The danger of poisonous plants depends on the plant’s prevalence, toxicity and palatability. Generally speaking, horses will avoid consuming most toxic plants if other forage is available. However, the risk of plant poisoning still exists if toxic plants are present. Good pasture management practices and honing your skills to be able to identify poisonous plants are important measures to prevent plant poisonings of your horse.
Jimsonweed (Datura stramonium) is a large summer annual (up to 5 feet tall) that typically emerges May through mid-June. Other common names include Jamestown weed, thorn apple, downy thornapple, devil’s trumpet, angel’s trumpet, mad apple and stinkwort.
Jimsonweed has long been known to be toxic all classes of livestock and to humans as well. Horses rarely consume Jimsonweed if other forage is available because of its foul odor and taste. All parts of the Jimsonweed plant are poisonous; toxicity is caused by tropane alkaloids.
Symptoms of poisoning in horses might occur within minutes to several hours and can include: seeking water to drink, dilated pupils, agitation, increased heart rate, trembling, convulsions, coma and possibly death.
Methods for controlling Jimsonweed can range from mechanical to chemical. For chemical control options you may refer to the 2014 Michigan State University Extension Weed Control Guide for Field Crops, Table 4B – Weed Response to Herbicides in Established Forage Grasses. It is important to remember that if you choose to use a herbicide for control method, be sure to carefully read the label for grazing restrictions that may apply.
Additional resources can be found online in this publication: Toxic plants of concern in pastures and hay for Michigan horses