Abstract
BACKGROUND AND AIM: Postpartum infertility remains a major challenge in tropical dairy systems, particularly in Zebu crossbred cows that experience delayed ovarian cyclicity and increased metabolic and environmental stress. Improving early conception is essential to reducing calving intervals and enhancing productivity. This study evaluated the effectiveness of an Augmented-OvSynch protocol incorporating a post-insemination gonadotropin-releasing hormone (GnRH) injection in improving reproductive performance and assessing associated endocrine and oxidative stress dynamics in postpartum Zebu crossbred cows under field conditions. MATERIALS AND METHODS: A total of 219 clinically healthy pluriparous Zebu crossbred cows (Holstein-Friesian × Sahiwal), 55-90 days postpartum, were randomly allocated to either a control group receiving the standard OvSynch protocol (n = 82) or a treatment group receiving the Augmented-OvSynch protocol (n = 137), which included an additional GnRH dose on day 5 post-artificial insemination (AI). Estrus response, follicular dynamics, pregnancy rate, and pregnancy loss were monitored through behavioral signs and ultrasonography. Blood samples collected on days 0, 5, 10, and 21 were analyzed for estrogen, progesterone, malondialdehyde (MDA), superoxide dismutase (SOD), and total antioxidant capacity (TAC). Statistical significance was set at p < 0.05. RESULTS: The Augmented-OvSynch protocol significantly improved estrus expression (83.94%) compared with the control group (37.80%). Pregnancy rate per AI was markedly higher in treated cows (68.61%) than in controls (29.27%). Progesterone concentrations increased significantly from day 5 to day 10 in treated cows, indicating enhanced luteal support and probable accessory corpus luteum formation. Estrogen levels declined more sharply in pregnant cows. Oxidative stress markers were more strongly associated with pregnancy status than protocol type: pregnant cows consistently showed lower MDA and higher TAC levels, whereas SOD exhibited moderate variation without strong treatment effects. CONCLUSION: The Augmented-OvSynch protocol substantially improved reproductive outcomes in postpartum Zebu crossbred cows, primarily by enhancing luteal activity and improving synchronization responses. Although oxidative stress markers were influenced more by pregnancy outcome than treatment, their integration provided valuable insight into physiological determinants of fertility. Owing to its simplicity, low-cost, and field suitability, the protocol offers a practical and scalable reproductive management strategy for tropical smallholder dairy systems.