Comparison of three approaches for synchronization of ovulation for timed artificial insemination in Bos indicus-influenced cattle managed on the Texas gulf coast

Our objectives were to compare the relative efficacies of three protocols designed to synchronize ovulation for timed artificial insemination (AI) of predominantly Brahman-influenced cows and heifers. In Exp. 1, 273 Brahman x Hereford (F1) cows at three locations were stratified by BW, body condition score (BCS), age, and days postpartum and assigned randomly to three treatments: 1) Syncro-Mate-B (SMB), 2) norgestomet-prostaglandin (NP), and 3) Ovsynch. The management goal required that cows have a minimum BCS of 5 and be at least 36 d postpartum (PP) at treatment onset. However, final results included 23 cows (8.4%) whose BCS fell below 5. In Exp. 2, 286 pubertal beef heifers were stratified by BW and BCS and allocated randomly to the three treatments. Heifers were predominantly Brahman crossbred (n = 265; Brahman x Hereford, F1; Santa Cruz) or purebred Brahman-influenced (Santa Gertrudis) with a smaller number (n = 21) of Hereford heifers also included. For both experiments, SMB treatment consisted of a 9-d norgestomet ear implant plus an estradiol valerate/norgestomet injection on d 0. Norgestomet-prostaglandin-treated females were implanted with a SMB implant without the estradiol valerate/norgestomet injection at the time of implant insertion and received 25 mg prostaglandin F2alpha (PGF) i.m. 2 d before implant removal. Ovsynch consisted of 100 microg GnRH i.m. on d 1, 25 mg PGF i.m. on d 8, and a second GnRH injection on d 10. Beginning on d 9, calves were removed for 48 h in Exp. 1. Cattle in SMB and NP groups in both experiments were timed-inseminated 48 to 54 h after implant removal and at 12 to 24 h after the second GnRH injection (Ovsynch). Timed AI conception rates did not differ between the SMB (45.1%) and Ovsynch (42.4%) groups; however, conception rate in the NP group tended (P < 0.12) to be lower overall than in the other groups due to a reduced (P < 0.05) conception rate in cows that were < 60 d PP at treatment onset. Conversely, timed-AI conception was greatest (P < 0.056) in NP (54.7%) compared with SMB (40.4%) and Ovsynch (39.1%) for heifers in Exp. 2. We conclude that in mature, suckled beef cows with Brahman genetic influence, SMB and Ovsynch perform similarly when cow eligibility relies primarily on BCS and minimum days PP. The NP treatment results in lower conception in cows < 60 d PP compared with SMB and Ovsynch. However, in nulliparous Brahman-influenced heifers that are confirmed to be pubertal, NP may be superior to the other