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A study seeking to estimate phenotypic and genetic parameters for sow body condition around lactation and their relationship with piglet growth and survival is described. It also investigates the genetic relationship between piglet growth and survival, including both direct and maternal effects for both traits. Records were available from 24 549 Swedish Yorkshire piglets born in 2198 litters. Sows had records of weight and backfat depth at farrowing and at weaning. Piglets had individual records for weight at birth and weaning and cause of death for those that failed to survive during lactation. Mixed linear bivariate models were used to estimate correlations between traits. The models for the sow traits took into account the random effects of permanent environment and the genetic effect of the sow, whereas the models for the piglet traits included a litter effect as well as direct and maternal genetic effects. Estimated heritabilities for sow weight and backfat at farrowing and change of weight and backfat during lactation were low to moderate (0·10 to 0·47). We found significant genetic correlations between change of weight and backfat during lactation and piglet survival and growth, indicating that sows with the genetic capacity for rapid early piglet growth and high survival rate may lose more body reserves during lactation. Negative direct-maternal correlations for early piglet growth and survival imply that both the piglet and the sow trait should be included in a genetic evaluation for these traits. Genetic correlations between piglet survival and growth were not clearly favourable. Selection for the direct effect of piglet survival may lead to a decrease in early growth rate. We conclude that in a selection programme aiming at improving piglet survival and growth, attention should be paid to the sow's body condition during lactation. A high enough level of body reserves needs to be maintained in the sow if the incidence of reproductive problems and involuntary culling is not to increase.

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