Cumulative Reproductive‐Lactation Burdens Induce a Systemic Immunometabolic Trade‐Off Between Milk Yield and Mammary Health of Dairy Cows

ABSTRACT Continuous lactation and repetitive reproductive cycles impose persistent physiological demands and cumulative systemic stress on dairy cows. However, the molecular mechanisms by which circulating blood profiles adapt to cumulative reproductive and lactational burdens to balance milk yield while preserving udder health remain poorly understood. In this study, we generated peripheral blood transcriptomes from 191 Holstein cows with parities ranging from one to nine across four physiological stages (dry, early, mid and late lactation), with the aim to characterize the immunometabolic adaptations modulated by the lactation stage and cumulative reproductive burden. Phenotypic analyses revealed that cows with higher parity produced greater milk yields, whereas the progressive accumulation of reproductive cycles led to increased somatic cell scores and a greater risk of mastitis. Pronounced transcriptomic differences were observed between low‐parity (LP) and high‐parity (HP) aged cows throughout all stages of lactation, with 49 immune‐related differentially expressed genes consistently detected. Functional enrichment analysis showed that LP cows exhibited consistent upregulation of pathways associated with ribosome biogenesis, RNA splicing, and DNA repair, whereas HP cows exhibited sustained activation of pathways associated with IL‐17, NF‐κB, cellular senescence, and mitochondrial oxidative stress. Weighted gene co‐expression network analysis and longitudinal trajectory clustering revealed divergent gene module regulatory networks between the LP and HP groups. Single‐cell transcriptomic deconvolution further demonstrated that the increasing reproductive burden was associated with remodeling of the peripheral immune cell composition, characterized by reduced proportions of T cells, increased proportions of eosinophils and NKT cells, and stage‐specific shifts in nonclassical monocytes. Collectively, these findings indicate that recurrent lactation and reproductive stress drive systemic immunometabolic reprogramming and circulating immune cell remodeling, ultimately leading to a physiological trade‐off between milk productivity and mammary immune homeostasis. This population‐scale transcriptomic resource provides novel mechanistic insights into the molecular basis of productive longevity in intensive dairy farming system.