Hi Ian, researchers at Iowa State University have been playing around with this idea for about 10 years now in a corn-soybean system. They tried kura clover at one of the research farms but it was poorly managed and caused significant yield drag in corn. They have tried several different clover and grass species and settled on Kentucky bluegrass as one that seems to work the best right now. The biggest challenge is early season competition from the understory crop because it absorbs far infrared light that would normally reflect off bare soil and onto the corn crop. This triggers a shade avoidance response (rapid vegetative growth) in corn and ultimately reduces yield. We will be starting a trial next year comparing perennial bluegrass understory to interseeded cover crops in corn. Much more detail is available here: https://www.mdpi.com/2073-4395/9/8/458/htm