Estrus synchronization is a game-changer for beef producers aiming to tighten calving windows, improve artificial insemination (AI) success, and streamline herd management. But when protocols don’t go as planned, it can lead to poor conception rates, extra labor, money, and frustration. Understanding the hormones involved, the timing required, and how to troubleshoot common mishaps is key to making synchronization work for your herd. 

The Hormonal Toolbox 

The estrous cycle in cows is characterized by hormonal changes that regulate ovarian activity, including the development of follicles and the corpus luteum (CL). This cycle typically recurs every 21 days, with ovulation occurring approximately 24 to 32 hours after the onset of standing estrus. 

Estrus synchronization protocols rely on a carefully timed sequence of hormones: 

• Prostaglandin F2α (PGF2α): Regresses the CL, allowing cows to return to estrus. Administration is effective between days 6 (6 days after estrus) and 16 of the estrous cycle. Prostaglandin does not induce cyclicity in anestrous (non-cycling postpartum) females.  
• Gonadotropin-Releasing Hormone (GnRH): Triggers ovulation or synchronizes follicular waves. Can induce estrous cycles in anestrous females that are close to cycling. If cattle have large, healthy follicles, ovulation will be induced approximately 24 to 30 hours after GnRH injection without estrus behavior.  
• Progesterone: Mimics the luteal phase, suppressing estrus until withdrawal. Can induce estrous cycles in anestrus females that are close to cycling and “jump start” them. Generally utilized in a 5-, 6-, 7- or 14-day protocol.  
• Controlled Internal Drug Release (CIDR): A T-shaped device that releases a constant dose of progesterone once inserted intravaginally. 
• Melengestrol acetate (MGA): An orally active progestin labelled for use in heifers to suppress estrus. For this protocol, the target feeding rates are 0.5 mg/day per heifer for 14 days, followed by a PGF2α injection 19 days later. 

Protocols that use these hormones can all be found at beefrepro.org and can be used with natural service breeding and AI. Estrus synchronization protocols available include options for heat detection, heat detection and timed AI, fixed time AI, and protocols for use with sexed semen. 

Make sure to follow proper hormone handling by storing them at the right temperature and following Beef Quality Assurance (BQA) guidelines.  

How important is timing? 

Timing of protocols with a timed AI component is very important and requires calculation of the specific time that CIDRs need to be removed, injections need to be administered, and AI completed. Heat detection only and natural service protocols require hormones to be administered and CIDR’s be inserted/removed on specific days but there are no specific time constraints on the specific day. 

When a hormone is given out of order in an estrus synchronization protocol, it can disrupt the entire reproductive sequence, leading to poor estrus response, missed ovulation, or failed conception. Here’s what typically happens depending on the hormone involved: 

 Giving PGF2α Too Early or Late 

• Too early: The CL may not be mature enough to respond, so luteolysis (regression of the CL) doesn’t occur, and the cow/heifer will not come into standing estrus  
• Too late: The cow may already be in estrus or ovulating, making the shot ineffective or even disruptive. 

Mistiming GnRH -- Giving GnRH at CIDR Out 

• Too early: The dominant follicle may not be ready to ovulate, so GnRH won’t induce ovulation. 
• Too late: Ovulation may have already occurred, and insemination will miss the fertile window. 

Incorrect CIDR (Progesterone) Timing 

• Inserted too late or removed too early: The cow may not have enough progesterone exposure to suppress estrus, leading to premature follicular development or ovulation. 
• Left in too long: Can suppress estrus excessively, delaying ovulation and reducing fertility. 
• Missing CIDRs: Occasionally, heifers or cows may remove CIDRs before the scheduled removal time. Visually assess the animal and be prepared to adjust your synchronization protocol as needed and/or may have to breed earlier depending on timing. You could cut CIDR strings to 2 inches to help reduce the chance of removal.  

Timing of insemination after an animal is detected in standing estrus 

• Too early: Risk inadequate sperm life and decreasing the fertilization rate. 
• Too late: The cow/heifer may have already ovulated leading to an aging oocyte and reduced embryo quality. 

What Can Go Wrong? 

1. Poor Estrus Response 

• Cause: Cows are not cycling (anestrous), often due to low body condition or postpartum status. 
• Fix: Ensure cows are in good nutritional condition and are an adequate number of days postpartum. A CIDR can be utilized to induce cycling in anestrus females that are close to cycling. 

2. Mistimed Hormone Administration 

• Cause: Giving PGF2α too early (immature CL) or GnRH too late (missed ovulation). 
• Fix: Stick to the exact timing of the protocol. Use timers, calendars, or apps to stay on track. 

3. Missed Heat Detection 

• Cause: Subtle estrus signs or poor observation. 
• Fix: Use heat detection aids (tail paint, Estrotect) or switch to fixed-time AI (FTAI) to eliminate guesswork. 

4. Low Conception Despite Good Estrus 

• Cause: Poor semen quality or handling, AI technique, or uterine health issues. 
• Fix: Evaluate semen handling, review AI technique, and check for reproductive tract infections. 

Best Practices for Success 

Producers can utilize the Estrus Synchronization Planner to create and print customized calendars for hormone treatments, CIDR insertion and removal, and AI timing. Success will depend on knowing your cows’ cycle status, following protocols precisely, and ensuring your team is trained for consistent hormone handling and semen handling if utilizing AI.  

Producers who successfully use synchronization and AI plan weeks or even months ahead of their target window. Early planning helps avoid last-minute surprises, ensures product availability, and allows time to address potential challenges—because we’ve all had that call: 'Can I AI next Tuesday?' 

Final Thoughts 

Estrus synchronization is a powerful tool—but only when used correctly. By understanding the hormones, respecting the timing, and troubleshooting issues early, producers can turn a frustrating experience into a reproductive win. 

Article by Kacie McCarthy, Nebraska Extension Beef Cow-Calf Specialist, Shelby Rosasco, University of Wyoming Beef Specialist, Becky Funk, DVM, Animal Health Teaching and Nebraska Extension Specialist.