Ticking the Estrous Cycle: A Review fo the Estrous Cycle and the Effects of a Timed A.I. Program

Dairy managers know the protocol, GnRH on day zero, prostaglandin on day seven and then another dose of GnRH on day nine followed by timed breeding. Of course, I am referring to the popular Ovsynch protocol, but what is the purpose of the order and timing of the injections? Sometimes we get so tied up in routine that we forget the purpose of the synchronization program, to trick the estrous cycle into producing a viable ova (egg) in the timeframe of our choice. Let's take a step back and review how the estrous cycle works.

A Review of the Estrous Cycle

The estrous cycle is the period of time from one estrus period to the next. The length of the estrous cycle varies depending on age, environment and production levels. The average length for a bovine is 21 days. However, high producing dairy cows tend to average closer to 23 or 24 days in length. The length of an average heifer's estrous cycle is 18 to 19 days.

Now that we have a better understanding of the variations in the length of the bovine's estrous cycle, let's take a look at what is going on during those 18 to 24 days.

The estrous cycle can be broken into two phases. The first phase of the estrous cycle is the follicular phase. This is the time period when the female shows estrous and ovulates. Starting on day zero (day zero being the first day signs of estrus or heat are expressed), the cow experiences increasing levels of estrogen produced by a dominant follicle. The spike in estrogen causes the physical signs of heat like standing to be ridden, mucus discharge and a swollen vulva. Inside the cow, estrogen causes contractions in the uterus which help move sperm cells from the uterine body through the uterine horn into the oviduct (the site of fertilization).

The estrogen also sends a signal to the hypothalamus gland which triggers the release of GnRH. GnRH is then directed to the pituitary gland which releases FSH (Follicle Stimulating Hormone) or LH (Lutenizing Hormone). At this point in the cycle, LH is released and directed to the ovary and causes the follicle to rupture resulting in the release of the egg (ovulation). The egg is caught by the infindibulum and funneled into the oviduct where it meets the awaiting sperm cells.

Back on the ovary, the crater left by the ruptured follicle fills with blood. The blood forms a structure called the corpus hemoragicum. After about five days, the corpus hemoragicum begins to transform into luteal tissue, resulting in a corpus luteum or CL. This marks the beginning of the second phase of the estrous cycle, the luteal phase. During this phase, the CL produces the hormone progesterone. Progesterone's responsibility is to maintain pregnancy. Around day 17 of the cycle, the cow makes a "gut check" to determine if she is, in fact, pregnant.

If she is pregnant, the CL will remain on the ovary and continue to produce progesterone. If she is not pregnant, the uterine lining begins to produce the hormone prostaglandin which causes CL regression. As the CL regresses, GnRH is released which sends a message to the pituitary gland that FSH should be released. The FSH stimulates a new group of follicles to begin growing on the ovary.

After FSH levels begin to decline, one follicle becomes dominant and the rest die off. At this point, the dominant follicle begins producing estrogen, starting the cycle over at day zero. Whew, that's a lot of different hormones in just three weeks!

The Effects of Timed A.I.

Now let's look back at the Ovsynch protocol and see how it sets up ovulation.

On day zero of the Ovsynch protocol, we inject the cow with GnRH. There are three things GnRH could do.

1-If the cow currently has a mature follicle, GnRH triggers the release of LH which causes the release of the egg and the cycle to start over.

2-If there is a CL, GnRH will trigger the release of FSH which creates a new group of follicles.

3-If the cow has just released an egg in the past four to five days, GnRH does nothing.

Basically, GnRH sets the cow up to have a CL between day seven and 13.

On day seven, Prostaglandin is injected. The injection of prostaglandin regresses the CL resulting in the emergence of a group of follicles and eventually a dominant follicle.

On day nine (48-56 hours after the prostaglandin injection), the final injection of GnRH is given stimulating LH release which will cause the follicle to ovulate and release a fresh egg.

Sixteen to 24 hours later, the cow is artificially inseminated.

With hectic schedules, it is easy to skip a shot one day because you can't find the cow you're looking for, to give the shot to the wrong cow because you didn't check the ear tag closely enough or to end up giving the shot later in the afternoon since there just wasn't enough time in the morning. But, in every synchronization protocol the accuracy of the injections is very important. Every published synchronization protocol has been studied extensively. The way it is published has been proven to give the best results. It is vital to follow the protocol as closely as possible in order to receive the best results.

Author Bio: Ross Leix grew up on a large dairy in southern Wisconsin. He graduated from the University of Wisconsin-Platteville with a degree in animal science. As a Genex Training Programs Manager, Leix conducts educational programs on reproduction and A.I.