Free Birth Control for Wild Horses

The wild or feral horses of the American plains are a hot topic for conservationists, ranchers, and the public at large. Ranchers would like for the horses to disappear because they compete with their cattle for grazing on public lands. The horses are an introduced species, brought here by the Spanish in the late 1400’s. This means they are not a native species and so some think they’re not worthy of protection. Here’s the twist, equines actually evolved in North America, dying out at the end of the last ice age, potentially at the hand of humans. So do they deserve a place in the ecosystem of the North American plains? Or should they be eradicated as a feral pest species?

The one thing that everyone can agree on is that the wild horse population must be kept in check. They have no real predators and there are limited resources available for them so if humans do not manage them they will become overpopulated and starve. Historically the methods of culling the herds have ranged from barbaric to, the public pleasing, rounding them up and adopting them out. The agency that manages the wild horses is the Bureau of Land Management, or BLM. But the BLM can’t keep up with the reproductive rate of the horses and the public has made it very clear that they do not want these horses hunted or slaughtered. So another idea is being tried.

The Porcine Zona Pellucida vaccine is a contraceptive that is being used on the horses. It works by introducing proteins extracted from pig eggs, include what is thought to be the sperm receptor, into the horses systems. This causes the animal to produce antibodies that target sperm, thus preventing the ova from being fertilized [3]. The PZP vaccine has proven to be effective in horses for only two years, which is less than ideal. This limitation has prevented the application of the vaccine to whole herds and caused the BLM to focus on animals being released after being caught. The BLM is funding more research to try and make a longer lasting vaccine, and potentially one that is easier to administer because trapping the horses in order to give them the contraceptive is stressful and expensive, and darting them with vaccines is difficult [1].

One potential cause for concern is that PZP could alter the behavior of the mares. Luckily the vaccine doesn’t alter the hormone levels of the mares like most contraceptives do [2]. A study was done in 1997 on wild ponies on Assateague Island to see if their behavior changed. They looked at the activity budget of the mares, their interactions with the stallions, and their aggressive encounters with other horses. The findings were that there were no statistical differences between the treated mares and the untreated mares and that the PZP caused no change in their behavior [4]. This is good news for managers concerned with how the contraceptive could impact the natural behaviors of the horses.

My conclusion on this topic is that there is potential for PZP for help control horse populations. But in order for it to be cost effective and not too stressful for the animals, we’ve got to find a way to make it last longer. This may mean looking for another drug to use or improving this one. In the meantime I think that managers should continue to give PZP to horses that are captured and released for other reasons. It can only help slow the population growth to do so and the populations will benefit.




1.“Fertility Control.” US Department of the Interior Bureau of Land Management.


  1. Kirkpatrick, J. F., Liu, I. K. M., and J. W. Turner. “Contraception of Wild and Feral Equids. Oct. 1993. USDA National Wildlife Research Center Symposia.


  1. “Porcine Zona Pellucida Vaccine.” PNC project for Wildlife Contraception.


  1. Powell, David. “Preliminary Evaluation of Porcine Zona Pellucida (PZP) Immunocontraception for Behavioral Effects in Feral Horses (Equus caballus).” 1999. Journal of Applied Animal Welfare Science.

14 thoughts on “Free Birth Control for Wild Horses

  1. sberg

    Wow I didn’t know that at all about the wild population rate of the old horses. The birth control thing is great idea if the population is getting out of hand. I agree we need to make it last longer to make it cost effective. How has the population risen? Like what are the actual numbers… That’s what I’m trying to say.

  2. ematth

    I had no idea that wild horses were causing any problems, but that completely makes sense. It’s tough to imagine horses as pests, but if they are competing with cattle and agriculture, I could see this being a huge issue. I agree, it sounds like PZP may be the best “humane” option of managing the horse populations. Although I think coming up with a strategic capture/reintroduction plan could be expensive, especially if PZP is expensive as well. Did you find any information on the production of PZP and whether it is cost effective in the long run?

