Part II: What You Need to Know About Feral Horses in the U.S.

Written by Kylie Field and Claudia Ingham

In this two part series, author Kylie Field, assisted by Oregon State University Senior Instructor Claudia Ingham, walk us through the issues surrounding feral horse populations in the United States.

Part II

In our previous article, you learned about problems associated with the overwhelming number of  feral horses in the U.S.  At this point, you’re probably wondering what the BLM is doing to reduce the estimated population of 95,114 feral horses and burros to their Appropriate Management Level (AML) of 26,715.  Because these horses are protected under the 1971 Wild and Free-Roaming Horses and Burros Act, population reduction, by euthanasia or hunting, is prohibited. Currently, BLM has three major strategies for population control: adoptions and sales, care in long-term holding facilities, and contraception.

 BLM regularly gathers range horses and removes them to off-range holding facilities to reduce their numbers. Some horses are taken to BLM adoption and sale corrals in hopes they are accepted into a loving home but the majority are sent to long-term holding facilities. There are approximately 52,832 horses and burros being kept in off-range holding facilities which includes corral and pastures. Have a look at this 2021 BLM facility report which shows where horses are located off-range across the country. Yes, these are in addition to the 95,114 animals still on rangelands (BLM, 2019; BLM, 2021a). Relocating horses provides the most immediate reduction of horse numbers compared to contraceptive techniques but this is not a sustainable solution to the problem. Care for these captive horses costs American taxpayers about $50 million each year (62% of the program budget) with half of this paid to hold horses on privately owned pastures in the Midwest (BLM, 2020a).  The fiscal year 2021 budget included $116.8 million for the BLM Wild Horse and Burro Program.  For the same period,  the Wilderness Management Budget (for 260 wilderness areas and 491 wilderness study areas) was $16.1 million and the Cultural Resource Management budget (which inventories, protects and stabilizes Native Americans and  Native Alaskan cultural sites) was $15.3 million.  As we consider the multiple use mandate for our federal lands, not all citizens will approve of the prioritization these expenditures represent. 

 It is worth asking whether tax dollars are best spent maintaining feral horses on leased pastures (see above) or maintaining them in corrals.  While purchases of many tons of hay is more affordable than it is for those of us who keep a few pleasure or sport horses, a ton of hay in Oklahoma costs at least $100 with the average horse eating between 3 and 4 tons per year.  Feed is always the largest single item in terms of cost but corrals and their maintenance, occasional vet care and other expenses mean that we are paying at least $1230 per horse/year to maintain animals which may never be sold or adopted and which will not return to range to prevent further damage to natural resources including soil, water and vegetation. 

Getting people to adopt or purchase feral horses is difficult. In 2020, only 3,270 horses were adopted and 1,094 horses were sold into private care (BLM, 2020a). With limited adoptions and sales and more than 50,000 horses waiting in holding facilities, BLM managers choose to use fertility control, developed with research partners at several universities, in conjunction with removals in order to achieve a stable population size.

What contraceptive methods are used in feral horses?

Contraceptive methods must be effective, long lasting, and safe while at the same time requiring little time and cost (Collins and Kasbohm, 2017). Limited funding for contraceptive research and testing puts pressure on BLM to use methods that show promising results in controlled trials and quickly apply them to horses on the range (Kane, 2018). Unfortunately, it’s common for certain contraceptives to be successful in limited settings but yield insufficient results when applied in the wild (Kane, 2018). BLM still has yet to find a contraceptive solution to become the standard for feral horses due to the limited success of the methods in the wild and the expectations of stakeholders for animal welfare (Morell, 2016; Kane, 2018; BLM, 2020b).

Fertility control techniques currently available range from short-term, reversible procedures such as immunocontraception to permanent procedures such as surgical sterilization of females (Kane, 2018).

Immunocontraceptive Vaccines

The most common, and most widely accepted, contraceptive used by BLM for feral horses are contraceptive vaccines. PZP (ZonaStat-H), PZP-22, and GonaCon-Equine are the vaccines the agency is currently using. Contraceptive vaccines are favored by many because they are minimally invasive and fully reversible for a number of years. The only known adverse side effects of the vaccines are transient injection site reactions like swelling and abscesses. (See Table 1 for advantages and disadvantages of each vaccine).


The porcine zona pellucida (PZP) vaccine is an immunocontraceptive vaccine which uses a glycoprotein antigen taken from the ovaries of pigs. When injected into a mare, an immune response causes the production of antibodies to the PZP protein. These antibodies bind to the female egg and prevent sperm from fertilizing it for up to one year, after which it must be re-administered annually (Kane, 2018). PZP has a specific target in the ovaries and leaves other body tissues and hormones unaffected, which means the mare can still behave and cycle normally, and it’s safe for pregnant mares and their foals as well.

PZP-22 is a type of PZP vaccine that is longer lasting than other PZP formulas. It’s effective for at least one year after the initial administration without a 30-day booster and may last up to two years with continued use (Kane, 2018).


