Based upon observations across a series of experiments that pregnancy rate to timed-AI was positively related to length of proestrus, the traditional 7-day CO-Synch + CIDR program was modified to allow an increased interval from PGF/CIDR removal to GnRH/timed-AI; resulting in the 5-day CO-Synch + CIDR program. This modification has been demonstrated to increase timed-AI pregnancy rates relative to the traditional approach. The impact of this modification on preovulatory estradiol concentrations, as a result of extending the period of gonadotropic stimulus provided to the follicle, initiation of proestrus at a time when ovulatory follicles are highly estrogenic and/or through reduction in the incidence of ovulation of very young follicles, are potential mechanisms for increased estradiol concentrations and enhanced fertility. Conversely, for females in these estrous control programs in which follicular growth is adequately controlled, differences in age of the ovulatory follicle may not be a significant contributor to variation in timed-AI pregnancy rate