Reports on Free-Ranging Horses and Burros
Unlike the Committee to Review the Bureau of Land Management Wild Horse and Burro Management Program, the committee for the 1980 and 1982 reports (Committee on Wild and Free-Roaming Horses and Burros) and the committee for the 1991 report (Committee on Wild Horse and Burro Research) were asked to develop and evaluate specific research programs and activities related to free-ranging horses and burros. In 1979, the Committee on Wild and Free-Roaming Horses and Burros was charged to develop a research program that would
- Develop data on the biology of wild horses and burros, including the population dynamics of wild horse and burro herds;
- Identify principles and procedures for managing populations of wild horses and burros in accordance with the policies and objectives of this Act;
- Develop information concerning the availability and use of forage and water resources, dietary and habitat overlaps, and other factors relevant to the determination of the number of wild freeroaming [sic] horses and burros that a herd area can sustain; and
- Provide the Secretaries of Interior and Agriculture with scientific information upon which to make the determination as to excess animals required by this Act. (NRC, 1980, pp. 13-14)
That committee fulfilled its statement of task in a three-phase study. The first phase reviewed the existing knowledge of free-ranging horses and burros and from that developed 18 research projects that it recommended that the Bureau of Land Management (BLM) pursue to fill gaps in knowledge that would contribute to the sound management of free-ranging horses and burros. In Phase II, the committee reviewed requests for proposals that were issued by BLM for research on census methods, habitat selection, range effects, comparative nutrition of range horses and cattle, pregnancy rates in horses and burros, and horse survival rates. The report published in 1982 summarized the results on the six
research topics and reiterated the necessity of a more thorough research program, as laid out in the Phase I report, to inform management decisions related to free-ranging horses and burros. The 1982 report completed the third phase of the study.
In 1985, the Committee on Wild Horse and Burro Research was asked to
- Review research on wild horses and burros completed since 1982;
- Assess the research recommendations of an earlier committee of the National Research Council in light of current issues, and update these recommendations if necessary;
- Develop guidelines to assist the BLM in contracting for additional studies;
- Monitor the progress of contracted research projects; and
- Evaluate the final reports of the research projects and prepare a final committee report. (NRC, 1991, p. 1)
That committee recommended research topics and guidelines that were based on BLM’s identification of high-priority research. BLM funded research on two subjects: free-ranging horse population genetics and control of fertility in free-ranging horses. The National Research Council committee reviewed the research proposals that were submitted on those topics after a BLM request for proposals. BLM ultimately funded one project in each. The committee’s report, published in 1991, reviewed the design and results of the two projects. The study committee formed in 2011, the Committee to Review the Bureau of Land Management Wild Horse and Burro Management Program, was not asked to evaluate specific current research projects funded by BLM or to design research activities and then review the results. Instead, it was charged to use the previous reports and later relevant research to inform an independent evaluation of the science, methods, and technical decisionmaking approaches of BLM’s management program.
COMMONALITY IN NATIONAL RESEARCH COUNCIL STUDIES ON FREE-RANGING HORSES AND BURROS
Although the Committee to Review the Bureau of Land Management Wild Horse and Burro Management Program was not tasked with designing a research program or reviewing specific research projects, its statement of task echoed many of the issues addressed in the earlier reports. Like the committee that prepared the present report, the Committee on Wild and Free-Roaming Horses and Burros examined issues related to genetic diversity, fertility control, population estimates, population growth rates, forage use, and societal opinions. The Committee on Wild Horses and Burro Research looked specifically at the results of a free-ranging horse genetics study and a project on fertility control.
The 1980 report flagged two management issues related to the genetic diversity of free-ranging horses and burros. First, it noted that a population that can sustain itself must have enough genetic variability to survive a multitude of environmental contingencies. Genetic information could be used to determine the size of a sustaining population for a given area that had particular environmental characteristics, and populations could be managed with that size as an objective. Second, the committee recognized that there was considerable disagreement about the origins of the free-ranging herds. One position held that the horses were the descendants of Spanish mustangs. Another took the view that the
horse herds were the result of released or escaped cavalry mounts and work animals. At the time that the 1980 report was written, no data existed to answer questions about the amount of genetic variation within and between populations and the relatedness of free-ranging equid populations to domestic horses and donkeys. Therefore, the report recommended that studies be carried out to determine the genetic variation between populations of free-ranging horses and burros and the genetic similarity between free-ranging equids and domestic breeds.
