early performance records from lambs on three New Zealand rese arch stations, involving 51 station-years of data (55 146 recorded lambs bo rn), were analysed in order to study genetic and environmental factors affe cting perinatal and preweaning survival. Mean birth weights at each site we re very similar, being 4.26 kg at Woodlands, 4.22 kg at Tokanui, and 4.32 k g at Rotomahana Station. Total survival rates to weaning (preweaning surviv al) across sites averaged 79.0, 78.9, and 80.1%, respectively. Analyses of variance for perinatal survival and preweaning survival showed significant effects of contemporary group of lamb, age of dam, and birth rank-sex combi nations, whilst linear and curvilinear effects of birth weight were also si gnificant. Optimal birth weights were 4.36 kg and 4.77 kg for maximal perin atal survival and maximal preweaning survival, respectively. Heavy singles and twins were at higher risk of not surviving, but light singles and twins were also at risk. At birth, survival was consistently lowest from lambs o ut of 2-year-old dams, whilst preweaning survival was lower amongst lambs f rom 2- and 5-year-old darns, and higher in lambs from 3- and 4-year-old dam s. Estimates of total heritability (direct + maternal + direct-maternal covari ance) for perinatal survival as a transformed (logit) trait were 0.055 (Tok anui and Woodlands data), and 0.105 (Rotomahana data). Corresponding estima tes for preweaning survival were 0.031 and 0.101, respectively. Maternal ge netic variances for perinatal and preweaning survival as logit traits were 1.5-5 times the size of the lamb's additive genetic variance. Our analyses confirm previous low genetic parameter estimates for lamb survival.