How do large orchards maintain grass under trees ?

University of California at Davis has determined that grasses produce a chemical that retards the growth of trees. Perhaps that is the reason for spraying.

I looked at local orchards the other day the one that's local is on a good 30% slope or more with alot of grass growing and looks like it's not maintained verywell.

I like the idea of using clover for the ground cover, however, it doesn't really release much of that nitrogen unless it is plowed under or harrowed.