  3. Lisa Angeloni

    This antibody form of birth control is an interesting one, and it seems like it would provide some benefits (compared to hormonal forms of birth control) of minimizing the additional introduction of hormones into the environment. It’s also really interesting to think about the controversy over wild horses as pests vs. reintroduced members of a native genus. There has been a lot of debate about the broader idea of “Pleistocene rewilding” or the restoration of North American megafauna with the same or closely related taxa (e.g., tortoises, horses, camels, elephants, cheetahs, lions, etc.). If you want to learn more about arguments in favor of this controversial idea, see this article: Donlan et al. 2006. Pleistocene rewilding: An optimistic agenda for twenty‐first century conservation. The American Naturalist 168: 660-681.

  4. reneev

    My question is, don’t the horses live on an island off the coast of one of the Carolinas? If the horses live on the mainland, I can understand the concern for population growth, but if the horses are confined to an island, the lack of space should be controlling the population, and people shouldn’t have to interfere.

    1. Amy D'Arcey Post author

      You’re thinking of the Assateague ponies, the post is discussing the mustangs that range BLM lands all over the U.S.

  5. amajor

    I love that pointed out the pros and cons of this birth control technique for wild horses, there’s not enough papers that talked about solutions that have both pros and cons. But I would point out that there is a native predator that can control the population of wild horses and that is the cougar or mountain lion. That species has been known for having a bit of a habit of going after horses, old and young, tame or wild. While I do believe that reintroducing horse-loving cougars to the locations of wild horse populations in need of control is a solution, it’s not the silver bullet for ALL locations and that’s where the PZP vaccine can work. All the same, great article.

    1. Amy D'Arcey Post author

      A lot of the states that have wild horse populations are east of the current range of mountain lions. Do you think they should be introduced further east?

  6. mplatt

    I didn’t know that these horses were a problem either. Has there been research done on the economic impact of the horses? Also if there is a nutrient limitation shouldn’t that be enough to keep the population in check? Also when it comes to giving male horses birth control the question arises which individuals do you give the birth control to? Selecting individuals could have in impact on genetic diversity and could possibly impact the population. I can see how this could be a useful solution however I believe a lot more research is needed to see the overall impacts.

  7. vgarduno

    We talked about this quite a bit in my NR History and Policy class. It’s a case of the government’s hands being tied as far as control is concerned, because people don’t like the thought of them being hunted or otherwise culled. I think the idea of birth control is an interesting one, but what would be the environmental effects? If human birth control can percolate into ecosystems through wastewater, can the horse version do the same?

  8. ssteele1

    This is an interesting and tricky topic. Of course you want to fix the problem, but you also don’t want to sterilize the population by accident and cause an extinction. I recently visited the Front Range Wildlife Research center where they are doing a study on oral vaccines for ungulates, they are testing various flavors and combinations to see which work the best! The only problem is that with oral vaccines being put out at random, there is no way to control how much one individual eats and no way to ensure all individuals get enough of the vaccine. Maybe if they find a better way for this oral vaccine to work, it could be implemented for the horses!

  9. leorah

    I’ve been aware of the horse issue for a long time, and gone through phases of varying different opinions on the subjects.
    I wonder, have they tried any sort of strategy similar to managing feral cats? If the horses can be rounded up and captured for adoption or culling, why not capture them and sterilize the males and release them?
    Also, in terms of the question posed in the first paragraph, I think it’s really interesting to consider when a species becomes a native? After something is introduced, how long does it have to be around before no longer being considered exotic? This is a key question for conservationist and mangers in a lot of situations, I think.

  10. mberne

    I have never heard of wild horse populations becoming an issue, probably because I myself have never seen a wild horse. It is very interesting that this form of birth control doesn’t effect the hormone levels of the mares, and therefore probably isn’t likely that its going to effect the environment. Does it seem to have any effect on the offspring that they may have in the future?

  11. Savimay

    I think the antibody system is a pretty good idea that won’t cause hormones to be poured into the environment like human contraception efforts. I agree that it would be more helpful if the antibodies lasted longer but I don’t see how that problem could be fixed. I think the only alternative would be to catch and permanently sterilize some selected individuals.

  12. Lacey Humphreys

    I totally agree with you about decreasing the population sizes instead of eradicating them all together. So they dart the horses with a dart gun in order to get the drug into their system? Is there an effect in the ecosystem like human birth control pills cause? I know you said it does not mess with the hormones in the horses, but when they process the drug and urinate it out do you think it can affect other animal or even plant species?

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