Sunday, September 12, 2010

Horse Breeding: When is The Best Time to Breed Your Mare?

Cascade effect is the hormones that control a mare's reproductive cycle are like a waterfall. They are produced in the horse's brain and flow through the horse's body to the reproductive organs. In this report we will endeavour to help you to understand this waterfall phenomenon: what is occurring within your mare and how you can plan your breeding season.

Termed polyoestrous is mares cycle of many times during the breeding season. One cycle is, on average, 21 days long. Within the cycle there is one fertile phase when the mare is considered 'in season'. This normally lasts around 5 days but varies greatly from mare to mare. Fillies usually will start cycling from about 12 months onwards.

The mare's reproductive year can be broken into some phases:

The first is Cycling Phase - when the mare is cycling (each cycle being, on average, 21 days) including the fertile period of approximately 5 days.
The second is Non-Cycling Phase - occurs in winter when most mares don't cycle and are not fertile.
The third is Transition - occurs twice in the year, late autumn and early spring. At these times, the mare can display unpredictable or unusual behaviour. She can also give the appearance of being in season during this transition period, although she is not.


A mare's 21 day cycle is measured from ovulation to ovulation. Day one is the day ovulation occurs. The cycle can be split into two phases:

1. The oestrus phase is the fertile period which lasts from day one until approximately day 5 or 6. It is at this time that the mare is receptive to the stallion.
2. The dioestrus phase is the non fertile period of the cycle. This phase lasts for approximately 15 or 16 days.


A mare's brain and reproductive tract both create hormones that control her cycle. These are sensitive to the amount of sunlight hours in a day. Melatonin is the 'commander and chief ' hormone responsible for a mare's cycle. Levels of melatonin rise when it is dark. This rise stops reproductive hormones being produced.

When it is light, generally for 16 hours or longer each day during the cycling phase, the level of melatonin drops. This allows the hypothalamus to release a hormone called Gonadotrophin Releasing Hormone (GnRH). Let's break this down a moment. Gonads is the descriptive term for the reproductive organs. Trophin is another descriptive word for growing or nourishing. 'Releasing Hormone' is descriptive of the function that GnRH has - namely to release the hormones from the pituitary gland. (The pituitary gland is located just under the hypothalamus, deep in the brain.) In summary, GnRH is the hormone which gives the command to other hormones to release and grow or nourish the reproductive organs. It's that easy!

The oestrogen is released into the blood stream and stimulates the anterior pituitary gland to release more LH, stimulating ovulation.

GnRH triggers the pituitary gland to release two very important hormones:

1. Follicle Stimulating Hormone (FSH) - this causes the ovaries to grow the follicle (the egg).
2. Luteinizing Hormone (LH) - this causes the follicle to mature and, in turn, start to release oestrogen.

The oestrogen is released into the blood stream and stimulates the anterior pituitary gland to release more LH, stimulating ovulation.


Internally, oestrogen changes the lining of the uterus so it is best able to support the fertilised follicle. Externally, oestrogen is the hormone responsible for your mare behaving in season. Signs can include:

- becoming more docile,
- raising her tail; often to the side,
- standing like she wants to urinate, often for many minutes,
- squatting and urinating frequently,
- heavy, sweet smelling urine,
- winking vulva, and
- leaning into horses, fences and other objects.

Of course signs vary from mare to mare. Some exhibit strong symptoms, others none at all. Mares who have never foaled before, or which have foals at foot, or are kept by themselves tend to show fewer signs. When an in season mare meets a new horse, her behaviour will often become more pronounced.


The follicle should burst through the side of the ovary leaving a hole. Despite sounding rather gory, this hole (called the corpus luteum) is very important. As it mends (around day 14), it produces progesterone which enables the final phase of the cycle. If the mare is not pregnant the uterus will produce a hormone called prostaglandin. This causes the corpus luteum to dissolve and the cycle to begin again.


When the follicle is successfully penetrated by a sperm, the follicle becomes an embryo. The embryo moves extensively around the uterus for the first 15 days. This is essential - as the embryo bounces off the walls of the uterus it stops the uterus from producing prostaglandin. If there is an area of the uterus where the embryo cannot get to (most likely due to a blockage of some sort), then that area will produce prostaglandin which will terminate the pregnancy and the mare will go back into season.

If the uterus doesn't produce prostaglandin then the embryo will implant into the wall of the uterus at about day 16. Your mare is now pregnant!

The mare is the key.....decide why you want to breed your horse and what will be the intended use of the foal? Consider size. Do you want your foal to be bigger or smaller boned than your mare? Consider height. Is your mare a good height for the activities you plan to do?

Consider your mare's conformation. Does she have a back that is too long or a neck that is too short or crooked legs? Find a stud with the ideal or opposite traits to improve or compliment your mare.

Understand your mare's personality. Is she hot, sensitive and spooky, or is she calm and laid back? If you like her personality traits breed her to a stud with the same characteristics; otherwise, find a stallion that has the personality traits you're wanting.

Determine how the horse will be bred. Will she stay at the stud farm for a certain amount of time or will she come home directly after the breeding? Consider the grazing fees - these often mount up quickly.

Determine if you want a live coverage, fresh, chilled or frozen AI? Determine the stud fee and what happens if you mare does not conceive or loses the foal during or following the pregnancy. For example many studs offer a live foal guarantee - find out what this means to the individual stud - it can vary.

Determine where the foal will be born. Do you have shelter for the mare in bad or cold weather? Does the stallion owner require that you have the mare at a place where she has a foal alarm on to be foaled down and an attendant for you to receive your live foal guarantee (LFG) if something goes wrong?


- Have the mare in great condition - not too fat or too thin as both will cause issues getting your mare in foal.
- Feet trimmed and shoes off.
- Good worming regime so she is in good internal health.
- No health issues i.e. temperatures or runny noses.
- Teeth filed.
- Up to date with all relevant vaccinations.
- Send her up with an old halter - often they don't come back with your one on!
- All breeding info - name of mare, breeding, registration number, vet identification completed if required.
- Certify the mare if you want to register the foal with an organisation such as the Hanoverian Society or Racing Thoroughbred Studbooks. Ensure your mare complies with their requirements.
- Make sure you are aware of all the costs involved.
- Read and sign the stud's contract - any questions, ask (and don't be shy about it)!


Ovulation usually occurs in the last 24 hours of your mare being in season but this can be variable from mare to mare. Ultrasound scanning has increased the reliability of picking the best time to serve your mare. Stallion semen will last for 48 hours in a mare's reproductive tract. Making sure the mare will ovulate within those 48 hours will increase your confidence that pregnancy should occur. If there is no way to regularly scan your mare, most studs will serve every second day while your mare is in season.

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