Op-ed: Wild horse contraception not without unintended consequences

By cassandra nunez, jim adelman and dan rubenstein as published in the Salt Lake Tribune

“The article below is shared unedited and in it’s entirety.  The content bears consideration but we at Straight from the Horse’s Heart and Wild Horse Freedom Federation take issue with those who interchange the words “wild” with “feral”.  Such lack of insight and sensitivity demeans the validity of the author’s knowledge of the prehistoric origin of North American equines but none the less, there are several good points made, here, that are worth sharing and further exploring.” ~ R.T.

Terry Fitch of Wild Horse Freedom Federation photographing the Wild Horses of Assateague ~ photo by R.T. Fitch
Terry Fitch of Wild Horse Freedom Federation photographing the Wild Horses of Assateague ~ photo by R.T. Fitch

In a recent op-ed, Jay Kirkpatrick suggested that our research on the side effects of contraception in feral horses was conducted at an “unusual location,” implying that it is uninformative for managing feral horses. Although we agree with Kirkpatrick that contraception is the best option for managing feral horses in the U.S., we disagree with several of his statements about our work and encourage readers to evaluate this science for themselves.

Our research focuses on Cape Lookout National Seashore, N.C. This region is ecologically similar to Assateague Island, where Kirkpatrick has studied contraception with porcine zona pellucida (PZP) for decades. Our writings argue that PZP is highly valuable and effective, but, as with any good tool, can be misused. We maintain that any “science-based and workable strategy for helping horses,” as Kirkpatrick puts it, should include analyses of PZP’s unintended consequences. Below, we address three of Kirkpatrick’s specific points, referencing relevant studies.

First, Kirkpatrick states that horse advocates have relied on “data regarding the small and unusual horse population at Cape Lookout…”

In actuality, this population may not be so “unusual.” PZP-treated females at Cape Lookout change social groups more often, display more reproductive behaviors and experience more male harassment. Similarly, treated females from three populations in the western U.S. (Colorado, Wyoming and Montana) received more reproductive behaviors from males. Such results have not been seen on Assateague. The fact that behavioral changes were documented in four out of five populations raises the question, is Cape Lookout “unusual”?

Second, Kirkpatrick challenges horse advocates to “identify wild horse populations where PZP has disrupted the social structure or social behaviors of the horses. (By definition, this means the disappearance of harem groups, bachelor groups, social hierarchy or other fundamental social behaviors.) Explain why this hasn’t even happened in … Cape Lookout … “

Although Kirkpatrick is a highly accomplished scientist, he is not a behavioral ecologist and his definition of disruption does not reflect a consensus among animal behaviorists. Behavioral ecologists have long considered mare fidelity, group stability and reproductive behavior crucial to the well-being of natural feral horse populations. As referenced above, if we include these behaviors, several populations have shown important behavioral changes with PZP treatment. Suggesting that important behavioral changes must include massive reorganizations of a species’ social system, as Kirkpatrick does, sets an unrealistic standard.

Finally, Kirkpatrick asks horse advocates to “identify any wild horse populations where PZP has increased the length of the foaling season and resulted in decreased foal survival. Include Cape Lookout … “

PZP has altered the foaling season in several populations. At Cape Lookout, mares treated repeatedly with PZP gave birth over a wider range of months and later in the year than did untreated mares. In western populations, previously treated mares also gave birth later in the season, even after stopping PZP treatment.

Kirkpatrick is correct that data on these foals’ survival is not available, reflecting an important and open question. It is reasonable, however, to hypothesize that animals born later in the season, when fewer nutritional resources are available, would not fare as well — an established principle across diverse species.

Again, we agree with Kirkpatrick that PZP is the best means currently available for managing feral horses in the U.S. His recent op-ed accurately highlighted several important benefits of PZP, including increased body condition, increased longevity and, critically, the need for fewer roundups. However, dismissing research that identifies PZP’s unintended consequences also dismisses opportunities to optimize wild horse management. Our response merely serves to clarify our research and reiterate our position that as with any valuable tool, PZP’s use should be carefully and continually evaluated when possible.

Cassandra Nuñez is adjunct assistant professor of Natural Resource Ecology and Management at Iowa State University. Jim Adelman is an assistant professor in the Department of Natural Resource Ecology and Management at Iowa State University. Dan Rubenstein is a professor at Princeton University who studies the behavior and ecology of horses, zebras and wild asses.


