Timed AI in Dairy Cows: The Ovsynch Core

Heat detection is a common but challenging practice that can limit reproductive management success in a dairy herd if not managed carefully. Finding effective strategies for addressing the challenges associated with this practice is critical for herd profitability. Fortunately, there are some treatment protocols (i.e., "tools in the toolbox") that can help consistently establish lasting pregnancies while lessening the need for heat detection. These protocols, of which there are many variations, involve a series of treatments that set heifers or cows up for insemination at a prescribed time, an approach commonly referred to as timed artificial insemination (TAI).

There are some drawbacks. Frequently needing to catch certain cows at a certain time to inject hormone products isn’t particularly enjoyable for anyone; the treatment schedule is not very flexible and needs to be completed as prescribed; and there is still some variability in the success rates, depending on the protocol used and a variety of environmental factors.

Accounting for the variables, the successful incorporation of TAI into a dairy herd’s reproductive management approach, whether as a small or large part of the program, can potentially yield impressive fertility results, lead to a surplus of calves being born, and boost milk production. Let’s look at a well-tested, foundational TAI protocol – Ovsynch.

Ovsynch

Ovsynch is the name that was given to a foundational TAI protocol first developed in the 1990s (Pursley et al., 1995). As its name suggests, it is designed to synchronize ovulation of follicles on the ovaries of cows. A few suggested refinements have been made over the years, but not a lot has changed with the core protocol. What has changed is the variety of supplemental treatments and protocols that can be added to Ovsynch, some of which improve success rates significantly. In the end, Ovsynch allows for AI at a fixed time for 100% of the eligible animals without any observation of estrous activity necessary.

The Ovsynch protocol calls for two different types of hormone treatments over the span of ten days, with a total of four times when the cow needs to be restrained for injection or AI. One treatment which is administered two different times is GnRH (designated as G1 and G2 on the calendar below), available with a variety of product names, including Cystorelin^®, Factrel^®, Fertagyl^®, and GONAbreed^®. The other injectable treatment is prostaglandin F_2α, (designated as PG below), available under product names such as estroPLAN^®, Estrumate^®, Lutalyse^®, and Synchsure^®. Ideally, G2 should be administered about 56 hours after PG and be followed 16 hours later by AI, disregarding the presence or absence of estrous activity.

G1

Ovsynch begins with an injection of GnRH, which is generally intended to cause ovulation of any large follicles (often just one) present on a cow's two ovaries. Successful ovulation will lead to the initiation of a new wave of small follicles starting to grow on the ovaries. If a functional corpus luteum (CL) was already present on one of the ovaries and producing progesterone, ovulation will cause an additional CL to form and likely increase progesterone concentrations in the circulation.

With ovarian dynamics always changing, there is some uncertainty regarding what the response will be to GnRH treatment. There are two possible outcomes to the treatment – ovulation of a sizeable follicle, or no ovulation because of no responsive follicle. Cows being treated fall into one of three categories - cycling normally with a CL at the time of treatment, cycling normally but without a fully functional CL at the time of treatment, or not cycling normally (see Table 1).

From the very beginning, progesterone makes a difference. If Ovsynch is started when circulating progesterone is low, there is a higher risk for double ovulations and twin pregnancies (Fricke, 2018). Also, for cows that start with low progesterone at G1, approximately 30% of them will not undergo complete CL regression after the PG treatment a week later. Ideally, cows will start Ovsynch with significant levels of progesterone already in their circulation. If this is accomplished, nearly 90% of them can be expected to undergo complete CL regression with higher odds for a normal, healthy pregnancy (Carvalho et al., 2018).

Almost as important as the progesterone environment at the beginning of Ovsynch is the responsiveness of follicles to G1. Most follicles on the ovaries never ovulate. Most of them never even grow large or acquire the ability to ovulate. Even many of the large follicles never ovulate because a high progesterone environment inhibits natural ovulation and forces them to eventually regress, triggering the onset of a new wave of follicle growth. Treatment with GnRH can usually (not always) override the suppression of ovulation by progesterone, but ovulation still only occurs if a follicle has reached an adequate stage of development.

Ovulatory response to G1 is less than 50% when G1 is administered at a random stage of the estrous cycle (Carvalho et al., 2018). Cows with a dominant follicle of the first follicular wave (i.e., days 5 to 8 of the cycle) are most responsive to G1 compared to other time points in the cycle (Carvalho et al., 2018; Vasconcelos et al., 1999), making those few days of the cycle a good target with presynchronization strategies.

When a large follicle successfully ovulates after G1, the odds of conception in response to Ovsynch are higher (Carvalho et al., 2018), even though the oocyte (egg) from that follicle is not the target for fertilization. Even if progesterone is low and a functional CL is not present at the start of Ovsynch, successful ovulation in response to G1 increases the odds for conception (Carvalho et al., 2018).

