Author Archives: Amy D'Arcey

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.