Simply put, synchronization advances estrous cycles so that all cycling cows come into heat at a predetermined time, usually designed to be at the beginning of the breeding season. Cows that breed and calve early wean older, heavier calves consistently and have a higher lifetime calf crop percentage. They have more time to rest and to recycle between calving and the next breeding.

Synchronization adds its own advantages to AI. The synchronization-based AI program results in a shorter breeding season with a shorter calving season. Both take less time away from other work and are likely to receive more attention than they would if they were spread out over several months.

When it comes to breeding, synchronization gives you two opportunities to settle a cow or heifer during a period of 21-25 days. And, if your replacement heifers are born to synchronized cows, they will be older and heavier at first insemination.

As far as calving, a shorter season with more uniform calves means your management can also be more uniform. For example, optimum feeding before and during breeding will be necessary for a shorter period of time, and cheaper too. You have the luxury of scheduling breeding and calving so that they fit around other work, and you can take full advantage of your area's peak nutritional season.

Don't worry that calves will all arrive on the same day. Even if every cow you own is cycling and bred in the synchronization program, calving will spread out over about 20 days - 10 days on each side of the due date.

How the Estrous Cycle Works

Before starting a synchronization program, remember that the percentage of animals in your herd that are cycling will be the single most important factor determining your program's synchronization success. And, to understand the process of synchronization, you must understand the estrous cycle.

The 21-day cycle of a cow is controlled by levels of the hormone progesterone. Decreased levels allow resumption of estrus activity; increased levels repress the reproductive system, prepare the uterus for pregnancy, and, if pregnancy occurs, maintain the uterus for that condition.

On the first day of the cycle, there is a blister-like structure on a cow's ovary called the follicle, which contains a mature egg. About 28 hours after standing estrus or "heat" begins, the follicle ruptures and the egg is released. During the next four days, the corpus luteum (CL) forms on the ovary at the site of ovulation. As the CL develops, it secretes progesterone, the hormone that supports pregnancy if a cow is mated and becomes pregnant.

During the next stage of the cycle (days 5 to 16), the CL continues to secrete progesterone. If the animal is pregnant, the CL will continue to produce progesterone in order to maintain development of the embryo and block further estrous cycles until after calving. However, if a pregnancy is not established, a hormone called prostaglandin is secreted from the uterus initiating regression of the CL.

In the third stage of the cycle (days 17 to 21), the luteal cells in the CL degenerate and stop secreting progesterone. This allows a new follicle to develop followed by the next estrus period and subsequent ovulation.

Manipulating the Estrous Cycle

With the brief explanation above, you can better understand the traditional approaches to manipulate estrus during the three stages described.

One approach to synchronization is to prevent ovulation until all cows in the herd have regressed their CL. This is usually accomplished with a natural or synthetic progesterone such as Norgestomet in Syncro-mate B. Once suppression of ovulation is removed, cows return to estrus and ovulate in a short period of time.

Another way to manipulate estrus is by shortening the cycle so that a large number of cows come into estrus at the same time. This is accomplished by treating cows with prostaglandin, the same hormone that causes CL regression during the normal estrus cycle. Examples of prostaglandins used today include Lutalyse and Estrumate. It is important to note that prostaglandin is effective only after day 6 or 7 of estrus.

The third concept employs both of the principles cited above. The aim with this system is to inhibit ovulation with a progesterone compound until most of the cows have regressed the CL with natural prostaglandin. Upon removal of the progestogen, animals will cycle within a few days but are not inseminated due to inherent low fertility of the herd. This system allows for more comprehensive management of the estrous cycle and even boosts fertilization over and above natural heat. Examples here include Syncro-mate B and the 14-day MGA-prostaglandin system.

Synchronization Tools Prostaglandins

Prostaglandin products are administered by injection and are available only by prescription from a veterinarian, who will also provide dosage instructions. Because prostaglandins function by regressing the CL, females must be cycling normally for the method to be effective. For that same reason, a prostaglandin injection will cause abortion in a pregnant animal. Because of potential side effects, prostaglandins should not be handled by women of child bearing age or by asthmatics.

Prostaglandins do offer you several management options. For instance, a one-injection system may be best if you are not sure the animals are cycling. In this system, you detect heat for five days as usual and breed the females detected. If 20 to 25 percent of all the animals show heat during those days, you can assume the herd is cycling normally. On the fifth day, give the remaining animals a single prostaglandin injection and continue to breed them as they come into heat. With this system, you will be able to breed almost all cycling animals within 10 days.

The advantage here is that if the percentage of cows showing heat by day 5 indicates there are some problems, you can stop and save yourself further time and labor as well as the cost of prostaglandin and semen.

In another method using prostaglandins, all animals are injected on the day you choose to start your program and then again 11 to 14 days later. After the second injection, you begin detecting and will find most animals in heat 2 to 4 days after that last injection. Breed accordingly.

Finally, if you want to synchronize but need to spread breeding and calving out more than the previous methods allow, you might consider a split breeding option. If you give one injection, you can expect about 60 percent of the normally cycling cows to come into heat two to four days later, and they can be inseminated 12 hours after being observed in standing heat. The animals not inseminated can be injected a second time on day 11 after the first shot. They'll come into heat two four days later and can be bred accordingly.

