Effects of parity on metabolic, oxidative status and reproductive responses in heat-stressed transition dairy cows supplemented with slow release boluses

Heat stress negatively impacts transition cow health, metabolism, and fertility, while parity further influences these physiological responses. Antioxidant supplementation is proposed to mitigate oxidative and metabolic stress during this critical period. This study evaluated the effect of parity on metabolic, oxidative, and reproductive parameters in heat-stressed dairy cows receiving antioxidant boluses. Primiparous and multiparous Holstein cows were supplemented with antioxidant boluses at calving. Measurements included calving-related traits, body condition score (BCS) changes, metabolic indicators (glucose, BHBA), oxidative status (TAC, GPX, SOD, MDA), hematology, uterine health, and reproductive performance over 42 days postpartum. Parity significantly affected postpartum physiology despite antioxidant supplementation. Multiparous cows exhibited greater BCS loss (P < 0.05), higher BHBA concentrations (P < 0.05), and elevated oxidative stress, along with stronger inflammatory responses indicated by increased WBC and neutrophil counts (P < 0.01). In contrast, primiparous cows experienced more calving difficulties (P < 0.05) but maintained a relatively favorable metabolic profile. Days open tended to be shorter in primiparous cows (P = 0.09), suggesting improved reproductive recovery. Antioxidant boluses improved oxidative balance but did not fully offset parity-driven metabolic and immune challenges under heat stress. These findings highlight the need for integrated, parity-adjusted nutritional and management strategies to optimize transition cow health and fertility in thermal environments.