Threshold-linear estimation of genetic parameters for farrowing mortality, litter size, and test performance of Large White sows J. Arango* 1,2 , I. Misztal*, S. Tsuruta*, M. Culbertson†, and W. Herring† *Department of Animal and Dairy Science, the University of Georgia, Athens 30602-2771; and †Smithfield Premium Genetics, Roanoke Rapids, NC 27870 ABSTRACT: Up to 109,447 records of 49,656 Large White sows were used to evaluate the genetic relation- ship between number of pigs born dead (BD) and num- ber born alive (BA) in first and later parities. Perfor- mance data (n = 30,832) for ultrasound backfat (BF) at the end of the test and days to reach 113.5 kg (AD) were used to estimate their relationships with BD and BA at first parity in a four-trait threshold-linear analy- sis (TL). Effects were year-farm, contemporary group (CG: farm-farrowing year-farrowing month) and ani- mal additive genetic. At first parity, estimates of herita- bility were 0.09, 0.09, 0.37, and 0.31 for BA, BD, AD, and BF, respectively. The estimate of genetic correlation between BD and litter size was -0.04 (BD-BA). Corres- ponding values with test traits were both -0.14 (BD- AD, BD-BF). Estimates of genetic correlation between BA and performance traits were 0.08 (BA-AD) and 0.05 (BA-BF). The two test traits were moderately nega- tively correlated (-0.22). For later parities, a six-trait (BD, BA in three parities) TL model was implemented. Key Words: Backfat, Farrowing Mortality, Genetic Correlations, Growth, Litter Size 2005 American Society of Animal Science. All rights reserved. J. Anim. Sci. 2005. 83:499–506 Introduction Litter size is the most important trait to define sow productivity and, despite its low heritability, an in- creased emphasis on it was expected in selection pro- grams (Ollivier, 1998). An antagonistic relationship be- tween litter size and farrowing survival has been found (Leenhouwers et al., 1999; Lay et al., 2001; Knol et al., 2002), in part associated with crowded and prolonged farrowing. Surviving pigs from large litters tend to grow slower than those from small litters (Hogberg and 1 Correspondence: 306 Dept. of Anim. and Dairy Sci. (phone: 706- 583-0250; fax: 706-583-0274; e-mail: arangoj@uga.edu). 2 On leave from Facultad de Ciencias Veterinarias, Universidad Central de Venezuela, Apartado, 4563, Maracay 2105, Aragua, Ven- ezuela. Received June 25, 2004. Accepted November 30, 2004. 499 The estimates of additive genetic variances and herita- bility increased with parity for BD and BA. Estimates of heritabilities were: 0.09, 0.10, and 0.11 for BD, and 0.09, 0.12, and 0.12 for BA in parities one to three, respectively. Estimates of genetic correlations between different parities were high (0.91 to 0.96) for BD, and slightly lower (0.74 to 0.95) for BA. Genetic correlations between BD and BA were low and positive (0.02 to 0.17) for BA in Parities 1 and 2, but negative (-0.04 to -0.10) for BA in Parity 3. Selection for increased litter size should have little effect on farrowing piglet mortality. Intense selection for faster growth and increased lean- ness should increase farrowing piglet mortality of first- parity sows. A repeatability model with a simple correc- tion for the heterogeneity of variances over parities could be implemented to select against farrowing mor- tality. The genetic components of perinatal piglet mor- tality are independent of the ones for litter size in the first parity, and they show an undesirable, but not strong, genetic association in second parity. Rydhmer, 2000), which indicates that survival should be considered when selecting for litter size to ensure greater productivity. On the other hand, farrowing sur- vival tends to decrease in later parities, which seems associated with larger litters, prolonged farrowing and decreasing uterus quality (Leenhouwers et al., 1999; Knol et al., 2002; Damgaard et al., 2003). Selection of maternal lines should include piglet survival due to the genetic correlation among components of reproduction (litter size) and growth (lean growth) with survivability in pigs (Johnson et al., 1999; Rydhmer, 2000). There- fore, it is important to know the genetic determination of piglet mortality and its relationship with the primary traits for which selection is practiced to incorporate survival ability in the breeding program. Most research has treated piglet mortality as a con- tinuous trait, thereby ignoring its categorical nature. The threshold model takes into account the asymmetry and extreme incidence of some categories, as in the case