Animal Industry Report

Extension Number

ASL R3056



Summary and Implications

The objective of this study was to determine howresponse to GnRH at the onset of the 5-d CO-Synch +CIDR protocol (5dCO) affected estrous response, follicular dynamics, and pregnancy success to timed-AI (TAI) beefcows that had calved. On 359 cows, GnRH response at theonset of the synchronization protocol was 54.6%. Estrous response prior to TAI was greater in cows that failed to ovulate after GnRH administration than those that ovulated. However, dominant follicle diameter at TAI was not influenced by GnRH response. Timed-AI pregnancy rates were greater in cows that did not respond to GnRH (65.0%) than those that did (51.5%). These results cannot be interpreted as removing GnRH at the beginning of the 5dCO would affect TAI pregnancy responses. These results, however, warrant subsequent field studies to determine the necessity of GnRH administration at CIDR insert in the 5-d CO-Synch + CIDR protocol.

Copyright Holder

Iowa State University