Using a large data set with nearly 40,000 weighed
piglets, the aim of the present study is to investigate the
question to what extend measures of the distribution of
the birth weight within litter are heritable. It was found that selection on high litter size was shown to decrease the mean piglet birth weight and to increase the within-litter variability of birth weight. These changes in the birthweight traits cause an increase in the number of stillborn piglets and in the losses from birth to weaning. Therefore, selection on litter size should be accompanied by selection on mortality traits and/or birth-weight traits. On the basis of the estimated heritabilities and genetic correlations, pre-weaning mortality and the minimal birth
weight in the litter are proposed as potential traits for selection against piglet mortality.