Managing Mares In Heat – Just for my Horse

Last Updated on February 22, 2022 by Allison Price

You can influence your mare’s breeding cycle for performance reasons or horse breeding.

Some mares don’t change much when they get into heat, while others can be unpredictable due to their sensitive sides and backs. (c) susanjstickle.com

When other horses pass her stall, she stamps and squeals. She is a total forgetter of her manners and will wiggle her tail, trying to nip you when you groom her and tie her up. She’s an absolute airhead when she rides, whining, dancing, and jigging all the while rubbernecking.

Your mare is in heat. How you feel about it will depend on what your plans are for her. You may be happy if this is the year that you intend to breed her. You might not want to breed her if she is days away from a major competition.

Timing is crucial, regardless of the outcome. Karen Wolfsdorf DVM, a specialist on equine reproduction at Hagyard Equine Medical Institute, Lexington, Kentucky, shares her tips and tricks to help your mare achieve her goal of a heat cycle that will result in top performance or a new foal. Below is a detailed explanation of what happens when she cycles.

Unwelcome Behavior
You may find your mare doesn’t change much when she goes into heat. Or she might behave in a way that is so bizarre you wish you had bought a gelding. The majority of mares fall somewhere in between these extremes. She may become more distracted, which you might notice. She might develop a strong attachment to one of her barn friends and whine constantly when that horse is not in sight. She might squeal or kick at any provocation. You may notice her sensitive back and sides. She might also be sensitive to pressure on her legs. As estrus approaches, some mares can be unusually scaredy.

Hormone swings can play a significant role in mare behavior. Dr. Wolfsdorf states that hormone swings are often blamed for a lot of problems, such as lameness or training issues. Talk to your veterinarian and trainer about the issues. Keep track of your mare’s behavior.

Dr. Wolfsdorf states that persistently abnormal behavior may be indicative of other underlying issues than your mare’s period. Ovarian tumors can also occur in mares. The most common, the granulosa cell tumor, can cause testosterone and other hormones to be released that may trigger aggression or “studdish” behavior such as taunting other mares. Sometimes your mare might appear to be always in heat, or not in heat at all. These tumors can be found by your veterinarian using ultrasound, rectal palpation and hormonal testing. They can then be surgically removed.

If your mare behaves in a normal, seasonal, cyclical heat pattern for two weeks, and then becomes agitated the following week, it is likely that her heat is to blame. What is the problem? If her behavior does not pose a risk to others or herself, then the answer will depend on how it affects your plans. You may choose to take it easy if she is difficult to ride or performs poorly during estrus. You may choose to use hormone therapy to regulate her heat cycle if she is unable to follow a training or competition schedule.

Hormone Therapy
Your veterinarian can prescribe medication to control the cycle. They will be able to help you decide what is best for your mare. These medications won’t cause long-term fertility problems and most of them mimic the hormones that your mare naturally produces. This means they don’t need to be regulated as competition drugs. Dr. Wolfsdorf says that every mare is unique, so some approaches might work better for yours than others.

Progesterone. This hormone is dominant during diestrus, and prevents the mare from going into heat. There are many types with different effectiveness. These medications should not be given to mares who have had a history or infection of the uterus.

Regu-Mate(altrenogest) is a liquid oral medication that provides a synthetic source of progesterone. It will not prevent your mare from going into estrus if she is given a daily dose. She will likely go into heat in five to ten days after you stop. It is easy to administer and works well with most mares.

Regu-Mate can be used to temporarily ensure your mare is not in heat for big competitions. You can also keep her on it from spring through fall.
This medication costs about $2.50 per day and can make it expensive to keep a mare on it.

Regu-Mate can be mixed into feed and is typically given as a dose syringe. The hormone can be absorbed through your skin so you should use nonporous gloves to handle it. You should not do this if you are pregnant.

Progesterone injections can be given to the muscle and take effect in 24 hours. Dr. Wolfsdorf states that the injections can delay heat for anywhere from five days up to a month, depending on the type and composition of progesterone as well as the administration vehicle (the substance that contains the active ingredient) used and the mare’s response. The human contraceptive Medoxyprogesterone Acetate

Although Depo-Provera can be given occasionally, studies have shown that it is less reliable than other forms progesterone in suppressing estrus in mares.

Progesterone injections can be used to control your mare’s season-long condition. They are more convenient and cost-effective than oral medication depending on how frequently you need them.
You need to know how long the product lasts in mares so that you can give her heat protection.
Dependent on the dosage and the administration vehicle, swelling at the injection site could be a side effect.

P&E. Estradiol 17B (a type of estrogen) can be combined with progesterone to help behavior problems that progesterone alone cannot solve. Dr. Wolfsdorf explains that this happens in mares because progesterone slows the onset heat, but doesn’t stop the ovarian activity, so follicles may still develop in the eggs. Adding estradiol suppresses follicular activity.

P&E can be ordered from compounding pharmacies. It is available intramuscularly as a daily injection or in a slow-release formulation that lasts for approximately 10 days.
Implants (Synovex), which are made up of progesterone, estradiol and other hormones, are designed to increase cattle’s weight. However, these implants don’t control estrus. They are placed under the skin and release very low levels of hormones for up to 100 days. Colorado State University researchers discovered that mares can still become heat-resistant even with multiple implants.

