EVALUATION OF 3 ESTRUS SYNCHRONIZATION REGIMENS FOR USE IN EXTENSIVELY MANAGED BOS-INDICUS AND BOS-INDICUS-TAURUS HEIFERS IN NORTHERN AUSTRALIA

Two herds of extensively managed yearling Bos indicus and Bos indicus x Bos taurus heifers (n = 169) were assigned to treatments consisted of 1) two injections of the prostaglandin (F2-alpha) analogue, luprostiol (7.5mg), given 12-d apart; 2) insertion of an intravaginal progesterone-releasing device (CIDR-B), followed by 400 IU i.m. pregnant mare serum gonadotrophin (PMSG) and 7.5mg luprostinol at CIDR-B removal or 3) subcutaneous progestagen implant (SMB) for 10-d, followed by 400 IU i.m. and 7.5 mg i.m. luprostiol at implant removal. A dual tail paint-raddle system was used to evaluate response to treatment. Each treatment group was inseminated according to the manufacturer's recommendations for fixed-time artificial insemination (A.I.; luprostiol, 72h after the second injection, SMB and CIDR-B 48h after implant/device removal). The overall 51-d pregnancy rates for the three treatments were 18.6% (luprostiol), 48.8% (CIDR-B), and 53.3% (SMB). There were significant differences in the pregnancy rate between CIDR-B and luprostiol (P = 0.004) and between SMB and luprostiol (P = < 0.0001), but there was no difference in pregnancy rate between CIDR-B and SMB treatment.