8 week old lambs will survive on grass alone, and the shorter the grass, generally (ie 1"+ in length), the more nutrition there is in it.
But they will do better if you give them a supplement as well. They will also do better with a mineral block, and only take what they need.
If they are mainly in the yard, then it's also an idea to give them some hay as they should have food available at all time.
By far the cheapest and easiest way to raise them is on grass.