The aim of this study was to determine the influence of clinical mastitis and time of first mastitis occurrence on reproductive and milk performance of Holstein cows. Data were collected in a dairy farm from 2008 to 2012 on 1725 cows, among which 464 cows with mastitis. To determine the influence of clinical mastitis on reproductive and milk performance, models included fixed effects of parity, calving season, calving year, and group (cows with and with no mastitis). To determine the effect of time of 1st mastitis occurrence on reproductive performance, the mastitic cows group was further reclassified into three groups: prior to 60 days, between 60 and 90 days and greater than 90 days postpartum. For milk performance, the mastitic cows group was divided into two groups: before and after peak milk yield. Clinical mastitis had significant effects on calving to first AI interval, milk yield, and fat yield, but a non-significant effect on days open, number of inseminations per conception, and milk fat percentage. Mastitic cows had a calving to first AI interval 6.1 days longer and 549.6 kg milk and 20.4 kg fat per 305 days of lactation lower than those with no mastitis. Time of 1st mastitis occurrence did not have any significant effect on reproductive performance. Further, milk and fat yields of cows diseased before peak milk yield were 506 kg and 23.9 kg, respectively, lower than those of cows affected after peak milk yield. Extra attention needs to be paid to mastitis during the early postpartum period.

Herd management and data recordingData were collected in a commercial dairy farm in northern Morocco. The mean annual temperature is 18.7 °C, varying from 0 °C in February to 41 °C in August. The mean annual rainfall is 600 mm, mainly occurring from November to March.Data on Holstein cows were recorded between July 2008 and December 2012. Average age at calving was 47.6 months, ranging from 23 to 82 months, and average lactation number was 2.31, ranging from 1 to 4. During the study, the mean milk production per cow was 8066 kg per 305-day lactation.All cows were housed in free-stall barns. They were observed for estrus twice daily, in the morning and afternoon. Cows with estrus were inseminated within 1 h of detection. All lactating cows were fed the same diet, which was formulated to meet the nutrient requirements