Forest Preserve District of Cook County, Illinois
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Plants Poisonous to Animals
Nature Bulletin No. 689   October 13, 1962
Forest Preserve District of Cook County
Seymour Simon, President
Roberts Mann, Conservation Editor

Originally, the title of this bulletin was intended to be "Poisonous Plants Eaten by Wildlife", but in all the available literature we found no information and few comments on that subject. One authority, admitting that not much is known about wildlife preferences and aversions for food, correctly states that browsing animals prefer soft succulent leaves to leathery ones, and often pass by plants with bitter or sticky milky juices; but then he adds: "Some poisonous plants seem to be avoided instinctively. " We question that.

In 1946 the University of Illinois published Circular No. 599 about "Illinois Plants Poisonous to Livestock", but it contains no information about their toxic effects upon wildlife. Most cases of livestock poisoning by plants occur in early spring before grasses have become plentiful, or in summer and fall when pastures are dry and brown.

A classic example is the white snakeroot responsible for the death of Abraham Lincoln's mother. It is a leafy waist-high plant that grows in woodlands and bears numerous heads of little white fuzzy flowers in late summer. It is extremely poisonous. If eaten in large quantities the animals die. When browsed continuously in small amounts, they develop "trembles" and their milk may cause death to nursing calves and lambs, or the fatal milk sickness to humans.

Bracken is another example. This tall fern with large coarse fronds is found in open woods and abandoned fields, especially upon sandy and gravelly soils. It is poisonous to cattle and horses that browse it when pastures are poor. Deer nibble on other ferns, and bracken is abundant in northern woods, but apparently no one knows whether they eat bracken, white snakeroot, mushrooms and other poisonous plants, nor what happens if they do.

On the Great Plains and in the Southwest, locoweeds -- several kinds of poisonous vetches -- cause serious losses among horses, cattle and sheep in dry seasons when good forage plants are scarce where locoweeds are abundant. Being legumes, they produce pods of seeds but those are harmless and provide food for quail, turkeys and rodents. Antelope and big game animals feed on the foliage to a limited extent and the authorities say, "Whether or not locoweeds are poisonous to such wildlife is not known. .

The large, glossy brown seeds of the buckeye and horse chestnut trees have been known to poison cattle and hogs but are relished by squirrels. They have poisoned children that ate them but, after roasting, were used as food by Indians.

Birds have a digestive mechanism including enzymes that apparently render certain poisons harmless. They are fond of berries such as those on poison ivy, the little blue ones on English ivy, and the white ones on mistletoe, that are poisonous to people; as well as the crimson berries on Japanese honeysuckle and bittersweet nightshade, that are nauseous to us. It takes a lot of strychnine to kill a bird. However, wild ducks and turkeys are killed sometimes by eating the blue-green algae that form a thick scum or bloom ' on the surface of stagnant ponds.

Most deadly of all native plants are Jimson weed and water hemlock. In the latter its poison is concentrated in the tuberous roots. The foliage is harmless but when cattle browse it in spring they sometimes pull up and eat the tubers -- with fatal results..

Only a hungry hog will eat Jimson weeds.

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