Gonacon is in a different category of contraceptive vaccines than PZP.  Unlike PZP, which blocks fertilization, GonaCon inhibits ovulation of the female egg. Antibodies produced by the body in response to the vaccine bind to Gonadotropin Releasing Hormone (GnRH) and prevent it from inducing the hormone cascade which causes ovulation. No ovulation, no fertilization, no new foals being born on the range, right? In reality, it’s not quite so simple. In feral horses, GonaCon has only been 30-40% effective in the first year, but studies show that fertility can be inhibited for 4-5 years if a 2nd dose is given 6-18 months after the 1st vaccination (BLM, 2020b). Only a few studies have been done following horses treated with GonaCon over multiple years, so the results are still inconsistent and more research needs to be done to figure out the best schedule for booster shots (Baker et al., 2018). The possibile multi-year efficacy of the GonaCon vaccine has caused BLM to pursue its expanded use as a temporary contraceptive treatment on the range.

Contraceptive vaccines can be administered by hand or darted into the body from far away (Kane, 2018). According to BLM (2020a), the timidness of feral horses towards humans, large herd sizes and vast management areas make identification and darting difficult in most instances and giving the vaccine by hand is often the most reliable method. Gathering and restraining feral horses for vaccinations by hand is time and labor intensive and becomes more difficult each following year as the horses learn to flee away from humans (Kane, 2018). More research needs to be done to increase the longevity of the vaccines so that the horses can be gathered less frequently. Despite minimal adverse implications for the horses, the performance level of these vaccines is still too low to be effective as the sole method for controlling the feral horse population (Bechert and Fraker, 2018; BLM, 2020b).

Surgical Sterilization

BLM uses permanent sterilization in some feral horses because it is the most effective method of controlling fertility and reducing population growth (BLM, 2020b). Long term costs for the management of feral horses would be lower using permanent sterilization and it may be more humane for the horses to be gathered and treated only once compared to the multiple gathers required by temporary contraceptive methods.

There are several methods of surgical sterilization that are performed on feral horses by BLM as a means of permanent contraception including vasectomy and castration for stallions and spaying in mares (Scully et al., 2015; HSUS, 2020).  Surgical vasectomy and castration are the only methods of contraception proven to prevent reproduction in stallions; however, they are only effective in treating individual males and not controlling reproduction at the population level, therefore, they are not considered a reasonable tool for BLM in population management (Eagle et al., 1993; Scully et al., 2015). Due to BLM’s need for an effective, long lasting and low cost method for reducing the feral horse population size, the agency is now putting more focus on restricting mare fertility through a surgery that removes the mare’s ovaries (Cureton, 2019; HSUS, 2020).

Sterilization is contested by some stakeholders, such as the Humane Society of the United States (HSUS, 2020), because they believe that mares will be held in corrals for longer periods of time than if immunized and that post-surgical complications will compromise these animals’ well-being. However, recent research conducted at the Sheldon National Wildlife Refuge in Nevada is teaching us otherwise. A multi-agency team of managers and researchers employed a combination of stallion vasectomies and mare ovariectomies (Collins and Kasbohm, 2017) which cost $125 and $150, respectively.  Horses were observed 3-8 days post-procedure with only 2% showing signs of complications. After release, the horses were observed by aerial survey several times and 88% of the animals were recaptured for examination in following years. No differences were seen in treated and untreated animals and social group associations of males and females were maintained. Thus, it was concluded that their well-being was not impaired while fertility rates did decline as desired.

In order to effectively reduce the numbers of feral horses on public rangelands, land managers will need to employ all the practical, humane and affordable tools at their disposal. Funding research and development efforts towards a sustainable fertility control method is critical to conserving America’s feral horses at an appropriate population size. Prioritizing the goal of reaching AML (appropriate management level) for this one species on rangelands would help us to achieve multi-use goals while conserving natural resources upon which we all depend. 

What are people doing to help?

Here are a few ideas of how people have taken action: 1) support programs which take in feral horses, or click here and look into adopting a horse for yourself (BLM, 2021b), 2) contact your elected officials and ask them to allocate funds for sustainable population management programs and, 3) talk with your friends and horse-keeping associates. The issues related to feral horse and burro populations and their management are not well-known by many, even amongst us “horse people”.  Educating one another is our most effective way to draw attention to and someday resolve this issue.

Kylie Field graduated from Oregon State University in 2021 with a Bachelor of Science degree in Animal Science. She plans to attend veterinary school and continue studying equine science. In her free time, Kylie enjoys riding horses and spending time with her pets at home.  
Claudia S. Ingham, PhD is a Senior Instructor in the Department of Animal and Rangeland Sciences at Oregon State University where she teaches ethics related to domestic animal use and rangeland ecology including the management of grazed landscapes.  Her research on targeted grazing to manage Himalaya blackberry and English ivy with goats is published in Invasive Plant Science and Management.

Claudia holds lifetime certification with the American Riding Instructors Association (ARIA) in the disciplines of Recreational Riding, Eventing and Dressage and teaches clients near her home in Marion County, Oregon. For riding and pasture consulting questions, she can be reached at:
Literature Cited in This Article (Parts I & II)
Provided by Kylie Field

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Scully, C.M., R.L. Lee, L. Pielstick, J. Medlock, K.M. Patton, G.H. Collins, and M.A. Kutzler. 2015. Comparison of chemical and surgical vasectomy on testicular activity in free-roaming horses (Equus Caballus). J. Zoo. Wildl. Med. 46: 815-824. doi: 10.1638/2014-0227.1.

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