Background: Xianan cattle, the first cross-bred beef cattle developed in China, are recognized for their rapid growth, tolerance to rough feed, and high meat yield. These characteristics make them a valuable model for studies aimed at improving beef production traits. Methods: In this study, two complementary gene mapping strategies, selection signature analysis and association analysis, were employed to identify candidate genes associated with body weight. The analyses utilized resequencing data comprising 16,250,950 high-quality single nucleotide polymorphisms (SNPs). Twenty independent variables showed significant correlations with body weight, with effect sizes ranging from 239 kg to 629.37 kg, while controlling for a false discovery rate (FDR) of less than 0.5. Results: The most prominent signal was identified in the 54.24–54.39 MB region on chromosome 9, which contains the MANEA gene. Furthermore, we investigated the functional role of the MANEA gene at the cellular level. siRNA-mediated knockdown of MANEA resulted in significant alterations in the expression of downstream genes, notably MGAT1, MGAT3, FUT8, and HK1. Among these, the expression of MGAT1 was markedly increased, showing an increase of up to 600-fold compared to the control. Conclusions: These results offer critical insights into the molecular mechanisms underlying body weight regulation and provide a foundation for developing strategies to enhance economically important production traits in beef cattle.