Parity-associated milk yield, disease occurrence and inflammatory-redox status in commercial Holstein dairy herds

Background Parity is routinely recorded in dairy herds and is often used informally in health management, but its relationship with production, common disease occurrence and physiological status under commercial conditions remains incompletely described. Methods We analyzed milk yield and health records from 20,144 Holstein cows in six commercial herds in Northeast China and measured serum redox-inflammatory biomarkers and mammary gene expression in 252 clinically healthy cows selected across parities 1 to 7. Results Milk yield increased to mid-parity and declined thereafter. Mastitis and hoof disease showed the clearest increase in older cows, whereas metritis and ketosis showed disease-specific nonlinear patterns. In sampled clinically healthy cows, higher parity was associated with lower antioxidant capacity, including lower T-AOC, total glutathione and SOD activity, and with higher IL-1β and IL-6 concentrations. P16 expression in mammary tissue also increased with parity. Exploratory aggregate-level plots suggested parallel parity-level patterns between biomarkers and observed mastitis incidence, but these plots were not interpreted as cow-level prediction. Conclusions Parity was associated with production, recorded disease occurrence and selected redox-inflammatory and mammary aging-related markers in commercial Holstein herds. The findings support parity-informed herd-health surveillance, while emphasizing the need for longitudinal cow-level studies that account for lactation stage, metabolic status and farm management.