Foal control


Nevada hosts more than half -- about 17,700 -- of the 33,700 wild horses that roam around federal lands. But Bureau of Land Management rangeland scientists estimate the state can support only 12,700 horses and burros. And if left alone, wild horse herds typically grow 20 percent annually, doubling in size every four years. "We know we have to manage the range,” says Alan Shepherd, state lead for the Nevada BLM horse program. “We have to do something population-wise." Current birth control measures last just one to two years, but longer-lasting birth control, now being studied in Nevada, could be one tool to help address the imbalance.

Wild horses are protected from slaughter under the 1971 Free-Roaming Wild Horse and Burro Act. But in order to protect habitat and wildlife, the BLM needs to get the horses off the range without hurting them. It then tries to find them homes. In 2010, the agency rounded up 10,255 wild horses and burros, but only 3,074 got adopted. The rest are held in corrals or sent to pastures in the Midwest. In 2010, just holding excess horses cost $36.9 million, nearly 60 percent of the agency's wild horse and burro budget.

"The fertility drug itself costs about $300 per dose to give to the mare." Shepherd says, "The way we look at it, every baby that we don't create and have to take off the range later saves us thousands of dollars for removal, adoption expenses, holding. That's money we can use for looking into other fertility control and doing management work."

Jay Kirkpatrick has been studying how birth control can be used to manage horses for more than two decades, starting with treatments to horses on Assateague Island off the coast of Maryland in 1988. Darting 65 percent of the adult mares on the island each year has, over time, reduced the herd from 175 to 112 horses. Without these measures, the herd's population would have increased significantly. It's harder for this approach to work for larger herds on uncontained ranges in the West, but some believe fertility control is still key. Kirkpatrick emphasizes that simply removing horses through gathers will never take care of the problems they cause on the range, because the remaining horses will just breed younger and faster. "Overpopulation, overgrazing, trampling vegetation, competing with livestock and other wildlife -- those are all symptoms of the problem," he says. "The problem is reproduction. The only way you can address any of those other symptoms is by doing something about reproduction."

This year, the BLM is preparing to step up its use of fertility treatments to manage wild horses. The agency plans to treat 2,000 mares, a four-fold increase over the 500 or so treated during the past several years, but still just a tiny fraction of the tens of thousands roaming the range. Meanwhile, researchers from the University of Toledo Medical School are partnering with the Nevada BLM Horse Program to do pen tests of a new version of fertility control that could keep mares from getting pregnant for four years, a treatment that would enable the agency to more effectively manage herds.

The new treatment builds on an existing one that lasts only about a year by adding an additional time-release dose of porcine zona pellucida or PZP, which blocks sperm from fertilizing eggs. The injection includes four pellets that look like quarter-inch-long pieces of colored pencil lead. Each contains a PZP booster wrapped in a special polymer that dissolves and releases the treatment after a set amount of time: 30 days, 90 days, 9 months and 20 months.

If the Nevada pen studies show that the new treatment effectively prevents pregnancy for three to four years without negative side effects, it will help overcome the main scientific hurdle for horse contraceptive treatments -- that is, the need for a single shot, multi-year treatment. But, Kirkpatrick concedes, other hurdles remain including advocacy group and agency opposition to treatments based on social, cultural, political and economic grounds.

Furthermore, while it has helped slow population growth in a few management areas, fertility treatment has yet to prove effective for large, free-roaming herds around the West. The treatments and application are advancing, but much work remains before the BLM will be able to reduce existing herds to appropriate levels and keep them there.

Emilene Ostlind holds an editorial fellowship at High Country News.

Photo of bony wild mare and foal in Nevada courtesy Flickr user James Marvin Phelps.

