Question of the Week: Birth Control for Mares


FoalQ: I own a five-year-old mare, and even though she’s special to me, I know she’s not breeding quality. I’ve heard a lot about overbreeding and unwanted horses, and I want to be a responsible owner. Is there any kind of safe and effective birth control method for mares that I could use for her?

A: It is true there is an unfortunately large portion of the horse population in the US that is unwanted, making reproductive responsibility a very important concept to impart on horse owners. The concept of birth control for females of the equine species is quite a bit different than what is encountered with other companion animals such as cats and dogs in that it is rarely utilized. This is primarily due to the fact that the most common form of birth control for female companion animals, the ovariohysterectomy (the surgical removal of the ovaries and uterus), is an expensive, invasive abdominal surgery for horses. Castrating male horses does not require a trip to an equine surgical facility, but rather field anesthesia and a relatively short procedure that is minimally invasive, requiring only the removal of the testicles, which are outside of the abdominal cavity. Alternatively, removal of a mare’s uterus and ovaries brings not only significantly higher risk of infection and lengthy recovery time, but is cost prohibitive. Therefore, means to control companion horse populations in the US rely solely on the sterilization of the male members of the species.

Having said this, there are chemical means of mare contraception that are available, although not commercially. The Bureau of Land Management (BLM) has been utilizing long-acting contraceptive vaccines to control herd sizes of feral mustangs for a few years now as part of an effort (in conjunction with increasing adoptions) to reduce the growing numbers of these animals on public land. There does not seem to be a push to have this contraceptive vaccine commercially available, most likely due to the differences in goals of the two equine populations: use in mustangs is strictly for long-term population reduction. The US companion equine industry does not yet seem to require this.

Occasionally, mares develop ovarian tumors that produce high levels of sex steroids that result in the mare acting overly masculine to the point of dangerous aggression. These mares are candidates to have their ovaries removed, thereby sterilizing them in the process.

In summary, the companion horse birth control issue (meaning, excluding feral mustangs) rests mostly on sterilizing the male population through surgical castration. For your mare, the best birth control is abstinence; just keep her away from stallions.

— Anna O’Brien, DVM

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  1. This was a great question the answer not so great. I too have a 5 yr old mare I choose not to breed now.I almost feel powerless if all I can do is keep stallions away as a means of birth control.Researchers please hear our cry for a safe and effective vaccine my vet can administor along with other seasonal vacines. If we are to control over breeding and stop the slaughter of those unwanted help those of us by starting there. It’s such a simple solution to both.

  2. I have a very healthy, 1,200 lb, 7 year old Haflinger mare that is the sweetest, most co-operative mare in the world – UNTIL SHE COMES IN ESTRUS – AND THEN SHE BECOMES THE HORSE FROM HELL !!! I am thinking I will start & keep her on Regumate, oral, everyday – rather than deal with her heat cycles. Why don’t they make an implant contraceptive for horses???

  3. It is a sterile (stainless steel I believe)marble that is inserted into the uterous of the mare. The idea is the body will tell the mare it is pregnant and will stop them from coming into heat. We have done a few here at the clinic where I work. I have seen it work great on one mare, did nothing for the second mare and the third mare “expelled” it within days so did not even stay in the uterous of that mare. I guess if it stays in and works, it is a great way to stop heat cycles. Some mare do terrific with a marble, others don’t…. Kinda like woman, some can use birth control pills, shots, etc some ca

  4. The stainless steel marble mentioned in the comments seems like the best idea. Pebbles are used to keep camels from becoming pregnant (same principle). Since several pebbles are used, perhaps several sterile marbles would work better than one. I’ve only read one study done in Nevada comparing two brands of injectable contraception and IUD. They used human IUD’s with less than spectacular results, same as reported by the comment, worked on some, expelled by others. We have feral horses here as well as free-ranging pet horses.


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