The first objective of the research was to estimate the heritabilities and genetic correlations of piglet survival and
birth weight of pigs kept under outdoor conditions. The second objective was to examine the influence of considering weights of stillborn piglets on the genetic parameters of survival traits and birth weight. In the third objective, the
influence of litter size on genetic parameters of survival traits and birth weight was studied. The results of the study showed that piglet survival at birth, at day 1 and during the entire nursing period can be attributed to genetic effects of the piglets, which can be efficiently used to improve their survival. These direct genetic effects explained a substantial higher proportion on the phenotypic variation of piglet survival under outdoor conditions than expected based on studies using data from indoor herds. Genetically, peri- and postnatal survival traits seem to be affected by different genes or gene effects, as indicated by correlations of less than or equal to 0.4. Therefore, a multiple trait model treating at least peri- and postnatal survival as different traits is recommended. The substantial direct genetic effects of birth weight are of great interest to be exploited for correlated improvement of piglet survival. The correlations of direct genetic effects of piglet survival traits and birth weight were all favourable but low. However, even low genetic correlations will have a substantial effect on piglet survival because birth weight showed an almost three times higher heritability and can be measured on the continuous scale. Its inclusion will therefore result in considerably more response to selection than the less informative categorical survival traits (Meuwissen et al., 1995). Recording of weights of stillborn piglets is shown to be informative because it increased the favourable correlation among survival traits as well as their correlations with birth weight. Furthermore, the use of birth weight together with survival traits in the multiple trait model provided important information for the estimation of survival traits.

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