America's Wild Horses and Burros—Research to Support Management: Home page

In 1971, the United States Congress passed The Wild Free-Roaming Horses and Burros Act of 1971 to protect, manage, and control wild horses and burros on public lands. This legislation declared these wild animal populations to be "living symbols of the historic and pioneer spirit of the West." It vested the U.S. Bureau of Land Management (BLM) and the USDA Forest Service with responsibility for their management and directed these agencies to manage wild horses and burros for a “thriving natural ecological balance.” Through its National Wild Horse and Burro Program, the BLM manages about 38,000 wild horses and 5,500 wild burros roaming in 179 herd management areas, comprising almost 32 million acres in ten western states. Largely unchecked by natural predators, wild horse populations can grow at rates of 18–25 percent per year and strongly influence their habitat. In addition, wild horses share rangelands with wildlife and seasonal cattle.  

Wild Horse History

The wild horses that roam the west are feral descendents of domestic animals that either escaped from or were intentionally released by early European explorers and later settlers. As a result of both origin and contemporary management, the Spanish or Iberian influence remains strong in some wild horse populations (e.g., the Kiger, Pryor Mountain, and Sulfur Mountain herds). In other populations, escaped or released military, saddle, and draft horses dominated by the Thoroughbred, Morgan, Quarter Horse, and draft breeds have formed broad zones of introgression (intermixing), sometimes with Spanish bloodlines. These populations of mixed ancestry increased to inhabit large areas of U.S. western rangelands.

The primary means of managing herd size has been periodic “gathers.” Most herds were gathered (rounded up) every 3–5 years. Wild horses and burros removed from the range are offered for public adoption. Animals that are not adopted or sold are maintained in long-term holding facilities where they can continue to be available for adoption, or they simply live out the remainder of their natural lives. But decreased adoption demand coupled with holding facilities reaching capacity has forced BLM to evaluate management options and strive for longer lasting solutions to high population growth.

In the late 1990s, the BLM entered into a partnership with the U.S. Geological Survey, Fort Collins Science Center (FORT) to design and implement a research program that would investigate alternative approaches to address population growth as well as other management challenges faced by BLM. The U.S. Geological Survey (USGS) is the research arm of the Department of the Interior (DOI), and in 2000 the Fort Collins Science Center hosted a series of expert panels to discuss the subjects of fertility control, population estimation, herd genetics, habitat assessments, and health and handling issues. Based on reports produced by these expert panels and information from a variety of other sources, BLM, FORT, and U.S. Department of Agriculture (USDA) Animal and Plant Health Inspection Service (APHIS) staff prepared a Strategic Research Plan for wild horse and burro management. The priority for management initially focused on fertility control and population estimation.

Wild Horse and Burro Research
Arial photo of Peaks Herd during a gather.
Aerial photo of the McCullough Peaks, Wyoming herd during a USGS population survey.

More recently, BLM needs have expanded to include broader ecological information to support wild horse and burro management. In 2013, the National Academies of Science (NAS) published a report, (National Research Council 2013) which recommended and prioritized research for the program. In response and support of the NAS report, USGS scientists proposed a number of new studies that are currently in various stages of proposal writing, peer-review, study approval, or project initiation. Proposed research projects include:

1. Non-invasive genetic sampling of free-roaming horses to estimate population size, genetic diversity, and consumption of invasive species.

2. Developing a suitable radio collar or radio tag for feral horses and burros.

3. Development of a population model and cost analysis for managing wild horses (“WinEquus II”).

4. Population demography and ecology of wild horses in two sentinel herds in the Western United States.

5. Demography of two wild burro populations in the western USA.

6. Developing and testing aerial survey techniques for wild burros.

7. Evaluating the efficacy and safety of Silicone O-ring intrauterine devices as a horse contraceptive through a captive breeding trial.

8. Effect of spaying females on the demography, behavior and ecology of a wild horse population.

9. Evaluating behavior and ecology of geldings among a breeding population.

10. Modeling carrying capacity of free-roaming horses.

11. Assessing effects of wild horses, cattle, and wildlife on sagebrush habitat and ecosystem processes.

12. Testing efficacy of contraceptives for female burros in a captive trial.

USGS seeks to continue the partnership and science support for BLM management of wild horses and burros. The resulting findings and products from these research studies will continue to provide BLM with the science necessary to guide decisions at the individual, population, and landscape level.


Sarah R.B. King, Ph.D., Research Scientist, Ecosystem Science and Sustainability, Colorado State University.

Randall Boone, Ph.D., Research Scientist, Ecosystem Science and Sustainability, Colorado State University.

Bruce Lubow, Ph.D., Natural Resource Ecology LaboratoryWarner College of Natural ResourcesColorado State University.

Dan L. Baker, Ph.D., Biomedical Sciences, Animal Reproduction and Biotechnology Laboratory, Colorado State University.

Jason E. Bruemmer, Ph.D., Department of Animal Sciences, Equine Sciences Program, Colorado State University.