Reproduction is a fascinating story. The story begins with the supply of oocytes that are present from before birth on both ovaries.
Ovaries, though small, bustle with rapid growth and regression of two key structures: follicles which contain individual oocytes and corpora lutea (CL) which form at the site of ovulation.
After ovulation, an oocyte is swept from its follicular home into the oviduct, where fertilization may take place if viable sperm cells have successfully been deposited in the right place at the right time. Fusion of an oocyte with a single sperm cell brings together a full set of unique genetic instructions.
Twelve days after fertilization, an embryo is 3 millimeters in size. A short four days later, it has grown to 250 millimeters and is signaling from the uterus to prevent regression of the CL and termination of pregnancy.
Eventually, the placenta develops cotyledons and is attached to the caruncles of the uterus to supply the growing fetus with nutrients until pregnancy is complete, at which point a combination of physical pressure and hormones cause the changes and muscle contractions necessary for the calf to make its first appearance.
Despite the beauty of the reproductive process, managing reproduction on a modern dairy farm can be complicated. Eavesdropping, you might hear a conversation like this …
Repro Manager 1: “We use an 11-day Presynch-Ovsynch 56 program. We then give GnRH on day 28 while also testing for PAGs.”
Repro Manager 2: “Have you thought about a CR advantage with Double Ovsynch or doubling up the final PG treatment? How about using CIDRs on anovular repeat breeders?”
It is a language of its own but one that many successful managers have learned well. There are seemingly endless variants of timed A.I. protocols – some with generic-sounding names, some with creative names, some that were uniquely designed for a specific dairy operation and some that have never been given a name.
Where does someone start if they don’t even know the lingo? Why do timed A.I. protocols feel like a complicated (and questionably necessary) list of chores for some?
With a foundation of knowledge about the basics of reproduction in dairy heifers and cows, estrus synchronization and timed A.I. can make sense and open the door for new conversations in striving for a high level of efficiency and success.
Though related, estrus synchronization and timed A.I. are not one and the same. Estrus synchronization refers to treatments designed to bring a group of animals that would normally display estrus over a range of more than 21 days into a narrower window of less than five days.
Timed A.I. typically builds on estrus synchronization, adding precision. Treatments are designed to synchronize ovulation within a narrow window of a few hours. As a result, A.I. can be performed at a prescribed time before ovulation, without estrus detection.
In open heifers and cows, it is important to understand the patterns of follicular development (Figure 1). During each estrus cycle, groups of follicles are recruited to grow.
Some only grow briefly and then regress. Others are selected to grow larger and more prominent on the ovary, but most of them also regress. Only a select few continue to grow and develop the capacity to ovulate.
Even then, not all of these follicles ovulate. If large follicles remain in an environment of high progesterone, they too will usually regress. It is only when CL regress and progesterone declines that a follicle can ovulate.
That is unless a gonadotropin-releasing hormone (GnRH) treatment is given which overrides progesterone’s suppression of ovulation. We’ll get to that point in a moment.
These patterns of follicular growth occur multiple times during each estrus cycle, a wave-like pattern as seen in Figure 1. Most cows will have two waves of growth during their estrus cycle, but some will have three.
Thus, if we were to look at a graph of specific ovarian dynamics from multiple cows over a 21-day period, it would become obvious how complicated it would be to make predictions at a herd level. But synchronization improves predictability.
Progesterone is a powerful hormone secreted from the CL. It is well-known for its essential role in the maintenance of pregnancy, but it also controls cyclicity in an open animal.
With high circulating concentrations of progesterone, follicular growth is limited and ovulation will not naturally occur, but when CL regression is triggered and progesterone concentrations plummet, the final stages of follicular development can proceed toward ovulation.
Using timed A.I., it is optimal to have high progesterone at the initiation of an Ovsynch protocol and low progesterone in the hours prior to insemination.
Luteinizing hormone is released in a pulsatile fashion from the pituitary throughout the estrus cycle, but a large surge at the end of the cycle is what triggers ovulation.
There are a number of synthetic GnRH products used to accomplish this event. GnRH is not luteinizing hormone, but it is what directly causes release of luteinizing hormone from the pituitary. Thus, treatment with GnRH is effective in causing ovulation, as long as there is at least one large, responsive follicle on one of the ovaries.
Notice the red-shaded areas in Figure 2, representing periods of the estrus cycle when follicles are not responsive to GnRH treatment. Given randomly, GnRH will not always cause ovulation, but with proven synchronization protocols, the odds become more favorable.
One nice assurance when using GnRH is that treatment will not adversely affect pregnancy.
Prostaglandin F2-alpha (PGF) is released from the uterus when there are no signals of pregnancy detected around day 17 of the estrus cycle. PGF targets the ovary and leads to the demise of any fully developed CL.
As with GnRH, there are a number of synthetic PGF products available that are commonly used for estrus synchronization. Two concepts to remember are: Never treat an animal with PGF that might be pregnant if you want to maintain pregnancy, and realize that young CL naturally present during the first few days of the estrus cycle, or young, accessory CL induced by GnRH treatment, may not fully respond to PGF treatment (see red-shaded areas on Figure 3).
Looking briefly at a standard Presynch-Ovsynch protocol (Figure 4), the first two PGF treatments can be expected to synchronize estrus in the majority of open, fertile cows.
A random, single treatment might be effective on 50 percent of them. The second treatment two weeks later may increase the percentage to 80 percent.
If estrus and ovulation occurred after the second PGF treatment, treatment with GnRH 10 to 14 days later will oftentimes cause ovulation in a high-progesterone environment, meaning an additional CL is formed. Though this often occurs, the primary goal of this treatment is to initiate a new follicular wave, tightening synchronicity.
The final PGF treatment should cause regression of all CL. If follicular wave development is effectively synchronized, the final GnRH treatment is administered when the preovulatory follicle should be at the proper stage of development.
At the end of the protocol, timing to ovulation is predictable, allowing for A.I. to be performed without any regard for whether or not estrus is displayed.
Though estrus often isn’t observed, proven timed A.I. protocols have proven to be quite successful, often exceeding conception rates achieved with a more traditional heat detection and A.I. approach.
If you want to improve your understanding of reproductive function, there are plenty of options. Find a related extension workshop, talk to a trusted A.I. representative, have a conversation with your veterinarian or spend some time online reading articles and watching videos developed by reputable individuals. You won’t regret building a good foundation of understanding. 
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