This study was carried out to determine the reproductive performance of Tehran Province Holstein dairy cows and some factors affecting it, while using 30678 records from 6913 cows. At estimation of means and standard deviation of calving interval, days open, calving to first service interval, and first service to conception interval were 397.50±60.50, 122.10±62.45, 93.20±41.00 and 24.60±12.00 days, respectively. Effects of herd, calving season, lactation number and milk yield on reproductive performance were significant (P<0.001). Effect of sex of calf on calving to first service interval was significant (P<0.01). The results suggest that, the most and the least days open pertained to spring and autumn calving respectively. Days open increased from the fifth lactation number. The reproductive performance decreased with increasing milk yield.
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