Hi John:
Thanks for reaching out to UI Extension Forestry!
As a general rule, I try to avoid providing site-specific forest management recommendations -- especially when it comes to black walnut management.
Therefore, my best advice to you is to contact a local Iowa DNR forester or a local Iowa consulting forester for an on-site property visit with site-specific management recommendations.
Iowa DNR Forestry: https://www.iowadnr.gov/Conservation/Forestry/Forestry-Landowner-Assistance
Best of luck!