Oestrous synchronisation in ewes is normally achieved using polyurethane sponges containing progestagen. The recent development of silicone elastomer controlled internal drug releasers (CIDR) offers an alternative synchronising technique. The appropriate duration of CIDR treatment and timing of ram introduction has yet to be determined. Romney ewes had a CIDR type G inserted on the same date and withdrawn either 11, 12, 13, 14 or 15 d later (Trial 1). The ewes were joined in single sire groups with 9 rams at CIDR withdrawal. This confounding exists between duration and days since ram matings commenced. Matings were recorded over days 1 to 7 and 15 to 21. Cheviot ewes were jointed in single sire groups with 5 rams at either 30 or 48 h after CIDR withdrawal (Trial 2). Matings were recorded on days, 2 3 and 15 to 21. Pregnancy status was assessed by ultrasonic scanning. In Trial 1 more ewes mated by day 1 following 11 and 15 d CIDR durations than 11 to 13 d durations (10% and 7% v 4%). Apart from this, longer durations were associated with fewer ewes subsequently mating. There was no association between duration of CIDR treatment and return matings, pregnancies to first service or multiple pregnancies. However, overall more ewes were pregnant following intermediate durations. In Trial 2 most of the ewes jointed at 30 h had mated by 48 h. By day 3, nearly all ewes had mated in both groups. Non-return rates (80% v 63%) and first service pregnancy rates (70% v 58%) were higher in ewes joined at 30 h than at 48 h. The final pregnancy rate was similar in both groups (88% v 90%). CIDRs should be timed to be withdrawn at 12 to 14 d to maximise pregnancy rates to 2 rounds of mating when ewes are progressively joined with rams. Joining rams at 30 rather than 48 h after withdrawal results in earlier lambing since more ewes are pregnant to matings at the synchronised oestrus.

TC, Reid, and PW Shannon

Proceedings of the New Zealand Society of Animal Production, Volume 47, , 123-124, 1987
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