Modern breeding methods: what are they and which one is right for your mare? *H&H Plus*

  • Modern reproductive methods may offer a better chance of breeding success, but what do the abbreviations mean – and which is right for your mare? Dr Charles Cooke MRCVS explains

    Breeding has always been based on decisions, from which stallion to use to the method of breeding and where to foal the mare. Recently, these decisions have become more numerous, with the emergence of a range of modern assisted reproductive technologies (ARTs).

    Careful planning remains a must, whichever method is chosen, so that you and your vet can maximise the chances of success. Certain questions still need to be asked beforehand. Is the mare healthy? Does she have any conditions that might prevent her from carrying the pregnancy and foaling? Consideration must also be given to when you want the foal to be born, and the logistics of using your chosen stallion.

    You may also wonder if you can still breed from your mare while she is competing, or whether it will take several years for her to produce the multiple foals you are hoping for. While much will depend upon your mare’s age, health and breeding history, ARTs may broaden your options.

    Mare preparation

    Any breeding process requires the mare to be managed with care. This usually involves pre-breeding health tests, including a swab for contagious equine metritis (CEM) and blood samples for equine viral arteritis (EVA) and equine infectious anemia (EIA), along with an understanding of her reproductive cycle.

    For most mares, there is a transition from the winter quiescent (inactive) state to a regular 21-day oestrus cycle in March/April, in response to increasing daylight hours. Part of the pre-breeding assessment is to check where the mare is in her cycle, so she can be bred at precisely the right time.

    The safety of the mare, handler and vet is paramount, so using stocks and occasionally sedation for examination is advisable.


    Artificial insemination (AI), the most commonly used ART in equine breeding, involves a series of ultrasound examinations and medications to ensure the mare is inseminated close to ovulation (pictured, right). Fresh semen must be used within three hours, but it can also be supplied chilled or frozen. Your choice will dictate how the mare is managed.

    Chilled semen is viable for 24 to 48 hours after collection, so must be ordered and couriered following careful monitoring of the mare. Ovulatory drugs help to ensure the follicle ovulates – releases a mature egg – within the correct time frame.

    Frozen semen must be stored appropriately (pictured, above right) and can be used when needed over a time period of many years. Increased veterinary input is needed with frozen semen, to time the insemination for the best pregnancy rates. The aim is to inseminate the mare within six to eight hours of ovulation, so repeated ultrasounds, possibly overnight, are required.

    Frozen semen doses vary from a single 0.5ml straw to the more usual four to eight straws. The use of the deep intrauterine insemination (DUI) technique is essential for a lower volume, to deposit the sperm at the tip of the uterus – next to the Fallopian tube – where the oocyte (egg) will be waiting.

    With appropriate veterinary management before and after insemination, pregnancy rates for chilled and frozen AI are similar – which increases stallion choice.

    Advantages: AI opens up a worldwide stallion market. However, if you are considering a non-UK stallion, it is advisable to use an experienced semen agent, due to the new Brexit regulations.


    Embryo transfer (ET) involves inseminating a donor mare, then “flushing” her uterus seven to eight days later to collect the embryo (pictured, right). This is transferred into the uterus of the recipient, or surrogate, mare who then carries the pregnancy.

    The reproductive cycles of the donor and recipient must be synchronised, so that the embryo remains in similar conditions. It is recommended that two or more synchronised recipients are ready, to ensure the “best fit” with the donor mare at the time of transfer. However, you don’t necessarily need your own recipient mares, as the embryo can be chilled and couriered promptly to a mare at another location.

    Advantages: ET can be repeated multiple times during the season, increasing the number of foals a mare can produce per year. It is worth considering for a competition mare, to avoid interrupting her career, or for a mare with fertility issues or an injury that would prevent her from carrying her own pregnancy.


    Intracytoplasmic sperm injection (ICSI) is used in conjunction with OPU (see box, right). In this lab-based technique, a single sperm is injected into the matured oocyte (pictured, below), which is held in specialised fluid medium and incubated to encourage the development of an embryo. This can take up to nine days.

    Not all of the oocytes will develop into an embryo, but those that do can be transferred immediately into recipient mares or frozen for future use.

    Advantages: ICSI is an efficient way of using semen, as a single straw can be used multiple times. The technique is advantageous for sub-fertile stallions or where semen stocks are limited, perhaps because a stallion has died.

    Also published in H&H 11 March 2021

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