Draft EIS Appendix 12 

APPENDIX 12—MANAGING WILD HORSE 

POPULATIONS IN THE RAWLINS RMPPA 

The Bureau of Land Management (BLM) monitors the wild horse population to ensure that the population 

meets the population management objectives set in the land use plan. Monitoring of the habitat and the 

wild horses has been ongoing within the Herd Management Areas (HMA) for several years and has

included some of the following methods and approaches: 

•

• 

• 

• 

• 

• 

Precipitation data

Rangeland trends (primarily on uplands)

Forage utilization data 

Licensed use by livestock grazing allotment 

Wildlife actual use and forage requirements 

Wild horse population data, including, but not limited to the following: 

– Population counts 

– Reproductive rates 

– Age/sex structure 

– Actual use sightings 

– Determining areas of highest horse use, or concentration areas 

If an evaluation of monitoring data were to indicate that wild horse management objectives in the land use 

plan were not being met, population adjustments in land use plans and Herd Management Area Plans 

(HMAP) could be necessary. Population adjustments would be analyzed prior to initiating maintenance 

actions in applicable HMAPs and Resource Management Plans (RMP). Monitoring and adjusting the

Appropriate Management Level (AML), as necessary, would ensure a thriving natural ecological balance 

is maintained. 

More information on specific management techniques used in managing wild horse populations and the 

nature of the horses being managed can be found in the Rawlins Field Office Wild Horse Management

Handbook. This document is an assemblage of all relevant policy and technical guidance that must be

considered in developing and implementing wild horse management plans and actions. The handbook is 

available for review at the BLM Rawlins Field Office, 1300 North 3rd Street, Rawlins, Wyoming. 

ESTABLISHMENT AND MODIFICATION OF HORSE MANAGEMENT 

AREAS AND LEVELS

The current AMLs were established in 1994 from a process that included 5 years of focused, intensive 

monitoring; evaluation of data; public input; and environmental analysis. Since that time, some boundary

adjustments and realignments to the HMAs have been made via the Land Use Plan (LUP) maintenance

process. 

AML/Population Expression in the Rawlins Field Office 

The existing AMLs in the Rawlins Field Office are expressed as the number of adults and yearlings, not 

including unweaned colts of the year. This definition was included in the 1994 AML evaluation and

decision. At the time of the development of these numbers, customary primary inventory times were in

the late winter or very early calendar year (typically February or March). The colts of the previous year 

had perished or survived to approach their first birthday, and the colts of the year had not yet been born. 

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Appendix 12 Draft EIS 

Thus, the number counted represented what the adult population would be for the following year. Lately, 

there has been a shift in inventory times to favor the month of July. When a herd is counted at this time, 

most of the colts of the year have been born and many are 2 months of age or older. Thus, an inventory

of the same herd in the same calendar year will yield a higher number, barring some unusual late winter 

event resulting in unusually high mortality. Inventory practices are under review pursuant to the Consent

Decree with the State of Wyoming. Inventory practices may be standardized and any such

standardization could well include a uniform method of expressing objective and existing populations. 

Increased interest in the genetic character of a particular herd has caused some differing views on the 

expression of population sizes and objectives. Geneticists usually define a population in terms of the 

effective population (NE

) that consists of the number of competent breeding age animals; therefore, colts

of the year, yearlings, a portion of the 2-year olds, and the very old would not be included as part of the 

effective population count. The AML necessary to maintain an effective breeding population of 100 

would be about 165 adult animals, not including unweaned colts. The exact number would vary

depending on the age/sex distribution of the particular herd involved. 

To change the AMLs and the upper and lower limits associated with the AML to a format that would

reflect the above-discussed competent breeding age approach would require adjustments in AML for the 

Rawlins Management Plan Planning Area (RMPPA) (see HMA Lost Creek (Alternative 3) in Table A12-

1). The table also shows the relationship between AML numbers and post-foaling populations of wild 

horses. 

Table A12-1. Relationship Between AML and Wild Horse Populations 

Population Pre-Foaling1 Population Post-Foaling2

HMA AML Upper 

Limit 

Lower

Limit At AML2 At Upper Limit 

At Lower

Limit 

Adobe Town 700 812 588 812 942 682 

Stewart Creek 150 177 123 177 209 145 

Lost Creek 

(Alternatives 1, 2, & 4) 70 83 57 83 98 68

Lost Creek 

(Alternative 3) 165 195 135 195 230 160

Notes: 

1 This number includes adult horses and yearlings, not including unweaned colts. This is the definition of 

appropriate management level included in the 1994 AML evaluation and decision. This definition of AML is 

conducive to wild horse inventories completed in late winter (February/March), because much of the winter 

mortality would have already occurred. 

