Seasonal changes in oestrus, ovulation and conception of Coopworth ewes treated with CIDRs and PMSG

Mixed-age Coopworth ewes (594) were allocated to 18 groups on the basis of age and live weight. Three groups (n=33) were treated and joined every second month from June 1986 until April 1987. Ewes were treated with controlled internal drug releasers (CIDR) for 14 d and either 0, 400, or 800 iu. :pregnant mare serum gonadotrophin (PMSG) at CIDR removal. Entire Poll Dorset rams (10%) were joined at CIDR removal and changed every 6 h during oestrus. Data on interval from CIDR removal to onset of oestrus, duration of oestrus, ovulation rate, pregnancy rate and lambing performance were recorded for all groups. The interval from CIDR removal to the onset of oestrus increased in the non-breeding season and this was modified by PMSG treatment which shortened the interval. The duration of oestrus was shorter in the non-breeding season in ewes with no PMSG and longer in ewes with the higher dose of PMSG. Both dose levels of PMSG prevented the out-of-season reductions in the incidence of oestrus and ovulation. Ovulation rate increased with dose of PMSG and all treatments showed a seasonal change. A decline in conception rate was a major problem even in PMSG treated ewes during the non-breeding season.