Oxytocin. Also used to suppress heat. Dr. Wolfsdorf states that it can delay estrus for up to 30 days.

Oxytocin extends the lifespan of the corpusluteum by continuing to produce progesterone and keeping the mare from heat if administered at the correct time.
It has few side effects and may be less expensive than other hormone treatments.

Your vet will help you to comply with the protocol. It calls for two intramuscular injections (small amounts) of hormone on the days 7-14 after ovulation.

Other Options
There are not many options to control heat-related behaviors other than hormones.

Dr. Wolfsdorf states that herbal supplements won’t prevent your mare from getting into heat but may help to calm her down. Some mares respond better to certain supplements than others. These supplements are not for everyone. To ensure that the product is compliant with medication regulations, you should pay attention to the ingredients. Some of these products contain herbs such as passionflower, vervain, and valerian. They are also banned by the International Equestrian Federation.

Ovariectomy–spaying–is a last resort. You can have the surgery done in one sitting with local anesthetic or sedation. Dr. Wolfsdorf warns that it is not recommended for behavioral fixes. It is usually used to treat tumors and other pathology. She explains that it may not resolve behavior problems because she is deprived of progesterone from her mare’s ovaries, which is the dominant hormone that helps her stay on track during diestrus. Most likely, you will need to give her supplemental Progesterone.

There may be new treatments in the future. British researchers discovered that small amounts of coconut oil can be used to extend the life span of a mare’s diestrus. This is similar to oxytocin injections. Researchers believe that the combination of fatty acids in coconut oil is the reason. Dr. Wolfsdorf says that although the method isn’t widely used, it could be useful in the future.

If you want to breed
Dr. Wolfsdorf suggests scheduling a breeding soundness examination before the season starts. This will allow you to check for any infections and other issues that could prevent a successful pregnancy. Monitor or manipulate your mare’s breeding cycle to determine when she will be ready.

Verify and confirm. A teaser stallion/colt is the best way to detect heat.

Take the mare to the teaser each day, and place a teaser rail, similar barrier, between them. If she isn’t ready, she might become aggressive and kick or strike at him.
When she is in heat, she will show interest and may squat or wink at her vulva. Some mares can be quiet or shy, while others may show a wide range of signs. You can tease your mare throughout the entire cycle and observe the changes in her behavior. Over time, you will be able to identify her signs.

You will need to have your veterinarian monitor your mare and determine where she is in her estrous cycles without a teaser.

Regardless of the reason, your veterinarian should confirm that your mare is in heat by performing transrectal palpation or ultrasound. The mare should be bred within a few days of ovulation. The timing of heat can be difficult to determine because heat lengths are dependent on the mare and the season. The veterinarian can assess the size of the developing foetus, its softness and swelling as well as the openness of her cervix by using ultrasound and rectal palpation. These findings will help you determine the time your mare will ovulate. Dr. Wolfsdorf states that a mare with a 40-millimeter diameter follicle will ovulate earlier than a mare with a 20-mm follicle or developing edema.

Short cycle. Using hormones for mare heat–shortcycling allows you to plan your breeding better. Dr. Wolfsdorf says that this can be particularly helpful when you need coordinate the delivery of semen and an appointment with your vet for artificial fertilization.

Prostaglandins are used in the treatment of ovarian cancer. They can be either a natural (Lutalyse), or a synthetic (Estrumate). They also break down the corpusluteum and stop progesterone production, similar to the prostaglandin that is produced from the mare’s endometrium. This allows the mare to become heat.

Before administering prostaglandin, your vet will inspect the mare to ensure she is in the right condition. This will usually take place six days after ovulation.
Dr. Wolfsdorf states that ultrasound and palpation are crucial during prostaglandin treatment to determine the size of the follicles. This allows the veterinarian estimate when ovulation will occur and the time that estrus will begin. You will most likely miss your chance if you give prostaglandins without knowing it and then schedule artificial insemination two days later.

Prostaglandins can cause some mares to sweat, while others may become colicky. Dr. Wolfsdorf claims that synthetic prostaglandins seem to have fewer side effect in these mares.
Intramuscular injections of prostaglandins can be given to mares. If the mare is given IV, she will convulse and collapse, which could prove life-threatening.

Get a Headstart
Thoroughbred mares are often bred in March or February to ensure their foals are born close to January 1, which is the official birthday for all Thoroughbred racing Thoroughbreds. Dr. Wolfsdorf explains that you cannot just take a mare out in the spring and expect her heat. The days are too short for her hormones to be activated. She requires at least 15 hours of sunlight each day.

If you want to breed in February, make sure she is under lights. You can increase the amount of light she gets at night by having lights in her stall from 11 p.m. to 11. Dr. Wolfsdorf says that this is necessary because she will be drawn to the darkest corners. After 60 days, most mares will start to cycle.

Hormones can also be used to induce the cycle. This is best done in the transition stage in early spring. Prostaglandins will not help in this stage if there is no corpus luteum. However, a combination progesterone-estradiol might. Follicle stimulating hormone, gonadotropin-releasing hormone and other hormones are also used. Dr. Wolfsdorf states that there are many protocols, which means that not all of them work perfectly.