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.