Breeding (Mares) in Horses

Breeding (Mares) in Horses - Symptoms, Causes, Diagnosis, Treatment, Recovery, Management, Cost

Most common symptoms

Aggression / Diarrhea / Frequent Urination / Increased Urination / Separation Anxiety


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Most common symptoms

Aggression / Diarrhea / Frequent Urination / Increased Urination / Separation Anxiety

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Breeding (Mares) in Horses - Symptoms, Causes, Diagnosis, Treatment, Recovery, Management, Cost

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What is Breeding (Mares) ?

Although mares only get pregnant in the spring, usually coming into heat several times between April and October, the preparation often begins months in advance. Horses typically come into their first heat shortly after reaching their first year. However, breeding is usually delayed until the horse is at least three years old for the health and safety of both the mare and the foal. Proper nutrition, good body condition, and preventative health care can go a long way in ensuring that a healthy pregnancy occurs.

Mares who are carrying a foal should be in the best condition possible. This generally includes evaluation and treatments that begin several weeks to months ahead of the actual breeding.

Symptoms of Breeding (Mares) in Horses

Although some mares show almost no signs when they come into heat, most will have indications that they are ready for breeding. Signs that your mare has come into heat and is conducive to cover can include:

  • Aggression
  • Anxiety
  • Frequent urination
  • Kicking
  • Laid back ears
  • Loose bowel movements
  • Movement of the vulva
  • Nervousness
  • Squatting or dropping hips
  • Squealing
  • Tail raising


  • Daylight - The estrus cycle in horses is triggered by lengthening daylight, so it is important that your mare gets enough light to trigger the cycle

  • Nutrition - Mares that are either under or overweight are less likely to conceive than ones who have a body condition score of five or six; this means that the ribs are not visible, but they are easily palpable
  • Preventative care - Mares that you are planning on breeding should ideally have preventative measures like vaccinations, dental work, and deworming several weeks prior to breeding; this is done in order to ensure that any parasites have had time to be eliminated, that there are no dental problems to interfere with the mare’s nutritional intake and to increase the number of antibodies transferred to the foal through the colostrum

Causes of Breeding (Mares) in Horses

There are several sexually transmitted diseases that should be checked for and eliminated before a horse is allowed to breed. Some of these diseases can include:

  • Contagious Equine Metritis (CEM) - This bacterial infection is passed to mares through the stallion, who has no symptoms; it can cause severe endometritis in the mare when it is present, preventing or complicating pregnancy

  • Equine Infectious Anemia (EIA )- A retrovirus that can cause high fever, severe anemia, and even death; once a horse is infected, either through sexual transmission or the bite of a mosquito, they can infect other horses
  • Equine Viral Arteritis (EVA) - This disease can cause flu-like symptoms and spontaneous abortion and can be spread by contact, particularly sexual contact; both mare and stallion should be tested for EVA before breeding

  • Equine viral arteritis (EVA) - This viral disease is highly contagious and causes fever, respiratory issues, and edema; these symptoms can lead to a spontaneous abortion of the foal, and this disease can be passed to the mare either through live cover or through artificial insemination as it is not destroyed by the cold

Diagnosis of Breeding (Mares) in Horses

A pre-breeding exam is essential to ascertain that the horse is fit for pregnancy. This will include several tests to verify the general fitness of the horse as well as her reproductive health. 

  • Physical examination

    - The examining veterinarian will check the horse’s physical confirmation as well as get samples of blood and urine for standard tests like a complete blood count, urinalysis, and a biochemical profile 

  • Vulvar exam

    - Poor confirmation of the vulva can increase the chances of infections and inflammation, which can reduce the chances of fertilization

  • Vaginal exam

    - The vaginal exam can help to determine where in the estrus cycle the mare is, if discharge or urine are caught in the area, and to confirm the ability of the cervix to close

  • Rectal exam

    - The rectal exam allows the veterinarian to evaluate the status of the ovaries, uterus, and cervix

  • Ultrasound

    - This test is utilized to better visualize the ovarian structure, as well to establish the length and positioning of the cervix

  • Uterine biopsy

    - Uterine biopsy may be used to further evaluate the health of the uterus and its suitability for carrying a foal


Treatment of Breeding (Mares) in Horses

In almost all cases, some form of preventative treatment will take place at the same time as the pre-breeding exam. This usually takes the form of vaccinations often accompanied by deworming treatments. Horses that are found to have intrauterine infections will be treated with antibiotic or antifungal medications, and any swelling is likely to be addressed with anti-inflammatory medications. The mare’s weight and diet will be evaluated to see if any adjustments need to be made in order for her to be at the optimal body condition for covering, and abnormal cycling may be able to be corrected with the administration of supplemental hormones. 

Mares who have been diagnosed with certain types of troubles with their vulva may have a surgical procedure known as a Caslick’s operation. This operation basically sews a portion of the the vulva shut to block out particles like mud, feces, or urine in order to prevent infection. In order to breed the vulva will need to be cut open again, and resewn after breeding. It will also need to be reopened for foaling. This procedure can only be done a limited amount of times.

Recovery of Breeding (Mares) in Horses

Once it is time to breed your horse there are several things you can do to improve your chances of pregnancy occurring:

  • Add Vitamins A and E to the mare's diet to maximize the reproductive condition
  • Choose a compatible stallion
  • Keep a good record of previous heats to better determine the most fertile days. 

If you are still having trouble producing a foal from your mare, you can consult a veterinary professional known as a theriogenologist. Theriogenologists specialize in animal reproduction and obstetrics and have an advanced knowledge of obstetrics, gynecology, and andrology.

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Breeding (Mares) Questions and Advice from Veterinary Professionals