en عمومى General پژوهشي Research <span style="font-size:11pt"><span style="line-height:120%"><span style="font-family:Calibri,sans-serif"><span lang="TR" style="font-size:12.0pt"><span style="line-height:120%"><span new="" roman="" style="font-family:" times="">Successful transfer of passive immunity (TPI) is essential for the health and long-term productivity of newborn calves. While colostrum quality and intake have been extensively investigated, the influence of calving time on TPI remains poorly understood. This study evaluated the effect of calving time on serum total protein (STP) concentration and passive immunity transfer in newborn Holstein calves. A total of 107 calves from a commercial dairy farm were assigned to four groups based on birth time: Group 1 (06:00&ndash;11:59 h), Group 2 (12:00&ndash;17:59 h), Group 3 (18:00&ndash;23:59 h), and Group 4 (00:00&ndash;05:59 h). Blood samples were collected 48 h after colostrum feeding, and STP concentrations were measured using a handheld optical refractometer. Data were analyzed using one-way ANOVA followed by Bonferroni post-hoc comparisons. Calving time significantly affected STP concentration (P < 0.05). Calves born during daytime hours had significantly higher STP concentrations (Group 1: 7.24 &plusmn; 0.51 g/dL; Group 2: 7.12 &plusmn; 0.72 g/dL) than calves born during nighttime hours (Group 3: 6.62 &plusmn; 0.66 g/dL; Group 4: 6.59 &plusmn; 0.47 g/dL; P < 0.05). No significant difference was observed between Groups 1 and 2 (P > 0.05). These findings suggest that calving time may influence passive immunity transfer in neonatal calves. Improved monitoring and standardized colostrum management during nighttime calving may help optimize calf health and passive immunity status.</span></span></span><span dir="RTL" lang="AR-SA" style="font-size:12.0pt"><span style="line-height:120%"><span style="font-family:"Times New Roman",serif"></span></span></span></span></span></span> Holstein calves, Colostrum, Passive immunity transfer, Serum total protein, Calving time. 111 117 http://injvr.com/browse.php?a_code=A-10-45-4&slc_lang=en&sid=1 A.R. Nazari Alireza.nazarii@ut.ac.ir 10031947532846001593 10031947532846001593 No Department of Internal Medicine, Faculty of Veterinary Medicine, University of Tehran, Tehran, Iran A.A. Avarseji Amirali.avarseji@ut.ac.ir 10031947532846001594 10031947532846001594 No Department of Internal Medicine, Faculty of Veterinary Medicine, University of Tehran, Tehran, Iran J. Abbasi jabbasi@ut.ac.ir 10031947532846001595 10031947532846001595 Yes Department of Animal and Poultry Health and Nutrition, Faculty of Veterinary Medicine, University of Tehran, Tehran, Iran S. Kashi Saeed.kashi@ut.ac.ir 10031947532846001596 10031947532846001596 No Faculty of Veterinary Medicine, University of Tehran, Tehran, Iran A. Sheikhi Ahoora.sheikhi@ut.ac.ir 10031947532846001597 10031947532846001597 No Department of Epidemiology, Faculty of Veterinary Medicine, University of Tehran, Tehran, Iran