under shallow snow depths and on palatable shrub species such as winterfat, bitter brush, and some
varieties of sagebrush. Water is often not avail-
able, forcing animals to rely on snow to meet their
needs. This works adequately when forage is lim-
ited along with water, but snow as a source of water
can be a fatal combination with hay or abundant
amounts of dry forage. When dry forage is fed to
horses that must rely on snow as their only source of
water, impactions usually develop because the
horses simply cannot consume enough snow to meet
their need for water. Improvement projects such as seeding, prescribed
fire treatments, emergency fire rehabilitation, or
shrub and tree removal have potential to increase
the forage available for WH&B use and to improve
habitat conditions. However, these improvements
may also cause grazing animals to concentrate their
use within project areas. Structural improvement
projects (e.g., fences, cattle guards, or water devel-
opments) are often constructed or removed to pro-
tect the wild free-roaming nature of WH&Bs and
provides for more normal herd distribution and
movement as well as genetic interchange over larger
groups of animals than may be possible otherwise.
Population Management
Wild horses and burros are managed in a manner
designed to achieve and maintain a thriving natural
ecological balance (TNEB) and protect the range
from the deterioration associated with WH&B over-
population. They are managed as self-sustaining
populations of healthy animals in balance with other
uses and the productive capacity of their habitat.
Some selected HMAs may be managed for nonrepro-
ducing wild horses to aid in controlling on-the-range
population numbers. To achieve a TNEB on the
public lands, WH&Bs should be managed in a man-
ner that assures significant progress is made toward
achieving the Land Health Standards for upland
vegetation and riparian plant communities, water-
shed function, and habitat quality for animal popu-
lations, as well as other site-specific or landscape-
level objectives, including those necessary to protect
and manage Threatened, Endangered, and Sensi-
tive Species.
9 Wild horse and burro herd health
and welfare are promoted by achieving and main-
taining a TNEB. The appropriate management
level (AML) is the number of WH&Bs (expressed as
a number range) that has been determined to be the
appropriate population size for a specific HMA over
time. The AML upper limit is established as the
maximum number of WH&Bs, which results in a
TNEB and avoids a deterioration of the range.
This figure should be below the number that would
cause rangeland damage due to excessive grazing.
The BLM must monitor the rangelands and WH&Bs
located on public lands to determine if there are an
excess number of animals present. In making this
determination, the field specialists analyze grazing
utilization and distribution, trends in the ecological condition of the range, actual grazing use of the
plants, weather data, current population invento-
ries, wild horse and burro locations (whether they
are inside the HMA or have strayed outside their
HMA), and other factors. The term “excess ani-
mals” is defined as those animals which must be
removed from an area in order to preserve and main-
tain a TNEB and multiple-use relationship in that
area. Annual monitoring objectives will include de-
termining whether or not forage and water are ad-
equate to support the animals in a healthy condition
throughout the year. Additionally, a visual assess-
ment and evaluation of animal condition will be
done as part of the monitoring. Population surveys
are done using helicopters or fixed-wing airplanes to
estimate the population size, growth rate, and dis-
tribution of animals within a specific area. At a
minimum, population surveys should be conducted
every 3 years, whenever possible, and within 6 to 12
months prior to establishing the need to gather and
remove excess WH&Bs. Management of WH&Bs also requires specific em-
phasis given to the animals themselves. The age
structure and sex ratio in the population can influ-
ence herd health, social interactions, and population
growth rates of reproducing WH&B herds. Typi-
cally, males and females are born in equal propor-
tions over time, but a herd’s sex ratio may have been
altered by previous BLM removal policies or natural
mortality. Similarly, age ratios will vary depend-
ing on previous BLM removals as well as the herd’s
survival characteristics and the typical environment
in a given HMA, particularly regarding droughts or
severe winters. Age structures are estimated from
the best available data, which may include annual
foal counts and the age distribution of captured an-
imals during a gather. Population growth rates
represent the net difference between births and
deaths and the numbers of animals that enter and
leave the area (ingress and egress) over time. Pop-
ulation growth rates can be highly variable and in-
fluenced by habitat quality, forage production, water
availability, and other factors. Typically, 60 to 75%
of breeding-age mares will foal each year and the
average population growth rate for populations
managed by the BLM is 15 to 20% a year. Survival
rates are lowest for animals less than one year of age
and over 20 years of age.
10 The foaling season for
wild horses on the range is generally March 1 to
June 30 for most herds. This has a great deal of
variability due to the location of individual HMAs
and past management practices and, in some areas,
newborn foals may be seen in any month. Wild
burros typically foal year-round.
Genetic Monitoring
The BLM uses hair follicle testing to monitor genetic
diversity in each herd to characterize the genetic
makeup of the herds and protect against inbreeding
depression. The breeds of horses most represented
in each herd’s genetic makeup varies, with about 6
414 2014 fiVol. 60fiAAEP PROCEEDINGS
THE AMERICAN MUSTANG