There is another one made by Spectricide that works as good as Roundup, but much faster, and it's MUCH cheaper. Either use the spray bottle, or in larger quantities, wet an old mop with it and drag it along the fence line. It will kill whatever it touches in about 24 hours.
Thanks - I'll think about that one of these times I'd rather be doing something else besides weedwacking!
It only really takes a long time when I've let it go all year like this. Much of that tall grass really doesn't like being cut short and buzzing it all the way to the ground weakens if from coming back very fast.