Paul, Roundup isn't the best choice for things that have to be chainsawed. Try Remedy. The best way I have found to use Remedy on a large tree is to bore holes in the trunk. Bore downward at an angle. Use an old time brace and bit or battery operated drill motor if you are away from electricity. I like to use a one inch bit. Then pour a mixture of 1/3 Remedy and 2/3 diesel into the new hole(s). That seems to work better than spraying as a foliar drench or on the trunk.