What kind of hay? Is the field chemically treated with herbicides and pesticides? Is it grazed, or is the hay cut, baled, and removed?
The best
answer (from your point of view) would be if it was an alfalfa field that had very little in the way of agricultural operations done except to cut and harvest it a few times a year. Even better if the animals are let in to graze it, because then the nutrients are retained to a greater degree. If that's the case, then the soil life
should be pretty intact and you won't have to do much preparation to plant your trees.
I've seen pecan orchards that grow a clover crop as a ground cover and the clover provides some seasonal grazing for
cattle. That's a fairly
sustainable permaculture type system, save for the fact that you don't have a lot of species diversity. What could be done with that to make it even more sustainable would be to (1) add some diversity to the cover crop so that there will be something to graze all year, and (2) as pecan trees need to be taken out, you replace them with a diversity of tree species.
