The basics of Artificial Insemination

The first thing to think about with AI is the logistics of getting the semen from somewhere (stud), in some manner (air/courier/personal pick up), to the inside of the mare’s uterus at the right time (vet), so the mare ovulates when the semen is at its most potent, so that the semen and the ovule (egg) meet at the right place, become fertilised, then arrive in the uterus and have a nice environment to land in. That’s the long and short of getting a mare pregnant through AI. Dead easy, isn’t it?

The second thing to consider with AI is that the semen has a limited life span from the time it’s collected. If it’s chilled semen you are working on approximately 48hrs of life before its fertility diminishes. If you’re working on frozen semen you may only have 12hrs of life before it dies. So you need to be fairly accurate.

The third thing is that getting the semen to the place where the mare is may be an issue. For example, you may order the semen this morning, go to the stud which may be an hour away, have them collect it, have it sitting there for you, go back to your place and have it in the mare in 3 hours. That’s a completely different scenario, in terms of preparation of the mare, to where you have to order the semen tonight, for the semen to be collected tomorrow morning, to be put on the plane from somewhere to arrive here early in the morning on the next morning, and to go into the mare a day and a half later, with a lifespan of possibly only 12 hours to get to the egg and get it fertilized. Obviously the timeline for the ovulation is critical in this case, as the semen can’t be held in suspended animation to wait for it.

And that’s where the work up for that kind of mating tends to be a bit more involved than for a natural mating. The stages of this process are:

  1. The mare needs to be evaluated for its reproductive health (clean uterus, vulval or conformational problems, etc), and also to determine the stage of its reproductive cycle, so that a plan can be formulated. At this examination any cysts found in the uterus need to be measured & plotted (description of where they are in the uterus) so that they are not confused with a pregnancy or presumed to be a cyst when they are actually a twin.
  2. The transport mechanism and timing of ordering of the semen needs to be clarified so that the mare can be examined and the semen ordered at the appropriate time. That is, when does the semen need to be ordered, how long does it take to get here and how can it arrive as early as possible to allow the maximum chance of pregnancy. Is it possible to have the semen collected or transported on a weekend, etc? Often this is not the case.
  3. The mare needs to be examined to determine when to order the semen. It may be necessary to use prostaglandins to manipulate the mare’s cycle to try to time the ovulation to fit in with the collection of the semen. It is often also necessary to use drugs such as Ovuplant or Chorulon to control the exact timing of the ovulation – once again to fit in with the semen.
  4. Once the semen arrives the mare is scanned to ensure that “everything is going to plan” prior to insemination. The pregnancy rate drops rapidly if the insemination occurs after ovulation.
  5. The insemination is carried out using sterile equipment.
  6. It is recommended that the mare is re-scanned within 2 days after service to ensure that the ovulation has occurred. If not it may be necessary to order more semen. The timing of this examination depends on the length of time between collection and insemination, and the type of semen used (chilled/fresh/frozen).
  7. Pregnancy testing is usually carried out at 15days and 45 days post-ovulation (or service). The 15 day test is to ensure pregnancy has occurred and there are no twins, and the 45 days test positive indicates that barring accidents and illness there is a greater than 90% chance of producing a foal at the end of the pregnancy next year. Additional testing at 30days checks for early embryonic loss allowing an earlier re-insemination if it has been lost.


Unless a vet has monitored a mare’s cycle through to ovulation previously, they can only go on averages when predicting a mare’s time of ovulation. On “average” a follicle may grow at 5mm per day & should ovulate at approximately 5 cm (it is not unusual for mares to ovulate either large or smaller sized follicles). However these parameters can vary greatly with the speed of growth changing for example when a second or more follicles are growing (may slow 1st follicle down).

You need to check for ovulation at around the time the semen is going to become unviable. This is usually around 48hrs post collection, because if the mare has not ovulated by then you may need to order more semen. Or you may opt to just forget about that cycle. Ovulation needs to occur about 6-8 hrs before the semen dies.

The next check that needs to be done is at 15-16 days post ovulation. This needs to be done for several reasons:

  1. It gives you an early indication if the mare is pregnant. If the mare is not pregnant you can cycle them earlier by the use of the hormone PG (see below for more info.), which would bring them into season earlier than they would naturally.
  2. This is the best time to check for twins. Twins are pathological in horses meaning most sets of twins do not make it to birth. The majority of them either abort or are born dead or get stuck during the birthing process. You will get a small proportion of mares that may absorb one twin, leaving the other to twin to go through to full term.
  3. The 15-16 day scan gives the vet the opportunity to remove one twin manually by squeezing. Also, at that stage embryos haven’t implanted so it gives the vet the opportunity to separate them and push them away from each other which allows the vet to squeeze one twin without affecting the other one and hopefully you will have one of those survive. The mare needs to be rechecked two days after this procedure to ensure there is only one viable pregnancy. If both pregnancies are lost she can be short cycled & inseminated again.
  4. At approximately day 19 the twin embryos start to get sticky and softer, so the vet generally finds them next to each other. This makes it more difficult to squeeze one without damaging the other, which may result in the loss of both embryos.
  5. Mares can ovulate separate follicles up to & over 48hrs apart. It is easier for a vet to find a 13 & 15day follicle or a 15 & 17 day follicle.
  6. Any cysts which were noted & plotted at the pre-mating examination need to be confirmed, measured & re-measured in 48 hrs time so they are not confused with a pregnancy. Cysts generally will not grow in 48hrs like a viable embryo would.


 Extra notes:
Understanding the use of PG to bring a mare on:

Prostaglandin (PG) which is the injection you give to bring mares on works in conjuction with the reproductive cycle & how this works is that a follicle begins to form in the ovary. A follicle is a little ball of fluid that contains, among other things, an egg. This gradually enlarges & matures until it gets to a certain size. In an average mare that size is approximately 5cm across, but it can vary in some individual mares where they will regularly ovulate early at 3.5 cm or sometimes 5.5 or more cm.

At times knowing how the mare cycles will help but there is no substitute for scanning the mare & making sure everything is going according to plan i.e.: the follicle is growing at a normal rate.

What happens after the ovulation occurs is that the egg is released to be fertilized in the fallopian tubes, there is bleeding into the space where the original follicle was & over a period of about 5-6 days that little blood clot turns into a hormone-secreting body, that releases hormones called progesterones and these hormones keep the mare pregnant, or keep the cycle shut down for the next 10 or so days until they start to cycle again. That little body that produces the hormones is called the Corpus Luteum. It is on this body that the prostaglandin (PG) works. What it does is to cause that body (CL) to lyse (deteriorate or break up), in other words it stops that body from working anymore by making it go away. Once it goes away progesterone then becomes withdrawn from the mare & then the horses’ body decides that as it is not pregnant it should come into season again (known as short cycling).

So that is an involved explanation why there is no point giving PG if a mare hasn’t ovulated. If the mare is springing (not going through a full cycle) then PG will often not work. Also if given too early in a mare’s cycle (3-4 days post ovulation) PG will not work as the Corpus Luteum will not have formed yet.

On the other end of the cycle, as a mare is just about to come into season, giving PG will often have little or no effect, or sometimes it may hasten the ovulation if the mare is very close to ovulation.