George,
They goats work really well for weeds and roughage. If you want them to mow the grass they will not. Sheep will though. I allow my goats to browse the banks for about 2 hours a day before I head off to work. The pond bank was entangled about 6 feet high with briars and saplings. You can see the end result. I burn my banks in the spring and let the goats trim it through the summer. I know manure run off with goats fenced around a pond would probably end up on the unwanted side. Although it would take quite a while to match the manure left by one cow mess. My pond is already to nutrient rich and very fertile so I did not fence them in there. I do not feed my goats until after they have done thier chores around the pond. They know excactly were to go and love it. They can mow down some serious vegetation in short time. I keep them penned all but the 2 hours to keep them safe for my goats are pygmy goats and can fall prey to the nice neighbor dog easily. The goats make easily cared for pets. The kids love them and they save me time on the trimming.
