Breeding Part 1- The Basics - before you breed, the mare's cycle and types of cover...

Breeding Part 1- The Basics - before you breed, the mare's cycle and types of cover...
Three foals from our bespoke breeding programme, left colt now in Denmark aimed at eventing; middle colt now in the UK for advanced all rounder plus family horse, left filly by Hos D'o retained for breeding and jumping.

So the saying goes, 'fools breed for wise men to buy'. Unfortunately this is very true, especially when you factor in the time, money and stress involved. However, nothing can beat breeding your own horse.


So now you have the time, you have the money, you have the horse but what important factors should you consider before going down this route. The most important factor - is the mare that you wish to breed from suitable? Will the foal, if born healthy, have a place in this life? Are you prepared to keep said foal and are you prepared to lose your mare and possibly the baby?

Breeding from nostalgia has its place, but in my opinion, it should probably only be done once unless the mare has some sort of breeding value. What I mean by this is if the mare has a nice temperament, is conformationally correct and sound and serves a purpose, then it is most likely her offspring will also serve a purpose. The aim of breeding should always be to improve on your stock - mares with bad confirmation, attitude and lack of ability or type should be excluded from any breeding programme.

Overview of Estrous Cycle

Most mares are seasonally polyestrous and cycle when the length of daylight is long. For thoroughbred breeders, their season starts in January and as daylight is short they will often use lights for the mare (from late November) either overhead or a light mask to encourage the mare to cycle early. For the rest of us who usually are happy with foals to be born from March onwards we are happy for our breeding cycle to begin later in the year. Providing a mare has access to daylight, she usually will begin to cycle in March. These cycles may be transitional in nature up until end of March or even April.

A mare's cycle on average last 21 days plus or minus 3 days. The estrous cycle is composed of two phases - the estrus phase (in heat) and diestrus phase (out of heat). The average length of the estrus phase is 6 days. Once the mare has come out of vernal transition and has undergone one ovulation, the estrous cycle then becomes more regular. It takes approximately 21 days from one ovulation to the next. When ovulation of a follicle occurs, the oocyte is released for potential fertilisation. The cavity left behind on the ovary by the evacuated follicle, fills with blood and eventually becomes a corpus luteum aka "CL". Approximately 5 days after ovulation, the CL is functional and secretes progesterone. This is known as the luteal phase. The presence of a CL will prevent a mare coming back into estrus therefore around day 13 post ovulation, the endometrium of the uterus secretes another hormone causing the regression and destruction of the CL. This permits the onset of estrus behaviour.

Follicles which have been sitting under 10 mm will then start developing. Three or four follicles will possibly reach over 25 mm before one or two will become dominant (usually over 35mm) due to FSH activity under the increasing influence of oestrogen levels. It is these one or two follicles that will then ovulate and the cycle will begin again.

Based on this very brief information your most important dates of the estrous cycle as someone new to breeding are - the start of estrus, ovulation which will occur in the last 24 to 48 hours of estrus and 5 days after ovulation (to allow for hormones if required to be given at the correct time).

To gain knowledge of your mares cycle if she doesn't have a change in behaviour, you can use a "teaser" or an ultrasound and physical examination. Done correctly, teasing is a great way to ascertain when the mare is coming in and then an ultrasound can confirm number, size and location of follicles and if repeated - ovulation.

Before Breeding

Before breeding, a mare should undergo a breeding soundness exam. Ask around to find the best vet involved with reproduction. The vet can scan and examine the mare to tell you whereabouts in the cycle she is, if she has any fluid, or cysts, or any other complications that may affect your mare's fertility. Ideally, a swab should be taken at this stage to check for any bacteria that would affect the mare's ability to conceive or to retain the pregnancy.

You should also have decided on a stallion by taking into account the mare's type, temperament, conformation, ability, the areas that need improvement and what you hope to achieve. The stallion should compliment the mare and improve on any areas that the mare may lack and most importantly should not exacerbate any conformational defects or flaws - physically or with temperament.

It can be hard sometimes to find all information about stallions especially rideability, trainability, temperament on the ground and under saddle. The best place to start is to ask the stallion owners, breeders of foals / young stock and those competing horses at a higher level by the stallion, to get their opinion on what traits the stallion tends to pass on. If you are breeding to sell on, you'll also need to try and find out if there are any genetically disposed defects such as OCD which can show up in x-rays and although may never affect the horse, can cause the horses value to fall dramatically.

The same goes for rideability and trainability. It is well known in some breeding lines that this line of horses are not for amateur riders. In general, breeding a hot mare with a hot stallion means the offspring may only be suitable for professionals and the larger market is the amateur market. Thus if the horse is not good enough for a professional and is unsuitable for an amateur it's only going to end up in one place….

Types of breeding

Natural, AI fresh / chilled / frozen, OPU & ICSI. (Embryo transfer is used after the covering itself).

The two main ways of breeding a mare are usually natural or AI.

OPU and ICSI (ovum pick up and intracytoplasmic sperm injection) are often prohibitively expensive and carry more risks.

The minimum number of scans a mare should have by a vet, if you value the health and welfare of your mare, in my opinion, should be 3.

1) Day 15/16 post ovulation to check for a) pregnancy or lack there of and b) twins.