Marybeth Devlin
Marybeth Devlin Subscriber
Apr 06, 2011 12:43 AM
There is actually a deficit of mustangs on the range. Using BLM's own data, birth-rate assumptions, and removal reports, C. R. MacDonald, a wild horse advocate, has calculated that there could be, at most, only 17,893 mustangs left on the range as of February 28, 2011. The 33,700 figure cited in your article is overstated by nearly double, but such are the numbers BLM references to give the false impression of "excess" mustangs. The only true excess is the number of wild horses in holding pens -- currently believed to be as many as 45,000. Even if BLM reduces its roundups by 25 percent as recently announced, there will be fewer than 10,000 mustangs on the range, and nearly 60,000 in holding, by the end of September 2012. BLM is creating a crisis that puts wild horses in danger of being euthanized or sent to slaughter due to the high cost of keeping them penned up. Cost to keep them on the range: $0.

PZP is both a biohazard and a sterilant. BLM plans to treat captured-but-to-be-released mares with the Porcine Zona Pellucida (PZP-22) fertility control drug. This plan endangers the herds' long-term survival. The PZP contraceptive is not without risk and can have unintended consequences. For instance, if mares are in excellent health and condition at the time they are treated, PZP can cause too strong an immune response, resulting in long delays in restoring fertility or outright sterilization after even the initial injection. Ironically, PZP works less well in mares that are in ill health or poor condition -- they are likely to conceive despite PZP treatment. Thus, the fittest mares don't reproduce while the least fit ones -- the immuno-compromised -- may. For these reasons, much care must be taken when administering PZP. It should not be given to fillies under two years old and never administered for four years consecutively. Multiple injections are more likely to result in loss of fertility, which may not be reversible. That could be one reason why PZP is not used in humans.

PZP does not prevent ovulation and does not change mare behavior toward stallions. This is not good. It results in repeated, stressful, futile breedings of the mares and ongoing battles among stallions. Out-of-season pregnancies and births can occur due to the wearing off of the drug at inopportune times. Foals born at the wrong time of year may not survive, and the mares' health may be endangered as well. Finally, there are reports of mares treated with PZP becoming masculinized, a phenomenon that occurs for reasons yet unknown. This is another cause for extreme caution in using the drug and for switching to a better fertility control agent -- such as GonaCon -- that suppresses ovulation temporarily, eliminates the negative behavioral manifestations, and thus avoids disruption to horse band behavior. In the meantime, if BLM is to use PZP, the safety procedures for administration of the drug need to be tightened and monitored by independent observers.

Here is the link to a study comparing and contrasting the use of PZP ("SpayVac") versus the GnRH agent ("GonaCon") versus an IUD (the latter was ineffective). The study concluded that GonaCon was reversible. PZP was deemed to evidence a permanent loss of fertility.


The meta-analysis linked below was published in the journal "Reproduction." Studies of the side-effects of different wildlife contraceptives, including PZP, were reviewed. [Once on the site, page down to the sidebar on the right of your screen next to "Abstract" and click on "Results" and then on "Discussion."] Among the findings with regard to PZP: Males lose body condition while the oft-claimed improvement in female body condition did not hold up. Further, females experienced increased irritability, aggression, and masculine behavior. The mares remained sexually active beyond the normal breeding season and had more "estrus events." The possibility of "selecting for immuno-compromised individuals" is raised. Finally, the analysis questions the supposed benefit of mares living much longer than their normal life expectancy.[…]/45

Here is the link to a study of PZP versus GonaCon in wild horses. Researchers note that mares in better body condition tend to produce more colts than fillies. Given that BLM's roundup methods tend to capture more females than males and to release more males than females, mass inoculation of mustang mares with PZP will likely lead to further imbalance in the resulting sex ratio of the herds.[…]/IND44438708.pdf

Here is a link to an article on wild horse contraception in New Zealand, with a discussion of PZP versus the GnRH agent GonaCon. The reporter states that the latter is not a biohazard, suggesting that the former is.

Here is the link to a fact sheet on GonaCon and its advantages over PZP. The beneficial attributes include suppression of estrus, thus eliminating the behavioral disruptions associated with PZP.


