Wim Gorssen ^1,2* Carmen Winters ³ Roel Meyermans ¹ Léa Chapard ^1* Katrijn Hooyberghs ¹ Jaro De Kort ^1* Alice Van Den Broeke ^4* Jürgen Depuydt ^5* Steven Janssens ^1* Nadine Buys ^1*

• ¹ Department of Biosystems, Faculty of Bioscience Engineering, KU Leuven, Leuven, Belgium
• ² Animal Genomics, Department of Environmental Systems Science, ETH Zürich, Zürich, Switzerland
• ³ Animal Physiology, Institute of Agricultural Sciences, ETH Zürich, Zürich, Switzerland
• ⁴ Flanders Research Institute for Agriculture, Fisheries and Food (ILVO), Merelbeke, Belgium
• ⁵ Vlaamse Piétrain Fokkerij vzw, Maldegem, Belgium

Reducing piglet mortality and congenital defects is crucial for improving piglet welfare and minimizing production losses in pig farming. Piglets with congenital defects often require additional attention and resources. While maternal lines have been the focus of balanced breeding efforts to improve reproductive capacity and survival traits, the influence of paternal genetics on piglet vitality and congenital defects has received less attention. This study evaluates the impact of a Piétrain sire breeding program, initiated in 2019, on these traits in crossbred piglets.The study utilized two datasets of litter records collected from 2016 to 2024 across four commercial sow farms. The first dataset, collected by the breeding organization, included 12,010 crossbred litters (Piétrain sire, hybrid dam; two farms) and recorded piglet vitality, congenital defects (splay leg and scrotal hernia), pre-weaning mortality, and stillbirths. The second dataset, collected by ILVO (Flanders Research Institute for Agriculture, Fisheries, and Food), comprised 5,380 litters (two farms) and used 426 Piétrain boars shared with the first dataset.A genetic sire-dam model estimated low heritability (h²=5.2–15.6%) for the studied traits. Vitality scores were genetically moderately correlated with pre-weaning mortality (rg=-0.59) and splay leg syndrome (rg=-0.52). Phenotypic and genetic trends since 2019 show positive outcomes from the breeding program. The prevalence of splay leg syndrome decreased from ~1% in 2019 to ~0.5% in 2024, while pre-weaning mortality dropped from ~17% to ~14%. A combined analysis of the datasets revealed moderate to high genetic correlations (rg=0.52–0.84) for splay leg syndrome and pre-weaning mortality, suggesting the breeding values for terminal sires are transferable to new farms for these traits.In conclusion, Piétrain sire breeding programs can significantly improve piglet vitality while reducing congenital defects like splay leg syndrome and pre-weaning mortality. These findings highlight the importance of paternal genetics in addressing societal demands for improved animal welfare and economic efficiency in pig production. Other breeding organizations are encouraged to prioritize paternal genetics to achieve these goals.