The increasing demand for efficiency in pork production requires great specialization of all sectors involved in this activity. In this context, the development of strategies that could reduce undesirable traits related with negative effects on piglet survival and postnatal growth and development are essential for the pig industry. Currently, special attention is given to variation in birth weight, as some evidences suggest an increased within-litter birth weight variation in modern sows. This variation has been shown to be associated with preweaning mortality, variable weights at weaning and deteriorated growth performance, which results in economic losses and lower efficiency. Therefore, understanding the factors that can influence the events that occur during gestation and that have an impact on the fetal growth and development are important to achieve better efficiency and also to develop strategies that can be used to achieve increased within-litter uniformity of piglet birth weight. This study concludes that even at a given placental size, fetal growth may vary because of differences in placental vascularization and efficiency. Feeding extra feed or energy during late gestation only marginally improves birth weight, and positive effects are not consistent between different studies. The detrimental effects of protein restriction on fetal growth during early gestation may be due to altered placental and endometrial angiogenesis and growth, which leads to a reduction in placental-fetal blood flow, nutrient supply from mother to the fetuses and ultimately to fetal growth retardation. The number of studies that attempted to influence within-litter birth weight variation by means of sow nutrition during gestation is limited. Therefore, more research concerning sow nutrition during gestation associated with the provision of balanced diets to meet requirements of the sows and fetuses are still required. This knowledge may subsequently provide starting points for the design of nutritional strategies that can influence within-litter birth variation.

Within-litter variation of piglet birth weight (BW0) is associated with an increased piglet mortality and a high variability in pig weight at weaning and weight or age at slaughter. Data collected in two experimental herds were used to quantify within-litter variability in BW0 and to assess the influence of factors mainly related to the sow. Within 24 h after birth, piglets born alive were individually weighed and stillborn piglets were collectively (first data set) or individually (second data set) weighed. The first data set was restricted to litters with no or only one stillborn piglet (3338 litters). It was used to assess the influence of genetic selection on BW0 variation by comparing litter characteristics before (1994 to 1996) and after (2001 to 2004) the development of hyperprolific sows in this herd. The second data set included all litters (n = 1596) from sows born between 2000 and 2004. For each litter, mean BW0 (mBW0) and its coefficient of variation (CVBW0) were calculated. Then, variance analyses were performed to test the influence of litter size, parity, year of sow birth and season at conception. Prolificacy improvement was associated with an increased CVBW0 in litters from pure Large White (LW) and Landrace × Large White (LR × LW) crossbred sows. The CVBW0 averaged 21% and was significantly influenced by litter size and parity. It increased from 15% to 24% when litter size varied from less than 10 piglets to more than 15 piglets. The proportion of small piglets (i.e. weighing less than 1 kg) increased concomitantly. The CVBW0 was not repeatable from a parity to the following. It was lowest for first and second parities (20%) and thereafter increased progressively. The CVBW0 was positively related to sow’s backfat thickness gain during gestation. Taking into account litter size, parity, year of sow birth and season at conception explained 20% of BW0 variation. Thus, major part of heterogeneity is due to other factors, presumably including embryo genotype, on the one hand, and factors that influence embryo and foetus development, such as epigenetic factors, on the other hand.

Genetics affects not only the weight of piglets at birth but also the variability of birth weight within litter. Previous studies on this topic assigned the sample standard deviation of piglet birth weights within litter as an observation to the sow. However, the contribution of the difference in mean birth weight per sex on the within-litter variance has been neglected so far. This work deals with the genetic effect on within-litter variance when different statistical models with different distributional assumptions are used and considers the sex effect and appropriate weights per trait. Traits were formed from the pooled sample variance of male and female birth weights within litter. A linear model approach was fitted to the logarithmized within-litter variance and the sample standard deviation. A generalized linear model with gamma-distributed residuals and log-link function was applied to the untransformed sample variance. Models were compared by analysing data from 9439 litters from Landrace and Large White of a commercial breeding programme. The estimates of heritability for different traits ranged from 7% to 11%. Although the generalized linear mixed model is preferred from a mathematical view, the rank correlations between breeding values of the linear mixed models and the generalized linear mixed model were relatively high, i.e. 94% to 98%. By residual diagnostics, a linear mixed model using the weighted and pooled within-litter standard deviation was identified as most suitable.