Read this Wikipedia article carefully:
http://en.wikipedia.org/wiki/Delphinium

Note the final sentence on larkspur:
Larkspur, especially tall larkspur, is a significant cause of cattle poisoning on rangelands in the western United States. Larkspur is more common in high-elevation areas, and many ranchers will delay moving cattle onto such ranges until late summer when the toxicity of the plants is reduced.

Thank you gardenbug. It's so nice to have help from the members of this forum.