It sounds like you're converting a forest to a pasture. Thanks for asking about organic methods before turning to chemical - the pasture will be much healthier for it.
If you're cutting down all of these trees then you'll be dealing with managing the stumps so they don't send out suckers and keep growing back. As a starting place, take a look at the
stump management link in the
Natural Organic Library.
Where are you? You can look in our
Products and Places page to search for organic vendors and services in your area that might help as a resource in this effort.
Good luck!