Wild Horses of America Foundation has partnered with the Bureau of Land Management (BLM) to administer the contraceptive Porcine Zona Pellucida (PZP) to select mares of the Onaqui Mountain Herd, which is located in the west desert of Utah.  The goal of this project is to control the herd’s reproduction rate in order to minimize (and hopefully eliminate) the use of the gathers that have been employed in the past to control herd size.

We feel that PZP is the best tool available right now that can effectively manage herd numbers while best preserving the horses’ natural behavior.  Please visit our PZP page to learn more about this important contraceptive.

Our partnership with the BLM is purely as a volunteer to help administer the contraceptive.  The BLM is not paying Wild Horses of America Foundation, our employees, or our volunteers for this work, nor is the BLM reimbursing us for any of our expenses (travel, dart rifle, darts, time, etc.).  Please consider making a contribution to help advance this work.



The Onaqui Mountain Herd Management Area is located near the Dugway Proving Grounds, and is about 90 miles southwest of downtown Salt Lake City.  It takes about 90 minutes to drive there on paved roads.

The two major roads within the herd area are gravel/dirt roads that are suitable for passenger cars when dry.  They are usually well maintained, but do take a beating during the winter.  There are also some secondary dirt roads that require high clearance and possibly four-wheel drive.   (Be advised that all roads in the herd area can become difficult, or impassable, to travel after rain or snow.  We know of four vehicles in the past year that have had to be towed out after getting stuck in the mud.)

There are no services (food, gas, etc.) within the herd area.  Please be properly prepared if you visit.   Depending on your service provider, cell coverage is usually acceptable when you are north and east of Davis Mountain, but is mostly nonexistent northwest, west and south of Davis.  There is also no service in the Simpson Springs area, or to the south.

To preserve the forage for the horses and other wildlife, PLEASE ONLY DRIVE ON ESTABLISHED ROADS.  Not only will your tires damage the range, your tracks may encourage others to follow.  The law enforcement ranger for this area will ticket violators.


Onaqui Herd map



The BLM has established an Appropriate Management Level (AML) of 121-210 horses for the Onaqui herd.  Currently, BLM estimates (Spring 2017) that there are about 420 horses in the Herd Management Area. The goal of the contraceptive program is to maintain a herd of approximately 165-175 adult horses. We are still working on getting an accurate count after the Fall 2019 gather and removal.

The BLM also allows up to 2,600 cattle AUMs on the range between November and April, and thousands of sheep who are passing through the transit corridor that follows the Pony Express Road.

We intend to prove that the herd’s reproductive growth can be controlled through the use of contraception, and then pursue discussions about whether the Appropriate Management Level (AML) can be increased.


Wild Horses of America Foundation is very concerned about maintaining at least the prescribed number of herd members.  This is important for genetic viability, long-term herd sustainability, and in proving to the public and other advocacy groups that the contraceptive program is beneficial for the horses.

Periodically, we count all of the horses that we can find in the field.  While accurately counting horses in the wild is difficult, especially when done from the ground, we feel that this is important information.  Our initial intent was to publish these counts, but some people have expressed concern about this.  So, we are going to keep the numbers private for now.  Our tallies vary greatly depending on the day.  Sometimes we see as few as 10 horses, and once we counted over 170.  The average day is usually between 50-130.

We did an aerial survey in the spring of 2016 since it is very difficult to find some of the groups of horses from the ground, and found quite a few horses we had not seen before.


We had quite the bumper crop of Onaqui foals in the spring of 2016.  You can see pictures here.   As much as we love seeing babies on the range, our goal is to reduce (not eliminate) the birthrate as the PZP program becomes more effective.

This spring (2017) you can see that the PZP treatments are starting to take effect, but we still have quite a few new foals.


The BLM has used helicopter gathers to reduce the size of the herd in the past.   During the 2005, 2009 and 2012 gathers some of the gathered mares were treated with PZP-22 (a PZP variant that was designed to provide a time-release aspect that was intended to counter contraception for about 2 years) and released. Currently, there are about 50 mares in the herd who have been previously treated with PZP.  You can identify them by the following brands:

All treated mares were given the BLM alpha angle neck freeze brand, and:
“BH” hip brand:  Mare was treated in 2005
“1” next to the neck brand and no hip brand:  Treated in 2009
“BZ” or “BH” hip brand and “3” neck brand:  Treated in 2012
“BZ hip brand and “1” and “3” neck brand:  Treated in 2009 and 2012
“BH hip brand and “1” and “3” neck brand:  Treated all three years (5 horses fall into this category)

Here is a link to a printable page regarding PZP Brands.

The brands are unsightly on a wild animal, and we now track treated mares using pictures and physical descriptions.

While one of the side effects of PZP is that it can cause permanent sterilization in some mares, it is fairly rare, and usually only has the potential to happen after 4-5 years of consistent treatment. The majority of mares who have been treated with PZP will return to having normal fertility levels. Many of the mares on the Onaqui range, who have been treated with PZP and marked with freeze brands, have delivered healthy foals.

Here is a picture of a “PZP mare” and her foal:

The young filly (Norma Jean) in the picture was born to the branded mare (Norma) in November 2015.  Her older brother (Norman/Frank – far left) was born in the Fall of 2014.  The mare was recently (March 2017) treated with PZP, but given her schedule we suspect she was already pregnant.  (Note:  Out of season foaling is one of the claimed possible side effects of PZP.  But, it is considered to be fairly rare, and difficult to determine since wild horses who have never been treated with PZP sometimes have “out of season” foals.  However, it is one of the reasons we plan to try to amend the PZP protocol currently being used.)  Update:  Norma Jean had her first foal in the spring of 2017.


We suggest you read the following BLM documents pertaining to the fertility control plan for the Onaqui herd as there is a lot of good information in them:

Environmental Assessment

Decision Record

Findings Of No Significant Impact (FONSI)

The primary goal is to reduce the reproductive rate of the herd so that helicopter gathers are not used to control herd size.  Selected mares who are at least 18 months of age will be treated until they are 5 years old.  Then, treatment will be suspended until each mare has produced a foal who has survived for at least one year.

The mares are being treated with ZonaStat-H which is an emulsion of PZP and an adjuvant.  The Science and Conservation Center in Billings, Montana manufactures it at a cost of about $26 per dose.  The BLM will be paying for the ZonaStat-H, but Wild Horses of America has offered to cover the cost if needed.

Each mare requires a primer and a booster for the PZP to reach peak performance.  The mares who the BLM previously caught, treated and released (branded mares) only require the booster.  If a mare has been treated previously (for example, the branded mares who were caught, treated and released) then only an annual booster is required.

We gave primers to 12 horses in the fall of 2015, and gave boosters to 37 mares in the spring of 2016.  We plan to treated additional mares with primer in the fall of 2016, and expect to treat approximately 50-70 mares with boosters in the spring of 2017.

Each dose will be delivered remotely, by dart gun/tube, and does not require that the mare be rounded-up and/or held captive.

Our president, Jim Schnepel, took the PZP training course from The Science and Conservation Center in October 2014 and will be the person initially darting the horses from our organization.

We are looking for more certified darters and encourage you to take the course.  Or, if you are certified, please get in touch with us.

Contact us if you have questions or concerns about this contraceptive program.

Please consider making a donation to help support our efforts.