I feed all of my chins unlimited timothy hay every day. I have one pair that came to me already eating orchard grass. SO I have kept the tradition going and they still get a small handful of their orchard grass every night. But they also get unlimited timothy hay.
Some of the time I give pinchfuls of the orchard grass to my other chins for a treat, they like a little variation now & then!
If it were me I would not substitute their timothy hay for orchard grass though.