PG

The response to PG treatment largely depends on the CL situation. At the time of treatment, high progesterone in the circulation is ideal (Bello et al., 2006; Carvalho et al., 2018), which is especially achieved when there are at least two CL of different ages – one that was present when Ovsynch began and one that was formed in response to the G1 treatment. However, there are some challenges posed when young CLs are present.

There are four possible scenarios that will impact the responsiveness and outcome of PG treatment, as shown in Table 2. Mature CLs generally regress without any problem, but young CLs, induced by G1 of Ovsynch, are sometimes resistant.

Young CL often do not fully regress in response to PG treatment during their first few days of development. Incomplete CL regression to a single PG treatment is associated with a decrease in conception rates, which is particularly apparent in cows in which an Ovsynch protocol is initiated in a low progesterone environment, resulting in only a young CL at the time of PG treatment (Carvalho et al., 2018).

Doubling up PG treatment by administering a second treatment 24 hours after the first treatment helps accomplish complete luteal regression nearly 100% of the time, also lowering progesterone concentrations leading up to the G2 treatment. Double PG treatment leads to an increase in conception rates by several percentage points (Borchardt et al., 2018; Brusveen et al., 2009; Heidari et al., 2017; Rheinberger et al., 2020; Tippenhauer et al., 2021), especially in older cows (Wiltbank et al., 2015). The positive effects are greatest for cows that had low progesterone at G1 (no CL) but ovulated in response to G1 (young CL at PG), since a single PG treatment is sometimes inadequate for causing regression of that one young CL (Carvalho et al., 2018).

There are additional benefits of double PG treatment that may show up later. In one research study, the proportion of cows that experienced pregnancy loss between 32 and 60 days after AI was significantly reduced in cows given a second PG treatment before TAI (Heidari et al., 2017).

G2

The timing of G2 is largely aimed to mimic the natural occurrence of GnRH and LH release in the cow, which leads to ovulation. The precise timing of treatment, however, is largely what controls physiological activity enough to allow for TAI without estrous detection. Though there are variations, the accumulated research results point to a 56-hour interval after PG treatment and 16 hours before TAI to be ideal (Pursley et al., 1998).

If a mature, responsive follicle is on one of the ovaries, ovulation will occur, on average, 28 hours after GnRH treatment (Pursley et al., 1995). Inseminating at the time of GnRH treatment is too early for optimal odds of conception. Waiting 24 or more hours is too late, going past the ideal peak for conception odds because of the time it takes sperm cells to migrate through the female reproductive tract to the site of fertilization and acquire the ability to effectively fertilize an oocyte.

The hope is that progesterone will be low at the time of G2, indicating that the PG treatment was effective in causing complete regression of CL. Lower progesterone concentrations at G2 are associated with lower conception rates (Carvalho et al., 2018).

Cows that start on an Ovsynch protocol with low progesterone (anovular or cyclic with no CL currently) have a high ovulatory response to G1, which results in a single 6-day-old CL at the time of PG treatment. Approximately one-third of those cows fail to fully regress this young CL in response to a single PG, resulting in slightly elevated progesterone levels at G2, which dramatically decreases conception rates (Fricke et al., 2015).

Two good goals for Ovsynch are to start cows that are cycling with a functional CL and double up on PG treatment to ensure CL regression, especially with those cows which don't meet the first goal. A more developed protocol with presynchronization treatments leading up to Ovsynch (e.g., Double Ovsynch) will help accomplish the first goal.

Resynchronization

Not every cow, or even heifer, will conceive and maintain pregnancy after the first insemination. Even the best protocols only work on about half of cows at first service after calving. Thus, there also needs to be a good plan for reinsemination, and TAI is still a good option to consider.

Ovsynch works well for resynchronization, and there are a few different strategies that can be employed to minimize the amount of time that elapses between services. One aggressive approach with Ovsynch is to administer G1 before pregnancy diagnosis, since GnRH will not harm a pregnancy, and complete the protocol only with those cows diagnosed as not pregnant before PG treatment a week later. Strategic treatments with GnRH or CIDRs (intravaginal progesterone inserts) can help especially with cows that don't have a CL at the time of pregnancy diagnosis and might not be cycling normally (Wijma et al., 2018).

Conclusion

Timed AI can be a valuable tool in any dairy herd open to using the treatments prescribed in the protocols. It is useful for making sure to inseminate every eligible cow within the desired timeframe. It can be paired nicely with heat detection in a multipronged approach. Advanced protocols can help elevate reproductive performance to levels that would have seemed impossible a few decades ago. In fine-tuning the program, consider a second PG treatment, make sure all farm personnel are on the same page with every detail of the protocol, and discuss potential adjustments with the herd veterinarian. Good luck!

The content of this document, including text, graphics, and images, is educational only and not intended to be a substitute for veterinary medical advice, diagnosis, or treatment. Always seek the advice of a licensed Doctor of Veterinary Medicine or other licensed or certified veterinary medical professional with any questions you may have regarding a veterinary medical condition or symptom.

Where trade names appear, no discrimination is intended, and no endorsement by Penn State Extension is implied.

References

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