It's apparently not that easy to separate Silphium integrifolium and Silphium radula. The key I have says that S. integrifolium has stems that may be smooth or hairy. If the stems are hairy, the hairs are less than 1mm long. It says S. radula typically has hairy stems and the hairs are1 to 2 mm long. I can see that your plant has hairy stems. Do you think the hairs are more or less than a millimeter?
To make things more fun, Silphiums are well known for being "promiscuous", they interbreed freely with each other producing intermediate forms. Just guessing from the pictures, it looks like the hairs are more than 1mm long? If so, Janet was right with Silphium radula.
