We've grown birdsfoot trefoil for a dozen years or more at Ames. We've watched it perform on different soils, in different parts of the state, with and without soil treatments, alone and with different grasses.
Hughes, H. D. and Heath, Maurice E.
"Birdsfoot Trefoil for Permanent Pastures,"
Iowa Farm Science: Vol. 3
, Article 2.
Available at: https://lib.dr.iastate.edu/farmscience/vol3/iss9/2