Response to prostaglandin F2alpha (PGF2alpha) and gonadotropin-releasing hormone (GnRH) in Bos taurus, and Bos taurus x Bos indicus cattle

A series of experiments were conducted to evaluate the response of Bos taurus, Bos indicus, and Bos taurus x Bos indicus cattle to PGF2alpha and GnRH treatments. Experiment 1 was replicated twice and cycling Angus (AN) and Brahman x Angus (BA) heifers were used to evaluate the effectiveness of a single injection (25 mg i.m.) of PGF2alpha administered during the early estrous cycle (d 6 or 7) to initiate corpus luteum (CL) regression as measured by progesterone concentrations. Breed had no effect on PGF2alpha induced luteolysis on d 6 or 7 of the estrous cycle. In the second experiment, Angus (AN), Brahman (B), Brangus (BR), and Brahman x Angus (BA) heifers were used to evaluate secretory patterns of LH and associated ovarian events in response to administration of GnRH (100 mug i.m.) on d 6 of the estrous cycle followed by a single injection of PGF2alpha 7 d later. There was a breed-dependent response to a GnRH challenge administered on d 6 of the estrous cycle. As the percentage of Bos indices breeding increased, the amount of luteinizing hormone released to GnRH decreased. In Experiment 3, AN, B, and BA heifers were used to evaluate the effectiveness of a single injection of PGF2alpha administered 7 d after a GnRH treatment on d 6 of the estrous cycle, to initiate luteolysis of an accessory CL, which had been exposed to either low (heifers treated with 15 mg PGF 2alpha i.m. at 12-h intervals on d 4 and 5 of the estrous cycle, followed by a single injection of PGF2alpha on d 6) or high (no PGF 2alpha treatment on d 4, 5 and 6) progesterone concentrations before PGF2alpha. Progesterone exposure before PGF2alpha had no effect on luteolysis. In Experiment 4, cycling and noncycling crossbred Bos indices lactating cows were used to evaluate the effectiveness of a single versus split (12.5 mg i.m.) dose of PGF2alpha in a 7 d GnRH/MGA/PGF2alpha protocol. Modifying the delivery of PGF2alpha from a single to two consecutive split injections 7 d after GnRH had no effect on subsequent estrous response and pregnancy rate.