A series of four trials, each involving 1200 ewes have been conducted over the period 1986 to 1990. Ewes have been treated with various synchronisation treatments and the time to onset of oestrus monitored each month of the year. Results indicate that while overall mean values for each year don't differ, there were significant seasonal effects within years with the interval to onset being shorter during the breeding season. However, the maximum difference between monthly means in any one year varied from 27 h to 17 h. Significant effects of synchronisation device were seen with ewes treated with sponges having a longer (+ 8 h) interval to onset than ewes treated with CIDR devices. While effects of ewe age and ewe liveweight were seen in some trials no consistent trend was identified. On most occasions significant interactions with month of year indicated inconsistency of effect within any one trial. The recorded variation in interval from device removal to onset of oestrus is in agreement with that reported between flock in the field. This variation is sufficient to effect conception rates to artificial insemination performed on a time basis and evaluation of contributing factors is required for the development of protocols for this activity.

DF, Hill, and TE Broad

Proceedings of the New Zealand Society of Animal Production, Volume 51, , 85-86, 1991
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