Timing single fixed-time inseminations in ewes: Some new concepts
W.H. McMillan
AgResearch Ruakura, Private Bag 3123, Hamilton, New Zealand.
Proceedings of the New Zealand Society of Animal Production 1994,
54: 45-50
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 CIDRĘ removal, provided a drop in flock pregnancy rate of up to 5%
is acceptable.
Keywords: NZSAPAB;
ewes; AI; oestrous synchronisation; fixed-time AI; interval to oestrus.
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Last Updated 25-01-1997
