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Copyright © 2015
Angus Journal

Physiological Principles of
Estrus Synchronization

FORT COLLINS (Dec. 2, 2008) — Development of technologies to increase reproductive efficiency and improve genetic merit has occurred at a rapid pace to include embryo transfer, ultrasonography, transgenics and cloning. Of all available reproductive technologies, University of Missouri animal scientist Michael Smith ranks estrus synchronization and artificial insemination (AI) among the most powerful and applicable.

Michael Smith of the University of Missouri emphasized the role of follicular waves and various reproductive hormones on estrus synchronization programs.

Successful application, however, depends on the understanding of physiological and hormonal mechanisms controlling the estrous cycle, Smith told attendees of the Robert E. Taylor Memorial Symposium in Fort Collins, Colo.

“While synchronization products and protocols have changed over time, the basic principles explaining why they work have not changed,” Smith said. “Understanding the biology helps us choose the best protocol for heifers or for cows. It can help us determine what went wrong if results are less than expected, and how to correct it.”

Three general approaches to estrus synchronization involve inhibiting ovulation with long-term progestin treatment, induction of corpus luteum regression with prostaglandin (PGF2a) treatment, or a combination of both, Smith said. Most protocols utilized today involve the “combination” approach. And with the ability to induce ovulation and synchronize follicular waves with hormonal (GnRH) injection, a new and important dimension was added by making fixed-time AI a viable option.

Smith cautioned producers to consider certain factors before implementing a synchronization protocol. With heifers, he recommends consideration of the previous heifer pregnancy rate. If that rate ranged from 85% to 90%, the operation is likely a good candidate for implementation of synchronized AI.

Producers should also consider whether heifers received growth-promoting implants. Implants administered at birth, or before 30 days of age, may be detrimental to reproductive development, he said. Producers should also select an appropriate breeding weight target (65% of mature weight) and have heifers in appropriate body condition (preferably body condition score 5) by time of breeding.

When implementing synchronization programs for cows, Smith advised producers to be aware of their previous pregnancy rate, decide on an appropriate breeding season length, and have a good estimate of the percentage of cows already cycling at the start of the season. Again, adequate body condition is a factor. Allow a reasonable length of time postpartum before starting synchronization treatments, with 40 to 45 days being recommended.

Smith also advised producers to consider how much time they can devote to heat detection before choosing a synchronization protocol. If time and labor for heat detection are limited, a protocol for fixed-time AI may be the best choice.

The Robert E. Taylor Memorial Symposium is conducted by Colorado State University every other year to provide current, research-based information for improving profitability in the beef cattle industry. 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. In 2008, CSU and the Task Force collaborated to provide the Dec. 2-3 symposium in Fort Collins. To listen to Smith’s presentation, view the accompanying PowerPoint or view other presentations from the symposium, visit the newsroom at

— by Troy Smith

Click here for accompanying PowerPoint as a pdf file (652 KB).
Click here for audio of the presentation (6 MB mp3).

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. For additional information visit the newsroom of