The research was not carried out in Phase II of the study by the Committee on Wild and Free-Roaming Horses and Burros, but it was one of the research projects overseen by the Committee on Wild Horse and Burro Research. Blood samples were collected from nearly 1,000 horses at seven trap sites in Nevada and Oregon from December 1985 and October 1986. The objectives of the research project were to
- Assess the average and individual heterozygosity in the populations to determine if there has been loss of heterozygosity or inbreeding through genetic drift, selection, removals, or management restrictions;
- Estimate the contributions of the original wild mustangs (descendants of animals released by the Spanish) and the current domestic lineages (13 breeds) to the present feral horse populations;
- Evaluate the several populations for possible divergence in gene frequencies and for the development of population substructure; and
- Determine parentage and particularly paternity within bands to evaluate the proportion of foals sired by the dominant band stallion. (NRC, 1991, pp. 3-4)
The data showed that there was less difference between populations of horses in the Nevada and Oregon herds than there was between domestic horse breeds. It also supported the hypothesis that the free-ranging horses in that region descended from escaped or released domestic draft, saddle, and cavalry animals. Dominant stallions did not sire about one-third of the foals born in intact harem bands. There was no evidence of loss of herterozygosity in the populations studied.
Census Techniques, Population Estimates, and Population Growth Rate
The committee that prepared the 1980 report identified many problems with counting free-ranging herd populations that raised questions about the accuracy of the population estimates and about the rate of growth of free-ranging horse and burro populations.
Herds on BLM land were not counted at a consistent frequency. The frequency of counts of each herd varied between 1 and 11 years, and the number of counts conducted ranged from 1 to 18 for each herd. The herds were not counted at the same time each year, and this negated the ability to compare counts of a single herd in different years. Horse herd population size varied with the time of the year, increasing in spring because of foaling and declining in the rest of the year because of deaths (NRC, 1980). Foaling in burros can occur throughout the year, although it is largely in spring. Counts made before or after foaling in different years are not comparable. In addition, the counting method used was not standardized among herds and was not tested for robustness. Most BLM surveys sought a complete count of the animals from the air, but the ability to count animals was strongly affected by terrain, vegetation cover, and the skill of the census counter and the pilot. To address the variation and inconsistencies, the committee recommended a research project to compare the accuracy and precision of three methods: complete counts, mark-resight
estimates, and strip–transect estimates. It suggested that the agency develop a set of criteria to determine the appropriate approach for a given habitat.
Research conducted during Phase II of the study confirmed that free-ranging horses and burros were usually undercounted, particularly in areas of tree cover and dissected topography. Burros are particularly difficult to see because they tend to stand still in response to an airplane or helicopter, and their coloring does not stand out from the terrain. The 1982 report concluded that mark-resight, capture-recapture, or counting animals immediately before and after a gather would be needed to improve accuracy. It also concluded that contemporary BLM counts of free-ranging horses and burros were conservative. The committee suggested that conducting a count of a herd every 2 or 3 years would be adequate for managing populations at a specified level.
Because herd populations are affected by the rate of reproduction, the Committee on Wild and Free-Roaming Horses and Burros extensively reviewed the available literature on free-ranging horse and burro reproduction. The 1980 report examined the information available about the age at which free-ranging females first successfully foal and compared these observations with those of domestic female horses and burros. It also looked at the fecundity of domestic and free-ranging females over their life span and at how the age distribution of a free-ranging herd affects its growth rate.
The committee found that data on reproductive rates and on survival rates of free-ranging horses were insufficient and resulted in a wide range of population growth rates of herds (NRC, 1980). Burro reproductive and survival rates were also uncertain because of deficiencies in data on foaling rates, foal survival, adult survival, and the response of reproductive activity to environmental conditions (NRC, 1980).
The research funded during Phase II of the Committee on Wild and Free-Roaming Horses and Burros was not sufficient to ascertain the population growth rate of free-ranging horse and burro herds. The committee concluded that growth rates probably vary because of a number of conditions: annual variations in forage conditions due to year-to-year weather changes or longer-term changes in climate, variations in forage conditions related to equid population densities, and variations in forage conditions related to the population density of other herbivores. The committee posited that, because of those factors, population growth rates vary spatially and temporally. The 1982 report noted that more research would be needed to ascertain the relationships between variations in forage conditions and population growth rates.
