For decades, estradiol-based synchronization protocols have been one of the foundations of fixed-time artificial insemination (AI) in beef cattle. Producers adopted them because they worked reliably, required relatively few handlings and fit the realities of large-scale cattle production. But as pressure grows globally to reduce the use of steroid hormones in food animals, researchers have increasingly been searching for alternatives that can maintain the efficiency of fixed-time AI without relying on estradiol.
Now, researchers believe they might have found one in an unlikely place: a human fertility drug called Cetrorelix. Jaswant Singh, professor of veterinary biomedical sciences at the University of Saskatchewan, and Carlos Leonardi, professor at the Federal University of Santa Maria, joined the most recent episode of “The Bovine Vet Podcast” to discuss their work with Cetrorelix and its potential in reproductive protocols for cattle.
The discovery did not begin as an attempt to build a new synchronization protocol. In fact, according to Singh, the project initially focused on basic reproductive physiology.
“We were looking at some fundamental mechanisms happening in the hypothalamus and pituitary gland, and then we discovered there was this applied application that we could take to the market,” Singh explains.
Why Is Follicular Wave Synchronization Important for Fixed-Time AI?
The work centered on follicular wave dynamics in cattle reproduction. Synchronization protocols depend heavily on controlling follicular waves so groups of cows ovulate within a predictable window.
“Starting a new follicular wave on our demand is the most important aspect in synchronization protocols in cattle,” Singh says. “If you don’t have synchronization of follicular waves in the beginning, we don’t have synchronization of ovulation at the end.”
For years, estradiol-based systems have effectively filled that role. The hormone suppresses dominant follicles and allows a new synchronized follicular wave to emerge. The approach became widely adopted because it fit practical production needs, especially in large beef systems.
There’s also a labor component to why these systems became so successful. Fixed-time AI allows producers to inseminate large groups of cattle without relying on heat detection.
“For example, if a farmer says, ‘Doctor, I want to inseminate my 200 animals next Tuesday at five o’clock in the evening,’ we can make that possible,” Singh says.
That kind of predictability is important for large operations, particularly in South America, where fixed-time AI has become deeply integrated into beef production systems.
Estradiol Use in Cattle Reproduction Facing Increased Scrutiny
The same estradiol protocols producers came to depend on are now facing increasing scrutiny internationally. The European Union prohibited the use of estrogenic compounds in food-producing animals and has long restricted estradiol use in reproductive programs. Although estradiol-based synchronization protocols remain common in major beef-producing countries, such as Brazil and Argentina, researchers and industry stakeholders recognize export market pressures could eventually force a shift toward non-steroidal alternatives.
The challenge is estradiol protocols became dominant for practical reasons. They are efficient, relatively short and require fewer handlings than many alternative systems. Any replacement technology would need to preserve those advantages while avoiding the regulatory concerns tied to steroid hormone use.
How Researchers Unexpectedly Discovered Cetrorelix’s Synchronization Potential
The turning point in the project came when the researchers began studying kisspeptin and GnRH signaling pathways. They needed a negative control for the work.
“So Carlos said, ‘Why don’t we give something to block the luteinizing hormone (LH_ release from the pituitary?’ So we used this existing human drug product, which is a gonadotropin-releasing hormone (GnRH) antagonist,” Singh remembers.
That drug was Cetrorelix.
What happened next surprised the team.
“We discovered that, in our animals, there was synchronization of follicular waves,” Singh recalls. “At that time we said, ‘What’s happening here? It’s wonderful.’”
The observation ultimately redirected the research program.
“It’s important for us to understand the fundamentals and mechanics of how hormones work,” Singh says. “Many times, applied research comes from your fundamental research, and that was exactly what happened in this particular discovery.”
How Does Cetrorelix Synchronize Follicular Waves in Cattle?
Cetrorelix works differently than estradiol, but the end result appears remarkably similar.
The drug acts as a GnRH antagonist, blocking GnRH receptors at the pituitary gland and suppressing LH release. Without LH support, dominant follicles regress. That suppression is then followed by increases in follicle stimulating hormone (FSH) and the emergence of a new synchronized follicular wave.
“The increase in FSH two days after giving Cetrorelix was actually an indication that the new wave was going to start,” Singh explains.
The researchers later tested the approach across multiple stages of follicular wave development and found synchronized wave emergence occurred consistently after treatment. The work eventually expanded into practical fixed-time AI trials in beef heifers, where researchers compared Cetrorelix-based synchronization systems against conventional estradiol protocols.
“The idea was not to prove that one protocol is always better than the other. The idea was to show that both protocols work in the same way,” Leonardi explains.
According to Singh, the results were encouraging.
“The two drugs work through exactly the same pathway, same mechanism of action,” he says. “Estradiol works indirectly on the pituitary, while Cetrorelix works directly on the pituitary.”
What Advantages Could Cetrorelix Offer Producers?
The practical side of the protocol might ultimately matter as much as the physiology.
Large beef operations often prioritize systems that reduce labor requirements and minimize cattle handling. In extensive production systems, adding extra days or extra trips through the chute can quickly become impractical.
Cetrorelix protocols could mean handling cattle only three times over a period of 10 days.
“That’s something that can fit into our production systems,” Singh says.
Singh and Leonardi emphasize any future synchronization alternative must function operationally.
“The protocol needs to work not just biologically, but practically,” Singh says.
What’s Next for Cetrorelix Research in Cattle?
This work remains in the relatively early stages, and is not yet available commercially.
Significant regulatory work remains before the technology could become commercially available for cattle use. Even though Cetrorelix has already been approved and used in human medicine, livestock approval pathways require additional residue, safety and regulatory evaluations.
Future work is already expanding into additional applications.
“We are looking at how to optimize this protocol for dairy cattle and also how we might reduce the use of progesterone devices,” Singh says.
Leonardi and Singh also note potential applications in embryo transfer systems and other livestock species.
For now, the work represents an intriguing possibility at a time when the cattle reproduction industry is beginning to think more seriously about what synchronization protocols may look like in a post-estradiol landscape. Fittingly, the project itself emerged from exactly the kind of curiosity-driven physiology work that often produces the most unexpected advances.
To hear more from Singh and Leonardi on the potential of Cetrorelix and the changing landscape of cattle reproductive management, listen to the full conversation on the latest episode of “The Bovine Vet Podcast.”
