Follicle Dynamics and Fertility at Fixed-Time AI of Bos Indicus-Influenced Beef Cows Synchronized with the 5-Day Bee Synch + Cidr Protocol with or without GnRH on Day 0

The 5-Day Bee Synch + CIDR (Bee Synch) protocol for Bos indicus-influenced cows utilizes a CIDR, GnRH (GnRH-1) and prostaglandin F2α (PGF) on d 0 to eliminate mature corpora lutea, with fixed-time AI (FTAI) and GnRH (GnRH-2) at 66 h after CIDR removal to yield pregnancy rates of ~ 50%. Objectives herein were to test the hypothesis GnRH-1 is not required to optimize follicle synchrony (Exp. 1) or FTAI pregnancy rates (Exp. 2). In Exp. 1, 71 cycling Brangus and Brahman x Hereford (F1) suckled cows were used in 2 replicates (35-36/replicate). Cows were stratified by BW, BCS, and d postpartum, and assigned randomly to a 2 x 3 factorial arrangement involving 2 truncated (no FTAI or GnRH-2) versions of Bee Synch (BS-I and II) begun at 3, 7 and 10 d postovulation (d 3, 7 and 10). Ovulation was pre-synchronized with PGF (Lutalyse; Zoetis). Cows in BS-I received 100 µg GnRH (Factrel; Zoetis) i.m., 25 mg PGF i.m., and a CIDR (Eazi-Breed CIDR; Zoetis) on d 3, 7 or 10 post-ovulation. Cows in BS-II did not receive GnRH-1. On d 5, CIDRs were removed and all cows received 50 mg PGF i.m. Estrus was detected with Estrotect patches (Rockway, Inc.). Daily ultrasonography confirmed ovulation. For Exp. 2, a group of 120 mature cows from spring and fall herds were assigned to full versions (FTAI + GnRH-2 at 66 h) of either BS-I or II in a switchback design over 4 yr. In Exp. 1, synchronized new follicular wave emergence (NFWE; d 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 d 0-4 was considered. Size of the large follicle at 66 h (13.5 ± 0.47 mm) did not differ by treatment or d postovulation. Interval from CIDR removal to ovulation was greater (P = 0.02) for BS-I (5.2 ± 0.2 d) than II (4.4 ± 0.2 d) and greater (P < 0.0001) for d 3 (6.35 ± 0.3 d) than 7 and 10 (4.31 ± 0.3 and 3.7 ± 0.3 d, respectively). A greater (P = 0.06) proportion (15.5%) of cows in BS-II ovulated by 72 h than in BS-I. GnRH-1 reduced variation in d to NFWE and incidence of early ovulations. However, pregnancy rates to FTAI (Exp. 2) did not differ (BS-I, 50.2 ± 3.3 and BS-II, 44.7 ± 4.8 %; P = 0.22).