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United States Department of Agriculture

Agricultural Research Service

Western Bracken Fern (Pteridium aquilinum)
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Bracken fern has broad, triangular leaves, or fronds. The plant reaches a height of 2 to 4 feet. It grows directly from stout, black, horizontal root stalks.

Where and When It Grows
Bracken fern is widely distributed in many places around the world. Bracken fern grows on burned-over areas, in woodlands and other shaded places, and on hillsides, open pastures, and ranges in sandy on gravelly soils.

The plant starts growth in the early spring and usually remains green until the leaves are killed by frost.

Bracken fern is poisonous to cattle, sheep, and horses; sheep, however, are more resistant. Bracken contains a thiaminase inhibitor that leads to the development of thiamine deficiency in horses that can be remedied by giving thiamine. Research has indicated that bracken fern is also carcinogenic. Milk from cows that graze bracken fern may be hazardous to humans.

Livestock losses have been high in the Pacific Coast States, as well as in the Eastern and Midwestern States and some areas of the Intermountain West.

All portions of the fern - both green and in garvested hay - are poisonous to livestock. The toxin is ptaquiloside, a nor-sesquiterpene glycoside.

How It Affects Livestock
Bracken fern produces different signs of poisoning in calle and sheep than it does in horses. Cattle poisoning often occurs during late summer when other feed in scarce, or when animals are fed hay containing bracken fern. The disease occurs after cows have consumed large amounts of the plant and is manifested in an acute, usually fatal, form. The disease is more cronic in horses.

The poison in bracken fern has a cumulative effect. Livestock are affected only after they have eaten considerable amounts of bracken fern for 2 to 4 weeks.

Cattle are affected by a nor-sesquiterpene glycoside called ptaquiloside, which causes bleeding and damage to the bone marrow. The disease has a delayed onset: Cattle may graze the plant for several weeks and then get sick and die. Poisoned animals seldom recover. If consumed over time, ptaquilside can also cause cancer in the urinary bladder and GI tract. These tumors often bleed, causing red uring (enzootic hematuria or redwater disease).

Bracken fern poisoning in horses can occur when they are fed hay containing about 20 percent bracken fern over a period of 30 days. Signs of poisoning include weight loss, incoordination, and lethargy. Horses may stand with their legs apart as though bracing themselves and may assume a crouching position with an arched back. Muscle tremors develop and the animals is unable to stand despite violent attempts to do so. Death will occur in several days to a week.

 Signs and Lesions of Poisoning

    In cattle and sheep:
    • High fever
    • Loss of appetite
    • Depression
    • Difficulty in breathing
    • Excessive salivation
    • Nasal and rectal bleeding; bloody urine and feces
    • Anemia, leukopenia, thrombocytopenia, and hemorrhagic syndrome
    • Hemorrhages on mucous membranes
    • Aplastic bone marrow
    • Bladder tumors in cattle
    In horses:
    • Loss of weight and condition; emaciation
    • Progressive incoordination
    • Marked depression
    • Crouching stance, back arched with legs apart
    • Twitching muscles
    • General body weakness
    • Weak, fast pulse
    • Inability to stand 
    • Convulsions or spasms
    • Pericardial and epicardial hemorrhage

How to Reduce Losses
Animals seldom eat bracken fern if sufficient forage is available, so grazing should be delayed until adequate forage is available. Young shoots are the most toxic and are relatively palatable in early growth stages.

To eliminate livestock losses, do not overgraze pastures and ranges. Make sure sufficient forage is available at all times to animals in infested areas. If necessary, supplement forage near the end of the grazing period. Do not feed hay contaminated with bracken fern.

Poisoning can be treated with thiamine hydrochloride, saline cathartics, and possibly activated charcoal. Few cattle have recovered after signs of acute poisoning appear; hoewver, horses in early stages of poisoning may be saved by intravenous injections of thiamine hydrochloride. The thiamine hydrochloride treatment should be given under the direction of your local veterinarian.

Bracken fern can be controlled. In areas where cultivation is practical, the plants can be destroyed by cultivating the soil for 2 to 3 years.


Last Modified: 2/24/2012