Abstract: We conducted a retrospective cohort study to investigate factors influencing the reproductive performance of dairy cattle in Khorosan Razavi province, Iran. Ten dairy herds, located within a 45 km radius of the city of Mashhad in Khorosan Razavi province took part in the study. Complete lactation records were collected for cows that calved between 21st March 2006 and 20th March 2007. Each cow was followed until the end of the study on 21st October 2007 or until the date of leaving the herd, either by culling, sale, or death. Median days open was 123 days (range 28-430 days). Cox proportional hazard models with and without a herd level frailty term were used to identify and quantify the effect of factors influencing days open. Parity and the presence of uterine infection, cystic ovarian disease, mastitis and lameness were positively associated with days open. The proportion of variance explained at the herd level was 0.33% suggesting that the herds that participated in this study were relatively homogenous in the distribution of unmeasured herd-level factors influencing days open. This study has provided starting point for defining benchmark estimates of reproductive performance in dairy herds in this area of Iran. Quantifying the effect of disease on reproductive performance provides a means for ranking disorders in terms of their effect on fertility, allowing intervention strategies designed to optimise herd health and production to be further refined

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