Timothy Hay is a high quality food for rabbits and some other rodents. You are fine with it as the main diet for your rabbits. If you want to make sure your pet rabbit has a very balanced diet, you may want to try to mix up some different hay.
From what I heard, Orchard grass is good too, so is Bermuda grass. Orchard grass is higher in fiber, and lower in protein. If it is up to me, I wouldn't feed any single type of grass though.
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