The aim of present study was to determine cut off point and to investigate milk loss caused by clinical and sub-clinical mastitis in Iranian Holstein dairy cattle. This research was conducted in large scale by using dataset collected by Animal Breading of Iran. The dataset consisted of 4932245 test day records of somatic calls count (SCC) gathered from 2002 to 2013 on 1281 dairy farms in Iran. Moreover, information of three herds with regular recording system in Isfahan province were collected to estimate milk loss due to mastitis during 2011-2013. Statistical analysis were done by using SAS software. Results show that cut off point occurs at approximately 10,000 cell counts and milk loss caused by sub-clinical mastitis for primiparus, multiparus, and overall were estimated 2.9, 5.21, and 4.8 kg/d respectively. Milk loss due to clinical mastitis calculated by mixed model, for primiparus, multiparus, and overall were 195, 406 and 276 kg for a lactation period. In addition, milk loss due to clinical mastitis were estimated by wood function. Milk loss compared to milk yield for lact=1, lact=2 and lact are 5.1 7.7 and 10.9 respectively. As the age at record increases, somatic cell count rises; however, milk loss doesn’t present a linear trend. Milk losses due to clinical mastitis were predicted Keywords: Cut off, Clinical Mastitis, Sub-clinical Mastitis, Milk Yield, and Holstein
