Cattle deaths from oleander poisonings near urban fence lines are a rising concern, according to agricultural researchers. Twenty-five Holstein cattle in one Northern California county died last year from consuming the popular landscaping plant. One 18-month-old heifer died recently after eating backyard oleander plants hanging near a pasture fence. Frank Galey, a toxicologist at the UC Davis-based California Veterinary Diagnostic Laboratory Systems and a specialist in large-animal poisonings, says oleander found throughout California is deadly to horses and cattle. Oleander leaves, when dried, undergo a chemical change and produce a toxin that fatally alters the animal's heartbeat rhythm. Because cattle are so sensitive to oleander, Galey cautions urban homeowners not to dump their yard clippings over the back fence, where they might be eaten by grazing livestock.