217 Effect of within-litter competition on lamb survivability

Abstract The objective of this study was to evaluate the influence of lamb and littermate competition on lamb survivability to weaning. Litter sex composition and litter birth weight variation were the primary considerations for influencing competition. Records from maternal dual-purpose (Polypay, Rambouillet, and Targhee) and terminal breeds [Suffolk and Paternal Composite (PC; 3/8 Columbia, 3/8 Suffolk, 1/4 Texel)] were used, with 18,654 single and 56,324 twin records from all breeds and 11,082 triplet records from the 3 maternal breeds. Records were reported from 1980 to 2023, except for the PC, which began inter se mating in 2009. Lamb survivability to 90 d was 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. The relationship among littermates was also accounted for. Least squares means were transformed from the log odds scale to the probability scale. 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 was also analyzed using the same model described above but including mean litter weight and individual birth weight deviation as linear covariates. Accounting for mean litter birth weight and individual birth weight deviation removed the birth weight differences between the sexes, leaving only true sex differences between lambs for survivability. For single births, females tended to have greater survivability than males when adjusted for birth weight for all breeds but was only significant (P < 0.05) for Rambouillet and Targhee. For maternal breeds, females in twin litters had significantly greater survivability (P < 0.05) than their male counterparts when adjusted for birth weight; differences in terminal breeds were not significant (Table 1). Males in twin litters had heavier birth weights than females (P < 0.05) for all breeds. Triplet born females for all litter-sex combinations had significantly higher survival than males (P < 0.05) for Polypay when adjusted for birth weight. Females in female-female-male (FFM) litters had 76.4% survival versus 58.9% for males while females in female-male-male (FMM) litters had 75.9% versus 66.7% for males. When lamb survivability was adjusted for birth weight, Rambouillet females in FFM litters had 72.2% survival compared with 59.9% for males (P < 0.05) and females in FMM litters had 73.0% survival compared with 63.8% for males. Similar significant differences were observed for FFM and FMM Targhee litters. This study suggests lamb performance is influenced by its littermate(s). Changes in postnatal management of mixed-sex litters may benefit overall lamb survival. USDA is an equal opportunity provider and employer.