Estimates of Genetic Parameters for Ewe Longevity and Lifetime Reproductive Performance Traits in South African Merino Sheep.

Ewe's functional longevity impacts flock profitability by reducing annual ewe replacement costs and increasing ewe outputs over the production cycle. In this context, the main objective of this study was to estimate variance components and genetic parameters of five ewe longevity indicators in South African Merino sheep. The traits evaluated included age at the last lambing (ALL), length of productive life (PL, measured as the number of days between the first and last lambing records), total number of litters (TNL), total number of lambs born (TLB) and total number of lambs weaned (TLW). These traits were recorded on a population of 39,011 ewes. Ewe's own birth type (singletons, twins, or triplets), the concatenated effect of birth flock-year and age at first lambing (3 classes: < 710, 710-743, 744-1095 days) were the fixed effects fitted in the animal models, while the animal and contemporary group (birth flock-year-season) were fitted as random effects. Variance components were estimated based on the Restricted Maximum Likelihood method implemented in the BLUPF90+ family of programmes. The sample size and mean (standard deviation) were 38,929 ewes and 1452.1 (559.3) days for ALL; 30,714 ewes and 918.9 (456.5) days for PL; 38,795 ewes and 2.98 (1.51) litters for TNL; 38,842 ewes and 4.26 (2.49) lambs born for TNB; and 19,322 ewes and 3.87 (2.39) lambs weaned for TNW. The heritability estimates were: 0.061 ± 0.0074 (ALL), 0.051 ± 0.0082 (PL), 0.061 ± 0.0073 (TNL), 0.059 ± 0.0072 (TNB) and 0.058 ± 0.0098 (TNW). The genetic correlation estimates ranged from 0.73 ± 0.056 (PL × TNW) to 0.99 ± 0.0023 (ALL × PL), while phenotypic correlation ranged from 0.82 ± 0.0019 (PL × TNB) to 0.99 ± 0.0001 (ALL × PL). Parents of ewes with phenotypic records had estimated breeding value average accuracy above 0.30 for all traits. Positive genetic trends were found for all traits, suggesting genetic gains per year of 0.34 days in ALL, 0.27 days in PL, 0.0082 litters in TNL, 0.0022 lambs born in TNB and 0.002 lambs weaned in TNW. Given similar heritability estimates and high genetic correlation among the traits, TNW is recommended as a selection criterion to improve Merino longevity, as this trait could improve both ewe longevity and reproductive performance.