What population control is superior to both PZP and GonaCon? Pumas. There can be no "thriving natural ecological balance" without apex predators. Mountain lions, wolves, and other such carnivores effectively control wild horse populations by targeting the weak, the sick, the young, and the old. BLM typically cites an absence of natural predators as one reason for excess mustangs.

BLM should stop all roundups, removals, and fertility controls until each wild horse herd's population reaches the level deemed adequate for genetic viability. PZP should be discontinued due to its many risks including sterilization. As mentioned above, PZP's other unintended consequences include: breeding selection for immuno-compromised individuals, masculinization of mares, deterioration of band fidelity, and prolonged battling among stallions over mares that continue to display estrus month after month. Should disaster strike a herd, fertility needs to be quickly restorable. Therefore, BLM should strive to transition to an improved contraceptive such as GonaCon as soon as possible. However, the best approach is for BLM to concentrate on promoting and then protecting native predators to permit natural control of the wild horse population on the range. A puma protection program would actually tend to strengthen the herds and would save costs. Concerned livestock operators should be encouraged to use guardian dogs to protect their animals. There are several specialty breeds that have been developed just for this purpose, and they are reportedly effective. BLM might even consider buying a number of trained guardian dogs, which could be placed, upon permit-holder request, with herds or flocks experiencing attacks.
Timothy R Baker
Timothy R Baker Subscriber
Apr 06, 2011 06:55 PM
I realize it's probably my bias as an ecologist showing, but how can there possibly be a deficit of individuals of a non-native (i.e. feral) animal? That's kind of like saying there isn't enough European beach grass on the west coast because we've been removing some of it for snowy plovers.

As for the rest of the lengthy comment above I'll just make 2 minor points: 1) genetic bottlenecks in feral populations most likely arise from the limited numbers of individuals released or escaped that began the population (it's a similar problem when trying to re-establish endangered animal populations). 2) While mountain lions and wolves are likely to do some predation, they probably aren't large enough animals to function as effective limits on populations of feral horses.

The BLM needs to continue to aggressively limit feral horse population expansion and reduce the impact that this non-native animal has on native wildlife populations.
Marybeth Devlin
Marybeth Devlin Subscriber
Apr 06, 2011 11:47 PM

There can properly be said to be a deficit of wild horses. According to BLM, the range can support 26,600 mustangs -- that is, in addition to the 5,000,000 livestock (non-native species) that graze the same public lands. There are no more than 17,893 mustangs left in the wild, with massive removals still planned. Thus, the wild horse population is 8,707 mustangs short of the miserly number BLM allows and it is declining. For instance, just this past December, Nevada's Clan Alpine herd was found to have 21 fewer members than it did three years ago. Yet BLM had planned to round up and remove hundreds of those horses, incorrectly assuming that the herd must have grown by 20 percent. In fact, the opposite was true.

The mustangs' genetic viability is threatened on many fronts. BLM keeps 75 percent of the herds below the minimum level needed to maintain gene pool diversity. Then it conducts massive removals leaving more males than mares on the range. Next it mass-inoculates the mares with a sterilant, leading to immuno-compromised breeding selection and compensatory reproduction. When a herd weakens and birth defects start appearing, BLM zeroes out that herd. BLM is managing the mustangs right into extinction.

Here is the link to a University of Nevada at Reno article concerning mountain lions and their prey. It begins: "As Nevada’s largest predator, mountain lions share complex interactions with species they prey upon including mule deer, elk, mountain sheep, and wild horses." The article notes that "...our preliminary data indicate that wild horses are a frequent prey item of mountain lions in western Nevada." It goes on to state: "With this project we hope to clarify many of the questions and possible misconceptions surrounding mountain lions and their prey in Nevada. In addition, the importance of mountain lions and interactions with the prey, especially wild horses, are important components of rangeland and mountain ecosystems."[…]/Research_Project.aspx?GrantID=650

Below is the link to a discussion of wild horses' natural predators. It identifies grizzly bears, wolves, and mountain lions as the apex predators that keep mustang populations in check. Eradication of such carnivores leads to overpopulations of prey animals. Elsewhere, I have found references to bobcats and coyotes as additional predators of mustangs, especially of young foals. Wolves hunt in packs and together can bring down animals much larger than themselves. Finally, mustangs tend to be smaller than the average saddle horse, with adults typically weighing between 800 and 1000 pounds.[…]/450-natural-predators

Here is the link to a site that identifies the many challenges facing wild horses as they are currently "managed" by BLM.

