Insemination at the second of two induced oestrous periods in anoestrous dairy cows increases conception rates to first service

Aim. To determine whether mating previously anoestrous cows on their second , rather than first, oestrus post partum could increase conception rates to that service. Methods. Cows diagnosed as anovulatory anoestrus were allocated to two grou ps, starting treatment on Day -17 (OES2) or on Day -7 (OES1) where the date of the planned start of mating = Day 0. All cows were treated with intrava ginal progesterone (CIDR device) for 6 days and an injection of 1 mg oestra diol benzoate 24 h after removal of the device. Cows in the OES2 group were also injected with prostaglandin F-2 alpha on Day 0. Results. During the first 7 days of mating, 61.8% (207/335) and 88.2% (328/ 372) of cows in the OES2 and OES1 groups, respectively, were inseminated on detection of oestrus (p < 0.001). Oestrus without ovulation occurred in 1. 5% of cows in the OES2 group and 6.2% of cows in the OES1 group during this period (p < 0.01). Of those cows inseminated in the first week of mating, conception rates were 54.0% in the OES2 group and 45.5% in the OES1 group ( p = 0.05). There was no significant difference in median interval from star t of mating to conception between the two groups (24 and 22 days for OES2 a nd OESI, respectively, p > 0.05). Conclusion. Mating previously anoestrous cows on their second, rather than first oestrus post partum increased conception rates to that service. Howev er, median intervals from start of mating to conception were not altered. K ey words. Dairy cattle, post-partum anoestrus, reproduction, progesterone t reatment.