The Colorado State University study focused on the start of the Lower Piney River trail in the White River Forest of western Colorado.
Graduate student Floye Wells and Professor William Lauenroth collected 12 horse-dung samples along the first 4000m of the trail, which is thought to be used by about 500 horse riders a year.
"We evaluated the seed content of each sample by applying standard methods for soil seed-bank analysis," they wrote in their findings, published in the latest issue of Rangeland Ecology & Management.
"We found 20 species and 564 seedlings. Twelve of the species were graminoids, six were forbs, one was a shrub, and one was a tree.
"The species were evenly divided between natives and aliens, but 85% of the seedlings were aliens. An average of 47 seedlings emerged per sample, but the range was from 4 to 192.
"Our results make it clear that horses, and very likely all pack stock used on recreational trails, represent a potentially important dispersal vector for alien plants into western wildlands."
The pair said that, as a rule, long-distance dispersal is rare and difficult to quantify, "but if we want to protect native communities that we know are at risk from particular alien species, we must gain a clearer understanding of how those plants travel long distances".
"There is increasing evidence that large mammals can play a crucial role in dispersing seeds over long distances. Many seeds are able to survive the passage through cattle, sheep, rabbits, and deer."
They pointed to a study in which horses were fed a mixture of weedy seeds, where the authors found that many species were able to pass through horses with little or no loss in viability.
"The large number of horses on public lands and the potential for them to carry alien seeds could make horses an important vector for alien plant dispersal into remote wildlands. It is common for horses to graze in 'weedy' pastures and for their feed to be contaminated with alien plants; therefore, one would expect that horses are capable of carrying in a wide variety of alien species."
The researchers did not apply any special germination techniques during the study, under the assumption that the seeds of interest are those that germinate easily given adequate water, nutrients, and light.
"Even though the alien species in our samples are common species that are not a priority for management, our most important result is that horses have the potential to disperse a large number of seeds from a wide variety of plant types."
Their study was entitled "The Potential for Horses to Disperse Alien Plants Along Recreational Trails".