Wild Horse and Burro

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Science and Research

BLM uses the latest science in its efforts to ensure that viable populations of wild horses and burros can thrive on healthy public rangelands. For example, the BLM uses science to monitor rangeland vegetation, soils, water, wildlife habitat, and the effects of wildfire, and the BLM relies on sound science and monitoring data to determine appropriate management levels for wild horse and burro herds. 

As part of its efforts to manage the population growth of wild horses on public rangelands, the BLM has supported the development of an effective contraceptive agent for wild horses since 1978. Over the years, attempts at different approaches -- such as hormone implants, chemical vasectomies, and intrauterine devices -- were tried, but abandoned as ineffective or impractical at that time.

A man prepares medical syringes for wild horses and burros. BLM photo.

Currently the most promising agents are porcine zona pellucida (PZP) vaccines that were developed starting in the 1990s. Two versions of the vaccine are currently in use – one version, known as Zonastat-H, is implemented through ground-darting programs and is only effective for approximately one year. The second version, known as PZP-22, is effective for 1-2 years but must be hand-injected into a captured wild horse. 

In addition to relying on science for its day-to-day management of wild horses and burros, the BLM is investing in a diverse portfolio of research projects to develop new, modern technologies and methods for wild horse and burro management.

The research is being conducted by university scientists as well as scientists from the U.S. Geological Survey.

Research with Universities

Research with Universities to Improve Fertility Control Tools and Methods

In its 2013 report to the BLM, the National Academy of Sciences (NAS) found that no highly effective, easily delivered and affordable fertility-control methods were currently available for use on wild horses and burros. The most promising vaccine, PZP, is limited in the duration of its effectiveness (1-2 years).

The BLM released a solicitation for research projects to develop new or improve existing population growth suppression methods for wild horses.  The following eight research projects were reviewed and recommended by an NAS panel of experts and are consistent with recommendations made to the BLM by its Wild Horse and Burro Advisory Board.

1. Evaluation of a minimally invasive method of contraception in wild horse mares

Recipient: Oregon State University
Summary: A one-year project that will aim to develop a minimally invasive contraceptive procedure for wild horse mares that requires no incisions.

2. Evaluation of minimally invasive methods of permanent contraception in wild horse mares

Recipient: Oregon State University
Summary: A one-year project that will aim to develop a minimally invasive permanent contraception procedure for wild horse mares.

3. Evaluation of a spay method for wild horse mares

Recipient: University of Kentucky
Summary: A two-year project to develop a different approach to spaying mares.

4. Assessment of an existing spay procedure in wild mares as an acceptable method of contraception and wild horse population control

Recipient: Oregon State University
Summary: A six-month project that will determine whether an accepted spay procedure commonly used for domestic mares can be safely conducted on wild horses.

5. Re-immunization of Free-Ranging Horses with GonaCon Vaccine: Effects on Reproduction, Safety, and Population Performance

Recipient: Colorado State University
Summary: A two-year project will focus on further study of Gonocon, an approved and labeled contraceptive vaccine for equids.

6. Development and evaluation of a permanent contraceptive vaccine in mares 

Recipient: Colorado State University
Summary: A two-year study to develop a new, permanent contraceptive vaccine for wild horse mares.

7. Improvements to long-lasting PZP vaccine delivery material for equids

Recipient: Ohio State University
Summary: A four-year project that will attempt to develop a new delivery capsule for porcine zona pellucida (PZP) – a temporary contraceptive currently used in some wild horse herds – that would increase the duration of the vaccine’s effectiveness.

8. Development of a new agent to reduce horse fertility

Recipient: Louisiana State University
Summary: A three-year project for the development of an injectable agent to control fertility in wild horses.

Research with U.S. Geological Survey

Research with the U.S. Geological Survey

Through its partnership with U.S. Geological Survey (USGS), the BLM is undertaking important research aimed at delivering better methods and tools for managing wild horse and burro herds on public lands.  These projects build upon on-going cooperation between the BLM and USGS that is implementing new methods to estimate wild horse and burro population size.

There are twelve USGS projects that have been approved or are on-going:

Collaring & radio marking (1 year): The aim is to develop safe GPS collars for tracking animals to determine habitat selection, movement ecology, population estimation, behavior, etc. GPS tracking might also help locating animals for contraceptive treatments.

Fecal DNA (genetics/population survey) (1.5 years): The study involves the collection and analysis of fecal DNA as a noninvasive method to determine genetic diversity and estimate population size.

Carrying capacity modeling (1 year): This project's aim is to develop a coarse model to evaluate changes in animal carrying capacity in response to changes in vegetation production. The resulting model may help BLM to adapt plans in response to climatic change.

Mare Contraception -SpayVac Pen Trial II (5 years): This pen trial will help determine the efficacy of alternative SpayVac contraceptive vaccine formulations that are potentially longer acting than conventional PZP vaccines.

Evaluating Behavior of Spayed Free-Roaming Mares (4 years): The study will determine the effects of spaying on behavior, interactions, and movement of spayed mares among a breeding herd. The study will also determine the population level effect on herd growth.

Evaluating Behavior of Neutered Males among a Breeding Herd (4 years): This study will determine any effects of neutering on behavior, movement, interactions and changes in habitat selection.

Two studies: Demography of Two Sentinel Horse Herd Management Areas (2 studies, each of 5 years): These projects will provide demographic data sets for use in new population models and serve as control HMAs for neutered male and spayed mare field studies.

Burro Sentinel HMA Demography Study (5 years): The study will involve collecting data on the survival, fertility, fecundity, recruitment, movements, range use, habitat selection and social behavior of wild burros. These data will be used in population modeling.

Burro Population Survey Method Development (2.5 years): This project will test two new population survey methods for wild burros. The existing simultaneous double-observer method, when applied to burros, tends to lead to underestimates of true burro population size.

Evaluate the Use of a Contraceptive Device in Mares (4 years): This work will determine any effects on mare health resulting from the long-term presence of the device. This device has effectively prevented pregnancy in domestic mares during one breeding season.

WinEquus II - Population Model with Cost/Benefit Outputs (1.5 years): This project will develop a model that compares population modeling outcomes and projects the costs, benefits and expected population growth resulting from management actions that involve PZP, removals, spaying, neutering and other population growth suppression tools.

The BLM has requested or is reviewing a proposal for the following project with USGS:

Testing Efficacy of Contraceptives for Female Burros (3-4 years): Contraceptive vaccines have yet to be used on wild burros due to limited research and unknown effects. This study will examine the efficacy of various existing vaccines.

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