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Toxic Weeds & Horses

 


 

 

    Upon a review of the published literature on equines and toxic plants, the impression remains that all weeds are toxic or dangerous.  However, this is not necessarily the case.  Weeds are no more toxic than native species, ornamentals, or garden plants.  Weeds are, however, costly: weeds in rangelands alone cost an estimated $2 billion loss annually in the United States and cost an estimated $137 billion annually in damage and control costs across the United States. 

 

    The word ‘weed’ is often used interchangeably with the term ‘problem plant’; this doesn’t necessarily mean that the plant is either invasive or noxious. This lack of division between weeds and other plants leads us to the misconceptions that weeds are poisonous, or that all poisonous plants are weeds. 

 

Noxious weeds vs. toxic weeds

A comparison of the compiled list of nearly 300 known toxic plants in the United States and four plants lists: noxious plants of Idaho, Oregon, and Washington, and the list Weeds of the West, shows that relatively few of the toxic plants are known weeds.  Of the state lists, 14 of the 40 legally noxious weeds in Idaho are known to be toxic, of Oregon’s 258 noxious weeds 24 are known to be toxic, and of Washington’s 149 noxious weeds, 22 are known to be toxic.  The comparison with the Weeds of the West list shows that 94 of the 344 invasive weeds are known to be toxic.  These numbers may be significant in dealing with the mistaken belief that noxious or invasive weeds are always a danger to equines.

The danger of concentrating on toxic weeds

The emphasis on weeds as the only dangerous plants to equines may result in a significant potential risk: a disregard for the presence of other seemingly harmless, highly toxic plants. For instance, some ornamental or garden plants that appear to be inoffensive can be critically toxic to equines.  These include red maple, onions, serviceberry, Japanese yew, oleander, lily of the valley, larkspur, delphinium (Delphinium spp.), persimmon (Diospyros virginiana), avocado (Persea americana), indigos, Indian paintbrush (Castilleja spp.), Easter lily (Lilium longiforum), daffodil (Narcissus spp.), tomato (Solanum lycopersicum), potato (Solanum spp.), chokecherry, pincherry (Prunus pensylvanica), and Alsike clover.

Toxic weeds do exist

Some noxious or invasive weeds are highly toxic to equines, however, and can cause tremendous problems if allowed to invade horse pastures.  This may be partially due to the extensive taproot in many broadleaf weeds that allow them to remain green longer into the dry season, thereby appearing potentially attractive to horses grazing in poor pastures. This list includes tansy ragwort, yellow starthistle, Russian knapweed, Canada thistle (Cirsium arvense), poison hemlocks, field bindweed, houndstongue, Scotchbroom (Cytisus scoparius), horsetails, leafy spurge, black henbane (Hyoscyamus niger), Klamath weed or St. Johnswort, kochia, yellow toadflax or butter-and-eggs (Linaria vulgaris), silverleaf nightshade (Solanum elaeagnifolium), and puncture vine.

 

While there is no mistaking the damage caused by invasive and noxious weeds in pastures and rangelands, it is not appropriate to automatically label all of them as toxic to equines.  Most native plant communities also contain some toxic plants.  If horse owners work only toward eradicating pastures of ‘weeds’, they may find that the plants they ignore as benign are the very ones that should have been cause for concern.

 

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