Genetic and phenotypic relationships of farrowing and weaning survival to birth and placental weights in pigs 1 H. Mesa, T. J. Safranski, K. M. Cammack, R. L. Weaber, and W. R. Lamberson 2 Division of Animal Sciences, University of Missouri, Columbia 65211 ABSTRACT: Data obtained during 4 generations of divergent selection for placental efficiency were used to determine factors influencing survival at farrowing and weaning in litters produced by first-parity females. Data were collected from 193 litters and included re- cords on 2,053 individuals. Farrowing survival (FS) and weaning survival (WS) were considered traits of the piglet and were scored 1 if the individual was alive at a time point or 0 if dead. Estimates of (co)variance components for direct and maternal additive genetic effects for FS and WS were obtained using an animal model and computed with the MTDFREML program. Estimates of direct heritability were 0.16 for FS and 0.18 for WS. Estimates of maternal heritability were 0.14 for FS and 0.10 for WS. Genetic correlation esti- mates between direct and maternal effects were high and negative for both traits. The direct genetic correla- Key words: birth weight, genetic parameter, pig, survival 2006 American Society of Animal Science. All rights reserved. J. Anim. Sci. 2006. 84:32–40 INTRODUCTION Litter size at weaning is a trait of major economic importance to swine producers (Tess et al., 1983). It may be possible to effectively increase litter size by selection (Lamberson et al., 1991; Bidanel et al., 1994; Johnson et al., 1999), although its heritability is low. A correlated response to selection for increased litter size has been decreased birth weight (BRW; Johnson et al., 1999; Mesa et al., 2003). Low BRW, and particu- larly low BRW within a litter, has been linked to high piglet mortality (Damgaard et al., 2003). A survey of US swine producers in 2000 revealed that average litter size was 10.9 pigs, of which 10.0 were born alive and 8.9 survived to weaning. Thus, of those pigs born alive, average preweaning mortality was 11%; more than 50% 1 Research supported by the Missouri Agricultural Experiment Sta- tion and National Pork Producers Council. 2 Corresponding author: LambersonW@missouri.edu Received December 17, 2004. Accepted September 13, 2005. 32 tion between FS and WS was 0.92. Variables associated with FS and WS were determined using logistic regres- sion procedures. Birth weight (BRW), placental weight, their interaction, and total born can be used as pre- dictors of survival at farrowing in the absence of esti- mates of genetic merit for survival. The same model, excluding total number born, was the best model for predicting WS. In the presence of BRW information, placental efficiency did not improve the prediction of survival. While it was clearly disadvantageous for a piglet to be below the litter mean in BRW, being above the mean did not provide a substantial advantage in survival. Results from this analysis suggest that it is possible to select for increased survival at farrowing and at weaning. Information on a piglet’s BRW, placen- tal weight, litter average BRW, and deviation from lit- ter average BRW can be used to optimize those values at levels resulting in high survival probability. of preweaning deaths resulted from being crushed by the sow (USDA, 2002). Previous results have suggested that direct selection for increased preweaning survival may not be effective because the heritability seems to be very low (Lamberson and Johnson, 1984; Grandinson et al., 2002), although the heritability of stillbirths may be somewhat greater (Grandinson et al., 2002). These traits are, in part, connected by prenatal growth of the piglet, which in turn is influenced by the placenta. Emphasis on placental efficiency, defined as the ratio of BRW to placental weight (PW) at birth, may be a mechanism through which litter size can be increased while maintaining adequate BRW (Wilson et al., 1999). Data for the current study were collected during 4 generations of divergent selection for placental efficiency (PE; Mesa et al., 2005). The objectives of this study were to determine the relationships between BRW and PW and factors influencing survival at far- rowing and weaning in swine. MATERIALS AND METHODS Selection procedures and results of the selection ex- periment that provided the data for this study have Published December 8, 2014