2 This number represents the total wild horse population being managed at AML following foaling. Because 

unweaned colts are not considered in the AML, the total wild horse population may be higher than the AML, 

although the population is still considered meeting the AML. This number is conducive to wild horse 

inventories completed in late summer (July/August), because many mares would have foaled, but some of the 

foals may not survive the upcoming seasons. 

Population post-foaling, shown in Table A12-1, represents the total population, including all age classes,

and represents the approximate population as of October 1 of a given year. To change the definition of

the AML would be an implementation decision and, therefore, appealable. 

IMPLICATIONS OF WILD HORSE GENETIC RESEARCH

Wild horses managed on the public lands have a variety of histories and originate from a variety of

backgrounds. The genetic roots of most of the horses are predominantly American, and some have

beginnings as recent as following World War II. Advances in genetic research have enabled the BLM to

identify the specific genetic stock from which a wild horse population originates, thereby assisting in

identifying the history of a population. Occasionally populations have been encountered in which their 

A12-2 Rawlins RMP 
Draft EIS Appendix 12 

genetic roots can be traced to the Spanish exploration period through the identification of genotypes 

associated with the New World Iberian (Spanish Colonial) breeds. Populations with this genetic 

similarity provide a genetic resource that the majority of wild horses on public lands do not provide. The

wild horses in the Rawlins RMPPA Lost Creek HMA are such a population. 

In 2001, blood samples from wild horses removed from the Lost Creek HMA were provided to Dr. E. 

Gus Cothran of the Equine Parentage Testing and Research Laboratory at the University of Kentucky. 

Results from the genetic analysis of these samples identified a clear contribution from New World Iberian 

breeds. The highest average genetic association of the blood samples provided was with these “Spanish

Colonial” breeds. The next highest average genetic association was with North American Gaited breeds,

most likely from the routine escape of domestic saddle stock from the surrounding areas. A full report of

the results of this genetic analysis can be obtained from the Rawlins Field Office. 

As genetic research continues with the wild horse populations throughout the Red Desert metapopulation,

the necessity of maintaining the population of wild horses in the Lost Creek HMA in genetic isolation

may vary. If populations adjacent to the Lost Creek HMA share the prevalence of New World Iberian 

genomes, intermingling these populations would be beneficial to maintain the genetic resource and 

isolating and maintaining an internally viable population in the Lost Creek HMA would not be required. 

However, if the New World Iberian genes are prevalent only in Lost Creek, further intermingling could 

cause this genetic resource to disappear. Continued monitoring and research may result in adjustments to

management decisions for the Lost Creek HMA. Adjustments would be implemented following

appropriate analysis and maintenance of management documents. 

WILD HORSE MANAGEMENT HISTORY IN THE STATE OF WYOMING 

AND THE RAWLINS RMPPA 

In 1971, in response to the passage of the Wild, Free-Roaming Horse and Burro Act, Wyoming BLM 

identified the existing wild horse habitats and populations in the state that would likely be subject to the 

provisions of the Act. These identifications were made using the best information and understanding 

available at the time. The result was 30 areas with populations totaling 4,411 horses. Of those, 1,049 

were estimated to be privately owned horses that would be claimed and removed from the range under the

provisions of the Act. Those 30 areas comprised a total of 6,557,160 acres of public lands, 389,112 acres

of land owned by the State of Wyoming, and 2,479,096 acres of privately owned lands. The 30 areas 

varied greatly in size and land ownership.

As soon as the Act passed, a number of activities that had served to limit the growth in horse numbers and

the expansion of their ranges ceased. Horse populations in Wyoming began to grow. 

Almost immediately, BLM personnel began to accumulate additional information about the horses and 

their habitats. Area boundaries were refined as more was learned about the seasonal needs and habits of

horses. By 1974 the list of 30 areas had increased to 40 areas comprised of a total of 6,820,749 acres of 

public lands, 406,103 acres of land owned by the State of Wyoming, and 2,355,852 acres of privately

owned lands. The 40 areas varied greatly in size and land ownership. 

