“Population Control”

This section is added to address the questions we receive about the subject of “population control.” (published in 2014, update required)

Please read” PZP, the Elephant in the Room https://wildhorseeducation.org/12928-2/

A key concept prior to any discussion on “population control” would be a discussion on it’s necessity. This article is not that conversation. This section has been added to address the questions on methods.

Another key component to discussion on use of any method of population control is genetic diversity. Genetic diversity is a complex subject and will have a section of it’s own added. Yet we feel it important to add a section of a BLM document on genetic diversity, added without edit:

“How does the BLM maintain genetic diversity when wild horse or burro herd size falls below the recommended minimum size?

If a recommended minimum herd size of about 150-200 wild horses (50 effective breeding age animals) cannot be maintained, a number of acceptable options exists to mitigate genetic concerns: maximize the number of breeding age in the herd (age 6-10 years); adjust the sex ratio in favor of males to increase the number of harems and effective breeding males; introduce 1-2 young mares every generation (about every 10 years) from other herds living in similar environments. A significant number of our HMAs are adjacent to other HMAs and interaction/movement occurs between them which allows for the maintenance of genetic diversity as well.”

our note: the above statement can only be accurate if data is available that supports each statement and that statements made adhere to the law. We have some specific issues with the statement above yet it is a good place to begin a discussion on “population control.” The information is supplied so that the public has access to basic information for discussion purposes. It does not imply endorsement of any of the noted methods.


Population control is a phrase used to describe the methods that are utilized by the BLM to achieve what they term as “Appropriate Management Level,” or “AML.”

Removal operations, such as helicopter roundups and bait trapping, are classified as population control methods.

Birth control methods, such as PZP, spayvac and gelding, are also classified as population control.

Jackson Mountain Roundup June 2012, TRO won to keep the agency accountable to their own policy on foaling season

Helicopter Roundups

Roundups are a familiar method used by BLM to achieve AML. They are controversial as the agency currently operates without any standard for humane handling.

Congress passed the Wild Free Roaming Horse and Burro Act (WFRH&B) in 1971 as a response to public outrage over the practices of “mustanging.” Mustanging often involved the use of aircraft to run horses to wranglers on the ground where they were roped or corralled. These animals were often left hog-tied in the desert and picked up by “canners” that took the animals off to be processed into fertilizer or chicken feed. Wild horses and burros were being taken unregulated and were literally fast disappearing from the public lands.

Many Americans are astounded that the agency is to protect these animals from “capture, branding, harassment and death.” Yet roundups are engaged by the government that “capture, brand” and do not have a policy in place to minimize risk of “harassment or death” during their own operations.

Public mustanging was curtailed. However many feel that the government simply engages in sanctioned mustanging with the “new profiteers” being those employed through government contracts for removal and warehousing wild horses and burros.

We also know that broad scale removals increase reproduction rates.


Birth Control

Birth Control used on wild horses and burros is less understood by the public.

Discussions about birth control in wild herds has stemmed around the use of PZP-22, spayvac, gelding and recently at the Wild Horse and Burro Advisory Board meeting in 2012 the possibility of spaying wild mares was raised.

PZP-22 is currently the most widely used form of birth control on wild horse populations.

From USGS contraceptive research:

In order for sperm to attach to the ovum and fertilize the egg, there must be complementary proteins on both the surface of the sperm and the zona pellucida (ZP) of the ovum. PZP acts as a foreign protein against which the treated mare produces antibodies (thus, the PZP fertility control agent is actually a vaccine). These antibodies attach to the mare’s zonae sperm receptors on the ovum and block fertilization. Domestic pig ovaries (obtained from slaughterhouses) are minced and the PZP is obtained from screening filtration. An adjuvant is mixed with the PZP to enhance its effectiveness when it is injected into mares intramuscularly. Once injected, it causes an immune response, making the mare infertile. Over time, the antibody titers fall and fertility returns. With the liquid vaccine, a booster injection can be given at 10 months to raise the titers back to the infertile range. This can be done each year for at least 4 years, after which time the effects may be more likely to become permanent. For this reason, current individual-level field trials involve only 1–4 years of treatment.

