en تخصصي Special پژوهشي Research <span style="font-size:12pt"><span style="background:white"><span style="line-height:120%"><span new="" roman="" style="font-family:" times=""><span style="color:black">Bovine Respiratory Disease (BRD) is one of the most significant health and economic challenges during the pre-weaning period, contributing substantially to calf mortality, reduced growth, and impaired future performance. Calves are born without maternal immunoglobulins due to the epitheliochorial structure of the bovine placenta, making passive immune transfer via colostrum essential for early protection. Serum Total Protein (STP) concentration is widely used as a simple and cost-effective indicator to assess the success of passive immune transfer. This study aimed to investigate the association between STP and the incidence of pneumonia in pre-weaning Holstein calves. The study was conducted on 183 calves under 15 days of age on a dairy farm in Alborz province from August to October 2024. Blood samples were collected at 48 hours of age, and STP was measured using a handheld optical refractometer. Respiratory symptoms were monitored, and data were analyzed using the Cox proportional hazards model. Results showed that higher STP was significantly associated with a reduced risk of pneumonia (P = 0.049). Each unit increase in STP corresponded to an approximately 39% reduction in disease risk. These findings highlight the importance of proper colostrum management and support the use of STP as a predictive tool for identifying high-risk calves. Monitoring STP can be an effective strategy to reduce BRD incidence and improve calf health and productivity.</span></span></span></span></span> Calf pneumonia, Serum Total Protein, Passive immune transfer, Colostrum. 103 109 http://injvr.com/browse.php?a_code=A-10-45-3&slc_lang=en&sid=1 A. Avarseji Amirali.avarseji@ut.ac.ir 10031947532846001587 10031947532846001587 No Resident in Large Animal Internal Medicine, Faculty of Veterinary Medicine, University of Tehran, Tehran, Iran A. Nazari Alireza.nazarii@ut.ac.ir 10031947532846001588 10031947532846001588 No Resident in Large Animal Internal Medicine, Faculty of Veterinary Medicine, University of Tehran, Tehran, Iran J. Abbasi jabbasi@ut.ac.ir 10031947532846001589 10031947532846001589 Yes Department of Animal and Poultry Health and Nutrition, Faculty of Veterinary Medicine, University of Tehran, Tehran Iran B. Pirali Barsampirali@ut.ac.ir 10031947532846001590 10031947532846001590 No Doctor of Veterinary Medicine Student, Faculty of Veterinary Medicine, University of Tehran, Tehran, Iran A. Sheikh Ahoora.sheikhi@ut.ac.ir 10031947532846001591 10031947532846001591 No PhD Candidate of Epidemiology, Faculty of Veterinary Medicine, University of Tehran, Tehran, Iran M.M. Mirzaee-dizgah mahdimirzaee@ut.ac.ir 10031947532846001592 10031947532846001592 No 3. Doctor of Veterinary Medicine Student, Faculty of Veterinary Medicine, University of Tehran, Tehran, Iran