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Simplifying Synchronization

It All Starts with Understanding the Estrous Cycle

1A. Estrous Cycle - The Basics


Estrous cycle definition: The series of changes that take place in the animal from one heat period to the next.
It is 21 days long on average. Actual range is about 18-24 days.

It is made up of two phases:
Follicular Phase - a drop in progesterone levels due to the destruction of the corpus luteum leads to an increase in GnRH which allows the animal to come into heat and ovulate resulting in the formation of a new corpus luteum; animal returns to the luteal phase.
Luteal Phase - an increase in progesterone levels maintain the pregnancy resulting from ovulation and prevents the animal from coming into heat and ovulating. If pregnancy does not occur, prostaglandin is released leading to the destruction the corpus luteum; animal returns to the follicular phase 

Key hormones involved and their function:
GnRH: produced by the hypothalamus - Increased levels lead to ovulation
Progesterone: produced by the corpus luteum - Maintains pregnancy, prevents animal from coming into heat and ovulating
Prostaglandin: produced by the uterine endometrium - Destroys corpus luteum allowing animal to come into heat and ovulate


1B. Estrous Cycle - The Basics: Waves in the Estrous Cycle

Follicle - The structure within the ovary which houses the egg.

Follicular waves - A phenonema occuring in the ovary throughout the estrus cycle that produces growing follicles capable of ovulating an egg.

Follicular waves begin with the growth of multiple follicles. Soon one follicle becomes dominant and continues to grow while the others die off. If the follicular wave approaches maturation in the luteal phase, ovulation will not occur due to elevated progesterone levels produced by the corpus luteum. However, if the follicular wave reaches maturation in the follicular phase, ovulation occurs due to increased levels of GnRH which occur in the absence of the corpus luteum.

The number of follicular waves varies with the type of animal. Dairy cows tend to display two wave follicular patterns (Figure 1). Dairy heifers tend to display three wave follicular patterns (Figure 2). Synchronization protocols are designed around the follicular wave pattern displayed by the animal.




2. Ovsynch -  How it Works

Ovsynch was developed in 1995 to increase service rates and improve the overall reproductive efficiency of a dairy. It allows a cow to be inseminated without observing signs of estrus and is particularly helpful in situations where heat detection is a challenge. Since its creation, Ovsynch has become an important component in many dairy producers' reproductive programs.

The Process
The term Ovsynch is short for ovulation synchronization. The process begins with the administration of an injection of gonadotropin-releasing hormone (GnRH) which causes ovulation and the start of a new follicular wave. The second injection, prostaglandin (PGF), is given seven days later and brings about regression of the corpus luteum. Two to three days after the prostaglandin shot (time is dependent on the protocol selected; see Table 1), a final injection of GnRH is given which will cause ovulation of the follicle contained in the new follicular wave. Ovulation will occur 24-32 hours after the second GnRH injection. Therefore, if working with a group of cows who have been subjected to this protocol, those that respond to the Ovsynch injections will experience synchronized ovulation; they will all ovulate within an eight-hour period. Insemination is completed at a prescribed time and allows semen to be present as the cows are ovulating. This leads to protocol success (see Graph 1).


Ovsynch Steps
1. Initial ovulation is caused by the first GnRH shot.
2. The corpus luteum is regressed on day 7 by PGF shot.
3. The second ovulation is caused by the final GnRH shot given 56 hours after PGF.
4. Timed A.I. (TAI) occurs 16 hours after final GnRH shot.

TABLE 1. Ovsynch Protocols and Calendar for Implementation
 *Table from Dairy Cattle Reproduction Council. 


3. Presynch: Leads to Higher Conception Rates

Presynch is short for Presynchronization. It consists of a series of injections given prior to Ovsynch. When Presynch is added to Ovsynch, the combination of the protocols is called Presynch - Ovsynch. The result of this combination is a protocol that is longer than the standard 10-day Ovsynch, and therefore is only recommended for use with first service breedings. Giving these extra injections prior to Ovsynch leads to higher conception rates versus using Ovsynch alone.

How Does Presynch Help Improve Fertility?
Presynch is a means of synchronizing the cycles of a group of animals. It places the cows between days 5 and 10 (see Figure 1) of the estrous cycle prior to administering the Ovsynch protocol. The advantage of grouping cows at this stage of the cycle is the high likelihood these animals will ovulate to the first shot of GnRH at the beginning of Ovsynch. This is the key to the increased fertility. Beginning Ovsynch at any other stage in the cycle results in lower rates of ovulation when GnRH is given. 


Presynch enables us to maximize the number of animals that ovulate to the first injection of GnRH in the Ovsynch protocol (as shown in Graph 1), and improves synchrony of emergence of new follicular waves. As a result, the animals will experience synchronized ovulation at a higher rate when the second injection of GnRH in the Ovsynch protocol is adminintered. This ensures that more animals are ovulating at the optimum time in relation to the arrival of semen and leads to increased fertility.


Ovsynch Steps
1. Initial ovulation is caused by the first GnRH shot.
2. The corpus luteum is regressed on day 7 by PGF shot.
3. The second ovulation is caused by the final GnRH shot given 56 hours after PGF.
4. Timed A.I. (TAI) occurs 16 hours after final GnRH shot.

Various studies have shown that Presynch - Ovsynch protocols increase fertility by approximately 7% to 9% over Ovsynch alone! 

Review of Potential Combinations of Presynch and Ovsynch
There are a variety of Presynch options available with the most highly recommended ones shown in Table 1. Any of the Presynch methods listed below can be used prior to any of the Ovsynch methods shown in Table 2.

TABLE 1. Presynch Methods Used Before TAI
*Table from Dairy Cattle Reproduction Council. 

TABLE 2. Ovsynch Methods for TAI
Dairy-SynchProtocol.jpg*Table from Dairy Cattle Reproduction Council. 


4. Resynch: Shortens the Days Between Inseminations

Resynch is short for Resynchronization. It is the process of setting a cow up on a timed A.I. program in conjunction with a non-pregnant diagnosis. It can refer to the second insemination or any number of re-inseminations that come after the first breeding. There are a variety of Resynch methods available with some beginning before pregnancy diagnosis and some beginning after the animal has been found open (see Table 1). Many dairies have adopted the use of Resynch programs because they are able to present open cows for re-insemination sooner than heat detection alone. 

TABLE 1. Resynch Methods Used for Animals Diagnosed as Non-pregnant
*Table from Dairy Cattle Reproduction Council.

How Does Resynch Shorten the Days Between Inseminations?
Many cows do not become pregnant after first insemination. This leaves a large number of animals open after initial breeding. The earliest time they can be re-inseminated is when they return to heat at around 21 days. Those that do express heat at this point are re-inseminated at the most optimum time. Here is where the challenge lies, however, because a significant number of cows do not display estrus before pregnancy diagnosis. Thus an alternative method, Resynch, must be employed if peak reproductive efficiency is to be obtained.

When properly implemented, Resynch permits timed A.I. to occur as soon as three days after a non-pregnancy diagnosis on all animals confirmed open. This leads to cows being rebred in a timelier manner and shortens the days between inseminations. As a result, Resynch improves the efficiency of a reproductive program and thus improves profitability of a dairy.

--In Conclusion--
This article series has been a brief overview of the estrous cycle and synchronization. Much more information could have been included in these articles, and there will always be new concepts and applications to learn as science continues to reveal more and more in this area. If you desire to gain additional knowledge on this subject, I would encourage you to deepen your understanding of the estrous cycle first. Why? Because a good comprehension of the cycle is the foundation of this pursuit and the first step in simplifying synchronization. Good luck!