How to remove blackberry bushes?

Hello. I rent property for my horses, and the property has extensive blackberry bushes growing among some trees. There is probably about 1 acre covered in brambles. There is no hose nearby or other water source. I have been researching how to remove/reduce the bushes, and have come across conflicting information about how to address the issue. What type of chemical should I use to treat the bushes? Should I use goats, or manually cut back the bushes? Thank you.