The original objective of a trial conducted in 3 herds of Friesian heifers was to compare pregnancy rates obtained when each animal was synchronised and then inseminated at about 50 h after removal of a subcutaneous ear implant (CrestarTM) or an intravaginal device (GenermateTM). Because most of the heifers in the CrestarTM group were in oestrus before any of the herdmates treated with the GenermateTM system, the CrestarTM treated heifers were inseminated at either 28 h or 50 h after implant removal. Over the 3 herds, 64% of 153 heifers treated with Crestar were in oestrus by 28 h after implant removal compared to 7% of their GenermateTM treated (n=155) herdmates (P<0.01). The average pregnancy rate to first insemination of 56.3% was similar for both synchrony treatments, and for the two insemination times in CrestarTM treated heifers. The earlier onset of oestrus in heifers treated using the CrestarTM system was possibly associated either with persistent and dominant ovarian follicles, or to the injected oestradiol valerate producing behavioural symptoms of oestrus. Nonetheless, inseminating heifers with a shortened interval to oestrus at 28 h rather than 50 h after implant removal may avoid reductions in pregnancy rate which occur with inseminations made after the end of oestrus, or with heifers which ovulate a persistent dominant follicle.
Proceedings of the New Zealand Society of Animal Production, Volume 56, , 347-349, 1996
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