THE REPRODUCTIVE CYCLE IN MARES

THE REPRODUCTIVE CYCLE IN MARES

The majority of mares cycle multiple times during the breeding season and do not cycle at all in the non-breeding season.
The average cycle length in the mare is 21 days.
In Australia some mares cycle all year round and this becomes more common as you get closer to the equator.

Fillies usually start to cycle at around 12 to 24 months of age depending on their age in relation to the onset of the breeding season.
Other factors such as nutrition and exposure to other horses can also affect when a filly will start to cycle.

THE SEASONAL PATTERN IN MARES

During the year there are three phases of the reproductive pattern:

Cycling phase – when the mare is experiencing regular cycles. This occurs during the longer days of spring, summer and early Autumn.

Non cycling phase – when the reproductive tract has shut down during the short days of the late autumn and winter months.

Transitional phase – when the reproductive tract is becoming active in the spring. The mare can often display prolonged or erratic signs of being in season during this time.

Hormonal control of the seasonal pattern
Reproductive hormones are produced in the brain and reproductive tracts.

The release of these hormones is triggered by the length of daylight the mare is exposed to.

Melatonin is produced by the mare during times of darkness, so the shorter the days the more melatonin produced.
The mares brain produces a hormone called gonadotropin releasing hormone (or GnRH for short).
Melatonin STOPS the production of GnRH in the brain.
GnRH acts on the pituitary glands in the brain to stimulate a whole range of other hormones that control the reproductive cycle of the mare.

In Summary: as the days grow longer (approaching Summer) the mare produces LESS melatonin, allowing the brain to produce more GnRH, in turn “activating” the mares cycling phase.

As many horse owners know longer days also promote the horse to drop its winter coat – for these two reasons many studs artificially increase the horses exposure to daylight by stabling their horses under lights.

THE CYCLING PHASE

The majority of mares will have a cycle lasting an average of 21 days.
Each 21 day cycle is broken into two phases

Oestrus (when the mare is fertile or “in-season”), usually lasting from 4 to 6 days depending on the length of daylight and
Diestrus (when the mare is not fertile or “in-season”), lasting the remaining 15 to 17 days

During oestrus one or more follicle/s matures and ovulates and the mare is receptive to the stallion.

HOWEVER Because oestrus is controlled by ovulation, and ovulation is controlled by hormones, which are controlled by day length – each oestrus may differ in length slightly, rather then using a set number of days its safest to say that oestrus usually ends 1 to 2 days AFTER ovulation.

How to recognize if your mare is in season
‘Normal’ mares who are in season show distinctive behaviour, often referred to as “showing” and includes:

raising their tail and possibly holding it to one side
showing a stance similar to urination but for prolonged periods of time
squatting and frequent urination of small amounts of urine. This urine is often very cloudy and has a strong odour.
Winking with the vulva.
Leaning into other horses, fences or other solid objects.

The FUN thing is that these signs will vary from mare to mare, maiden mares and mares with foals at foot may show no signs at all, Mares with exposure to other horses are more likely to show signs, sudden exposure to a new horse can cause a mare in season to show, and contact with a stallion can cause the mare to be more obvious.

Others may show signs of being in heat when they aren’t!

HOWEVER having made the decision to breed MINI equines we have the added problem that some mini mares will have “silent heats”

Silent heats
While all horse breeds can have “silent heats” it is a lot more common in the minis and basically means the mare comes into Oestrus but does not show any signs.

Many of these mares are having endocrinological oestrus - in other words, the hormones are doing what they should be doing but she just won't show to the stallion or "stand" to be bred.
Some of these mares may show signs with heavy teasing (for up to 15 minutes) – so a quick walk by of the stallion will miss it.

There are mares that will not show any signs of oestrus except for the day they are ovulating, and even then only after heavy teasing. The hand breeding of these mares could well be missed.

For mares that are undergoing oestrus but not showing it, hand serving can be tricky to pull off without your vets assistance in monitoring ovarian activity.

If the vet detects ovulation, and the mare will still not "stand", then A.I. could be an alternative.

