We have three on about 3/4 acre and we had to cut it, so we could check the sheep were still there! I think the problem is balancing winter needs with the summer abundance of grass. If you stock tightly enough to keep the grass bowling green length in summer (or at least not needing mown), you'll have to feed a of of hay in the winter.
I know thsi isn't realy an answer to your question but we have the same issues.