A follower, Randy W, recently sent me this article for Horse Science News. This issue has been under discussion many times in the Boulder area and here is a recent study that disproves the old question: Do horses spread weeds?

Do Horses Spread Weeds Along Trails?

By Liz Osborn, HorseScienceNews.com

Horses have been blamed for scattering unwanted weeds across the landscape as they travel down public trails. This accusation has now been scientifically investigated. The results reassure trail riders that horses in the United States are not guilty as charged.

Research probed from several different angles the possibility of horses causing non-native plants to take root alongside trails. The raw material examined was horse hay, manure and hoof debris. Samples were collected at endurance riding events in five states: North Carolina, Kentucky, Illinois, Wisconsin, and Michigan.

The study investigated whether weeds would germinate from the material when carefully cultivated in pots under ideal conditions. Researchers also looked at what grew from the horse debris left alongside trails.

They found that only a tiny portion of the potted hay samples actually sprouted any non-native plants. Meanwhile, no exotic species emerged among the cultivated manure or hoof debris.

Alongside the horse endurance trails, nearly 300 plots of hay, manure and hoof debris that got left behind were monitored to see what plants they produced. Only three of these plots sprouted anything, and all were native plants. By the end of the second growing season, no plants had survived in the trailside leavings.

A third angle of this study compared plant species growing near trails used by horses with plants along trails used only by hikers. Botanists detected no differences between these two trail uses in the number of exotic plant species.

This study concludes that horse hay does contain some non-native plant seeds. But because of the harsh environmental conditions near horse trails, it’s rare for any plants to establish from the seeds that horses leave behind.

Reference
Stith T. Gower. 2008. Are horses responsible for introducing non-native plants along forest trails in the eastern United States? Forest Ecology and Management. 256(5): 997-1003