Wild horse strategies ineffective and costly, committee finds

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An independent report has condemned the strategies used by the Bureau of Land Management to control wild horse numbers, saying it promotes a high population growth rate and is economically unsustainable.

The major report by the National Research Council, set to play a key role in the shaping of future wild horse management strategies, warned that continuing the “business as usual” approach would become increasingly expensive and unproductive for the bureau.

Free-ranging horses from the Onaqui Herd, near Dugway, Utah. © BLM/Utah

The report provided support for the stance of many wild horse advocacy groups, which have long been critical of the bureau’s mustang management strategies, but some sounded caution over plans to markedly increase the use of contraception to control herd numbers.

The report said removing wild horses from their western rangelands and maintaining them in long-term holding facilities was both economically unsustainable and incongruent with public expectations.

It said tools already existed for the bureau to better manage horses and burros on healthy ecosystems, enhance public engagement and confidence, and make the program more financially sustainable.

The committee that wrote the report determined that most free-ranging horse populations were growing at 15 percent to 20 percent a year, meaning the populations could double in four years and triple in six years.

With no intervention by the bureau, the horse population would increase to the point of self-limitation, where both degradation of the land and high rates of horse mortality will occur due to inadequate forage and water, committee members said.

In addition, periodic droughts, many of them severe, in the western public lands cause immediate and often unpredicted impacts.

There is little if any public support for allowing these impacts on either the horse population or the land to take place, and both go against the bureau’s program mission, the committee said.

However, the current removal strategy perpetuates the overpopulation problem by maintaining the number of animals at levels below the carrying capacity of the land, protecting the rangeland and the horse population in the short term, but resulting in continually high population growth and exacerbating the long-term problem.

To manage populations without periodic removals, widespread and consistent application of fertility control would be required, the committee determined.

Three methods in particular – porcine zona pellucida (PZP) and GonaCon for mares, and chemical vasectomy for stallions – were identified as effective approaches.

Overall population-size estimates of horses (A) and burros (B) on the range (which may be underestimates), the number removed from the range, and the number in holding facilities from 1996 to 2012 (for years available). Source: Population-size estimates and numbers of animals removed and in holding facilities provided by BLM.

“The committee recommended these approaches based on the evidence of their efficacy with other populations, notably the horses on Assateague Island, but cautioned that scaling up use of these methods to the larger and more disseminated horse populations in the western US will be challenging,” said Guy Palmer, a veterinarian with Washington State University and chairman of the study committee.

The committee also strongly recommended that the bureau improve and standardize its methods to estimate population size, stressing the importance of accurate counts as the basis for all management strategies.

A large body of scientific literature suggested that the proportion of animals missed in current surveys ranged from 10 percent to 50 percent.

Additionally, an examination of the genetics and health of population groups as well as of the rangelands they occupied could be used to assure that both the animal populations and the ecosystem were being appropriately managed.

Developing a process whereby public participants could engage with bureau personnel on data gathering and assessment would increase the transparency, quality, and acceptance of the bureau’s decision-making process, the committee found.

The study was sponsored by the US Bureau of Land Management. The National Academy of Sciences, National Academy of Engineering, Institute of Medicine, and National Research Council make up the National Academies. They are private, nonprofit institutions that provide science, technology, and health policy advice under a congressional charter. Panel members, who serve pro bono as volunteers, are chosen by the Academies for each study based on their expertise and experience and must satisfy the Academies’ conflict-of-interest standards. The resulting consensus reports undergo external peer review before completion. A committee roster follows.

Key committee messages:

