Mastitis is one of the most common diseases affecting dairy cows and it causes major economic losses to the dairy industry including reduction in milk production and reproductive performance. Recently, survival analysis was used to assess the effects of diseases on reproductive performance. Maizon et al. [1] showed that survival analysis allows for a more appropriate management of censored data and time-dependent covariates, which are very common in the study of variables such as days from first breeding to conception (DFBC). The objective of this research was to study impact of clinical mastitis on DFBC and first 200 days cumulative milk production in Iranian Holstein dairy cows. A data set of 13,000 lactations during 1996 to 2003 from five large dairy herds was used for this study. Mastitis definitions included mastitis treated by systemic therapy or mastitis treated by local therapy, acute mastitis or chronic mastitis, all cases of clinical mastitis, treatment for mastitis, and clinical mastitis diagnosed by a veterinarian or a technician. According to the parturition date, DFBC and mastitis data were analyzed by survival analysis. Survival statistical analysis was performed using the statistical software package JMP (SAS Institute Inc., NC, USA). Mastitis was considered if it occurred after calving up to conception or until censoring. So, for the uncensored data, mastitis occurred before first breeding or also during breeding but not after the conception. Conception was defined by rectal palpation and subsequently confirmed by calving. The DFBC was defined for an uncensored cow as the interval from first breeding to conception and for a censored cow as the interval from first breeding to either culling or 350 d after calving, whichever came first. The first 200 cumulative milk data were analyzed by Mixed models in SAS. The results showed that DFBC (44.93 and 77.55 d, respectively in the healthy and mastitic cows) was different among groups (P<0.001) and cows with clinical mastitis had a prolonged interval compared with the healthy cows. The median DFBC almost in doubled in the mastitic compared with the healthy cows (21 and 46 d, respectively). The Log-Rank and Wilcoxon tests showed significant difference between the two groups (P<0.0001). The cumulative 200 days milk production was less in cows with clinical mastitis (6021 and 5870 kg, respectively in the healthy and mastitic cows, p<0.001). This result is similar to finding of Santos et al. [2]. The results demonstrated that clinical mastitis before first breeding or also during breeding was one of the effective factors on milk production and reproductive performance.

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