2) Day 24-26 to check for heartbeat and again double check for twins (definitely before day 34 as the endometrial cups form from day 35 and any pregnancy lost after this date usually means a mare will be out for the season depending on the time of year and the use of hormones).

3) Day 45-90 to check the pregnancy is progressing.

Ideally as mentioned before there should be a breeding soundness examination, scan coming into / during estrus and a scan for ovulation. As an extra, the October/November scan is great to see that pregnancy is still ongoing or else you can start the season early the following year. For AI frozen, there will be multiple scans during estrus and up to ovulation as the window for insemination is narrow (more on that later).

Natural is usually the most cost effective and involves taking the mare to the stallion within 48 hours of ovulation and allowing him to cover her, either in hand or loose. Mares will generally only accept the stallion when in estrus. Cross cover may also be required (covering twice in the estrus phase). Again scans need to and should be done as yes, you may get lucky that the mare takes first time but eventually, you will run into an issue and it will be costly in terms of time, money and possibly the health and welfare of your mare.

AI fresh is usually the next best option followed by chilled and then frozen semen. However, if you have a very knowledgeable vet who is proficient at AI (including both chilled and frozen semen) and providing semen quality is good, resulting conceptions are quite similar.

So let's go on to AI, probably the most common way of getting a mare in foal in the sport horse world.

Why AI?


  • generally safer for the mare and stallion as there is less chance of injury
  • may be less chance of sexually transmitted disease being transmitted
  • greater choice of stallions and thus access to a global market
  • allows difficult stallions to be collected or those that are competing
  • allows for evaluation of semen quality
  • can be more beneficial for a problem mare as insemination can be timed as close as possible to ovulation
  • can be safer for the stallion handlers and barn staff
  • Allows more mares to bred as one collection is usually split
  • Reduces the pressure on the stallion for multiple 'coverings'


  • More expensive than natural cover
  • Covering on weekends can be difficult if shipping in chilled semen.
  • Possibility of spreading EVA
  • Specialised equipment and technical expertise are needed. (You must have a good vet!)
  • Some mares react to the extenders in chilled semen or frozen semen
  • Some stallions do not freeze/thaw well

AI fresh - Can be used when the stallion is fairly local and the semen can be kept cool for a couple of hours during transport before insemination. This in my opinion is the best form of AI - the semen is collected from the stallion and inseminated into the mare... no extenders are needed. Ideally, fresh semen should be inseminated within 24-48hours of ovulation. The closer to ovulation the better. It has been very rarely known for certain stallions semen to last in the mare for up to 7 days and still provide fertilisation.

AI chilled - chilled semen is where the stallion is collected, the dose is split and extenders are added to prolong the life of the sperm. This is then transported in a cooled container. There are many different types of extenders and each stallion semen will react in a different way to each one. Therefore, each AI centre or stud will experiment with the semen and different extenders to find the one that is most suitable for that stallion.

Chilled semen can be transported for a longer duration than fresh semen. It is usually inseminated within 24-48hrs of collection and within 24hrs of ovulation. It is again best to have the semen shipped and inseminated in the shortest possible time and within 24hrs (which is possible with a lot of EU studs). Insemination up to 48hrs before ovulation may provide a pregnancy - however, the positive percentages drop rapidly.

Personally, I have witnessed positive pregnancies from chilled semen that was inseminated 4 days before ovulation and the collection would have been 24 hours previous to insemination...I have a mare that loves to hang on even with the use of hormones!). Obviously, the earlier the insemination the better as semen quality degrades as time passes.

Chilled semen is transported in special containers with ice packs. The semen would usually be in a syringe which is then inseminated directly into the mare through the cervix using a semen insemination catheter.

AI frozen - the most versatile in terms of longevity, resistance to the environment (hot weather and transport of chilled semen from the continent can cause havoc), and ability to be available exactly when the mare is ready. Yet it has the narrowest window for insemination and the need for a highly experienced and dedicated vet. Frozen-thawed semen has a relatively short life span and therefore requires mares to be inseminated immediately before or after ovulation (within a few hours either side). This requires extra mare management in terms of scans and usually medications. You will need a vet who is available to scan your mare multiple times sometimes every 6-8 hours. Hence the need for a dedicated center. More modern management and the use management protocols that include the use of either hCG or Deslorelin enables the veterinarian to achieve good pregnancy rates by palpating the reproductive tract only once every 12 to 24 hours. (More info in part 2 of this breeding series!).

Some stallion's semen does not freeze or thaw well, others do not travel well. It is very important to find this information out as it can become very costly very quickly. The grapevine and vets, should be able to assist you on the fertility of certain stallions.

Now your mare has been inseminated (and hopefully scanned to confirm ovulation) what do you do? Wait…..and keep everything crossed / pray / think positive thoughts / shower her with love in the form of carrots and apples? Whatever it is you want to do, work away…but inevitably you have to wait!! Those two weeks or in the case of embryo transfer 6/7days before the first scan/flush are the longest.

That's a very brief overview of breeding. I will delve more into breeding, AI, embryo transfer (ET) and the mare's cycle in part two.

If you have any questions on breeding please send them to me and I will try my best to answer them in this series of articles.

Heartbeat scan also showing blood flow
Emily Fletcher

Emily Fletcher

30+years experience with horses. I now have a smallholding breeding sport horses as a hobby in Ireland. Much experience in purchasing & export of horses. You can find us at Virtuoso Sport Horses!