• Management of free-ranging horses and burros is not based on rigorous population-monitoring procedures. At the time of the committee’s review, most herd management areas did not use inventory methods or statistical tools common to modern wildlife management. Survey methods used to count animals were often inconsistent and poorly documented and did not quantify the uncertainty attached to counts.
• On the basis of information provided to the committee, the statistics on the national population size cannot be considered scientifically rigorous. The links between the bureau’s estimates of the national population size and its actual population surveys – the data that underlie these estimates – are obscure. The procedures used to develop population estimates for the herd management areas from counts of animals are not standardized and frequently not documented. It seems that the national statistics are the product of hundreds of subjective, probably independent judgments and assumptions by range personnel about the proportion of animals counted during surveys, population growth rates, and other factors. As a result, the bureau’s reported annual population statistics, which are based on the assumption that all animals are detected and counted, probably underestimate the actual number of animals on the range.
• Developing and using a centralized relational database that captures all data on animal counts and removals generated by bureau’s field offices and by animal processing and holding facilities would provide a clear connection between the actual data collected and the reported statistics. To improve transparency and public confidence, survey data at the herd management area level, as well as procedures used to modify it to generate population estimates, should be made available to the public.
• The majority of free-ranging horse populations on public rangelands in the western United States are growing 15 to 20 percent a year. The committee reviewed the ages of horses removed from the range during the years 1989 to 2011 and found that these data can provide a reasonable assessment of the general growth rate of the horse populations. That growth rate was supported by the published literature the committee reviewed.
• Management practices are facilitating high rates of population growth. Free-ranging horse populations are growing at high rates because the bureau’s removals hold populations below levels affected by food limits. If population density were to increase to the point that there was not enough forage available, it could result in fewer pregnancies and births and lower young-to-female ratios and survival rates. Decreased competition for forage through removals may instead allow population growth, which then drives the need to remove more animals.
• Predators will not typically control population growth rates of horses. Because predators such as mountain lions and wolves are not abundant in herd management areas, the potential for predators to affect free-ranging horse populations is limited. Mountain lions require habitats different from those favored by horses, and the committee was unable to find any examples of wolf predation on free-ranging horses in the United States.
• The most promising fertility-control methods for free-ranging horses or burros are porcine zona pellucida (PZP) vaccines and GonaConTM vaccine for females and chemical vasectomy for males. This conclusion is based on criteria such as delivery method, availability, efficacy, duration of effect, and potential for side effects. Although applying these methods usually requires gathering horses and burros, that process is no more disruptive than the current method of population control – gathering and removal – without the further disruption of removing animals. Considering all the current options, these three methods, either alone or in combination, offer the most acceptable alternative to removing animals for managing population numbers.
• Management of horses and burros as metapopulations is necessary for their long-term genetic health. Genetic studies of horses on 102 herd management areas show that the genetic diversity for most populations is similar to those of healthy mammal populations, although genetic diversity is not static and could change over time. Little is known about the genetic health of burros; the few studies that have been conducted reported low genetic diversity compared to domestic donkeys. To achieve an optimal level of genetic diversity, managers could consider the collective populations of several herd management areas as a single population. Management options include intensively managing individuals according to their genetic makeup within herd management areas, translocating horses and burros among these areas, or both.
• Recording the occurrence of diseases and clinical signs and the age and sex of the affected animals would allow the bureau to monitor the distribution and prevalence of genetic conditions that affect population health. Such data have not been recorded and integrated to date. Some conditions that cause genetic mutations in horses are not lethal, so it is possible for the mutations to increase in frequency in herd management areas, especially if inbreeding occurs. Surveillance of these mutations would be possible if blood or hair samples are collected from horses during gathers. Over time, regular sampling would reveal whether a particular herd management area has a higher occurrence of a given mutation that might affect the fitness of the herd.
• The Wild Horses and Burros Management Handbook lacks specificity. Issued by the bureau in 2010, the handbook provides some degree of consistency in goals, allocation of forage, and general habitat considerations. Currently the handbook lacks the specificity needed to adequately guide managers on establishing and adjusting appropriate management levels – the number of horses and burros the bureau deems appropriate for a given herd management area. It does not provide sufficient detail on how to conduct various kinds of assessments. In addition, the handbook does not clarify the important legal definitions related to implementing and assessing management strategies for free-ranging horses and burros, leaving these concepts uninformed by science and open to multiple interpretations.
• How appropriate management levels are established, monitored, and adjusted is not transparent to stakeholders, supported by scientific information, or amenable to adaptation with new information and environmental and social change. Appropriate Management Levels are a focal point of controversy between the bureau and the public. Standards for transparency, quality, and equity are needed in establishing these levels, monitoring them, and adjusting them. The public should be able to understand the methods used and how they are implemented, and to access the data used to make decisions. In addition, data and methods used to inform decisions must be scientifically defensible. Appropriate management levels must be adaptable based on environmental change, changes in social values, or the discovery of new information.
• Resolving conflicts with polarized values and opinions regarding land management rests on principles of transparency and public participation in decision making. Participatory decision-making processes foster the development of a shared understanding of the ecosystem, an appreciation for others’ viewpoints, and the development of good working relationships. Thus, the bureau should develop an iteractive process between public deliberation and scientific research and co-design the participatory process with representatives of the public.

This map shows Herd Management Areas managed together or with U.S. Forest Service Wild Horse (or Burro) Territories as complexes. Aggregating neighboring Herd Management Areas on which free movement of horses or burros is known or likely into Herd Management Area complexes can improve data quality and enhance population management. Herd Management Areas shown in white are not managed as part of a complex. Source: Mapping data and complex information provided by the Bureau of Land Management.