Insight into the Fertility of the Friesian Mare

The steps below will provide more insight into the fertility of Friesian mares. Take a moment to carefully read through the list of questions. With the answers the mare owner can map out a good breeding strategy.

1. What is the history of the mare?   a. First pregnancy b. Mare kept open last season c. Mare did not take last season What measures were taken at that time to still try to get the mare pregnant?

2. What is the age of the mare? Mares over 14 years old are part of the category ‘older brood mares.’ Take a good look at the position of the vulva. The top of the vulva should  not come up over  the pelvic basin. If that is the case then the mare could be ‘sucking air’ with the consequence that  dirt enters the uterus, which may prevent the mare from conceiving. The only option here is to ask the veterinarian to put in a caslick up to two centimeters under the pelvic basin.

3. What is the condition of the mare?  In a skinny mare the anus will draw inward, which positions the vulva more horizontally (when the vulva needs to have a vertical position). When the vulva has a more horizontal position the mare will be more prone to sucking in air, which can have dirt enter into the uterus. Therefore make sure that the mare is in good condition!

4. How does the mare cycle?   If the cycle lasts longer than normal, longer than 23 days, this is usually an indication that the mare is not clean inside.   A mare can also have a short cycle, shorter than 23 days. This can point to a tumor on the ovaries. Often a mare with a tumor on the ovaries will show ‘stallion behavior.’  Teasing is still an important aid. It will tell you a great deal about the cycle of the mare.

5. Check on the uterus and the ovaries! Have your veterinarian scan the mare, so he/she can check the uterus and ovaries for fluid, cysts, and other abnormalities. The veterinarian can then put together a treatment plan to bring the uterus to its best condition.

Procedures and choices

Those who would like to have their mare bred will go look for the right partner and a stallion is chosen. Then comes the moment to determine when the mare is in heat and when she needs to be bred. Detecting the heat can be a problem. A good aid that is not used enough is teasing the mare with a stallion. Teasing stimulates the mare and a mare in heat will really show where she is in her cycle with a stallion nearby. A gelding in the vicinity can also do wonders. In order not to distract the stallion too much it is good to let him tease in a quiet and familiar setting. This also goes for the mare but since not everyone has a suitable stallion or other effective horse in the barn of the mare the advice is to take the mare to the stallion. A good tease with a good determination of the heat can save much in bills, because the procedure for a successful insemination then only needs to be performed once. 


There are a number of differences between warmbloods and Friesians. Friesians, e.g.,  have a longer heat with often more edema, one of the items a veterinarian will have to pay extra attention to when ultrasounding the mare. In earlier days one could only feel up a mare to see how it was going. Depending on the softness of the cervix and the size and softness of the follicle, because the follicle has to be soft, it was determined where the mare was at. Now we do that by means of ultrasounding so that also the degree of edema can be observed. Edema is a physiological response influenced by hormones and can be describes as ‘fluid in the uterine wall. It is, therefore, not ‘loose fluid,’ which occurs when there is a defensive response.  Only when there is no more edema production can one inseminate. When the uterus is wet from edema and an follicle of six centimeters—which is in itself favorable—one can still not inseminate. Many breeders have a hard time with that and do let the mare get bred. In the end without result.


Edema production has three stages: round and wet, round, dry. Not until between round and dry does the mare ovulate. It is therefore useless to inseminate before that time. The advice for a Friesian is to inseminate the day after the second stage, the round stage is determined. Many hasty breedings are based on the size of the follicle. This determination means nothing since Friesians quickly have follicles of 55 millimeters or larger. For warmbloods the max is around forty millimeters. Once the insemination has taken place we have to check for pregnancy, usually at 18 days later. For those who worry about twins have the mare checked at fourteen days.  Should there be two follicles then the veterinarian can still pinch one off. Day 18 offers an advantage.


Should the mare not be pregnant then the start of the new heat will already be visible. After day 35 of the pregnancy the chance of absorption goes down significantly.

Maiden Mares

Maiden mares have never been bred and have, therefore, never had a foal. This is technically the easiest group as you can assume there have never been any procedures performed inside the mare. The uterus should, therefore, be clean Normally horses in this category are pregnant from one to two breedings. There is, however, always a chance of physical deviations that can prevent a mare from becoming pregnant or carrying a foal to term. An example is if the connection between ovary and oviduct is missing. Only with an internal exam with a scope, with which the veterinarian enters the uterine horn, can this physical problem be determined. Another, less problematic item is the fact that there is a group of mares that at the moment of insemination, e.g., in the year they turn three, are too young. Many of these horses do take a year later.


Young mares in general show their heat less easily. When the environment is familiar and there is no stress a mare will show heat better. When purchasing a mare it is important that a buyer always checks if the mare has had a foal before or if she was bred to have one. The potential buyer is advised to see if the vulva of the mare is nicely vertical and tight. With three year olds the buyer, seller or owner can other than that not observe much else.


Open (not in foal) on Purpose
Mares that are open on purpose have had a foal before but are for one reason or another, such as sports activities, not being bred. In that case the uterus has had over a year to ‘clean up’ i.e. recover from the last pregnancy. Often these are mares that can, therefore, become pregnant again quickly.