Sometimes an issue needs to be addressed at length, even though it involves a lot of research and writing -- and then reading. My reply is mercifully shorter!
Tim Baker
Tim Baker Subscriber
Apr 07, 2011 07:49 AM
I appreciate the passion you show for the horses, however none of the sites or links actually have the science behind them to support the position that we should simply allow the horse populations to expand unmanaged or that they deserve a status equivalent to native wildlife.

The DNA argument that they're native (from one of the sites you linked) is akin to suggesting we should manage golden retrievers as a wild population since they are essentially genetically identical to wolves and actually points to the idea that any genetic bottleneck is from a limited number of feral progenitors not from management actions since the actions do not focus on specific lineages.

The article from UNR is simply a preface for research that is going to be conducted, not conclusions of the research and not apparently designed to assess the importance of horses as prey for mountain lions. The other articles (from horse advocates) assume that wolves, grizzlies and mountain lions can limit horse populations but don't have any evidence to support the assumptions. And given the problems with wolf introductions in the Northern Rockies, it's unlikely that either wolves or grizzlies will be allowed to expand into current horse ranges --leaving just mountain lions which will not be effective limits without dramatic increases in lion numbers -- something else that's very unlikely to occur and would lead to increased depredation on deer and bighorn sheep as well.

To get a different view, check out The Wildlife Professional, a journal from the Wildlife Society and look for the issue before last (I don't have it handy at the moment for a proper citation) for a discussion on horses and their impact. By the way, the assertion that there are 5 million cattle or sheep on the same range as horses seems far fetched at best -- the last data I saw was that there were only 9.6 million AUM's on the entire western public grazing lands though I'll readily admit I don't have the current hard numbers in front of me.

Marybeth Devlin
Marybeth Devlin Subscriber
Apr 08, 2011 01:29 AM

I extend my heartfelt appreciation for your taking the time to read the information I provided and to share your viewpoint. My purpose in responding to the HCN article was to counter the misinformation disseminated by BLM regarding mustangs.

1. BLM's claim of excess wild horses on the range is false. There is no surplus. There are nearly 9,000 fewer mustangs than the absurdly low number that BLM has arbitrarily set. Tellingly, even the Kearns & West review, commissioned by BLM, cited a National Research Council Committee finding of "the pressures which many district and area personnel feel to depict range, population, and other conditions in an antihorse and antiburro context." (Page 10 at link below.)[…]/KW_Final_Plan_5-10-10.pdf

2. BLM's removal and sterilization program does not constitute true management and does not achieve self-sustaining herds as required by the Wild Free-Roaming Horse and Burro Act. Instead, it actually constitutes a back-door extinction program.

3. There can be no "thriving natural ecological balance" without apex predators. I agree that it is politically challenging to re-introduce carnivores that had been intentionally eradicated in the interests of livestock operators. Nevertheless, to restore nature, predators must be given their rightful place.

Here is the link to an article by Dr. Craig Downer, like yourself a Wildlife Ecologist, on the subject of America's wild horses and BLM's mismanagement thereof.

With regard to mustangs being non-native or merely feral, here is the link to a scholarly discussion by Drs. Jay F. Kirkpatrick and Patricia M. Fazio, showing that horses originated here and have just been re-introduced. Thus, wild horses should be treated as wildlife.

