The objective of this Univ. of Missouri experiment was to compare pregnancy rates resulting from fixed-time AI after administration of either one of two controlled internal release (CIDR)-based heat synchronization protocols (CIDR Select or CO-Synch + CIDR). Heifers assigned to CIDR Select received a CIDR insert (1.38 mg of progesterone) from day 0 to 14 followed by an intramuscular injection of GnRH (100 micrograms) 9 days after CIDR removal and PGF2 (PG, 25 mg i.m.) 7 days after GnRH (day 30). Heifers assigned to CO-Synch + CIDR were administered GnRH and received a CIDR insert on day 23 followed by PG and CIDR removal on day 30. Artificial insemination was performed at predetermined fixed times for heifers in both treatments at 72 or 54 hours after PG for the CIDR Select and CO-Synch + CIDR group, respectively. All heifers were injected with GnRH at the time of AI.

Estrous response during the synchronized period was significantly greater (87 vs. 69%, respectively), and the variance for interval to estrus after PG was significantly reduced among CIDR Select compared with CO-Synch + CIDR heifers. Furthermore, fixed-time AI pregnancy rates were significantly greater after the CIDR Select protocol compared with the CO-Synch + CIDR protocol (62 vs. 47%, respectively). The authors concluded that the CIDR Select protocol resulted in a greater and more synchronized estrous response and greater fixed-time AI pregnancy rates compared with CO-Synch + CIDR (Busch et al. 2007. J. Anim. Sci. 85:1933).