Until recently the actual application of PZP-22 on most wild horse populations was ineffective due to a multitude of factors. The primary factor being insufficient number of mares treated.  The trials were done in areas where populations were smaller and more easily identified and tracked. When application of the drug was used in more vast areas both data collection and application were insufficient.

Application of the drug was used whenever a roundup occurred, not as recommended, causing an increase in “out of season” births. The significant factor here is that foals were being born at times of year when not only severe weather impacted survival but the ability of the mare to recover resource was impacted. In other words she was giving birth and nursing when she should have been reserving body mass to survive the winter.

Application in the field was never done with any rigorous follow up. Only about 4% of wild populations were actually treated. Most often this had no follow up vaccine. PZP is an immune contraceptive (non hormonal) and reapplication is key to effectiveness.

As PZP is most effective the second year after application, and the rate at which the current formula wears off is variable, this method of birth control requires roundups to occur at two-three year intervals to facilitate application.

In the field, when used appropriately, we have seen birth rates drop to around 6%, mares retaining healthy weight in winter and births occurring at normal intervals. PZP may be a short term solution to slow population growth as the data to appropriately determine AML, boundary lines, corridor systems, genetic diversity are determined.

(PZP native is NOT used in anyway to include appropriately in this discussion, although efficacy and safety appear to be demonstrated. Most often associated with the term “darting.”. This drug may be the logical, cost effective choice. NO costly helicopter roundups and the ability to collect additional data required to make management decisions is a necessity)

Sex-Ratio skewing

Sex Ratio skewing has been regularly implemented with PZP use (remember usually done on less than 4% of population). The natural ratio of males to females on the ranges is often an unknown prior to the commencement of a removal operation. Sex ratio skewing will be done based on estmates of populations left on the range. A 60-40 (males to females) is often attempted. This practice  may in fact contribute to over-breeding. Mares, capable of estrus, regardless of age, will become ‘prime real estate’, including the very young and the very old. Very young fillies may be flighty and lack “calm mind” necessary to be good mothers; moreover, while capable of estrus, they are still physically immature, and the stress of pregnancy could kill both mother and infant. We also know that “nature” creates the balance required for each specific circumstance and “skewing” is “unskewed” naturally over time.

Spayvac™ (source) is another form of birth control that is being mentioned in planning documents.

The antigens in SpayVac™, porcine (pig) zona pellucida (PZP) proteins, cause a treated female mammal to produce antibodies (immune response) that adhere to the surface (zona pellucida) of her own ova (eggs) and prevent sperm from binding, thus blocking fertilization. SpayVac™ is made up of three components: the antigen (PZP extracted and purified from pigs ovaries), liposomes (cholesterol and lecithin), and an adjuvant, to stimulate the immune response. 

In practical terms, the most important difference between SpayVac™ and the conventional PZP vaccine is performance. SpayVac™ is the only vaccine to achieve multi-year contraception with just one administration. 

Adjuvants are materials that are incorporated into vaccines to enhance the immune response. Trials of the conventional PZP vaccines have routinely used Freund’s Complete Adjuvant (FCA), which is recognized as the “gold standard” of adjuvants and is used widely in vaccine research. FCA has also been used in trials of SpayVac™. Although FCA is a very powerful adjuvant, some individual animals may respond adversely, and vaccines containing FCA cannot obtain regulatory approval. Fortunately, SpayVac™ performs well with another adjuvant, AdjuVac, which has regulatory approval. Although we may continue to use FCA for research, other adjuvants, with regulatory approval, such as AdjuVac, would be used in SpayVac™ formulations intended for routine use. 

The National Academies of Science had a webinar on the use of Spayvac. AWHPC reported that there are serious side effects that leave animals vulnerable to infection.

In April of 2012 the BLM began a study (in conjunction with USGS) at a long-term holding facility in Pauls Valley on SpayVac.

“The goal is to see if SpayVac®, a novel formulation of a glycoprotein called porcine zona pellucida (PZP), will provide a longer-term effect than other PZP vaccines currently used by the BLM.  If the vaccine is found to reduce foaling in this controlled setting, it will be considered for use with free-roaming horses to help control population growth rates on the range.”

