Cattle Physiology and the CO-Synch + CIDR Synchronization Program

Introduction
The most commonly used approaches to timed-artificial insemination (AI) in beef cattle in the USA are based upon the CO-Synch program. In the USA, three hormones are available to synchronize cows; progesterone (usually a vaginal insert; CIDR), prostaglandin F2α (PGF; or its analog) and gonadotropin-releasing hormone (GnRH). The original CO-Synch program consisted of an initial GnRH treatment, PGF seven days later to induce luteolysis, and a second GnRH treatment 48 h after PGF to induce ovulation for timed-AI. The timing of the second injection of GnRH determines the length of “proestrus”, or the interval between the initiation of regression of the corpus luteum (CL) and the luteinizing hormone (LH) surge. A CIDR is usually inserted into beef females between the initial GnRH and the PGF treatment, resulting in a CO-Synch + CIDR program. Each exogenous hormone used in this program has specific actions and the efficacy and accuracy of these actions are crucial for synchronization. The first GnRH treatment is used to induce ovulation and reset follicular growth. In other words, approximately one to two days after GnRH a new follicular wave should be initiated in a majority of cows. The efficacy of the initial GnRH, however, varies among animal class and stage of the estrous cycle. The second GnRH will induce an LH surge and subsequent ovulation of the dominant follicle that results from the new wave induced by the first GnRH. [...]