Ovarian follicular and luteal characteristics in Bos indicus-influenced beef cows using prostaglandin F2 with or without GnRH at the onset of the 5-day CO-Synch + controlled internal drug release (CIDR) protocol.
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A modification of the standard 5-day CO-Synch+CIDR procedure (5-day Bee Synch+CIDR; Bee Synch), developed for use in Bos indicus-influenced cows, utilizes the addition of prostaglandin F2 (PGF) on Day 0 of the protocol to eliminate mature corpora lutea (CL) and fixed-time AI (FTAI) at 66h. Objectives were to test the hypothesis that elimination of GnRH on Day 0 (GnRH-1) does not impact significantly the synchronized development of a dominant follicle for presumptive FTAI. Seventy-one estrous cycling Brangus and Brahman x Hereford suckled cows were used in two replicates (35-36/replicate). Following stratification, cows were assigned randomly to a 23 factorial arrangement of treatments involving two truncated (no FTAI or GnRH-2) versions of Bee Synch (Bee Synch It and IIt), each begun 3, 7, and 10 days post-ovulation. Cows in Bee Synch It received 100g GnRH (GnRH-1), 25mg PGF, and a CIDR on Day 0, whereas cows assigned to Bee Synch IIt received the same treatment but without GnRH-1. All cows received 50mg PGF on Day 5 at CIDR removal. Synchronized new follicular wave emergence (NFWE; days 1-4) was observed in 68.6 and 38.9% of Bee Synch It and IIt, respectively (P= 0.01). This increased to 93.3% and 72.2%, respectively, if days 0-4 were considered. Inclusion of GnRH at CIDR insertion improved synchronized NFWE but size of the largest follicle at 66 h, the normal time of FTAI, did not differ due to treatment or day of the estrous cycle.