Yet on September 4, 2012 the Lander field office in Wyoming recorded a Record of Decision that includes the use of SpayVac prior to the trial completion being done at the above mentioned facility. This decision effects the horses of the North Lander Complex. The operation is expected to begin on November 4, 2012 according to the BLM schedule.

This demonstration of a planning process that occurs without sufficient information to make any conclusion based on “the health and safety” of the horses seems to appear commonplace within the agency. The BLM study will not be complete for five years, yet the horses existing in the wild will essentially be an experiment in practice that will most likely not involve appropriate monitoring and data collection.


Gelding, or castration, is when the testicles of a stallion are removed. This eliminates the ability to breed yet it also eliminates the behaviors associated with a breeding animal.

There have been recent Court filings on using gelding as a population growth inhibitor. Those cases were filed by AWHPC and to date have been successful in their efforts to keep the agency from releasing large numbers of geldings to the range. These cases are based on the premise that “wild horses” are to be managed in their natural state and geldings do not exhibit natural behaviors. Also introducing a large number of geldings inhibits a wild population from “reproducing itself,” a basic accepted definition.

Geldings, by their nature, will not contribute to a ‘thriving ecological balance’; ‘thriving’ infers growth, which geldings will not contribute to. In addition, the presence of geldings further stresses AML limitations. They count against population numbers and forage allocations, while contributing little to herd health and social structure.

In a BLM document available online gelding is classified as inefficient: “Research has shown that while neutering males can slow population growth to a minor extent, a single intact stallion can breed a large number of mares. Therefore, the BLM continues to concentrate its research on finding an effective and long-lasting fertility control agent for mares.”

Spaying of mares

This procedure has not been studied in a wild horse population.

Spaying has occurred in the field on wild mares under jurisdictions other than BLM, such as USFWS (Fish and Wildlife) in areas such as the Sheldon National Wildlife Reserve. The procedures done at Sheldon vary from removal of ovaries to hysterectomies done through the rectum. No data or tracking of animals was done to demonstrate the mortality rates of such procedures.

Our data shows that spaying of mares creates a stressor that has no relief and is detrimental to health and safety of the herd. Our observations were made over a period of time at Sheldon National Wildlife Refuge where vasectomies were also utilized. This created a significant population of males that believed they were reproductive but were not. The constant stress of competition for the reduced number of fertile females was intense.


There are other methods of birth control that have been studied. Yet either due to ineffectiveness, expense, or other reasons have not made the sphere of public scrutiny.

GonaCon or GnRH, primarily used to control deer populations.

“Gonadotropin-releasing hormone (GnRH) is part of a pathway that signals the body to produce sex hormones. GnRH is produced by the hypothalamus, a major organ in the brain. Without GnRH, very little estrogen, progesterone, or testosterone are made.

The aim of a GnRH vaccine is to bind to or “tie up” the GnRH produced within an animal’s body so that it does not trigger reproduction. The vaccine induces the body to make antibodies against its own GnRH. To do this, GnRH is synthesized and hooked to a foreign protein. This new material (called a conjugate because it is made up of two components) looks like a giant new molecule that the animal’s immune system has never encountered. As a result, when the GnRH vaccine is injected into the animal’s body, the body’s immune response neutralizes the hormone’s function, resulting in infertility in both males and females.”

Opinion: In 2009 we attended a seminar that had GonaCon as a focus. We did not understand why this method was not under more scrutiny. We felt this had promise when a strong data component was available and used with a strong directive of genetic diversity, showed promise.

IUD’s, or IntraUterine Devices (from Killian, APHIS)

“Although the 380 copper “T” IUD performed respectably during the first year of study with an 80% contraception rate (Figure 1), its performance as a contraceptive was poor during Years 2 and 3. Because we were able to visualize by ultrasonography the IUD in infertile mares, but in Years 2 and 3 we were not able to visualize the IUD in mares that were <50 days pregnant, we believe that the IUD was expelled from the reproductive tract.”


The above information is supplied so that the public has access to basic information for discussion purposes. It does not imply endorsement of any of the above methods.