The period of 1976 to 1984 saw a great deal of activity in land use planning. For Wyoming, this can be 

called the Management Framework Plan (MFP) era. During this period, the 40 areas previously identified 

were combined into 24 areas. The MFP process resulted in identifying 14 of those 24 areas that, in one

way or another, failed to meet the suitability for maintaining a long-term healthy population of horses in 

accordance with the intent of the Act. The two most prevalent criteria were that the area contained 

significant amounts of private land and that the horse population was too small to continue to thrive when

Rawlins RMP A12-3 
Appendix 12 Draft EIS 

isolated from customary sources of new genetic stock. The remaining 10 areas were then designated as

HMAs. The HMAs comprised 3,322,776 public acres, 152,551 acres of land owned by the State of 

Wyoming, and 861,022 acres of privately owned land. This process also identified population objectives 

for these herds, totaling 2,673 horses. 

Land use plans and the decisions that they generate are dynamic, adjusting in response to changing 

environmental conditions or new information. With respect to horses in Wyoming, this has resulted in

the 16 HMAs currently recognized in Wyoming. These 16 areas comprise 3,664,002 acres of public land, 

154,737 acres of land owned by the State of Wyoming, and 846,243 acres of privately owned lands. The 

16 areas still vary in size and land ownership, though not to the extent that they once did. Particularly

worthy of note is the significant amount of private land now included within designated HMAs. Much of

the private acreage consists of land owned or controlled by the Rock Springs Grazing Association of

southwestern Wyoming. It has made its lands available to an agreed-upon number of wild horses since 

1979. Without access to those lands, approximately 1.5 million acres of adjoining and commingled

public lands would be unavailable for inclusion in HMAs. This would, in effect, eliminate one-third of 

the free-roaming horses in Wyoming. The current, combined population objective (AML) for wild, freeroaming horses in Wyoming is 3,263. That is 18 percent more than it was in 1980. Without the access to 

the private lands noted above, the combined AML would be only 2,038. 

As noted above, an effective breeding population of 100 horses is necessary to maintain a genetically

viable herd of wild horses. Nine of the 16 HMAs in Wyoming do not have AMLs that would indicate 

genetically stable long-term populations. However, wild horse herds in these HMAs are usually part of a 

larger metapopulation comprising adjacent HMAs. The metapopulation is the entire gene pool available 

to a specific herd. 

When originally identified and reviewed through planning, HMA boundaries were designated to reflect 

common herd location, as well as to simplify administration and management of wild horses. As a result, 

several HMAs may be designated adjacent to one another in different BLM Field Offices, or simply

separated by geographic features such as watersheds. The individual populations in each HMA may be 

separated for most of the year, but both may share the same winter range. Sharing resources allows for

regular interaction between the two populations. Interaction allows for horses from each herd to be

recruited by and assimilated into the other. Thus, although the AML of the individual HMAs would 

appear to be genetically deficient, each population is periodically infused with new genetic material and 

the genetic diversity of both herds is enhanced. In any given year, only a very few bands from each herd

may actually exchange members, but over time, the normal behaviors of each herd cause the mixing to 

become widespread. 

From the standpoint of genetic viability, the required level of exchange of animals and the related

introduction of new genetic material is not high. In small populations of less than 150 animals, the 

introduction of one or two competent breeding animals per generation (~10 years) will insure the

maintenance of the genetic resource. To be members of a metapopulation, individual animals need not

experience frequent, large-scale contact with one another. 

Table A12-2 identifies the Rawlins RMPPA wild horse HMAs and the metapopulations in which the

HMAs’ horses interact. 

