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Extension > Agriculture > Livestock > Horse > Nutrition and forages > Poisonous plants

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Poisonous plants

Krishona Martinson, PhD, University of Minnesota

The key to preventing problems with poisonous plants is proper identification and avoidance of these plants. Examine pastures, hay fields, roadsides and fence rows for poisonous plants. In a drought year, or a year when feed is short, take extra precautions, and look for these plants in new areas planned for grazing or haying. Horses, under conditions of adequate feed, will avoid most poisonous plants. However, when feed is short, or horses are hungry, plants normally avoided become a tempting source of feed, thus a potential poisoning problem.

When a horse goes off feed, loses weight, colics, or appears unhealthy, poisonous plants may be the cause. Poisonous plants contain toxic compounds which can injure horses or kill, even in small doses. Others contain substances which cause a reduction in performance, such as weight loss, weakness, rapid pulse, or recumbency. Poisonous plants should be considered as the potential cause of disease, especially if the following situations exist:

  1. Forage supply in a pasture is sparse due to overgrazing, drought, or poor early season growth.
  2. Animals have recently been moved into a new pasture.
  3. Animals have been released into a new pasture when hungry.
  4. Herbicides have recently been used to control weeds.
  5. Pasture has recently been fertilized with nitrogen.
  6. A new forage source (i.e. hay or pasture) has been fed.

Some herbicides may increase the palatability of these weeds. Therefore, it is important to read the herbicide label and follow all grazing restrictions. Also, if there are poisonous plants in the pasture, it is best to keep all livestock out until the plants have died and the grazing restriction has passed. Other management tips to avoid problems include:

Additional information can be found in the book "Plants Poisonous or Harmful to Horses in the North Central United States". The book is available at:

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