The mean interval from the end of synchrony treatment to the onset of oestrus can be highly variable and as a consequence 'single-time' inseminations may occur at other than optimal times. This study was undertaken to identify some of the factors influencing the interval to oestrus following synchrony treatment, and to develop a spreadsheet model to assist with determining the timing of AI in ewes under a range of oestrous onset patterns. Synchrony device, PMSG, time of year, and live weight tended to have important effects on the interval to oestrus. Ewe genotype and year effects were generally small. Three new concepts have arisen from the modelling exercise. Firstly, the actual timing of AI to achieve near maximum fertility depends on the mean interval to oestrus in a given flock, but not the precision of synchrony or the incidence of ewes that are synchronised. Secondly, the 'tolerance' about this optimum time of insemination depends on how much is known about the mean interval to onset (more information means more tolerance). Thirdly, the 'tolerance' about this optimum time of insemination depends on the maximum reduction in flock pregnancy rate that is acceptable to the flock owner (higher reduction means more tolerance). If the onset pattern in a particular flock is unknown, the 'universal' timing for a single fixed-time AI during the breeding season is 42- 46 h after CIDRÆ removal, provided a drop in flock pregnancy rate of up to 5% is acceptable.

WH, McMillan

Proceedings of the New Zealand Society of Animal Production, Volume 54, , 45-50, 1994
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