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Draft EIS Appendix 12 

Table A12-2. Wild Horse Regional Metapopulations Associated With the Rawlins 

RMPPA 

Rawlins RMPPA HMAs Metapopulation 

Name AML Name AML

HMA(s)in the 

Metapopulation 

Type of 

Interaction

Points of Contact 

Adobe Town 700 Stateline 1,250 

Adobe Town

Salt Wells 

Sand Wash (Co)1

Male migration, 

female exchange

Haystacks, 

Alkali, 

Sand Creek, 

Powder Wash 

Stewart Creek 150 Red Desert2 790 

Stewart Cr

Lost Creek 

Antelope Hills 

Divide Basin 

Male migration, 

female exchange

Hay Reservoir,

Bare ring, 

Hadsell, 

Osborne Draw

Lost Creek 70 Red Desert2 790 

Stewart Cr

Lost Creek 

Antelope Hills 

Divide Basin 

Male migration, 

female exchange

Hay Reservoir,

Bare ring, 

Hadsell, 

Osborne Draw

Notes: 

1 Sand Wash HMA is located entirely in Colorado within the BLM’s Craig Field Office. While managed by Colorado BLM, 

horses from the Sand Wash HMA provide biologically and genetically important interactions with horses from the other 

HMAs in the metapopulation.

2 Wild horses from the Sweetwater metapopulation (Green Mountain HMA & Crooks Mountain HMA) occasionally mix with

wild horses in the Red Desert metapopulation.

In the 30 years since the passage of the Act, the following has occurred in the State of Wyoming: 

• 

• 

• 

• 

• 

The average herd size has increased from 147 to 197. 

The area of public land available for use by horses has increased slightly since 1980. 

The area of private land occupied has decreased from 2.5 million to 846,243. 

The area of private land available for legal use has increased from 0 to 846,243. 

Of the 16 herds, 14 are parts of metapopulations greater than 300. 

The same trends that are described for the State of Wyoming also occurred in the RMPPA. In 1971, an

estimated 435 horses subject to management under the Act occupied 2,116,095 acres in six areas. Today,

an estimated 1,540 horses subject to management under the Act occupy approximately 929,000 acres in

three areas. 

It should be noted that the BLM has routinely removed excess and stray horses from the range since 1978. 

During that period, more than 17,000 horses have been removed from Wyoming rangelands and placed 

through the BLM's Adopt-a-Horse program. And yet the population is still in excess of 6,000 animals, an 

important indicator that the cornerstone principle of the Act, a thriving ecological balance, is not 

imperiled by BLM's management of the horses. 

POPULATION MANAGEMENT ACTIONS IN THE RMPPA 

It should be noted that population management actions in the RMPPA take place as part of a state and 

national undertaking, which allocates scarce resources and equally scarce space for horses to be removed

from the public rangelands so that effective planning and scheduling can take place, bureauwide. No

single office controls the fiscal and logistical resources necessary to effect the desired management of

horses in its jurisdiction. Instead, each office is part of the BLM-wide wild horse management program.

A key part of this program is the identification of a gather cycle for a state. Once established, the gather

cycle needs to be adhered to even when this results in gathers taking place in less than ideal conditions 

within a particular HMA. 

Rawlins RMP A12-5 
Appendix 12 Draft EIS 

AMLs are established to allow for a range of fluctuation in the population and still meet the criteria for a 

valid AML. In evaluating the AML, a lower limit is identified and then examined to ensure that the 

particular herd will remain genetically viable if periodically reduced to that level. This is a crucial

consideration in many of the smaller herds in the state. Concurrently, the upper limit is evaluated to

determine that, under normal climatic conditions, resource damage or other significant conflicts would 

not be likely to occur if the population were allowed to increase to this level cyclically. The AML will 

equate to the average population level during a management cycle. AMLs in the Rawlins Field Office do

not include the unweaned colts of the year. 

Analysis of various gather cycles (occurring outside the scope of this RMP) is occurring as part of the 

statewide wild horse management strategy. Three- and four-year gather cycles for the state are being 

evaluated and compared. Cycles longer than 4 years are also being evaluated as part of a management

scheme that would employ fertility control to limit population increases. 

The fluctuations suggested in Alternative three were determined based on the historically observed rates

of increase for the individual herds. As of August 2004, the determination of the gather cycle to be 

employed in Wyoming is still tentative and awaits further evaluation. When a gather cycle is chosen for 

implementation in the state, part of the evaluation leading to the choice will be the ability of Wyoming 

BLM to remain in substantial conformance with the consent decree of August 28, 2003, and all other

relevant law and policy. The upper and lower limits would be reevaluated and adjusted to ensure that a 

thriving ecological balance would be maintained. Because the AMLs were evaluated considering the 

potential for adverse effects from a 4-year gather cycle and the associated level of population fluctuation, 

a shorter cycle and lower average population levels would still serve the purposes of the AML

determination process. 

A12-6 Rawlins RMP