Selection aimed at increasing litter size in pigs has been accompanied by a higher proportion of piglets with low birth weight. These piglets have limited survival prospects without management interventions, as their reduced vitality is further compromised by competition with littermates for colostrum and milk. Although caretaker interventions can improve survival, they also increase labor requirements and rearing costs. The need for robust and vital piglets is expected to rise with the anticipated shift toward loose farrowing systems. Therefore, we investigated whether traits derived from piglet birth weights could be used to achieve a more balanced selection between litter size and piglet vitality.
The objective of this study was to estimate the genetic parameters of piglet birth weight traits and their relationships with litter size in Czech Large White and Czech Landrace sows. The analysis included 86,241 piglets from 5,536 litters and was conducted using genetic and genomic models (BLUP and ssGBLUP). Traits were calculated at the litter level: mean birth weight (MBW), standard deviation of birth weight (SDBW), coefficient of variation of birth weight (CVBW), and the proportion of small piglets in the litter (SMALL). Heritabilities of birth weight traits were low, with the highest estimates for MBW (0.06–0.10) and SMALL (0.08–0.17). Variability traits (SDBW, CVBW) showed lower heritabilities (0.03–0.05). Heritabilities of litter size traits – number of piglets born alive (NBA) and number of piglets weaned (NW) – ranged from 0.05 to 0.07. The ssGBLUP method improved prediction accuracy by 4–15 % compared to the classical BLUP model.Genetic correlations indicated that larger litter size at birth (NBA) was negatively associated with MBW and positively or weakly associated with within-litter variability (SDBW, CVBW, SMALL), whereas the number of weaned piglets (NW) showed positive correlations with MBW and negative correlations with variability traits. All correlations between birth weight traits and litter size traits were statistically nonsignificant. The results confirm that selection solely for litter size may have an adverse effect on within-litter variability. Incorporating birth weight traits, particularly SMALL or MBW, into selection criteria may help improve litter uniformity and positively influence piglet welfare.