Synchronization of ovulation and conception rates in Holstein heifers given an intravaginal progesterone-releasing device (CIDR), and estradiol cypionate, porcine LH or gonadotropin releasing hormone

ne objective of Experiment I Was to compare a low dose of estradiol cypiona te (ECP) or porcine LH (pLH) to a standard dose of GnRH, for initiating fol licular growth, LH surge and synchronization of ovulation. On Day 0 (withou t regard to the stage of oestrous cycle) postpubertal Holstein heifers rece ived an intravaginal progesterone-releasing device (CIDR) and an IM injecti on of 0.5 mg ECP (n=6), 100 mug GnRH (n=6), 5 mg pLH (n=6) or saline (n=7). All heifers received 25 mg prostaglandin F-2 infinity. (PGF) IM on Day 7. The CIDR was removed on Day 3. Heifers that received ECP on Day 0 received a second injection on Day 8, concurrent with CIDR removal. Heifers that rec eived pLH, GnRH, or saline on Day 0 received a second injection 48 h after PGF (Day 9). Days to establishment of a new dominant follicle did not diffe r by treatment. Mean interval (h) from CIDR removal to ovulation was greate r in pLH (102, P=.04) than in ECP (66), GnRH (56) or saline (70) treated he ifers. Mean interval (h) to LH peak after second injection was shorter (P < .01) for GnRH (1.6) than ECP (38.1), pLH (8.2) or saline (20.0). The objec tive of Experiment II was to compare conception rates in two of the timed i nsemination protocols of Experiment 1, that is, using GnRH or ECP in the pr esence of CIDR. Heifers were assigned to either GnRH (n=57) or ECP (n=53). Heifers were timed-inseminated approximately 16 or 44 h after the second in jection of GnRH or ECP, respectively. Conception rates were 61 % for GnRH-t reated heifers, and 66 % for ECP-treated heifers. A low dose of ECP was as effective as a standard dose of GnRH in synchronizing ovulation in dairy he ifers.