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Field horsetail grows in wet soils and is hard to control.
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Brackenfern grows in open pastures and woodlands and is controlled by multiple herbicide treatments.
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A 20 to 25 percent diet of field horsetail and brackenfern for three weeks can cause neurological problems in horses.
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Neurologic signs include unsteady gait, twitching, depression and constipation.
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An initial 0.5 to 1 gram dose of thiamine and decreasing daily doses can treat poisoning.
Field horsetail
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Hollow, wiry and jointed stems
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Has 8 to 12 small, scale-like whorled leaves
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No flowers
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Cone-like structure is borne at the top of the stem and contains spores
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2 to 4 inches tall
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It’s a perennial
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Reproduces by rhizomes (underground stems) and spores (reproductive structures)
Where is it found?
Field horsetail grows throughout the United States except for the southeastern region. You can find it in moist to wet soils that are usually sandy or gravely in texture.
Horsetail is relatively slow-spreading, noncompetitive weed. Once established, field horsetail is hard to control. Few herbicides provide adequate control. Tillage may actually increase plant density by spreading the rhizomes.
Correct drainage problems or fence horses out of wet areas populated with horsetail for the best control.
Brackenfern
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Fern-like leaf
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1 to 4 feet in height
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Spores are borne under each leaf in brownish bands
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It’s a perennial
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Reproduces by rhizomes and spores
Where is it found?
Brackenfern grows throughout the United States. You can find it in open pastures and woodlands, particularly on acid soils
Some herbicides suppress and control brackenfern but may need multiple treatments. Always carefully follow all grazing restrictions and pertinent information on herbicide labels.
Toxicity
Brackenfern and horsetail are toxic when horses eat it fresh (in pasture) or dried in hay. If horses eat a diet with 20 to 25 percent brackenfern or field horsetail for about three weeks, neurological signs may occur. Clinical signs may occur a week to ten days if horses eat a diet nearly 100 percent brackenfern or field horsetail.
Brackenfern contains an enzyme that causes neurological syndrome in horses. Field horsetail affects horses similarly.
Signs of poisoning
Depression
Constipation
Unsteady gait (usually in one to two days)
Clinical signs progress to:
Unsteady gait
Muscle twitching
Going down
Paddling
Seizing
These signs occur for a period of a week or more.
Treatment
Horses will need an initial dose of 0.5 to 1 gram of thiamine. They will then need decreasing daily doses for three to five days.
Reviewed in 2021