Necropsies confirmed that nine horses from a Carbon County farm died after eating large quantities of a toxic plant that was cut from their pasture and baled into hay.

The saga started about 10 days ago when a couple of horses that lived on a farm in Wellington died unexpectedly, said Dan Harmer, a veterinarian with Price’s Animal Hospital of Eastern Utah.

“I treated quite a few of them,” Harmer said. “We’ve been watching horses die for a couple of weeks now.”

The man who owns the Wellington farm asked Harmer and another veterinarian to investigate. Walking in the alfalfa field from which the horses’ hay was cut, Harmer said, they found the culprit: equisetum.

The plant — also known as horsetail and scouring rush — commonly grows along streams and thrives in moist or wet soil, according to Utah State University Extension’s plant database.

While horsetail is one of about 25 poisonous plants in Utah, finding it in an open field, Harmer said, is unusual. He theorizes it might have spread from a now-dry canal near the field.

However it got into the alfalfa field, it did. And when the farmer cut the grass for hay, the horsetail came with it. Horses won’t normally eat the toxic plant — Harmer said they don’t like the taste — but when it’s dried into the hay they’re eating, they don’t have much of a choice.

Although an antidote exists and Harmer caught the symptoms “about as soon as possible,” he said the horses that ate the largest amounts of equisetum ultimately died.

The farmer told Harmer he wanted others to learn from the experience.

Harmer said the best way to keep grazing animals safe from harmful plants is simple: Check the fields.

“When somebody cuts up hay or puts animals out on pasture, if they don’t know what they’re getting into, they could be poisoning them,” he said.