Considerable potential exists to improve lambing performance traits in sheep through breeding

The high demand for resources at lambing necessitates strategies to reduce such requirements; one such strategy is breeding programs that enable selection for fewer lambing events requiring assistance as well as less perinatal mortality. The objective of the present study was to estimate the genetic (co)variances for a range of lambing traits in sheep, based on alternative trait definitions that considered differences in mating type and litter size. A total of 158,561 lambing events from both crossbred and purebred Irish sheep were available. Four lambing traits were considered in the analysis: lambing difficulty score, lambing dystocia, lamb perinatal mortality and birth weight. Variance components for each trait were estimated using animal linear mixed models either in the entire data as a whole, or within sub-populations stratified by litter size (i.e., single versus multiple) or stratified by mating type (i.e., crossbred versus purebred animals). The presence of significant re-scaling and re-ranking across population strata were investigated to determine if genotype-by-environmental interactions existed. Considerable genetic variation existed for all lambing traits investigated. Irrespective of the dataset analysed, direct heritability estimates for lambing difficulty score (0.04 ± 0.02 to 0.12 ± 0.03), lambing dystocia (0.04 ± 0.01 to 0.09 ± 0.03), and lamb mortality (0.01 ± 0.01 to 0.09 ± 0.02) were all low. In contrast, lamb birth weight (0.11 ± 0.02 to 0.20 ± 0.04) exhibited a low to moderate direct heritability. Using the entire dataset, the maternal heritability ranged from 0.01 ± 0.005 (lamb mortality) to 0.15 ± 0.02 (birth weight). All direct genetic effects for the lambing traits were positively correlated with each other, except between lamb birth weight and perinatal mortality (-0.15; SE=0.16). Significantly different genetic variances in different environments and significant re-ranking between environments was detected for lamb mortality (across different mating types and litter sizes), and for lambing difficulty score (only across different litter sizes). Results clearly indicate that ample genetic variation exists for the lambing-related traits investigated.