The committee also investigated whether free-ranging horse and burro populations will self-limit. It noted that there was little information to demonstrate whether populations limit themselves at densities below those which would affect ecosystems or to support the theory that populations will not limit themselves until severe ecosystem damage has occurred. The committee cited a few examples in which members of a population had starved over the winter but added this caveat
We do not cite these examples to imply in any way that these kinds of severe impact are widespread or common in the wild horse and burro ranges of western United States. In fact, we have seen very few areas with heavy vegetation impacts, although we have asked the BLM to show them to us.
Our purpose here is simply to convey our impression that, while there may be some density-dependent tendencies in the demography of these equids, they do not appear effective enough to prevent populations from increasing to the point of significant impact on other ecosystem components. What population control policy this dictates depends on the management goal for any given piece of land. If the goal is solely equid management that is experimental and “natural” as possible, a laissez-faire approach may be appropriate. The
equids and other ecosystem components could be allowed to seek their own balance. But where the goal is a multiple-use one, as set forth in PRIA [Public Rangelands Improvement Act of 1978], and there is concern for the values of other ecosystem components, it seems likely to us that horse and burro populations will need to be limited artificially by human action to avoid undesirable effects on other ecosystem components. (NRC, 1982, pp. 17-18)
The 1980 report did not advocate the use of contraception in free-ranging horses and burros because such a decision was related to policy rather than to science. However, that committee did review the research conducted on equine contraception up to that time and recommended research that could advance the use of contraception in free-ranging horses and burros. Research on a population of burros that included vasectomized dominant male burros had reported that female burros were later bred by younger males. The committee found that reversible endocrine contraception was feasible for both mares and stallions. The report noted that tests looking at the efficacy of fertility suppression in free-ranging and captive stallions over the course of a single breeding season were underway at the time of the committee’s review. When reviewing the options for females, the committee determined that hormone manipulation, intrauterine devices (IUDs), and surgery could be used as methods for equine contraception. However, at that time, hormone manipulation research on female equids was in its infancy, treatments were effective for only a year, IUDs required skilled fitting and frequently became dislodged, and surgery on mares in the field posed a risk of inflection and required too much time. Therefore, the committee recommended a research project that would develop a method of reproductive inhibition in mares that would be at least 95-percent effective, require only one treatment in field conditions, could last for up to 7 years, could be reversed, and would not adversely affect the health or behavior of the animals. Such research was not undertaken in Phase II of the study and remained a recommendation of the committee in its 1982 report. The 1982 report also concluded that sterilization of only the dominant stallions in a harem was unlikely to successfully control populations.
The 1991 report reviewed fertility-control results of steroid treatments in corralled and free-ranging mares and vasectomies in free-ranging dominant harem stallions. Three series of experiments were conducted on corralled mares. In the three series, 18 combinations of hormone treatments were administered to 500 mares via silicone rods. The first series of steroid implants were found to be ineffective in preventing pregnancy. The results of the other two series demonstrated greater success: some hormone treatment combinations contracepted more than 90 percent of the mares through two breeding seasons.
Two of the hormone treatments used on the corralled horses were implanted into free-ranging mares on two different Herd Management Areas (HMAs) in Nevada. Foaling rates were determined with aerial surveys. The combined foaling rate in observed mares treated with either hormone implant on the two HMAs in 1988 and 1989 was 9 percent, (22 of 255 mares) compared with 51 percent (114 of 222) of observed mares given placebos (NRC, 1991).
Dominant stallions in 20 bands in northwestern Nevada were vasectomized in December 1985, and 20 more dominant stallions were vasectomized on a less mountainous HMA in Oregon in February 1986. Aerial surveys were conducted 2 years later to observe the effects on foaling rates. The efficacy of treating only dominant stallions was questionable. Reductions in foaling rates appeared to depend not only on the vasectomized stallion but on the stability of the harem band and its isolation. Fewer foals were observed in the stable
bands on the more mountainous HMA in Nevada than on the fatter HMA in Oregon, where bands mingled more. The findings of the genetic study that one-third of foals were not sired by the dominant stallion also raised doubts about the efficacy of vasectomizing only dominant harem stallions as a fertility-control method.
