You often see them on the side of the highway: tall, bright colored weeds engulfing the hillsides. But these little weeds are causing big problems for Oregon's livestock.

"A plant like this can kill a horse," says weed expert Carol Mallory-Smith, pointing to a tansy ragwort flower. "It can harm animal health if it's poisonous."

According to agriculture experts, many of Oregon's pastures have noxious weeds such as spotten knapweed, bull thistle and the highly toxic tansy ragwort. Experts say some plants can cause choking and even death if consumed by livestock like horses and cows.

"We receive reports of about 100 animals each year," adds weed expert Andy Hulting. "So we know it's a continuing problem."

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