Dr Andrew Hoffman
Do you know what's in your pasture other than grass? Foreign plants can colonize a pasture from remote sources of seed, sometimes conquering large flats in a single year. Fields that are poorly managed, previously flooded, overgrazed, or surrounded by overgrown fields may be prone to weed infestation. Horses that are grazed next to woodlands, roads, ornamental gardens, lawns, or orchards are particularly prone towards toxicity. Take a walk around your pasture and get a sense of what's new? Pasture is the main source of toxic plants, but hay can be another. Hay is viewed as suspect in cases where there is no pasture.
Most horses won't eat toxic plants since they are distasteful and unconventional. Furthermore, it is rare that a mouthful of a toxic plant will be dangerous. Exceptions include Yew (Taxus) or Water Hemlock (Cicuta douglasii) which are quite deadly – fortunately, horses rarely have access to these plants. Goats, on the other hand, seem to get into trouble with them, while satisfying their curious appetites.
Horses often resort to eating toxic plants in times of drought, when pastures are sparse, grazing on recaptured land, with access to woodlands, and in malnourished states, horses. Horses denied forage, roughage, salt, minerals, horses that are fed 'complete feed' pellets instead of hay (old horses with bad teeth), and horses that are bored may start nibbling on toxic plants.
Plant toxicosis should be considered in any horse with unexplained signs of slobbering, laminitis, sudden onset of tremors or weakness, behavioral changes, bloody diarrhea, red urine, cardiac arrhythmias, jaundice or other signs of liver disease, severe anemia, and in cases of sudden death, plant toxicity.
Treatment is rarely specific for individual plants – anecdotes are not the norm. In general, your veterinarian will want to treat all suspected plant intoxications with mineral oil and/or activated charcoal to absorb the toxins, and intravenous fluids to speed the excretion of the toxic principle through the kidneys.
Identification of the plant or testing of hay samples for toxic plants can also be pursued, but more often than not, no plant is found and the intoxication difficult to prove. Extension agents and specialists should be called upon to identify toxic plants if there is any question. Take a photograph and send the plant to a specialist if the whole plant is available.
Plant toxicities are discussed by the symptoms they produce.