Abstract: [Objective] With the rapid development of large-scale dairy farming,the traditional estrus identification method can no longer meet the production demands. Synchronized estrus-timed artificial insemination technology has been widely popularized to improve the reproductive management efficiency of dairy farms.[Method] Lactating Holstein cows from a large-scale dairy farm in Yunnan were selected as experimental animals. Three treatment groups were set up,including double synchronization group,natural estrus group and resynchronization group for open cows. The number of mated cows,pregnant cows and conception rate were recorded monthly throughout the year,and all data were analyzed by SPSS 26.0.[Result] The average conception rate of the natural estrus group was 50.63%,and that of the double synchronization group was 47.04%,with no significant difference between the two groups （P>0.05）. The conception rate of open cows in resynchronization group was only 33.15%,which was extremely significantly lower than the other two groups （P<0.01）. Due to herd transfer and heat stress,the overall conception rate decreased obviously in late winter,early spring and summer.[Conclusion] The double synchronization technology can achieve 100% mating rate,and its comprehensive effect is equivalent to natural estrus mating,which is suitable for large-scale dairy farms in Yunnan plateau area. Reproductive disorders of open cows are serious and cannot be solved merely by estrus synchronization. It is necessary to strengthen postpartum care,reproductive disease prevention and environmental management in daily production.

Key words: cows, large-scale dairy farm, synchronized estrus, double synchronization, timed artificial insemination, pregnancy rate