The Committee on Wild and Free-Roaming Horses and Burros focused much of its thesis on identifying “excess” horses or burros because excess could be related to the number of equids that exerted detrimental effects on their own welfare, the number of equids that adversely affected the ecosystem, or the number of equids that interfered with other management objectives for public rangeland. The 1980 report’s research projects were designed to help BLM to determine what excess meant. It also suggested projects to assist BLM in determining the value of “excess” equids and the most cost-effective ways of managing them.
The 1982 report reviewed the merits of removing some sex-age classes from a herd to decrease population growth. The committee found that the theory had potential but that more thorough analysis of this management approach than had been conducted was needed.
Forage Utilization by Free-Ranging Horses and Burros
The Committee on Wild and Free-Roaming Horses and Burros reviewed the forage practices, preferences, and requirements of equids and reported that hardly any information existed on daily forage intake by horses or on the nutritional value of range-plant communities as relative to equid dietary needs. It also found that possible competition for forage between free-ranging equids and cattle or between free-ranging equids and wildlife species was poorly documented. Similarly, little empirical research existed on the effects of free-ranging equid grazing on range vegetation or hydrology. The committee determined that such information would be needed to ascertain whether free-ranging horses and burros can sustain themselves on the land without adversely affecting an ecosystem or to ascertain at what quantity free-ranging horses and burros should be managed to ensure a thriving natural ecological balance. The information was particularly necessary because most of BLM’s forage allocations for livestock and wildlife had been determined before the agency became responsible for free-ranging horses and burros and few forage areas for equids had been established. The committee recommended three research projects to address the information gaps on the interaction of free-ranging equids and range ecosystems.
The research conducted in Phase II of the study was an insufficient response to the committee’s call for information about the vegetation, soil, and water potential of herd areas: the number of herbivores with varied feeding types that could be carried on an area without diminishing the area’s potential; the kinds and amounts of forage required by the herbivores; and the effects of herbivores on vegetation. However, three projects were undertaken that provided some data on those topics. BLM-funded research projects examined the distribution of and habitat use by cattle, free-ranging horses, and pronghorn near Rock Springs, Wyoming; the specifics of diet selection and grazing effects on forage plants in the presence of known densities of horses and cattle in the same area; and the quantity of forage consumed by free-ranging horses compared with cows and in relation to animal size and physiological status. Many questions remained to be answered, but the research provided greater specificity as to horse diets, including variation by season and nutrient-use
efficiency, and as to horse dietary overlap with cattle. It also provided evidence that short grazing periods have fewer adverse effects on plant communities.
The committee also investigated research that had taken place since Phase I was completed that was not part of Phase II, particularly on the interaction on the range between free-ranging horses and burros and wildlife. Although the results were far from definitive, the committee found that because free-ranging horses are grazers, they generally did not compete with ungulate browsers even if they occupied the same area temporally. However, vegetation may still be adversely affected on degraded rangeland if herbivore density is high. Adverse competition between bighorn sheep and free-ranging burros seemed likely on the basis of the number of observations, but the evidence was often circumstantial. The committee recommended that conclusive research be done to determine the degree and effects of competition between bighorn sheep and free-ranging burros (NRC, 1982). It also reiterated its conclusion from the 1980 report that research on semiarid and arid rangeland needs to be conducted for 7-10 years to capture the biological and climate variation in those regions.
The committee that prepared the 1980 report recognized that little information existed about the direct costs of free-ranging horse and burro management and the indirect costs of free-ranging horse and burro management as related to other range management objectives. Range managers needed data to guide them in managing the free-ranging horses and burros in the context of these other objectives. The committee also noted that BLM should have information on the attitude of the public toward the free-ranging equids, including the public’s awareness of the issue and its nonmarket valuation of the animals. The 1980 report therefore recommended six research projects on socioeconomic and political issues that included surveying interested parties and the general public for their opinions on free-ranging horses and burros. However, no requests for proposals on socioeconomic or political issues were issued by BLM after the committee recommendations. Those issues were not within the mandate of the Committee on Wild Horse and Burro Research and therefore were not addressed in the 1991 report.
NRC (National Research Council). 1980. Wild and Free-Roaming Horses and Burros: Current Knowledge and Recommended Research. Phase I Final Report. Washington, DC: National Academy Press.
NRC (National Research Council). 1982. Wild and Free-Roaming Horses and Burros. Final Report. Washington, DC: National Academy Press.
NRC (National Research Council). 1991. Wild Horse Populations: Field Studies in Genetic and Fertility. Washington, DC: National Academy Press.