Genetic parameters for litter size of different parities in Duroc, Landrace, and large white sows.

Univariate and multivariate analyses were performed for litter size born alive and at 21 d in Duroc, Landrace, and Large White sows raised in southern Brazil. Variance and covariance components were estimated for first, second, and third parities, and for different sets of parities using DFREML under animal models with direct and maternal genetic effects. Additive direct genetic variances were different for breeds and for parities within breeds. Most estimates of heritability of direct effects were higher than the average value of .10 reported in the literature, and were two to five times the size of their standard errors, indicating that opportunities for genetic improvement of litter size in these breeds may be greater than assumed on the basis of previous reports. Maternal additive genetic variances were higher in second than in first or third parities, but were significant only for Large White sows. Genetic correlations for all breeds were high between first and third, and between second and third parities, but were low, in the range of .32 and .48, between first and second parities. For this reason, multiple-trait genetic evaluation of animals for litter size should be used to estimate breeding values when first and later parity records are involved.