Comparison of Follicular and Luteal Dynamics in Protocols Developed for Synchronization of Ovulation in Bos indicus-Influenced Beef Cows: The Role of Gonadotropin-Releasing Hormone at Treatment Onset

Abstract

The 5-day Bee Synch + CIDR (Bee Synch) protocol for Bos indicus-influenced cows utilizes CIDR, GnRH (GnRH-1) and PGF on day 0 to eliminate mature corpora lutea, with FTAI and GnRH (GnRH-2) at 66 hours after CIDR removal to yield pregnancy rates of ~ 50%. The objective was to test the hypothesis that GnRH-1 is not required to optimize follicle synchrony. Seventy-one cycling Brangus and Brahman × Hereford suckled cows were used in 2 replicates (35-36 per replicate). Cows were stratified by BW, BCS, and days postpartum, and assigned randomly in a 2 × 3 factorial arrangement involving 2 truncated (no FTAI or GnRH-2) versions of Bee Synch (BS-I and II) begun 3, 7 and 10 days post ovulation. Ovulation was pre-synchronized with PGF. Cows in BS-I received 100 µg GnRH i.m., 25 mg PGF i.m., and a CIDR on treatment onset. Cows in BS-II did not receive GnRH-1. On day 5, CIDRs were removed and all cows received 50 mg PGF i.m. Estrus was detected with Estrotect patches. Daily ultrasonography confirmed ovulation. Synchronized new follicular wave emergence (NFWE; days 1-4) was observed in 68.6 and 38.9% (BS-I vs. BS-II; P = 0.01) of cows and increased to 93.3 and 72.2%, respectively, if days 0-4 were considered. Size of the largest follicle at 66 hours (13.5 ± 0.47 mm) did not differ by treatment or day post ovulation. Interval from CIDR removal to ovulation was greater (P = 0.02) for BS-I (5.2 ± 0.2 days) than BS-II (4.4 ± 0.2 days) and greater (P < 0.0001) for day 3 (6.35 ± 0.3 days) than days 7 and 10 (4.31 ± 0.3 and 3.7 ± 0.3, respectively). Progesterone concentrations did not differ between treatments (P = 0.12). The LH peak at 120 minutes after GnRH-1 was higher (4.28 ± 0.71 ng/mL, P < 0.05) for day 3 than days 7 and 10. A greater (P = 0.06) proportion (15.5%) of cows in BS-II ovulated by 72 hours than in BSI. GnRH-1 reduced variation in day to NFWE and incidence of early ovulations.