Mares who have silent heats may have a better chance of being served by being paddocked with the stallion.

Lactational anestrus is one situation where silent heats are seen, a mare who has foaled may not “show” for 3 months after foaling or sometimes until after the foal is weaned.
Sometimes the mare will show oestrus on the "foal heat" 5 to 15 or so days after foaling, and not at all from there on, and sometimes she will only show oestrus in the absence of the foal and under heavy teasing.

Non cycling mares
Some lactationally anestrus mares genuinely ARE "anestrus" - that is they have no hormonal activity. Often these mares will resume their regular oestrous cycles once the foal is weaned.

Obviously if your mare does this and you want two foals in a row you must breed her very early in the season to have time to wean her foal and re-serve her after weaning.

Besides having foals at foot there are other reasons a mare may not cycle such as:

Poor nutrition (too fat OR too thin)
Hormone imbalance
Irregular weather conditions at the beginning of the season may increase the time taken for proper cycling.
The mares age
The mares general health

Another reason is ovarian cancer, if you suspect your mare is not cycling, get your vets advice.

THE TRANSITIONAL PHASE

The transitional phase occurs in early to mid spring (when day length is increasing). Reproductive hormone levels are increasing but are still not sufficiently high to cause ovulation and regular 21 day cycles.
The ovaries develop numerous follicles which grow and regress without ovulating. These follicles produce oestrogen which causes the mare to show signs of being in season.
Finally, when hormone levels reach a high enough level, one of the follicles will mature and ovulate.
The mare then starts cycling at regular 21 day intervals.

How to recognize the transitional mare
Transitional mares often display constant or erratic signs of being in season. One mare may show signs of being in season for 3 weeks. Another mare may show signs of being in season for 2 days, not in season for 6 days, then back on again! These mares are receptive to the stallion but are not fertile as the follicles on their ovaries are still not maturing to ovulation.

Because over breeding mares can increase the risk of uterine inflammation and infection and as these mares are not fertile, breeding is futile.

MANIPULATING THE REPRODUCTIVE CYCLE

Some studs like to manipulate their breeding mares to have foals born earlier then they could be if the mares were left to their natural cycles.

Light
Day length is very important for determining the seasonal pattern in which mares cycle. By exposing a mare to 16 hours of light for 8 weeks prior to the start of the breeding season, a higher proportion of mares are cycling and able to be bred earlier in the year.

Prostaglandin
Background info: After the egg is released from the follicle, it collapses to become a crater. This crater fills with blood and forms a clot. This clot becomes the corpus luteum (CL). The purpose of the CL is to produce progesterone.

This drug induces a mare to come into season by causing destruction of the Corpus Luteum (CL). By destroying the CL the production of progesterone is stoped and a new follicle develops, causing the mare to start her cycle sooner then she would normally.

This drug will only work in mares that are cycling. If there is no CL there is nothing for the injected prostaglandin to act on.
Also prostaglandin only works if given at the right stage of the cycle. If a prostaglandin injection is given seven days after heat, the mare will start showing the early signs of a new heat in three days. This is six days earlier than if she was left to have a natural heat.

Another very important use of prostaglandin is scheduling the mares' heat for visiting busy stallions or using AI.

Ovulating drugs
The horse breeding industry now uses a lot of ovulating drugs. These drugs mimic the production of the normal reproductive hormones and ensure that ovulation occurs at a predictable time.

The two drugs most commonly used are GnRH (eg, Ovuplant) and human chorionic gonadtropin or hCG (eg, Chorolon). Both drugs trigger follicle maturation and ovulation.
It is important that these drugs are used in conjunction with ultrasound scanning. They do not work unless a follicle is adequately developed to respond to the hormones and scanning the ovaries is the best way to determine this.

Progesterone
Progesterone can be administered to mares to mimic the phase of the cycle where a CL is present. This is useful for stopping mares coming on heat or sending them off. We also use progesterone in transitional mares. It is thought that sending the mare off allows the pituitary hormone levels to build up. When the mare is removed from the progesterone therapy she will come back into season and hopefully go on to ovulate and start normal cycling.