A San Diego County ranch owner says 23 show horses have been poisoned by someone who fed them highly toxic oleander leaves.

Debbie Tomin, owner of Rockridge Farm in Rancho Santa Fe, says three horses are seriously ill and are undergoing treatment at an equine hospital.

The other horses, including two pregnant mares, are being treated at the ranch.

Tomin says workers discovered the sick American Saddlebred horses Thursday morning and a feed mix of sliced apples and carrots, which disguised the bitter oleander.

Tomin says someone broke into the ranch overnight. The sheriff's department is investigating.

The ranch has about 30 show horses worth about $2 million. Four are owned by the ranch; the rest are boarded for clients.