Noxious Weeds Germinating in Horse Dung the World Over

Botanists in Australia are expressing concern after learning that more than a dozen non-native plants considered to be noxious weeds are easily dispersed via horse manure around the country.

Catherine Pickering of Griffith University led the research, which reviewed 15 studies of seed germination from horse dung from around the world. The research team looked at six studies from Europe, four from North America, three from Australia and one study each from Africa and Central America. After analyzing the studies, Pickering and her team learned that non-native, noxious weeds are being propagated through horse dung around the world.

"Not only are the seeds dispersed through dung but the manure provides the means by which the introduced plant to take hold," Pickering said of the noxious weeds.

Noxious weeds are those considered by state or federal governments to be injurious to crops, habitats, ecosystems, humans or livestock. Most noxious weeds are non-native to the areas where they are considered noxious.

"Of the 2739 non-native plants that are naturalized in Australia, 156 have been shown to germinate in horse dung. What is very concerning is this includes 16 of the 429 listed noxious weeds in Australia and two weeds of national significance," Pickering said.

A similar situation was observed in North America, where 105 of 1,596 invasive or noxious plants were found to be germinating in horse dung.

"Habitat disturbance from trampling has been demonstrated to further facilitate the germination of seedlings from dung in both natural and experimental studies," Pickering said. "Additional threats come in the form of trampled soils and vegetation, nutrient addition via dung and urine, and changing hydrology via damage to riverine systems."

The extent to which each the disbursement of noxious or invasive plant species is harmful depends on individual species, the researchers found. Some invasive plants that germinate in dung do not survive to mature and flower.

Pickering stressed the importance of considering factors such as horse dung when undertaking conservation efforts.

"To maintain the conservation value of protected areas, it is vitally important to understand and manage the different potential weed dispersal vectors, including horses," she said. "Legislators everywhere should take these into consideration before opening parks to this recreational activity."

The research is published in the journal Ecological Management and Restoration.