Keep in mind, with any prostaglandin synchronization protocol, if a high percentage of the herd is not cycling, you're wasting your time and money. For this reason, prostaglandin programs typically work better on virgin heifers than on cows.

All cycling animals can be treated with Syncro-Mate B and will respond at the same time, regardless of their cycling status when you start.

This system involves three steps:

Step 1 -- On day 1, each cow is implanted (behind the ear) with a synthetic form of progesterone called Norgestomet.

Step 2 -- At the same time cows are implanted with norgestomet, they are given an intramuscular injection containing a combination of the hormones estradiol valerate and norgestomet. The injected norgestomet prevents ovulation in any animal close to ovulation, and the implanted norgestomet prevents ovulation over the next 9 days. The estradiol causes CL regression in any animal that happens to be in the luteal phase when injected. Over a 9-day period, all cycling stops.

Step 3 -- The implant is removed on day 9.

Once the implant is removed, the animals start releasing hormones that will bring them into heat; those that respond to Syncro-Mate B will be in heat between 36 and 48 hours after implant removal.

If you want to inseminate all cows at once, do so 50 to 54 hours after implant removal. If not, inseminate about 12 hours after each animal is observed in standing heat. The detection method likely will result in a higher conception rate and will not waste semen on females not cycling.

You must remember cows must be cycling before they can be synchronized with either prostaglandins or Syncro-Mate B. That means cows must be healthy and receiving adequate nutrition. Even if they are, sometimes a nursing calf can suppress cycling. In that case, a 48-hour separation can help to overcome the problem and doesn't adversely affect the calves as long as they have access to plenty of water and good feed during the separation.

If you are planning to synchronize with prostaglandins, the ideal time for calf removal is when the second injection is given. If you are using Syncro-Mate B, separate cows and calves when the implants are removed.

MGA/Prostaglandin

Typically used on heifers, MGA is an orally active synthetic progesterone that has been used successfully to suppress heat in feedlot heifers. Research has shown that feeding MGA for 14 days (0.5 mg/head/day), removing it, then injecting prostaglandin 16 to 18 days later to regress the CL will result in the majority of the heifers coming into heat within five days.

Keep in mind that most females will show estrus two to five days after withdrawal of MGA; this is a subfertile heat, and females should not be bred at that time. Most females will exhibit estrus 48 to 72 hours after the prostaglandin injection. Females should be bred 12 hours after observing estrus.

An advantage to this system is that cattle only need to be handled once, besides insemination time. Research also indicates that this method stimulates cyclicity in anestrous animals.

Beyond Estrous Synchronization

While estrous synchronization traditionally focused on controlling the CL on the ovary, researchers in recent years have developed new programs aimed at synchronizing estrus by manipulating ovulation.

In basic terms, ovarian follicles grow and regress continuously during the 21-day estrous cycle. At the beginning of each new wave of follicle growth, a group of follicles, similar in size, are present and begin to increase in size.

As the cyclical wave continues, one follicle, the dominant follicle, grows to a much larger size than the others. Remember that ovulation occurs only when progesterone levels are low. If the correct physiological signals aren't present, the dominant follicle doesn't ovulate, regresses, dies and is replaced by a new dominant follicle from a subsequent wave. High levels of progesterone secreted by the CL during the middle of the estrous cycle (days 5 to 15) send signals that inhibit ovulation.

Conversely, cows produce a brain hormone called Luteinizing Hormone-releasing hormone (LH-RH). This hormone serves as the primary signal for ovulation to occur. If a cow's reproductive cycle has been manipulated so that a dominant follicle is present, an injection of a synthetic form of LH-RH called Gonadotropin Releasing Hormone (Gn-RH) will cause the dominant follicle to ovulate when progesterone is low.

In one system using GnRH, the hormone is injected into all females on day 0, followed by a prostaglandin injection seven days later. Although some cows will exhibit estrus the same day they receive the prostaglandin, peak estrus will occur two to three days after the prostaglandin injection. Animals are observed for five days following the prostaglandin injection and are inseminated 12 hours after standing heat is observed.

Good Management First

Since the first prostaglandin product (Lutalyse) was approved in 1979, synchronization has proven to be an important management tool in many herds. We listed some of its advantages earlier, but if you are considering a synchronization program, you also should keep in mind the things it won't do.

Estrus synchronization is not a cure-all for breeding and management problems. It will not replace good management and will not be successful under poor management.

Synchronization will not cause cows to come into heat earlier after calving. Nor will the calving season automatically shorten - late calvers will still be late calvers. And unless you use one of the fixed time breeding methods, synchronization will not eliminate heat detection, only shorten it.

If you are going to synchronize, facilities must be adequate to handle the extra load, and you must be sure you have enough trained and experienced help on hand when breeding starts. It will get intense, so be prepared.

If you are interested in starting a synchronization program, you should consult with your veterinarian and your AI representative as well as cattle breeders that have experience. They will be able to give you more information and help you decide which will be the most practical and beneficial method for you.