  1. Interesting but i too agree that feral horses is not the term that should be used.. I too take exception to it… Feral is for stay cats, not for horses.


    • I agree with you on not calling the horses feral they’re wild as in wildlife. But, I do take offense at you calling all stray cats feral, they are NOT! People leave pet cats behind when they move or just toss them out when they don’t want them any more, etc., etc. I take care of a TNR colony, which started from one cat my neighbors left behind. She was very mistrustful of humans because of being tossed out. Some are lost pets that we try to return to their homes, and the kittens we come across and can socialize we find homes, barn homes, etc.


  2. I don’t think anyone doubts that PZP is one of the tools that can be used for reducing some wildlife populations – including wild horses and burros. But we are not talking here about the need to control the populations of abandoned and scavenging and starving dogs on the streets of Tijuana and other cities.

    We are talking about wild horses and burros who have legally designated lands set aside for them under the laws of Congress. In 1971, when Congress passed the Wild Free-Roaming Horses and Burros Act, these animals were found roaming across 53,800,000 million acres. That amount of acreage could support more than about 250,000 wild horses and burros but even after 22,200,000 acres were stolen from the American people by government agencies the remaining 31,600,000 acres could support more than 100,000 wild horses and burros today. Although wild horses and burros may not be re-located to other public lands where they were not found roaming when the law was passed, there is no law that states that wild horses and burros cannot be moved from current locations onto other legal herd lands. It is currently independently estimated that less than 20,000 wild horses and burros are living on their legal land today and yet the government continues its aggressive removal and destructive management toward total wild horse and burro extermination.

    There is NO reason for these wild horse and burro removals and destruction procedures … because there are NO excess wild horses and burros on their legally designated land.


    • I agree, vehemently. There is no reason to remove them. Everyone Federal who makes these decisions sees and hear ONLY tired, repetitive rhetoric without questioning it’s validity. All the standard language and data is based on no sound science that we can find, and until someone looks at the false figures and failed policies with an unjaded eye, population ‘control’ will continue to be nothing more than snake oil…
      It may not be a kind way to view this particular assertion when it comes to managing wild horses with PZP, but really, how advantageous is the ‘longer life’ gifted by PZP here in the West, when the expectation of dying on your own range is whittled down by a management policy that continually expresses – there isn’t enough food and water to support that long life?


  3. First, the Public must be given the TRUTH
    There is NO overpopulation of Wild Horses OR Wild Burros

    The following is a public statement issued by International Society for the Protection of Mustangs and Burros.(ISPMB)

    ISPMB is the oldest Wild Horse & Burro protection organization in the United States.
    Velma Johnston (Wild Horse Annie) was the first president.
    ISPMB has been working with Princeton University to study Wild Horse herd and reproductive behavior

    There is NO overpopulation of wild horses and burros

    Wild horses and burros are NOT the cause of habitat degradation of public lands.

    Wild horse and burro populations have been nearly CUT IN HALF since 1971 when Congress declared they were “fast disappearing from the American scene.”

    Approximately 75% of the herds have LESS than viable numbers of animals for future sustainability. Each herd must have 150 effective breeders according to Dr. Gus Cothran, leading equine geneticist in the world. (effective breeders constitute only those animals breeding which omits the very old and the very young in the count)

    In 2009, Dr. Cothran reported studying 123 wild herds which 20% of THEM showed a critical or near critical loss of genetic diversity.
    We must STOP the BLM AT ALL COSTS from suppressing wild horse and burro populations any further and especially through permanent sterilization.

    For the PAST 100 YEARS, due to overgrazing by livestock, our public lands habitat has been in static to downward trend with few areas improving.


  4. From an interview with Karen Sussman

    In the International Society for the Protection of Mustangs and Burros

    At one time you administered PZP to the mares in two of your herds? What is the most startling thing you learned about PZP to cause you to stop using it and to speak out against it? Do you plan to research what happens to mares when they are no longer given PZP?

    : The BLM must remove nearly all the horses (approximately 90 percent) within a Herd Management Area to give the drugs to the mares, which actually destroys the family units once again defeating any purpose of fertility control. Secondly, instead of nature deciding who breeds and who does not, it now lies within the realm of humans to determine who carries on the genes. This has not worked in the domestic horse industry and this will create failure within the realm of wild horses because only the horses and nature knows who should survive and who should not. By this intrusive management of PZP, we will eventually destroy the true nature of wild horses.

