Watch for Poisonous Weeds in Hay

Watch for Poisonous Weeds in Hay

Photo: The Horse Staff

Most weeds are not palatable and will be avoided by pastured livestock if adequate forage is available. However, in hay, most livestock cannot differentiate weeds from beneficial long-stemmed forage, which can result in accidental ingestion and possibly a loss in performance or even death.

Three weeds commonly found in the upper Midwest that remain toxic when dried in hay are hoary alyssum, wild parsnip, and poison hemlock.

Hoary alyssum is toxic only to horses. It's a perennial weed that is commonly found in pastures and hay fields after areas experience drought. Hoary alyssum is light green to gray in color with white flowers. The seeds are small and oblong and easily seen in baled hay. Horses that ingest hoary alyssum might experience stocking-up or swelling of the limbs, founder, and even death. The toxic dose of hoary alyssum is estimated at 20% (of the plant ingested) in hay, but is known to affect some horses differently. Some horses have a zero tolerance to hoary alyssum.

Wild parsnip is a biennial that is toxic to cattle, horses, and sheep and is infesting ditches and fields in Minnesota. Leaves are course, with saw-toothed edges. Flowers are yellow, and umbrella-shaped. Wild parsnip can contain a toxin called furanocoumarins and can create severe skin irritations. High levels of the toxin have been found in all parts of the plant, including the seeds. The toxic dose of wild parsnip is not known. Signs and effects of toxicity include severe sunburn (photosensitivity). If you suspect wild parsnip toxicity, remove the plant source. Move all affected animals to a shaded area. A topical treatment can be applied to skin lesions. Since wild parsnip is commonly found in ditches, hay harvested from ditches containing wild parsnip should not be fed to livestock.

Poison hemlock is found in wet sites or along streams. Poison hemlock is a biennial that produces a rosette of leaves near the ground in the first year of growth, followed by an erect, flowering stalk the second year. Leaves have a lacy appearance and smell like parsnip when crushed. Flowers are white and are borne in umbrella shaped clusters. The tap root resembles a small white carrot. Poison hemlock contains alkaloids that are toxic to cattle and horses when 0.5% and 0.25% respectively of body weight are ingested. Clinical signs include death, salivation, and excitement. Treatment is rarely possible.

Remember to work with a veterinarian if you suspect a plant poisoning. Several clinical signs associated with plant toxicity can also be associated with other severe and deadly disorders. Evidence of poisonous plant ingestion, clinical signs, and diagnostic tests should be used to confirm poisoning from a toxic plant.

More information and photos of toxic plants.

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