by Mary Knetter

Created on: September 11, 2008   Last Updated: June 25, 2009

There are hundreds of toxic or poisonous plants that affect horses. Even plants considered good for an animal can be toxic if eaten in excess. Luckily, most horses will not eat toxic plants under a normal, healthy lifestyle.

This article features five common plants, along with their scientific names, that are poisonous to horses. A list of websites providing more information about these and other poisonous plants, as well as photographs to help identify them, are available at the end of the article.

FIVE COMMON PLANTS

-BRACKEN FERN (pteridium, aquilinum spp.) grows in acidic soil and is consumed when other food sources are scarce, such as a drought or in burn zones. They are commonly found in woods where poor, dry soils do not support other vegetation. The young, tender shoots are most desirable to animals and acute poisoning occurs most often in a drought. Bracken is toxic in all seasons and maintains its toxicity even when found dry in hay.

Animals exposed to bracken fern show loss of appetite and depression first. Toxicity is cumulative, often developing over months and horses may show symptoms long after they are removed from infested pastures. Symptoms of bracken fern toxicity are similar to thiamine deficiency and should be distinguished from a neurological disorder as the symptoms are displayed through staggers, increased heart rate and convulsions. Death occurs within 12-72 hours.

Preventing bracken fern poisoning is easier than treating it because poisoning can only be treated symptomatically. Applying lime to the roots or cutting the plant several times during the summer months prevents animals from consuming the plant. Remove dry fronds from hay or don't offer the hay as a food source. Alternating between bracken-infested and non-bracken pastures every three weeks also prevents the toxicity from accumulating.

-COCKLEBUR (Xanthium strumarium) can be found in fields and waste areas as well as the shores of ponds and rivers. Poisoning is most frequent in spring or fall when plants rejuvenate after a drought. Seedlings are most poisonous between seed germination and the two-leaf seedling stage. Seeds are also toxic and can sometimes contaminate grain. Burrs are rarely eaten but are equally toxic. Mature plants are less palatable and believed to be less toxic.

Consumption of cocklebur is toxic in small amounts. Just 1.5 percent of an animal's weight can be consumed with fatal results. If half that amount is consumed, animals will show symptoms often develop

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