The Value of a Resynch | UGA Dairy Team

Dr. Jillian Bohlen, Associate Professor and Dairy Extension Specialist | 706-542-9108 | jfain@uga.edu

Resynchronizing cattle is an effective way to shorten the breeding interval and reduce days open. However, resynch programs remain one of the least evaluated components of reproductive management. They are often understudied in the literature and lack the precision achieved with presynchronization or synchronization programs. Much of this is the variability in the timing of ovulation after initial breeding, alterations in follicular dynamics and early pregnancy losses, all of which make it more difficult to standardize cow profiles for resynchronization. As a result, pregnancy per AI (P/AI) is typically lower for resynchronization programs compared with initial fixed time AI (FTAI). Of greatest influence on P/AI during a resynch is the progesterone concentration at program initiation. Variations in follicular wave synchrony likely contribute to reduced fertility, but the most consistent and well-documented factor is low progesterone at the start of the resynchronization protocol.

The most widely used and well-supported resynch program is Ovsynch-56. Some operations implement a 5-day Cosynch as their resynchronization program. Figure 1 contains the steps for each of these protocols. With the mandatory double PGF in the 5-day Cosynch, there is no reduction in handling times but is a reduction in protocol duration. Additionally, 5-day Cosynch with duration between activities is generally easier to implement when compared with Ovsynch-56. While this document focuses on Ovsynch-56, relevant notes regarding the 5-day Cosynch are included where applicable.

Figure showing potential resynch programs Ovsynch-56 and 5-day Cosynch. — **Figure 1:** Timelines for potential resynch programs Ovsynch-56 and 5-day Cosynch

Determining When to Initiate a Resynch

A major decision when implementing a resynch program is selecting the day post AI to begin. This choice is often influenced by when pregnancy can be accurately diagnosed, the heat-detection goals of the farm, and overall management logistics. Physiologically, however, there is an optimal window.

With an average estrous cycle length of approximately 21-23 days, initiating a resynch on day 32 post AI places most cows roughly 9-11 days into a new cycle. Research on Ovsynch demonstrates that beginning the protocol during the early luteal phase yields the highest P/AI. This understanding drove the development of presynchronization programs, which were designed to increase the likelihood of a cow starting a synchronization program with a functional CL and high progesterone. This timing also corresponds to a period when a larger, dominant follicle is present (Figure 2).

line graph showing the estrous cycle of the cow with two follicular waves. — **Figure 2**: Representative estrous cycle of the cow with two follicular waves. Note the alignment of elevated progesterone (orange dashed line) with the peak size of the dominant follicle of wave 1.

Initiating the resynch at day 32 has the advantage of reducing the breeding interval; however, it also shortens the window for heat detection and increases the likelihood of administering GnRH to cows that are already pregnant—adding cost. Delaying resynch initiation to the time of non-pregnancy diagnosis—around day 39—extends opportunities for heat detection and can improve efficiency in herds where visual or automated heat detection is a priority.

Optimizing Luteal Regression: The Value of Double PGF

In any Ovsynch based resynch, administering a double PGF (a second dose 24 hours after the first) helps ensure complete luteal regression. This is especially valuable in cows that may not have achieved adequate progesterone levels at the start of the program. While optional in Ovsynch-56, double PGF is essential when using a 5-day Cosynch, as the shortened interval between GnRH injections results in a younger CL that is more resistant to a single PGF dose.

Additional Strategies to Improve Resynch Fertility

Beyond the basic protocol, several research-backed strategies may further increase P/AI in resynchronization programs:

Use of CIDR in Cows Without a CL at Resynch Initiation
- For cows lacking a CL at the first GnRH of the resynch, inserting a CIDR until the PGF injection can help elevate progesterone and support follicular synchrony. This adds cost and requires the use of double PGF but can meaningfully improve outcomes in certain herds.
Presynchronization with GnRH Prior to Resynch Initiation
- Giving GnRH seven days before starting the resynch increases the likelihood of cows having elevated progesterone at resynch initiation. This strategy can improve synchrony and P/AI. However, it also reduces the proportion of cows showing heat before the resynch, making it a less desirable option for operations prioritizing heat detection and breeding off observed estrus.

Final Considerations

Selecting a resynchronization method must align with the goals, labor capacity, and management style of each individual operation. Even the most effective protocols will fail if they cannot be executed consistently and correctly. While physiology provides clear guidance on “ideal” approaches, the best resynch program is the one a farm can implement with high compliance and from which the outcomes are acceptable.