Synchronization of lactating dairy cows with prostaglandin F-2 alpha: Insemination on observed oestrus versus timed artificial insemination

The effect of two insemination policies after synchronization of oestrus on reproductive performance in two groups of cows was investigated. Oestrus w as synchronized by two treatments with prostaglandin F-2 alpha (PGF(2 alpha )) at a 14-day interval. Cows in Group 1 (n = 234) were inseminated twice o n appointment 66 and 90 h after the second treatment. Cows in Group 2 (n = 222) were intensively watched for signs of oestrus after the synchronizatio n protocol and inseminated on observed oestrus. Cows with abnormal discharg e after synchronization were excluded from breeding and inseminated later d uring the study period. Service rate within 1 week after synchronization wa s higher in Group 1 than in Group 2 (89.3 versus 67.1 %). Conception rates on first service did not differ between groups (33.2 versus 30.0 %). Days t o first service and days open were shorter in Group 1 (P < 0.05). The numbe r of cows culled for infertility did not differ between the groups. Endomet ritis 14-20 days post-partum decreased the percentage of cows pregnant at t he end of the study period in both groups but did not have a significant ef fect on conception rates and days open. It is concluded that additional ins eminations required in the timed artificial insemination protocol were econ omically justified by the reduction in days open in comparison with insemin ation on observed oestrus after synchronization.