Supplies of birdsfoot trefoil seed haven't met the demand in Iowa. Seed is scarce and relatively high priced. Seed production can be profitable for those willing to take the precautions necessary for good seed yields.
Thompson, H. E. and Metcalfe, D. S.
"Producing Birdsfoot Trefoil Seed,"
Iowa Farm Science: Vol. 9
, Article 3.
Available at: https://lib.dr.iastate.edu/farmscience/vol9/iss12/3