Generation-Specific Heterosis in Lactation, Reproduction, and Blood Transcriptomic Profiles of Chinese Simmental × Holstein Crossbred Cows

This study investigated generation-specific heterosis in Chinese Simmental × Chinese Holstein crossbred cows by comparing lactation performance, reproductive performance, hematological traits, and blood transcriptomic profiles among Chinese Simmental (SIM), Chinese Holstein (HOL), F1 crossbreds, and first-generation backcrosses (BC1). Lactation analysis was based on 17,005 valid records, reproductive analysis was based on 5481 valid records, and transcriptomic analysis was conducted using blood samples from 31 cows. F1 showed the most favorable reproductive profile, whereas BC1 showed relatively strong milk-yield performance. Hematological analysis revealed group-level differences in leukocyte-related indicators. Main-text transcriptomic interpretation focused on SIM, F1, and BC1 comparisons because the HOL RNA-seq group included only three samples. These comparisons suggested that F1-related DEGs were mainly associated with immune and inflammatory processes, whereas BC1-related DEGs showed broader immune- and metabolism-related enrichment patterns. Candidate-gene expression heterosis and d/a-ratio analyses are presented as exploratory supplementary results with bootstrap uncertainty estimates because qPCR validation was not available. Overall, these results indicate that heterosis in this crossbreeding system is generation-specific rather than uniform. The integration of phenotypic and blood transcriptomic analyses provides candidate pathways and associated systemic expression patterns for future validation rather than definitive mechanistic evidence.