Selecting a broodmare

There are several important factors that anyone contemplating breeding should consider:

  • Is your mare suitable for breeding? Traits such as conformation and temperament are highly heritable and should be important considerations when selecting a mare. Mares with serious conformational faults or temperament issues are not ideal candidates. Mares considered suitable would have shown a high quality of performance during their competing career and have an amenable and reliable temperament.
  • Is your mare in good health? Health, body condition, vaccination and worming status can play an important role in the fertility of your mare.
  • How old is your mare? Mares should ideally be bred before 12 years of age. After this, fertility rates reduce significantly and there may be more problems with your mare maintaining the pregnancy and foaling.
  • Has the mare bred before? If yes, have you seen previous foals of the mare? Did she foal with any problems? These questions should be asked if buying a broodmare with the intention of further breeding.
  • Where do I foal the mare? Consideration must also be given to the facilities required. You will need a large foaling box, ideally with foaling cameras available, a ‘nursery’ field that is suitably fenced and good quality pasture. Turnout with a similarly aged mare and foal is recommended.
  • Can I afford it? Stud fees and AI charges, in addition to livery, routine and unexpected veterinary bills can add up to a substantial sum and there is no guarantee that a healthy foal will be produced or if that foal will mature into a quality horse. So careful thought is required from the outset.

Mare Reproductive Cycle – General Information

Most mares have a 21 day oestrous cycle. This is divided into:

  • Oestrus (average 5 – 7 days)
  • Diestrus (14 – 16 days).

The cycles begin at puberty (approximately 18 – 24 months) and continue throughout the mare’s life.


The mare is receptive to the stallion and is said to be ‘in season’. Typical signs of oestrus include:

  • Adopting a urinating stance with the tail raised and passing small squirts of urine
  • Opening and closing the vulval lips. This is known as ‘winking’

During this time one or more follicles within the ovaries increase in size and rupture to release an egg (ovulation). The mare normally ovulates approximately 24 hours before the end of oestrus. The exact timing of ovulation can only be determined by performing repeated ultrasound scans of the ovaries. The maturation of the follicle can be monitored by the vet by successive rectal ultrasound examinations.


The mare is no longer receptive and may behave aggressively towards the stallion. She is likely to put her ears back, swish her tail and may squeal or lash out.

Control of the oestrous cycle:

Mares have a seasonal breeding period which is influenced by factors such as daylight length, temperature and nutrition.

Environmental and other effects (e.g. nutrition, lameness, worming etc) can exert a significant effect on the mares’ reproductive cycle, especially during the ‘transitional period’. The transitional period is the period between winter anoestrous (when the mare does not cycle) and the onset of cyclic activity in the spring. The transition period also occurs in autumn when mares cease cycling and return to winter anoestrous. There are exceptions to this rule, as some mares (up to 30%) will cycle all year round.

The production of hormones that trigger the mare to start cycling is controlled by daylight. Therefore, the natural breeding season in the mare is strongest from May until August, when the days are longest and the weather is warmest.

During the spring months i.e. Feb-April the weather can be very variable and daylight hours are still relatively short. This in turn creates very variable oestrous activity in the mare, with one or more of the following scenarios often occurring:

  • No oestrous behaviour seen
  • Erratic and often confusing signs of oestrus behaviour
  • Oestrous behaviour seen, but no dominant follicle growing and hence no ovulation
  • Long or unpredictable length of cycle

During this period it is often difficult to predict when the mare will ovulate and thus determine the precise time to cover a mare. Because it is not cost effective to use stallions or stored semen during this unpredictable period, it is often wise to artificially manipulate the mare to encourage normal cycling and shorten the transitional period as much as possible. This can be done in a number of ways:

  • Artificially increasing daylight – Studied have shown that increasing daylight to 14-16 hours (natural plus artificial) daily for a minimum of 8-10 weeks can induce normal cycling.
  • Progestogens – progesterone treatment suppresses the release of luteinising hormone (LH) during administration. This allows LH to build up which is stored. When progesterone treatment is stopped, a huge surge in LH induces a strong oestrous which usually results in ovulation. ReguMate® (Altrenogest) given orally once daily for 10-15 days is commonly used during the transitional period. Mares come into season 4-7 days and ovulation occurs 7-12 days after treatment has stopped. We also use progesterone releasing devices which we place in the vagina for 10-12 days. These drugs are shown to encourage ovulation and result in more successful insemination.

Gestation Length:

The gestation (pregnancy) length is 11 months (340 days) but considerable variation occurs with a range of 320 – 360 days, and sometimes even longer.

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