Comparison of the Timing of the Second Dose of Prostaglandin F2 Alpha in a Five-Day Progesterone-based Timed AI Synchronization Protocol in Angus Cross Beef Cows

Abstract

Artificial insemination (AI) is a highly effective reproductive technology for promoting genetic improvement in beef cattle. The most commonly used timed-AI program in beef cattle is the CO-Synch program. Pregnancy rates achieved through this program have ranged from 31 to 66% across a series of published reports, a rate that is acceptable to many beef producers. It has been shown that reducing the interval from the initial GnRH treatment to PGF2a (and withdrawal of a progestin) improves the outcome of timed-AI. With this protocol, termed the 5-day Co-Synch+ CIDR protocol, cows are given GnRH and have a CIDR device placed intravaginally, have the device removed after five days, and are given PGF2a, and then are time inseminated at 72 hours. However, this protocol requires the injection of two doses of PGF2a with an inconvenient 12-hour wait between the two doses. Reducing the interval between the two doses of PGF2a will allow cows to be handled twice with only a single penning and sorting to remove calves. The objective of this study was to compare reproductive performance of Angus cross beef cows synchronized with 5-day Co-Synch+ CIDR protocol and two doses of dinoprost tromethamine (PGF) given at CIDR removal and either at 2 h or 8 h following CIDR withdrawal.