Do Horses Spread Non-Native Plants on Trails?
    by: Stith T. Gower, PhD
    February 11 2007 Article # 8846
   
   
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    Can plant and weed seeds contained in horse manure, hooves, and hay
    cause nonnative plant species (plants that were introduced to that
    ecosystem but do not grow there naturally) to spread along trails and
    into parks and forests? This is an ecological question that often
    arises. Stith T. Gower, PhD, of the Department of Forest Ecology and
    Management at the University of Wisconsin-Madison, has determined that
    while there are seeds from weed and non-native plant species in horse
    manure and hay, the plants that result don't survive or spread on
    trails. Therefore, horses do not appear to be a major source for the
    introduction of nonnative species.
   
    "Nonnative plant species pose a serious ecological and economic threat
    to managed and natural ecosystems," said Gower. "Therefore, there is a
    great need to identify major sources for the introduction of
    non-native species and implement management plans to reduce or
    eliminate their introduction. Horses have been suggested to be an
    important source for the introduction of nonnative plant species along
    trails, but the data are largely anecdotal."
   
    The objectives of two studies were to determine if horse hay, manure,
    and hoof debris samples contained seeds from nonnative species, and if
    so, whether their seeds would germinate and establish on the trails.
   
    Gower took samples of horse hay, manure, and hoof debris from 12-24
    horses at each of five American Endurance Ride Conference (AERC) rides
    held in North Carolina, Kentucky, Illinois, Wisconsin, and Michigan.
    Subsets of these samples were sown in potting buckets and grown
    outdoors in ideal conditions in Madison. In addition, samples from the
    horses at each ride were placed back on the horse trail.
   
    On average, nonnative plant species germinated from 5.2% of the hay
    samples placed in the potting buckets, indicating horse hay contains
    seeds from nonnative plant species. However, only three of the 288
    hay, manure, and hoof debris samples placed on the horse trail
    contained plants after the first growing season, and no surviving
    plants were observed after the second season.
   
    Gower also did vegetation surveys along 50-meter (150-foot) survey
    lines perpendicular to horse and hiking (horses not permitted) trails
    at three of the five sites to compare species composition of native
    and nonnative plants. He wanted to determine if newly introduced
    plant species were moving into ecosystems adjacent to the horse trails
    and trails that prohibited horses.
   
    Species composition and percent of total nonnative plant species did
    not differ between horse and hiker trails, and the nonnative plant
    species always occurred within 2 meters (6 feet) of the trail. Other
    potential sources for nonnative species might be birds, he said. "Many
    studies have observed that trails and roads (of all uses) facilitate
    the establishment of nonnative species," added Gower.
   
    "The results of this and other studies demonstrate that horse hay and
    manure contains seeds of nonnative plant species, but native and
    nonnative plant species rarely become established on horse trails
    because of the adverse effects of harsh environmental conditions and
    frequent disturbance on seedling establishment," concluded Gower.
   
    For more information about the study, e-mail Gower at stgower@wisc.edu
    <mailto:stgower@wisc.edu>. The study was funded by the American
    Endurance Ride Conference (www.aerc.org <http://www.aerc.org/>).