The objectives of this study were to estimate genetic parameters for stayability in Chinese Holstein cattle and to estimate its genetic correlations with routinely collected traits. Although knowledge and genetic breeding technologies for dairy cattle have advanced, the lifespan of dairy cows has been declining, mainly due to increased production stress and associated health problems. This makes the genetic improvement of Holstein cattle crucial for sustainable production. Stayability is an indicator trait reflecting longevity, measured as the survival probability of dairy cows at specific stages. To evaluate temporal differences in herd retention, we defined 7 stayability traits as the ability of cows to remain in the herd for 36 (S36), 42 (S42), 48 (S48), 54 (S54), 60 (S60), 72 (S72), and 84 (S84) months after first calving. Data from 56,630 cows between 2011 and 2020 across 14 dairy farms in Ningxia were considered for stayability analyses. Survival analysis using non-parametric methods showed that cattle with sterility had the longest survival time, while those with abomasal displacement had the shortest survival time. The heritability estimates (±standard error) for S36, S42, S48, S54, S60, S72 and S84 were 0.048 ± 0.006, 0.063 ± 0.006, 0.074 ± 0.007, 0.099 ± 0.007, 0.115 ± 0.007, 0.088 ± 0.007, 0.118 ± 0.008, respectively. The genetic and phenotypic correlations among stayability traits ranged from 0.382 to 0.975 and from 0.090 to 0.799, respectively. Furthermore, the approximate genetic correlations between stayability and routinely collected traits (8 production, 20 conformation, 13 fertility, 13 health traits) were assessed. Stayability showed low to moderate genetic correlations with most of these traits. In summary, developing a selection index that incorporates stayability is expected to improve the longevity of dairy cows. All unfavorable genetic relationships observed between stayability and other routinely collected traits should be accounted for in a selection index, thereby enhancing the lifespan of Holstein cattle while maintaining or improving productive performance.

Lactation traits are important indicators for evaluating the production performance of dairy cows and the economic efficiency of dairy production. Identifying potential regulatory genes is essential for elucidating the molecular mechanisms underlying lactation and facilitating molecular breeding. This study aimed to identify candidate genes and regulatory pathways potentially associated with lactation traits in dairy cows by integrating transcriptome expression profiles of primary bovine mammary epithelial cells (BMECs) from eight Holstein cows with lactation phenotypic data. A gene co-expression network was constructed using weighted gene co-expression network analysis (WGCNA). Co-expression modules associated with lactation traits were identified, and candidate genes were further screened by integrating gene significance, module membership, functional enrichment analysis, random forest analysis, gene-phenotype association analysis, single-gene gene set enrichment analysis (GSEA), ROC curve analysis, and tissue expression profiling. Four co-expression modules, namely MEdarkturquoise, MEsteelblue, MEbrown, and MEskyblue3, were significantly associated with lactation traits (p < 0.05). Genes in these modules were mainly enriched in biological processes and pathways related to ribosome biogenesis, protein translation, the cell cycle, oxidative phosphorylation, and the PI3K-Akt signaling pathway, which may be involved in lactation regulation. Through multi-strategy cross-screening, four candidate genes were ultimately identified. MDM2 was associated with daily milk yield (DYM), RPL37A with total milk solids (TMS), and SLC25A25 and CCNE2 with milk fat percentage (MFP) and fat-to-protein percentage ratio (FPP). Reverse transcription quantitative real-time PCR (RT-qPCR) validation showed that MDM2 and RPL37A were relatively highly expressed in mammary tissue, suggesting that they may be involved in lactation-related biological processes in dairy cows. These findings provide potential candidate regulators and theoretical support for further studies on the molecular mechanisms of lactation traits; however, their functional roles require further validation through in vitro and in vivo experiments.