Bracken fern is poisonous to cattle, sheep, and horses, but sheep are more resistant. Bracken contains a thiaminase, which leads to the development of thiamine deficiency in horses that can be remedied by giving thiamine.

Ruminants are less susceptible to the effects of the thiaminase, but other bracken toxins (such as ptaquiloside) affect them. Cattle poisoning often occurs in spring when young shoots sprout and during late summer when other feed is scarce. Livestock may also be poisoned when animals are fed hay containing bracken fern. The disease occurs in cattle after they have consumed large amounts of the plant and is manifested in an acute, usually fatal, form. The disease is more chronic in horses.

All portions of the fern, both green and dry stages, are poisonous to livestock. Bracken fern has broad, triangular leaves, or fronds. The plant reaches a height of 0.5 to 1.5 meters. It grows directly from stout, black, horizontal rootstalks. Bracken fern is a perennial in the fern family that can dominate certain plant communities, especially on shaded hillsides.

Where and When It Grows
Bracken fern is widely distributed in many parts of the world. One toxin that has been identified is ptaquiloside, a norsesquiterpene glucoside. Bracken fern grows on burned-over areas, in woodlands and other shaded places, and on hillsides, open pastures, and ranges in sandy or gravelly soils. Livestock losses have been high in the Pacific Coast States, as well as in the Northeastern, Southeastern, and Midwestern States. It starts growth in the early spring and usually remains green until the leaves are killed by frost.

How It Affects Livestock
Cattle are affected only after they have eaten considerable amounts of bracken fern for 2 to 4 weeks. Some animals will continue to develop signs and die even after they are removed from infested pastures. Poisoned cattle and sheep initially appear normal; clinical signs and often death occur suddenly. The disease is of short duration and no treatments have proven effective in reversing the process. Most poisoned cattle die. 

Horses that eat bracken fern develop signs more slowly, and the disease becomes chronic. Thiamine therapy has been effective and if affected horses are not treated, they will eventually die as a result of bracken fern poisoning.

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
• Animal unable to stand
• Convulsions or spasms
• Pericardial and epicardial hemorrhage

How to Reduce Losses
Animals seldom eat bracken fern if sufficient forage is available. To eliminate livestock losses, do not overgraze pastures and ranges. Delay grazing in the spring until grass is abundant. 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. Few cattle have recovered after signs of acute poisoning appear; however, 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.