Open (not in foal) in Spite of Breeding

Open in spite of breeding This is a mare that can or does not want to get pregnant. In the context of this article we are talking about the largest problem group for which we need to ask ourselves three main questions:

•  Has the mare ever had a foal? Or has this been attempted for years and she just does not take or aborts? Especially when a mare aborted is important.

•  How long has the mare been open?

•  What has happened the past year? Has she been treated with antibiotics, flushed or was she cultured or a biopsy done? 


As described above a clean uterus is a must for a healthy pregnancy. Regularly older mares, we talk about horses from fourteen years of age here, have difficulty with the independent, natural cleansing of the uterus. This cleansing happens by  working out matter through the vulva. Flushing will easily solve this problem. Cleansing issues can be the cause that (older) mares do not want to take but a hormonal or age-related cause is also possible.


A large group of older mares becomes skinnier. Their back starts to sway and the anus draws in. This makes it that the vulva is more horizontal and sinks while at the same time the uterus sinks below the vulva. This makes it harder for the mare to expel loose fluid from the uterus. Because the anus draws in the vulva is drawn in too. This puts it above the pelvic basin, which gives her room to suck in air and filth such as bacteria. To get these mares pregnant the air sucking has to be prevented. For that reasons these mares have to be ‘sown up.’ The nutritional condition is of great importance, because if the mare gains weight the anus and vulva will also be pushed out again and nothing will have to be sown up. The problem is that these brood mares often don’t gain good weight until during the pregnancy. Cysts in the uterus, a problem often seen in older mares,  can also be an obstacle for pregnancy. In three-yearold mares such cysts are rarely seen. When the follicle, which swims through the whole uterus during the first 18 days , continuously bumps into cysts it will die. This also happens when the follicle nestles against cysts, because then too little nutrients can be absorbed. In the case of twins the same problem occurs; when the follicles are loose from each other they usually survive. If they are right next to each other, however, they lack part of the contact with the uterine wall and thus part of the nutrient intake. Cysts can, in the uterus, be burnt off. As mentioned before, there can be more reasons for a mare staying open in spite of breeding. It would go into too much detail to cover all causes in this article, but one has to be mentioned: lacking yellow body (corpus luteum).

Foal a Foot (or on the side)

Foal on the side The group of mares with a foal on the side is for both breeders and veterinarians a beautiful group of horses. They have (recently) proven to be able of pregnancy and were able to carry to term.


Many breeders inseminate during the foal heat, the first heat ten days after the foal is born. Mares will become pregnant but the follicle will often be absorbed six weeks later. The clear visibility of the foal heat is usually a reason to inseminate the mares anyway. Shortly after the birth the uterus is, logically, out of proportion. It is obvious that it first needs to recover to be able to carry the next pregnancy especially if the mare retained the placenta it is not recommended to inseminate her during the foal heat. In addition it needs to be noted that a mare that was hooked up to an i.v. has an advantage over a horse that was cleaned up and flushed. In the breeding of warmbloods it is meanwhile reasonably normal to give the mares a shot to bring them into heat on day 20 after the birth of their foal. Around day 26/27 you can then with reasonable security safely inseminate.


One of the arguments to take this approach is the fact that mares with a foal show their heat less well and it is more difficult to tease them with a stallion.


Image of 21 year old Friesian mare with a sewn up Vulva (caslick)

Image of 23 year old Friesian mare with a 'not' sewn up Vulva 


A biopsy is a bit of tissue taken from the uterine wall from which the veterinarian can study the pattern of lymph and blood vessels and cellular structure. In short: if the structure of the uterine wall is normal.

Flushing with salt solution

Flushing means that the uterus is completely filled with a physiological salt solution by the veterinarian. Especially older mares often have problems with the self cleansing of the uterus. The salt solution makes for it that the cleansing process is better started up. Flushing takes place six hours after the insemination, because the semen has then swam up into the oviduct and there is not yet a natural defense response in the uterus. Objective is that the veterinarian flushes out  the remaining fluids. Especially in the case of a sunken uterus self-cleansing is difficult and flushing will help.

Flushing with antibiotics

This is a cleansing of the uterus by injecting antibiotics. Before the veterinarian applies the treatment he has to be sure that the follicle is gone.

Sowing up/Caslick

A veterinarian will only then sow up the vagina if the vulva is situated above the pelvic basin. If a mare has been sown up then this will have to be repeated every year after the birth of a foal and possibly for the next year if there will be another insemination. A veterinarian will only then again sow up a mare when the follicle is gone, because otherwise the wound will tear back open when within a few days you have to inseminate again. A seller needs to mention when a mare has been sown up and a buyer who lifts the tail will be able to observe this. In any case it is always important to ask when purchasing a brood mare.

Early Season foals

Some breeders swear that foals need to be born early in the year. Others don’t want that at all. The former raises some questions. A horse is a grazer and moves around. For him or her it is important that right from the start it can develop well with much exercise and fresh air. Early foals often have to stay in their stall due to winter weather and wet pastures. This increases the chance of OC/OCD, lung problems and diarrhea. The best part of the year for foals to be born is the months of April and May. Mares will then have to be bred in May or June

An ultrasound can, among other things, determine how much fluid there is in the uterus,  the size and placement of follicles, cycle term, ovulation, twins, pregnancy and is envaluable in obtaining and maintaining a pregnancy.