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Using a unique sterile design intended to eliminate outside seed contamination of horse
feces, we investigated whether weed seeds germinate after digestion by horses. Feces were collected
from selected National Parks and other locations in central and northern California. All potted fecal
samples were irrigated and grown in an enclosed sterile nursery environment. Thirty-two plant species
emerged from these fecal samples, 24 of which were not native to California. None of these were
identified on the California Department of Agriculture’s Noxious Weed List, which is used as a basis to
certify equine feed as weed free. However, seven of the non-native species are identified as moderately
invasive on the California Invasive Plant Council’s (Cal-IPC) list. These species are: Hirschfeldia incana,
Hordeum marinum, Lolium multiflorum, Mentha pulegium, Rumex acetosella, Trifolium hirtum, and
Vulpia myuros. In addition, the following four non-native plants are listed at the limited invasiveness
level on the Cal-IPC list: Hypochaeris glabra, Lythrum hyssopifolium, Medicago polymorpha, and Poa
pratensis. Because we did not survey invasive plant cover in locations from which we sampled, we
cannot guarantee that species identified in our samples would have also germinated in the field. Our
results add to a growing body of literature documenting germination of seeds after passing through the
digestive system of horses and suggest that conscientious horse owners should select feed sources that
are free of weeds. We also find that the current list of noxious weeds used to certify weed-free feed for
equines should be comprehensive. |
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