“One of the plants that often causes livestock loss is larkspur, and there are several species,” says Jerry Volesky, range and forage specialist at the University of Nebraska Lincoln’s West Central Research and Extension Center. “Here in Nebraska, we have one called prairie larkspur that causes the most problems when blooming. That’s when cattle seem to like it and seek it out.” This is one of the taller larkspur varieties (2 to 3 feet tall), but the flower is light-colored, almost white, rather than blue.

Low larkspur (6 to 24 inches high) has deep blue to purple flowers and grows in dry to moderately moist soil on plains, foothills and mountains. It blooms in early spring and is most poisonous when young. Cattle may eat it even when good forage is available.

Tall larkspur (wild delphinium) has blue flowers and grows 3 to 7 feet tall, with a deep, woody root. It grows in thick stands in moist, shaded areas along streams, springs or aspen groves. It matures and flowers later than low larkspur; poisoning is most common in midsummer.

Poison content is high during early growth and drops when the plant matures, but it is high again in the dry plant. 

“Toxic effects vary from year to year, depending on growth conditions. The amount a cow has to eat before she shows signs of poisoning could vary from half a pound to 2 pounds,” Volesky says.

“Toxic compounds in larkspur are alkaloids. Signs of poisoning include muscle tremors, twitching and collapse,” he says. Animals tend to bloat because the gut shuts down, and gas builds up in the rumen.