Swine

A decade of untangling the genetic background of litter size phenotypic variability in pigs

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Abstract

Litter size (LS) is a key trait determining the efficiency of pig breeding; any deviation from the expected number of piglets causes concern for breeders. This is particularly important given the positive genetic correlation between the average LS and its variability. The aim of this work is to explore the knowledge on phenotypic variability of LS and search for the optimum LS. Data were obtained for Large White and Landrace pigs from Topigs Norsvin, and Polish ZÅ‚otnicka breeds. The focus of studies was primarily on the heterogeneity of residual variance; however, quantitative genetics tools were also employed to demonstrate the "parity curve" or lack of biological background associated with the second litter syndrome (SLS). Method comparison study of the Double Hierarchical Generalized Linear Model (DHGLM) and a simpler two-step approach based on log-transformed residual variance (LnVar) indicated that the DHGLM is a significantly more accurate method for estimating the phenotype of trait variability. Furthermore, the variability of LS obtained with the two methods is not phenotypically or genetically identical. GWAS analysis (data from 2015 and 2022) confirmed that the most promising candidate genes for LS variability are located on chromosome 7 in Large White pigs. It was also demonstrated that in Landrace pigs, different genomic regions are associated with LS variability and that environmental and genetic sire effects play a greater role in determining LS than in the Large White breed. Further differences between breeds (at the phenotypic level) were observed when LS variability was studied in Polish breeds. Importantly, in each breed, the variability of LS could be reduced through selection, making it possible to select for optimum LS. It also indicated that the occurrence of SLS in a herd is primarily related to management practices and should not be treated as a "syndrome", suggesting reproductive abnormalities. Individual cases of SLS do not pose a problem for breeders if their occurrence does not affect the average "parity curve". Such cases should be considered a statistical artifact of the normal distribution. This parity curve could be used to extend existing models for variance component estimation, allowing for the accounting of average LS in consecutive parities. As evaluated, the selection aimed at modifying the shape of this "curve" is possible; however, reducing differences between the litters of sows resulting from this curve can only be achieved at the cost of lowering the overall average LS. Moreover, such a modification does not improve the accuracy of estimating LS variability expressed as LnVar. In conclusion, selection can be used to reduce LS variability and maintain the optimal level of this trait. However, such a process will not be easy due to the complexity of the trait.

Keywords: 2026

How to Cite:

Sell-Kubiak, E., (2026) “A decade of untangling the genetic background of litter size phenotypic variability in pigs”, World Congress on Genetics Applied to Livestock Production Digital Archive 2026(1): 2283894. doi: https://doi.org/10.31274/wcgalp.23533

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