Evaluation of Crestar, a synthetic progestogen regime, for synchronising oestrus in maiden heifers used as recipients of embryo transfers.

Crestar consists of an ear implant containing 3 mg norgestomet combined with an intramuscular injection of 3 mg norgestomet and 5 mg oestradiol valerate. Its effectiveness for synchronising oestrus in embryo transfer recipients was evaluated in comparison with a progesterone-releasing intravaginal device (PRID) and prostaglandin regimen, using 334 maiden heifers. The treatment devices were inserted on day 1, prostaglandin was administered to the PRID-treated heifers on day 8 and the devices were removed on day 10. High proportions of the heifers were seen in oestrus within five days of the removal of the devices after both the PRID prostaglandin (90.4 per cent) and Crestar (86.2 per cent) treatments. The interval from the removal of the device to the onset of oestrus was significantly shorter for Crestar than for PRID prostaglandin-treated heifers (45 vs 51 hours, P < 0.001), and the duration of oestrus was significantly longer (13 vs 10 hours, P < 0.01). The PRID prostaglandin treatment resulted in a higher degree of synchrony than the Crestar treatment (74.1 per cent vs 61.8 per cent, P < 0.05). There were no significant differences between the treatments in the proportions of the heifers selected as embryo transfer recipients or in the proportions which became pregnant after embryo transfer.