Carrie S Wilson ^* Natalie L Cherry J. Bret Taylor

• Range Sheep Production Efficiency Research Unit, Agricultural Research Service (USDA), Dubois, United States

Lamb survivability is an important trait from both an economic and animal welfare perspective. Five breeds at the U.S. Sheep Experiment Station were evaluated for survival to weaning and included 74,448 records from 43 years. Objectives were to evaluate 1) the influence of lamb and littermate competition on lamb survivability to 3 d and 120 d (weaning); 2) the influence of withinlitter competition on birth weight; 3) the relationship between within-litter birth weight deviation and lamb survivability, 4) the influence of ewe age on lamb survivability and birth weight, and 5) reasons for and timing of lamb loss. Lamb survivability to 3 d and 120 d were analyzed using a Bayesian logistic regression model with the logit link function. Lamb survivability was assumed binomially distributed and influenced by age of dam, birth year, lamb sex, and sex of littermates as fixed effects. Litter was included as a random effect. Birth weight was assumed normally distributed and influenced by the same fixed effects as lamb survival and was analyzed using a linear mixed model. Lamb survivability to 3 d and 120 d were also analyzed using the same model described above but included mean litter weight as linear and quadratic covariates and individual birth weight deviation as a linear covariate. Sex of littermate(s) had an impact on lamb survivability, which was more pronounced for triplets than for twins. Although not significant, female birth weights were lighter in mixed-sex litters than all-female litters. Within-litter birth weight deviation had an impact on lamb survivability with lambs from below mean weight litters that were more than 1 kg lighter than the litter having less than a 50 % chance of survival. Both young and old ewes had lower lamb survival than mid-age ewes. Reasons for lamb loss differed depending on the age of the lamb with weak and trauma categories resulting in early loss and predation resulting in later loss. This study provides insight into within-litter competition for use in future genetic evaluation.