Genetic parameter estimates for lamb survivability as separate traits based on birth type for Polypay, Rambouillet, Targhee, and Suffolk sheep

Lamb survivability is a primary determinant of profitability in sheep production. Most published heritability estimates for this complex trait are low (< 0.05). A comprehensive analysis of lamb survivability using five random effects models of increasing complexity and three fixed effects models for combined (“Combined”) and separate birth types (“Multivariate”) for Polypay, Rambouillet, Targhee, and Suffolk U.S. sheep breeds was performed along with birth weight in a linear-threshold model. Records from 1980 to 2024 from the U.S. Sheep Experiment Station were used and included 22,284 Polypay, 25,085 Rambouillet, 20,004 Targhee, and 3,965 Suffolk lamb survivability observations. For the final models for all breeds, direct heritability for each birth type was higher than when birth types were combined and analyzed as a single trait. The final model for Suffolk did not include maternal permanent environment. Heritability estimates for the Combined data set were low for direct (0.011 for Polypay to 0.152 for Suffolk) and maternal components (0.038 for Polypay to 0.102 for Suffolk). For the Multivariate data sets, each birth type had a higher direct heritability estimate than the Combined data set, with twins being generally lower within breed (0.056 for Rambouillet to 0.158 for Suffolk) compared to singles (0.111 for Targhee to 0.311 for Suffolk) and triplets (0.183 for Rambouillet to 0.199 for Polypay). Estimated breeding values (EBV) were computed for all pedigree animals and converted to the 80% probability scale and genetic trends were derived. There were sires that had above average EBV for both direct and maternal lamb survivability, suggesting genetic progress can be made for both components of the trait. Evaluating single, twin, and triplet lamb survivability as separate traits is warranted because the direct and maternal influences differ by birth type and heritability estimates were substantially increased over considering all birth types in a single trait.