Most cases of plant toxicity occur when the horse is able to digest large quantities of certain undesirable weed species over a long period of time. Effects may include hyper-salivation, colic, diarrhea, photodermatitis, neurological problems, lameness, anemia, or sudden death. In many cases only a certain part of the plant or a certain stage of growth or reproduction produce toxic compounds; depending on the species stems, leaves, roots, flowers, berries, fruits, or nuts can be hazardous. Toxic species common to the Northeast include white snakeroot (Eupatorium rugosum), nightshade (Solanum sp.), yew (Taxus sp.), milkweed (Asclepias syriaca L.), groundsel (Senecio vulgaris L.), St. Johns Wort (Hypericum perforatum), and jimsonweed (Datura stramonium L.); hardwood species such as oak (Quercus sp.), black locust (Robinia pseudoacacia L.), red maple (Acer rubrum L.), and black walnut (Juglans nigra) can be both toxic and attractive to horses, and should be removed. There is also the possibility of physical injury during investigation of plants with bristles, spines, or stinging hairs. Occasional ingestion usually does not cause any major ill effect. However, it is still important to monitor populations of toxic weeds in your pasture and keep them from becoming an appreciable part of your horse's diet.
The following sites offer more information on toxic pasture plants.
Tall Fescue Endophyte
The tall fescue endophyte Acremonium coenophialum produces ergot alkaloids responsible for hormonal interference in broodmares. The national percentage of infection of tall fescue is at least 85%. Seed transmission is the only mechanism for endophyte infection so planting an endophyte-free variety in an isolated area should eliminate the potential for infection. Endophyte-free varieties are commercially available but have lower seedling vigor, and less resistance to drought, insect pests, disease, and frequent grazing. Screening forage for Acremonium Endophytes is a service offered by the Plant Diagnostic Laboratory at Rutgers University.
More information about tall fescue toxicity can be found at these web sites: