Inzicht in en verbetering van de toomuniformiteit van het lichaamsgewicht bij varkens door genetische en omgevingsfactoren

Over the past decades, selective breeding and improved sow management have significantly increased litter sizes in pigs. Larger litters typically show lower average birth weights and greater variation in the body weights of the piglets. Within-litter body weight uniformity, defined as the degree of similarity in body weights among piglets within the same litter, influences piglet survival, growth performance and overall production efficiency. This body weight uniformity is generally assessed using the coefficient of variation, standard deviation or the proportion of piglets below a certain weight threshold. The lightweight piglets, of which a higher proportion is present in those large litters, face many challenges including hypothermia, limited access to colostrum and milk due to increased competition and a higher risk for being crushed by the sow, resulting in increased mortality rates. If they survive, lightweight pigs typically have lower growth performance and feed efficiency, resulting in more time needed to reach the same market weight as their batchmates. Consequently, extended finishing periods, suboptimal housing occupancy and feed efficiency raise production costs and complicate pig farming logistics. Also, slaughtering these lightweight pigs together with their batchmates, would lead to financial penalties for the pig farmers, reducing their income. Recognizing the complexity of this issue, this PhD thesis investigated how sow-, litter- and piglet-related features, targeted management strategies and the genetic merit of the sire impact the within-litter body weight uniformity from birth to slaughter. The research was structured into four studies, each addressing a specific aspect of the problem. In a first study, six metrics were compared to assess within-litter birth weight uniformity: the coefficient of variation, standard deviation, weight range (maximum - minimum body weight body weight per litter), average Z score and the proportion of piglets below 0.8 and 1kg. The coefficient of variation, standard deviation and weight range were found to be highly correlated, indicating that they can be used interchangeably to a large extent. The percentage of lightweight piglets (<0.8 or <1 kg) was proposed as a labor-efficient alternative, showing strong correlation with the coefficient of variation. In contrast, the coefficient of variation based on only the birth weights of the six piglets closest to the mean was not an accurate metric of birth weight uniformity of the whole litter, making it unsuitable. Individual sow features explained 8 to 63% of the variation in birth weight uniformity. Specifically, a higher percentage of mummified or stillborn piglets and an older age at first insemination were associated with increasing within-litter birth weight variation. These findings emphasize the importance of standardizing uniformity metrics and considering sow characteristics in breeding and management decisions to improve within-litter birth weight uniformity in pigs. In a second study, 924 piglets were tracked from birth to finishing phase to examine how sow-, litter- and pig-related features affect within-litter body weight uniformity, growth performance and survival. Major shifts in within-litter body weight uniformity were observed around weaning, with only low correlations between the uniformity at birth and post-weaning, suggesting that birth and post-weaning body weight uniformity are two distinct traits which might require separate management and breeding strategies. Piglets with a larger weaning dip (lower average daily gain during the first week post-weaning) showed more growth deviations later. Furthermore, within-litter body weight uniformity decreased with higher sow parity, larger litter sizes and larger chest circumference of the sow before farrowing but increased with a higher number of functional teats. Body weight and growth performance was positively associated with higher sow parity, longer cumulative birth interval, greater proportion of male piglets, higher relative birth order and older weaning age. In contrast, female piglets, larger litters and greater condition loss of the sow during lactation were linked to lower body weights and average daily gain. The survival odds of lightweight piglets (< 1 kg at birth) were higher in litters with more functional teats and greater sow condition loss during lactation, but lower in larger litters and with longer birth intervals. These findings highlight the major impact of sows on within-litter body weight uniformity, body weight, average daily gain and survival. Notably, larger litter sizes were associated with reduced uniformity, growth performance and survival of lightweight piglets. In a third study, two post-weaning feeding strategies were researched to reduce body weight variation: 1) using the same feed bowl as in the farrowing house during the first week post-weaning and 2) offering liquid pre-starter feed in the familiar bowl during the first week post-weaning. Both strategies increased feed intake shortly. . In the first trial, piglets with access to the familiar bowl consumed more total feed but had a lower feed efficiency and reduced average body weight gain during the second week post-weaning compared to piglets without the familiar bowl. Furthermore, liquid pre-starter feed increased body weight uniformity, feed intake and average daily gain but also reduced feed efficiency. However, these effects did not persist and neither of the strategies led to long-term improvements during the finishing phase. In a fourth study assessedwhether the estimated breeding values for survival and uniformity of terminal Piétrain sires could predict body weight uniformity and survival in their crossbred offspring. Six sires (three with a high and three with a low survival index) were selected based on available progeny testing information from the breeding program. Their offspring (N = 1421) were followed up from birth to slaughter. Litters of sires with a high survival-uniformity index had a lower birth weight standard deviation and coefficient of variation and tended to have more stable growth trajectories. Conversely, these litters also showed increased pre-weaning mortality. Selecting sires based on favorable and unfavorable estimated breeding values for uniformity had only limited effects on within-litter body weight uniformity and survival in this study. In this PhD thesis, both environmental and genetic influences on within-litter body weight uniformity were assessed throughout the lifetime of the pigs. Maternal traits such as age at first insemination, parity, litter size, chest circumference and number of functional teats were associated with uniformity. Furthermore, this PhD thesis has demonstrated that there are two distinct within-litter body weight uniformity traits: birth weight uniformity, especially important for survival shortly after birth and during lactation and post-weaning body weight uniformity, which persists during the finishing phase. Furthermore, associations between within-litter body weight uniformity and the current estimated breeding values of a Piétrain line were investigated and showed limited predictive value. Based on these findings, we advise to improve the phenotyping methods for body weight uniformity, prioritize sows with good condition before farrowing and an adequate number of functional teats, target the two uniformity traits separately and continue research on alternative estimated breeding values to increase body weight uniformity in pig farming.