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and Implications The objective of this study was to determine how response 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) beef cows that had calved. On 359 cows, GnRH response at the onset 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. Introduction The administration of GnRH at CIDR insertion is done to induce ovulation and reset follicular waves, which allows for synchronized follicular growth patterns during timed-AI protocols. It has been reported that responses to GnRH at CIDR insertion is variable and in many cows, follicular dynamics may not be controlled. Depending upon protocol used, failure of follicular turnover can result in ovulation of a follicle at TAI that is not optimal and produce altered reproductive steroid concentrations. When using the 7-d CO-Synch + CIDR protocol, it has been demonstrated that failure to respond to GnRH at CIDR insertion results in detrimental effects to estradiol and progesterone concentrations. Within the 5dCO protocol, however, endocrine responses are not impacted by response to GnRH at CIDR insertion. Therefore, the objectives of this study were to determine how response to GnRH at the onset of the 5dCO protocol in beef cows impacted 1) estrous response, 2) dominant follicle size at timed-AI, and 3) pregnancy success to timed-AI.

Incidence of Ovulation to GnRH at Onset of 5-d CO-Synch + CIDR Protocol and Impact on Reproductive Responses