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Copyright © 2015
Angus Journal

Physiological Principles Underlying Synchronization of Estrus

by Troy Smith for Angus Productions Inc.

SAN ANTONIO, TEXAS (Jan. 28, 2010) — Estrus synchronization and artificial insemination (AI) rank high among technologies for genetic improvement of beef breeding herds. However, according to University of Missouri (MU) animal scientist Mike Smith, the development and application of synchronizing estrus and ovulation depends on an understanding of the physiological and hormonal mechanisms controlling the estrous cycle. Smith discussed the underlying physiological principles during the Applied Reproductive Strategies in Beef Cattle (ARSBC) workshop, which was in conjunction with the 2010 Cattle Industry Annual Convention and NCBA Trade Show in San Antonio, Texas.

Mike Smith

"It is essential to pay attention to details throughout an estrus synchronization and artificial insemination program," Mike Smith warned. "Success hinges on many factors and a fault in one area cannot be made up by success in another."

Smith reviewed the characteristics of the estrous cycle, which may vary from 17 to 24 days in length. He also noted the variability of the duration of estrus, or standing heat, which typically ranges from 10 to 18 hours. Some cows, however, exhibit estrus for less than 10 hours. Ovulation occurs approximately 28-32 hours after the onset of estrus, or 12-20 hours following the end of estrus.

Following an explanation of the estrous cycles’ three phases (follicular phase, estrus and luteal phase) and the cycle’s regulation by hormones, Smith discussed specific protocols for administering hormones (progestins, prostaglandins and gonadotropin-releasing hormone) for estrus synchronization. Smith also cited management considerations for selecting and preparing heifers or cows for synchronized AI. He advised producers to consider genetic and environmental (management) influences that may affect reproductive development and readiness for breeding.

“What has the pregnancy rate of your heifers been over the past few years? Have your heifers received growth promoting hormones? Have heifers achieved adequate target weight for breeding? And what proportion of your heifers have a reproductive tract score of 4 or higher?," Smith asked, noting how these factors can affect breeding success.

When contemplating synchronized AI of cows, Smith said producers should also consider historical pregnancy rate, the proportion of cows that are cycling by the start of breeding season, the body condition cows exhibited at calving and at the start of breeding season, and whether sufficient time has elapsed between calving and the time a synchronization protocol is begun. Producers should also consider time necessary for detection of estrus and whether their working facilities allow for low-stress handling of the number of animals they intend to breed.

“It is essential to pay attention to details throughout an estrus synchronization and artificial insemination program,” Smith warned. “Success hinges on many factors and a fault in one area cannot be made up by success in another.”

Click here for speaker's PowerPoint. (5.1 MB pdf)
Click here for audio to presentation. (5.2 MB mp3)

The ARSBC program was developed by the Beef Cattle Reproduction Task Force to improve understanding and application of reproductive technologies, including AI, estrus synchronization and factors affecting male fertility. For additional coverage — including summaries, proceedings and audio for each presentation — visit the newsroom at For API coverage of the Cattle Industry Convention, visit the newsroom at

Editor’s Note: This article is available as a news release to redistribute per an agreement between the symposium hosts and Angus Productions Inc. Click here to submit a request for a high-resolution photo of the speaker.