Applications of Physiological Principles to Optimize Timed AI Pregnancy Rates with the 5-d CO-Synch + CIDR Protocol in Cattle

Abstract
The 5-d CO-Synch + CIDR protocol was developed to compliment fundamental research that identified physiological and endocrine factors that impact the probability of pregnancy in cattle. Initial studies in beef cows demonstrated that reducing the interval from controlled internal drug release (CIDR) insertion and gonadotropin-releasing hormone (GnRH) to CIDR removal from seven to five days and extending the period from CIDR removal to timed artificial insemination (AI) from 60 to 72 h increased timed-AI pregnancy rates in beef cows. Although initial studies with the 5-d approach utilized two doses of prostaglandin F2α (PGF)given at a 12 h interval at CIDR removal, subsequent investigations in beef cows indicate that although two doses of PGF2αare necessary, these doses can be delivered simultaneously at CIDR removal without affecting timed-AI pregnancy rates. The 5-d approach to estrous synchronization has also been identified as an effective method for facilitating AI in heifers. Although investigations are still ongoing to develop the ‘optimal’ approach when using the 5-d protocol in heifers, in its current form the 5-d CO-Synch + CIDR and the 5-d Select Synch + CIDR and timed-AI protocols have been demonstrated to yield increased AI pregnancy rates in beef heifers compared to the 7-d approaches and similar AI pregnancy rates to the longer 14-d CIDR protocols. In summary, the 5-d approach to estrous synchronization in cattle is an effective protocol that results in greater AI pregnancy rates than most protocols currently available. [...]