EVALUATION OF CRESTAR, A SYNTHETIC PROGESTOGEN REGIME, FOR SYNCHRONIZING ESTRUS IN MAIDEN HEIFERS USED AS RECIPIENTS OF EMBRYO TRANSFERS

Crestar consists of an ear implant containing 3 mg norgestomet combine d with an intramuscular injection of 3 mg norgestomet and 5 mg oestrad iol valerate. Its effectiveness for synchronising oestrus in embryo tr ansfer recipients was evaluated in comparison with a progesterone-rele asing intravaginal device (PRID) and prostaglandin regimen, using 334 maiden heifers. The treatment devices were inserted on day 1, prostagl andin was administered to the PRID-treated heifers on day 8 and the de vices were removed on day 10. High proportions of the heifers were see n 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) tr eatments. The interval from the removal of the device to the onset of oestrus was significantly shorter for Crestar than for PRID prostaglan din-treated heifers (45 vs 51 hours, P<0.001), and the duration of oes trus was significantly longer (13 vs 10 hours, P<0.01). The PRID prost aglandin treatment resulted in a higher degree of synchrony than the C restar treatment (74.1 per cent vs 61.8 per cent, P<O.05). There were no significant differences between the treatments in the proportions o f the heifers selected as embryo transfer recipients or in the proport ions which became pregnant after embryo transfer.