    ISPMB will have the future to determine the effects of PZP on the last two herds in our conservation program and information will be forthcoming. We know that permanent infertility happens as early as five consecutive years of treatment. We also know that 4.5 years of treatments produced infertility for two years in our Virginia Range herd.

    We have noticed that in several of the mares who cannot have foals, they have actually stolen foals from mares who may be having their first foal and can’t guard against a more dominant mare. We have had one mare who has stolen four foals in the Catnip herd alone. It is heartbreaking to see how badly a mare really want to have a foal. Nature designed the females to carry their genes forth through birth.

    We have noticed in the Catnip herd that when mares cannot get pregnant that they have “loose” social bonds where there is not group formation. When a foal is born, the mare and a stallion with whom she approves become a family. Without foals, we don’t see that close family bond developing. We are now monitoring the health and well-being of foals born to mares that have received the drug for nearly five years.

    We have also noticed that coming off birth control, the general foaling cycle is interrupted and we are having late season births. Our other two stable herds have foals from March through July. We don’t see the number of late season births in these herds as we have seen in the PZPed herds.


  5. 569.008. “Feral livestock” defined
    “Feral livestock” means any formerly domesticated livestock or progeny of domesticated livestock which have become wild and are running at large upon public or private lands in the State of Nevada, and which have no physical signs of domestication. The term does NOT include horses or burros that are subject to the jurisdiction of the Federal Government pursuant to the Wild Free-Roaming Horses and Burros Act, 16 U.S.C. §§ 1331 to 1340, inclusive, and any regulations adopted pursuant thereto, or any other federal statute or regulation.


  6. I’m for careful application of PZP (PZP here is in reference to Native vs. 22 month PZP). Understanding age, color, how many offspring the mare has out on the range etc. I’m not for since it doesn’t have a sheath were free to dart…You have to keep and maintain daily records if your out in the field.

    I understand branding is against the Wild Horse and Burro Mandate. But I hope you’ll hear me out.

    The reason I’m for branding ALL remaining horses on the range is this. We know BLM says they’re approximately this number of horses and they do a roundup. But they don’t stop when they have their allotted 112 horses (or whatever the number was). They round everyone up and then select a few to be returned back out.

    By branding everyone it becomes clear who has passed and who hasn’t. Who has foaled (or possibly)and who hasn’t. On lesser known HMA’s I believe it just makes tracking easier.

    This way when you dart in the column under XIO123 you mark an X for having darted this horse on such and such date. (That brand no. was an absolute example). By updating this schedule daily the next day another team that comes across this mares family knows not to dart her but maybe another mare who didn’t get darted.

    This way you can also tell if horses are moving from one HMA to another and what the actual count is.

    Within a smaller herd like the Pryors which are followed mostly year round I don’t believe this is necessary. But not every HMA has horses so well known, where everyone has a name, their ancestry has been built and where people stay so much in tune.


    • I guess the really frightening aspect of PZP is giving BLM the go-ahead to use it. If (and that’s a big if) they used it in an intelligent way (think about this – BLM) and actually did enough research & tracking of individual horses to know how many and what horses were out there – it would be one thing. Honestly, I believe that the majority of BLMers (local & Washington) don’t want that to happen because THEN the actual numbers of horses would be right out in the public! And we all are well aware that there are way way less horses in the wild than the BLM will admit to! And I’m betting that at least at the local levels – they know this too.

      Liked by 1 person

      • My brother lives in Utah, and yes you’re right, he believes all the propaganda that state puts out. We got in a big argument about it recently. He told me I was listening to much to the BLM treehuggers, isn’t that a joke!


  7. Perhaps some of the horses are indeed feral where negligent humans have abandoned them, but I would imagine that most of the free roaming horses are wild ones, descendants of the mainly the Spanish horses.


  8. “The Cloud Foundation recognizes the unintended side effects, both positive and negative, of PZP. We believe the vaccine is far better and more human than the alternative—roundup, removal and life-long incarceration. Our Foundation goal is clear—every foal born wild is allowed to live their life in precious freedom with their family. At the same time, we advocate for the protection of the primary predator of
    wild horse foals, the mountain lion. Nature knows best how to maintain that delicate balance between predator and prey. With PZP, we try to mimic the role that
    predators play.”

    Liked by 1 person

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