The discussion linked below cites 17,617,986 as the number of animal unit months (AUMs) available. If correct, that would equate to just under 1.5 million year-round cow-plus-suckling-calf equivalents for all forage users -- wildlife, livestock, and wild horses. However, most permit holders do not keep their cattle and/or sheep on the range all year. For instance, say a beef producer holds 1,200 AUM slots -- the equivalent of 100 cows grazing all year. Such a permit-holder might actually graze 300 cows-with-calves for four months. Also, remember that there are five sheep per AUM. So, given the same 1,200 AUMs, the corresponding numbers would be 500 sheep grazing all year or 1,500 sheep for four months. Thus, the livestock numbers are higher than might be inferred from the national AUM total and year-round equivalents. Such concentrations of animals, even for shorter periods, have a tremendous impact on the environment. Livestock overgrazing is universally cited by wildlife biologists as being largely responsible for degrading the range. Mustangs are a minor player. But BLM pits permit-holders against wild horse advocates, scaring the former with a potential reduction in AUMs unless the mustang herds are decimated.

The report linked below is, in my estimation, the definitive dissertation on BLM's faulty figures and mismanagement. In the report (pdf-page 12), the authors present the total number of AUMs as 18,366,000 -- which is very close to the figure referenced above. They provide the following BLM distribution: 9,465,000 to wildlife, 8,600,000 to livestock, and 301,000 to wild horses and burros.


Further, as HCN reported just days ago (link below), BLM doesn't appear to care about wildlife that stand in the way of energy exploration / exploitation. The air pollution and water contamination caused by gas wells are widely known. Note how BLM dances around "adaptive management" and twists its meaning. But BLM staffers are only reflecting senior management's direction. Government workers follow the Administration's mandates. I am sure many of them -- the ones who really care -- are just as frustrated with the Bureau as wild horse and wildlife advocates are.[…]h-despite-mule-deer-decline

I accessed The Wildlife Professional journal on-line but haven't yet found the article you recommended. However, if the authors relied on BLM data to base their conclusions, all hope is lost. In "BLM World," mustangs reproduce like rabbits and horses never die. Per "BLM math," no matter how many thousand wild horses are removed from the range, the posted population figures remain the same and may even increase, with the growth characterized as "exponential." No wonder wildlife ecologists fear being overrun by wild horses!

Wildlife and wild horse advocates should be united because we share the same goal: a thriving natural ecological balance. The true threats to wildlife and wild horses include urban encroachment, livestock overgrazing, and mining/drilling/energy development projects. As long as BLM can keep wildlife advocates focused on blaming those few mustangs left on the range and thereby distract them from noticing the damage done by the real culprits, then it can continue business as usual, ruining the range with its irresponsible management hidden in full view.

Looks like I've gone on again too long. But it has been a pleasure having this dialogue with you. Take care. Mb.
Janet  Schultz
Janet Schultz
May 04, 2011 02:39 PM
I only have one thing to say about this article. Alan Shepard - state lead for the Nevada BLM program - states the PZP drug costs $300 per dose to give the mare. Dr. Kirkpatrick in an article published by NY Times on 4/21/09 states the doses actually cost $21 per dose plus $2 for the dart. I hope that Mr. Shepard has acutalized the dosage with the cost of the roundup and personnel and is not quoting the price BLM pays for the drug. Any clarification coming on that?
Emilene Ostlind
Emilene Ostlind
May 05, 2011 09:45 AM
Hi Janet. Good question. I emailed Alan to find out the explanation. Here's his response:

"The commentor is partly right on the price. As I described to you, our primary PZP treatment (PZP-22) involves 2 parts. The first part is the liquid dose which primes the system and it costs about $23. However the second part, which is the time-release pellets, has a cost of about $275. Combined you have a single vaccine dose costing approximately $300. It's
actually more than this due to handling, shipping and surcharges where it is made for us but we generally only use the $300+ to identify our cost of it."

So he's not including the cost of the roundup, just the cost to the BLM of the treatment dose.