Estimation of genetic parameters for litter size in Canadian Yorkshire and Landrace swine with each parity of farrowing treated as a different trait.

Genetic variances and covariances for the number of pigs born in total (NOBT), the number of pigs born alive (NOBA), and the number of weaned pigs (NOW) were estimated by REML under an animal model. Data on 30,357 and 42,041 litters born between 1977 and 1992 from Yorkshire and Landrace sows, respectively, were obtained from the Quebec Record of Performance sow productivity program. Data of the first four parities of litter size were used for four different analyses under an animal model: univariate analyses with direct genetic effects only, univariate analyses with maternal and direct genetic effects , seri0s of bivariate analyses with each parity treated as a different trait, and a series of bivariate analyses between NOBT, NOBA, and NOW within each parity. Heritabilities of different parities from univariate analyses under a direct genetic effects model ranged from .10 to .15, .09 to .14, and .06 to .08 for NOBT, NOBA, and NOW, respectively. Estimates of direct heritability from bivariate analyses between parties were consistent with estimates from univariate analyses in Landrace but not in Yorkshire. Genetic correlations between first and secondary parity in Yorkshire were .59, .49, and .17 for NOBT, NOBA, and NOW, and in Landrace were .90, .93, and .81, respectively. Influence of maternal effects on moderate correlations between first and secondary parity in Yorkshire was suggested. Genetic correlations averaged over all parities between NOBA and NOBT or NOW were .97 and .65 in Yorkshire and .97 and .82 in Landrace. A multiple-trait animal model with parities treated as different traits was recommended.